/src/tomlplusplus/include/toml++/impl/parser.inl

Source
//# This file is a part of toml++ and is subject to the the terms of the MIT license.
//# Copyright (c) Mark Gillard <mark.gillard@outlook.com.au>
//# See https://github.com/marzer/tomlplusplus/blob/master/LICENSE for the full license text.
// SPDX-License-Identifier: MIT
#pragma once

#include "preprocessor.hpp"
//# {{
#if !TOML_IMPLEMENTATION
#error This is an implementation-only header.
#endif
//# }}
#if TOML_ENABLE_PARSER

#include "parser.hpp"
#include "std_optional.hpp"
#include "source_region.hpp"
#include "parse_error.hpp"
#include "date_time.hpp"
#include "value.hpp"
#include "array.hpp"
#include "table.hpp"
#include "unicode.hpp"
TOML_DISABLE_WARNINGS;
#include <istream>
#include <fstream>
#if TOML_INT_CHARCONV || TOML_FLOAT_CHARCONV
#include <charconv>
#endif
#if !TOML_INT_CHARCONV || !TOML_FLOAT_CHARCONV
#include <sstream>
#endif
#if !TOML_INT_CHARCONV
#include <iomanip>
#endif
TOML_ENABLE_WARNINGS;
#include "header_start.hpp"

//#---------------------------------------------------------------------------------------------------------------------
//# UTF8 STREAMS
//#---------------------------------------------------------------------------------------------------------------------

TOML_ANON_NAMESPACE_START
{
  template <typename T>
  class utf8_byte_stream;

  TOML_INTERNAL_LINKAGE
  constexpr auto utf8_byte_order_mark = "\xEF\xBB\xBF"sv;

  template <typename Char>
  class utf8_byte_stream<std::basic_string_view<Char>>
  {
    static_assert(sizeof(Char) == 1);

    private:
    std::basic_string_view<Char> source_;
    size_t position_ = {};

    public:
    TOML_NODISCARD_CTOR
    explicit constexpr utf8_byte_stream(std::basic_string_view<Char> sv) noexcept //
      : source_{ sv }
    {
      // skip bom
      if (source_.length() >= 3u && memcmp(utf8_byte_order_mark.data(), source_.data(), 3u) == 0)
        position_ += 3u;
    }

    TOML_CONST_INLINE_GETTER
    constexpr bool error() const noexcept
    {
      return false;
    }

    TOML_PURE_INLINE_GETTER
    constexpr bool eof() const noexcept
    {
      return position_ >= source_.length();
    }

    TOML_PURE_INLINE_GETTER
    explicit constexpr operator bool() const noexcept
    {
      return !eof();
    }

    TOML_PURE_INLINE_GETTER
    constexpr bool peek_eof() const noexcept
    {
      return eof();
    }

    TOML_NODISCARD
    TOML_ATTR(nonnull)
    size_t operator()(void* dest, size_t num) noexcept
    {
      TOML_ASSERT_ASSUME(!eof());

      num = impl::min(position_ + num, source_.length()) - position_;
      std::memcpy(dest, source_.data() + position_, num);
      position_ += num;
      return num;
    }
  };

  template <>
  class utf8_byte_stream<std::istream>
  {
    private:
    std::istream* source_;

    public:
    TOML_NODISCARD_CTOR
    explicit utf8_byte_stream(std::istream& stream) noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) //
      : source_{ &stream }
    {
      if (!*this) // eof, bad
        return;

      const auto initial_pos = source_->tellg();
      char bom[3];
      source_->read(bom, 3);
      if (source_->bad() || (source_->gcount() == 3 && memcmp(utf8_byte_order_mark.data(), bom, 3u) == 0))
        return;

      source_->clear();
      source_->seekg(initial_pos, std::istream::beg);
    }

    TOML_PURE_INLINE_GETTER
    bool error() const noexcept
    {
      return !!(source_->rdstate() & std::istream::badbit);
    }

    TOML_PURE_INLINE_GETTER
    bool eof() const noexcept
    {
      return !!(source_->rdstate() & std::istream::eofbit);
    }

    TOML_PURE_INLINE_GETTER
    explicit operator bool() const noexcept
    {
      return !(source_->rdstate() & (std::istream::badbit | std::istream::eofbit));
    }

    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
    {
      return eof() || source_->peek() == std::istream::traits_type::eof();
    }

    TOML_NODISCARD
    TOML_ATTR(nonnull)
    size_t operator()(void* dest, size_t num) noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
    {
      TOML_ASSERT(*this);

      source_->read(static_cast<char*>(dest), static_cast<std::streamsize>(num));
      return static_cast<size_t>(source_->gcount());
    }
  };

  struct utf8_codepoint
  {
    char32_t value;
    char bytes[4];
    size_t count;
    source_position position;

    TOML_PURE_INLINE_GETTER
    constexpr operator const char32_t&() const noexcept
    {
      return value;
    }

    TOML_PURE_INLINE_GETTER
    constexpr const char32_t& operator*() const noexcept
    {
      return value;
    }
  };
  static_assert(std::is_trivially_default_constructible_v<utf8_codepoint> && std::is_trivially_copyable_v<utf8_codepoint>);
  static_assert(std::is_standard_layout_v<utf8_codepoint>);

  struct TOML_ABSTRACT_INTERFACE utf8_reader_interface
  {
    TOML_NODISCARD
    virtual const source_path_ptr& source_path() const noexcept = 0;

    TOML_NODISCARD
    virtual const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) = 0;

    TOML_NODISCARD
    virtual bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) = 0;

#if !TOML_EXCEPTIONS

    TOML_NODISCARD
    virtual optional<parse_error>&& error() noexcept = 0;

#endif

    virtual ~utf8_reader_interface() noexcept = default;
  };

#if TOML_EXCEPTIONS
#define utf8_reader_error(...)        throw parse_error(__VA_ARGS__)
#define utf8_reader_return_after_error(...) static_assert(true)
#define utf8_reader_error_check(...)    static_assert(true)
#else
#define utf8_reader_error(...)        err_.emplace(__VA_ARGS__)
#define utf8_reader_return_after_error(...) return __VA_ARGS__
#define utf8_reader_error_check(...)                                                                                   \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(err_)                                                                                         \
      return __VA_ARGS__;                                                                                        \
  }                                                                                                                  \
  while (false)

#endif

#if defined(__APPLE__) || defined(__MINGW32__) || defined(__MINGW64__)
#define TOML_OVERALIGNED
#else
#define TOML_OVERALIGNED alignas(32)
#endif

  template <typename T>
  class TOML_EMPTY_BASES utf8_reader final : public utf8_reader_interface
  {
    private:
    static constexpr size_t block_capacity = 32;
    utf8_byte_stream<T> stream_;
    source_position next_pos_ = { 1, 1 };

    impl::utf8_decoder decoder_;
    struct currently_decoding_t
    {
      char bytes[4];
      size_t count;
    } currently_decoding_;

    struct codepoints_t
    {
      TOML_OVERALIGNED utf8_codepoint buffer[block_capacity];
      size_t current;
      size_t count;
    } codepoints_;

    source_path_ptr source_path_;

#if !TOML_EXCEPTIONS
    optional<parse_error> err_;
#endif

    bool read_next_block() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
    {
      TOML_ASSERT(stream_);

      TOML_OVERALIGNED char raw_bytes[block_capacity];
      size_t raw_bytes_read;

      // read the next raw (encoded) block in from the stream
      if constexpr (noexcept(stream_(raw_bytes, block_capacity)) || !TOML_EXCEPTIONS)
      {
        raw_bytes_read = stream_(raw_bytes, block_capacity);
      }
#if TOML_EXCEPTIONS
      else
      {
        try
        {
          raw_bytes_read = stream_(raw_bytes, block_capacity);
        }
        catch (const std::exception& exc)
        {
          throw parse_error{ exc.what(), next_pos_, source_path_ };
        }
        catch (...)
        {
          throw parse_error{ "An unspecified error occurred", next_pos_, source_path_ };
        }
      }
#endif // TOML_EXCEPTIONS

      // handle a zero-byte read
      if TOML_UNLIKELY(!raw_bytes_read)
      {
        if (stream_.eof())
        {
          // EOF only sets the error state if the decoder wants more input, otherwise
          // a zero-byte read might have just caused the underlying stream to realize it's exhaused and set
          // the EOF flag, and that's totally fine
          if (decoder_.needs_more_input())
            utf8_reader_error("Encountered EOF during incomplete utf-8 code point sequence",
                      next_pos_,
                      source_path_);
        }
        else
        {
          utf8_reader_error("Reading from the underlying stream failed - zero bytes read",
                    next_pos_,
                    source_path_);
        }
        return false;
      }

      TOML_ASSERT_ASSUME(raw_bytes_read);
      std::memset(&codepoints_, 0, sizeof(codepoints_));

      // helper for calculating decoded codepoint line+cols
      const auto calc_positions = [&]() noexcept
      {
        for (size_t i = 0; i < codepoints_.count; i++)
        {
          auto& cp  = codepoints_.buffer[i];
          cp.position = next_pos_;

          if (cp == U'\n')
          {
            next_pos_.line++;
            next_pos_.column = source_index{ 1 };
          }
          else
            next_pos_.column++;
        }
      };

      // decide whether we need to use the UTF-8 decoder or if we can treat this block as plain ASCII
      const auto ascii_fast_path = !decoder_.needs_more_input() && impl::is_ascii(raw_bytes, raw_bytes_read);

      // ASCII fast-path
      if (ascii_fast_path)
      {
        decoder_.reset();
        currently_decoding_.count = {};

        codepoints_.count = raw_bytes_read;
        for (size_t i = 0; i < codepoints_.count; i++)
        {
          auto& cp  = codepoints_.buffer[i];
          cp.value  = static_cast<char32_t>(raw_bytes[i]);
          cp.bytes[0] = raw_bytes[i];
          cp.count  = 1u;
        }
      }

      // UTF-8 slow-path
      else
      {
        // helper for getting precise error location
        const auto error_pos = [&]() noexcept -> const source_position&
        { //
          return codepoints_.count ? codepoints_.buffer[codepoints_.count - 1u].position : next_pos_;
        };

        for (size_t i = 0; i < raw_bytes_read; i++)
        {
          decoder_(static_cast<uint8_t>(raw_bytes[i]));
          if TOML_UNLIKELY(decoder_.error())
          {
            calc_positions();
            utf8_reader_error("Encountered invalid utf-8 sequence", error_pos(), source_path_);
            utf8_reader_return_after_error(false);
          }

          currently_decoding_.bytes[currently_decoding_.count++] = raw_bytes[i];

          if (decoder_.has_code_point())
          {
            auto& cp = codepoints_.buffer[codepoints_.count++];

            cp.value = decoder_.codepoint;
            cp.count = currently_decoding_.count;
            std::memcpy(cp.bytes, currently_decoding_.bytes, currently_decoding_.count);
            currently_decoding_.count = {};
          }
          else if TOML_UNLIKELY(currently_decoding_.count == 4u)
          {
            calc_positions();
            utf8_reader_error("Encountered overlong utf-8 sequence", error_pos(), source_path_);
            utf8_reader_return_after_error(false);
          }
        }
        if TOML_UNLIKELY(decoder_.needs_more_input() && stream_.eof())
        {
          calc_positions();
          utf8_reader_error("Encountered EOF during incomplete utf-8 code point sequence",
                    error_pos(),
                    source_path_);
          utf8_reader_return_after_error(false);
        }
      }

      TOML_ASSERT_ASSUME(codepoints_.count);
      calc_positions();

      // handle general I/O errors
      // (down here so the next_pos_ benefits from calc_positions())
      if TOML_UNLIKELY(stream_.error())
      {
        utf8_reader_error("An I/O error occurred while reading from the underlying stream",
                  next_pos_,
                  source_path_);
        utf8_reader_return_after_error(false);
      }

      return true;
    }

    public:
    template <typename U, typename String = std::string_view>
    TOML_NODISCARD_CTOR
    explicit utf8_reader(U&& source, String&& source_path = {}) noexcept(
      std::is_nothrow_constructible_v<utf8_byte_stream<T>, U&&>)
      : stream_{ static_cast<U&&>(source) }
    {
      currently_decoding_.count = {};

      codepoints_.current = {};
      codepoints_.count = {};

      if (!source_path.empty())
        source_path_ = std::make_shared<const std::string>(static_cast<String&&>(source_path));
    }

    TOML_PURE_INLINE_GETTER
    const source_path_ptr& source_path() const noexcept final
    {
      return source_path_;
    }

    TOML_NODISCARD
    const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) final
    {
      utf8_reader_error_check({});

      if (codepoints_.current == codepoints_.count)
      {
        if TOML_UNLIKELY(!stream_ || !read_next_block())
          return nullptr;

        TOML_ASSERT_ASSUME(!codepoints_.current);
      }
      TOML_ASSERT_ASSUME(codepoints_.count);
      TOML_ASSERT_ASSUME(codepoints_.count <= block_capacity);
      TOML_ASSERT_ASSUME(codepoints_.current < codepoints_.count);

      return &codepoints_.buffer[codepoints_.current++];
    }

    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS) final
    {
      return stream_.peek_eof();
    }

#if !TOML_EXCEPTIONS

    TOML_NODISCARD
    optional<parse_error>&& error() noexcept final
    {
      return std::move(err_);
    }

#endif
  };

  template <typename Char>
  utf8_reader(std::basic_string_view<Char>, std::string_view) -> utf8_reader<std::basic_string_view<Char>>;
  template <typename Char>
  utf8_reader(std::basic_string_view<Char>, std::string&&) -> utf8_reader<std::basic_string_view<Char>>;
  template <typename Char>
  utf8_reader(std::basic_istream<Char>&, std::string_view) -> utf8_reader<std::basic_istream<Char>>;
  template <typename Char>
  utf8_reader(std::basic_istream<Char>&, std::string&&) -> utf8_reader<std::basic_istream<Char>>;

#if TOML_EXCEPTIONS
#define utf8_buffered_reader_error_check(...) static_assert(true)
#else
#define utf8_buffered_reader_error_check(...)                                                                          \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(reader_.error())                                                                              \
      return __VA_ARGS__;                                                                                        \
  }                                                                                                                  \
  while (false)

#endif

  class TOML_EMPTY_BASES utf8_buffered_reader
  {
    public:
    static constexpr size_t max_history_length = 128;

    private:
    static constexpr size_t history_buffer_size = max_history_length - 1; //'head' is stored in the reader
    utf8_reader_interface& reader_;
    struct
    {
      utf8_codepoint buffer[history_buffer_size];
      size_t count, first;
    } history_          = {};
    const utf8_codepoint* head_ = {};
    size_t negative_offset_   = {};

    public:
    TOML_NODISCARD_CTOR
    explicit utf8_buffered_reader(utf8_reader_interface& reader) noexcept //
      : reader_{ reader }
    {}

    TOML_PURE_INLINE_GETTER
    const source_path_ptr& source_path() const noexcept
    {
      return reader_.source_path();
    }

    TOML_NODISCARD
    const utf8_codepoint* read_next() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
    {
      utf8_buffered_reader_error_check({});

      if (negative_offset_)
      {
        negative_offset_--;

        // an entry negative offset of 1 just means "replay the current head"
        if (!negative_offset_)
          return head_;

        // otherwise step back into the history buffer
        else
          return history_.buffer
             + ((history_.first + history_.count - negative_offset_) % history_buffer_size);
      }
      else
      {
        // first character read from stream
        if TOML_UNLIKELY(!history_.count && !head_)
          head_ = reader_.read_next();

        // subsequent characters and not eof
        else if (head_)
        {
          if TOML_UNLIKELY(history_.count < history_buffer_size)
            history_.buffer[history_.count++] = *head_;
          else
            history_.buffer[(history_.first++ + history_buffer_size) % history_buffer_size] = *head_;

          head_ = reader_.read_next();
        }

        return head_;
      }
    }

    TOML_NODISCARD
    const utf8_codepoint* step_back(size_t count) noexcept
    {
      utf8_buffered_reader_error_check({});

      TOML_ASSERT_ASSUME(history_.count);
      TOML_ASSERT_ASSUME(negative_offset_ + count <= history_.count);

      negative_offset_ += count;

      return negative_offset_
           ? history_.buffer + ((history_.first + history_.count - negative_offset_) % history_buffer_size)
           : head_;
    }

    TOML_NODISCARD
    bool peek_eof() const noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
    {
      return reader_.peek_eof();
    }

#if !TOML_EXCEPTIONS

    TOML_NODISCARD
    optional<parse_error>&& error() noexcept
    {
      return reader_.error();
    }

#endif
  };
}
TOML_ANON_NAMESPACE_END;

//#---------------------------------------------------------------------------------------------------------------------
//# PARSER INTERNAL IMPLEMENTATION
//#---------------------------------------------------------------------------------------------------------------------

#if TOML_EXCEPTIONS
#define TOML_RETURNS_BY_THROWING [[noreturn]]
#else
#define TOML_RETURNS_BY_THROWING
#endif

TOML_ANON_NAMESPACE_START
{
  template <typename... T>
  TOML_CONST_GETTER
  TOML_INTERNAL_LINKAGE
  constexpr bool is_match(char32_t codepoint, T... vals) noexcept
  {
    static_assert((std::is_same_v<char32_t, T> && ...));
    return ((codepoint == vals) || ...);
  }

  template <uint64_t>
  struct parse_integer_traits;
  template <>
  struct parse_integer_traits<2>
  {
    static constexpr auto scope_qualifier  = "binary integer"sv;
    static constexpr auto is_digit       = impl::is_binary_digit;
    static constexpr auto is_signed      = false;
    static constexpr auto max_digits     = 63;
    static constexpr auto prefix_codepoint = U'b';
    static constexpr auto prefix       = "b"sv;
    static constexpr auto full_prefix    = "0b"sv;
  };
  template <>
  struct parse_integer_traits<8>
  {
    static constexpr auto scope_qualifier  = "octal integer"sv;
    static constexpr auto is_digit       = impl::is_octal_digit;
    static constexpr auto is_signed      = false;
    static constexpr auto max_digits     = 21; // strlen("777777777777777777777")
    static constexpr auto prefix_codepoint = U'o';
    static constexpr auto prefix       = "o"sv;
    static constexpr auto full_prefix    = "0o"sv;
  };
  template <>
  struct parse_integer_traits<10>
  {
    static constexpr auto scope_qualifier = "decimal integer"sv;
    static constexpr auto is_digit      = impl::is_decimal_digit;
    static constexpr auto is_signed     = true;
    static constexpr auto max_digits    = 19; // strlen("9223372036854775807")
    static constexpr auto full_prefix   = ""sv;
  };
  template <>
  struct parse_integer_traits<16>
  {
    static constexpr auto scope_qualifier  = "hexadecimal integer"sv;
    static constexpr auto is_digit       = impl::is_hexadecimal_digit;
    static constexpr auto is_signed      = false;
    static constexpr auto max_digits     = 16; // strlen("7FFFFFFFFFFFFFFF")
    static constexpr auto prefix_codepoint = U'x';
    static constexpr auto prefix       = "x"sv;
    static constexpr auto full_prefix    = "0x"sv;
  };

  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(node_type val) noexcept
  {
    return impl::node_type_friendly_names[impl::unwrap_enum(val)];
  }

  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const std::string& str) noexcept
  {
    return std::string_view{ str };
  }

  TOML_CONST_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(bool val) noexcept
  {
    using namespace std::string_view_literals;

    return val ? "true"sv : "false"sv;
  }

  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const utf8_codepoint& cp) noexcept
  {
    if (cp.value <= U'\x1F')
      return impl::control_char_escapes[cp.value];
    else if (cp.value == U'\x7F')
      return "\\u007F"sv;
    else
      return std::string_view{ cp.bytes, cp.count };
  }

  TOML_PURE_GETTER
  TOML_INTERNAL_LINKAGE
  std::string_view to_sv(const utf8_codepoint* cp) noexcept
  {
    if (cp)
      return to_sv(*cp);
    return ""sv;
  }

  struct escaped_codepoint
  {
    const utf8_codepoint& cp;
  };

  template <typename T>
  TOML_ATTR(nonnull)
  TOML_INTERNAL_LINKAGE
  void concatenate(char*& write_pos, char* const buf_end, const T& arg) noexcept
  {
    if TOML_UNLIKELY(write_pos >= buf_end)
      return;

    using arg_type = impl::remove_cvref<T>;

    // string views
    if constexpr (std::is_same_v<arg_type, std::string_view>)
    {
      const auto max_chars = static_cast<size_t>(buf_end - write_pos);
      const auto len     = max_chars < arg.length() ? max_chars : arg.length();
      std::memcpy(write_pos, arg.data(), len);
      write_pos += len;
    }

    // doubles
    else if constexpr (std::is_same_v<arg_type, double>)
    {
#if TOML_FLOAT_CHARCONV
      const auto result = std::to_chars(write_pos, buf_end, arg);
      write_pos     = result.ptr;
#else
      std::ostringstream ss;
      ss.imbue(std::locale::classic());
      ss.precision(std::numeric_limits<arg_type>::max_digits10);
      ss << arg;
      concatenate(write_pos, buf_end, to_sv(std::move(ss).str()));
#endif
    }

    // 64-bit integers
    else if constexpr (impl::is_one_of<arg_type, int64_t, uint64_t>)
    {
#if TOML_INT_CHARCONV
      const auto result = std::to_chars(write_pos, buf_end, arg);
      write_pos     = result.ptr;
#else
      std::ostringstream ss;
      ss.imbue(std::locale::classic());
      using cast_type = std::conditional_t<std::is_signed_v<arg_type>, int64_t, uint64_t>;
      ss << static_cast<cast_type>(arg);
      concatenate(write_pos, buf_end, to_sv(std::move(ss).str()));
#endif
    }

    // escaped_codepoint
    else if constexpr (std::is_same_v<arg_type, escaped_codepoint>)
    {
      if (arg.cp.value <= U'\x7F')
        concatenate(write_pos, buf_end, to_sv(arg.cp));
      else
      {
        auto val      = static_cast<uint_least32_t>(arg.cp.value);
        const auto digits = val > 0xFFFFu ? 8u : 4u;
        constexpr auto mask = uint_least32_t{ 0xFu };
        char buf[10]    = { '\\', digits > 4 ? 'U' : 'u' };
        for (auto i = 2u + digits; i-- > 2u;)
        {
          const auto hexdig = val & mask;
          buf[i]        = static_cast<char>(hexdig >= 0xAu ? ('A' + (hexdig - 0xAu)) : ('0' + hexdig));
          val >>= 4;
        }
        concatenate(write_pos, buf_end, std::string_view{ buf, digits + 2u });
      }
    }

    // all other floats (fallback - coerce to double)
    else if constexpr (std::is_floating_point_v<arg_type>)
      concatenate(write_pos, buf_end, static_cast<double>(arg));

    // all other integers (fallback - coerce to (u)int64_t)
    else if constexpr (std::is_arithmetic_v<arg_type> && std::is_integral_v<arg_type>)
    {
      using cast_type = std::conditional_t<std::is_unsigned_v<arg_type>, uint64_t, int64_t>;
      concatenate(write_pos, buf_end, static_cast<cast_type>(arg));
    }

    else
    {
      static_assert(
        impl::always_false<T>,
        "concatenate() inputs are limited to std::string_views, integers, floats, and escaped_codepoint");
    }
  }

  struct error_builder
  {
    static constexpr std::size_t buf_size = 512;
    char buf[buf_size];
    char* write_pos       = buf;
    char* const max_write_pos = buf + (buf_size - std::size_t{ 1 }); // allow for null terminator

    TOML_NODISCARD_CTOR
    error_builder(std::string_view scope) noexcept
    {
      concatenate(write_pos, max_write_pos, "Error while parsing "sv);
      concatenate(write_pos, max_write_pos, scope);
      concatenate(write_pos, max_write_pos, ": "sv);
    }

    template <typename T>
    void append(const T& arg) noexcept
    {
      concatenate(write_pos, max_write_pos, arg);
    }

    TOML_RETURNS_BY_THROWING
    auto finish(const source_position& pos, const source_path_ptr& source_path) const
    {
      *write_pos = '\0';

#if TOML_EXCEPTIONS
      throw parse_error{ buf, pos, source_path };
#else
      return parse_error{ std::string(buf, static_cast<size_t>(write_pos - buf)), pos, source_path };
#endif
    }

    TOML_DELETE_DEFAULTS(error_builder);
  };

  struct parse_scope
  {
    std::string_view& storage_;
    std::string_view parent_;

    TOML_NODISCARD_CTOR
    explicit parse_scope(std::string_view& current_scope, std::string_view new_scope) noexcept
      : storage_{ current_scope },
        parent_{ current_scope }
    {
      storage_ = new_scope;
    }

    ~parse_scope() noexcept
    {
      storage_ = parent_;
    }

    TOML_DELETE_DEFAULTS(parse_scope);
  };
#define push_parse_scope_2(scope, line) parse_scope ps_##line(current_scope, scope)
#define push_parse_scope_1(scope, line) push_parse_scope_2(scope, line)
#define push_parse_scope(scope)     push_parse_scope_1(scope, __LINE__)

  struct parse_key_buffer
  {
    std::string buffer;
    std::vector<std::pair<size_t, size_t>> segments;
    std::vector<source_position> starts;
    std::vector<source_position> ends;

    void clear() noexcept
    {
      buffer.clear();
      segments.clear();
      starts.clear();
      ends.clear();
    }

    void push_back(std::string_view segment, source_position b, source_position e)
    {
      segments.push_back({ buffer.length(), segment.length() });
      buffer.append(segment);
      starts.push_back(b);
      ends.push_back(e);
    }

    TOML_PURE_INLINE_GETTER
    std::string_view operator[](size_t i) const noexcept
    {
      return std::string_view{ buffer.c_str() + segments[i].first, segments[i].second };
    }

    TOML_PURE_INLINE_GETTER
    std::string_view back() const noexcept
    {
      return (*this)[segments.size() - 1u];
    }

    TOML_PURE_INLINE_GETTER
    bool empty() const noexcept
    {
      return segments.empty();
    }

    TOML_PURE_INLINE_GETTER
    size_t size() const noexcept
    {
      return segments.size();
    }
  };

  struct depth_counter_scope
  {
    size_t& depth_;

    TOML_NODISCARD_CTOR
    explicit depth_counter_scope(size_t& depth) noexcept //
      : depth_{ depth }
    {
      depth_++;
    }

    ~depth_counter_scope() noexcept
    {
      depth_--;
    }

    TOML_DELETE_DEFAULTS(depth_counter_scope);
  };

  struct parsed_string
  {
    std::string_view value;
    bool was_multi_line;
  };

  struct table_vector_scope
  {
    std::vector<table*>& tables;

    TOML_NODISCARD_CTOR
    explicit table_vector_scope(std::vector<table*>& tables_, table& tbl) //
      : tables{ tables_ }
    {
      tables.push_back(&tbl);
    }

    ~table_vector_scope() noexcept
    {
      tables.pop_back();
    }

    TOML_DELETE_DEFAULTS(table_vector_scope);
  };
}
TOML_ANON_NAMESPACE_END;

#if 1 // parser helper macros

// Q: "what the fuck is this? MACROS????"
// A: The parser needs to work in exceptionless mode (returning error objects directly)
//    and exception mode (reporting parse failures by throwing). Two totally different control flows.
//    These macros encapsulate the differences between the two modes so I can write code code
//    as though I was only targeting one mode and not want yeet myself into the sun.
//    They're all #undef'd at the bottom of the parser's implementation so they should be harmless outside
//    of toml++.

#define is_eof()     !cp
#define assert_not_eof() TOML_ASSERT_ASSUME(cp != nullptr)
#define return_if_eof(...)                                                                                             \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(is_eof())                                                                                     \
      return __VA_ARGS__;                                                                                        \
  }                                                                                                                  \
  while (false)

#if TOML_EXCEPTIONS
#define is_error()          false
#define return_after_error(...)   TOML_UNREACHABLE
#define assert_not_error()      static_assert(true)
#define return_if_error(...)    static_assert(true)
#define return_if_error_or_eof(...) return_if_eof(__VA_ARGS__)
#else
#define is_error()        !!err
#define return_after_error(...) return __VA_ARGS__
#define assert_not_error()    TOML_ASSERT(!is_error())
#define return_if_error(...)                                                                                           \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(is_error())                                                                                   \
      return __VA_ARGS__;                                                                                        \
  }                                                                                                                  \
  while (false)
#define return_if_error_or_eof(...)                                                                                    \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(is_eof() || is_error())                                                                       \
      return __VA_ARGS__;                                                                                        \
  }                                                                                                                  \
  while (false)
#endif

#if defined(TOML_BREAK_AT_PARSE_ERRORS) && TOML_BREAK_AT_PARSE_ERRORS
#if defined(__has_builtin)
#if __has_builtin(__builtin_debugtrap)
#define parse_error_break() __builtin_debugtrap()
#elif __has_builtin(__debugbreak)
#define parse_error_break() __debugbreak()
#endif
#endif
#ifndef parse_error_break
#if TOML_MSVC || TOML_ICC
#define parse_error_break() __debugbreak()
#else
#define parse_error_break() TOML_ASSERT(false)
#endif
#endif
#else
#define parse_error_break() static_assert(true)
#endif

#define set_error_and_return(ret, ...)                                                                                 \
  do                                                                                                                 \
  {                                                                                                                  \
    if (!is_error())                                                                                               \
      set_error(__VA_ARGS__);                                                                                    \
    return_after_error(ret);                                                                                       \
  }                                                                                                                  \
  while (false)

#define set_error_and_return_default(...) set_error_and_return({}, __VA_ARGS__)

#define set_error_and_return_if_eof(...)                                                                               \
  do                                                                                                                 \
  {                                                                                                                  \
    if TOML_UNLIKELY(is_eof())                                                                                     \
      set_error_and_return(__VA_ARGS__, "encountered end-of-file"sv);                                            \
  }                                                                                                                  \
  while (false)

#define advance_and_return_if_error(...)                                                                               \
  do                                                                                                                 \
  {                                                                                                                  \
    assert_not_eof();                                                                                              \
    advance();                                                                                                     \
    return_if_error(__VA_ARGS__);                                                                                  \
  }                                                                                                                  \
  while (false)

#define advance_and_return_if_error_or_eof(...)                                                                        \
  do                                                                                                                 \
  {                                                                                                                  \
    assert_not_eof();                                                                                              \
    advance();                                                                                                     \
    return_if_error(__VA_ARGS__);                                                                                  \
    set_error_and_return_if_eof(__VA_ARGS__);                                                                      \
  }                                                                                                                  \
  while (false)

#endif // parser helper macros

TOML_IMPL_NAMESPACE_START
{
  TOML_ABI_NAMESPACE_BOOL(TOML_EXCEPTIONS, impl_ex, impl_noex);

  class parser
  {
    private:
    static constexpr size_t max_nested_values   = TOML_MAX_NESTED_VALUES;
    static constexpr size_t max_dotted_keys_depth = TOML_MAX_DOTTED_KEYS_DEPTH;

    utf8_buffered_reader reader;
    table root;
    source_position prev_pos = { 1, 1 };
    const utf8_codepoint* cp = {};
    std::vector<table*> implicit_tables;
    std::vector<table*> dotted_key_tables;
    std::vector<table*> open_inline_tables;
    std::vector<array*> table_arrays;
    parse_key_buffer key_buffer;
    std::string string_buffer;
    std::string recording_buffer; // for diagnostics
    bool recording = false, recording_whitespace = true;
    std::string_view current_scope;
    size_t nested_values = {};
#if !TOML_EXCEPTIONS
    mutable optional<parse_error> err;
#endif

    TOML_NODISCARD
    source_position current_position(source_index fallback_offset = 0) const noexcept
    {
      if (!is_eof())
        return cp->position;
      return { prev_pos.line, static_cast<source_index>(prev_pos.column + fallback_offset) };
    }

    template <typename... T>
    TOML_RETURNS_BY_THROWING
    TOML_NEVER_INLINE
    void set_error_at(source_position pos, const T&... reason) const
    {
      static_assert(sizeof...(T) > 0);
      return_if_error();

      error_builder builder{ current_scope };
      (builder.append(reason), ...);

      parse_error_break();

#if TOML_EXCEPTIONS
      builder.finish(pos, reader.source_path());
#else
      err.emplace(builder.finish(pos, reader.source_path()));
#endif
    }

    template <typename... T>
    TOML_RETURNS_BY_THROWING
    void set_error(const T&... reason) const
    {
      set_error_at(current_position(1), reason...);
    }

    void go_back(size_t count = 1) noexcept
    {
      return_if_error();
      TOML_ASSERT_ASSUME(count);

      cp     = reader.step_back(count);
      prev_pos = cp->position;
    }

    void advance()
    {
      return_if_error();
      assert_not_eof();

      prev_pos = cp->position;
      cp     = reader.read_next();

#if !TOML_EXCEPTIONS
      if (reader.error())
      {
        err = std::move(reader.error());
        return;
      }
#endif

      if (recording && !is_eof())
      {
        if (recording_whitespace || !is_whitespace(*cp))
          recording_buffer.append(cp->bytes, cp->count);
      }
    }

    void start_recording(bool include_current = true) noexcept
    {
      return_if_error();

      recording      = true;
      recording_whitespace = true;
      recording_buffer.clear();
      if (include_current && !is_eof())
        recording_buffer.append(cp->bytes, cp->count);
    }

    void stop_recording(size_t pop_bytes = 0) noexcept
    {
      return_if_error();

      recording = false;
      if (pop_bytes)
      {
        if (pop_bytes >= recording_buffer.length())
          recording_buffer.clear();
        else if (pop_bytes == 1u)
          recording_buffer.pop_back();
        else
          recording_buffer.erase(recording_buffer.begin()
                         + static_cast<ptrdiff_t>(recording_buffer.length() - pop_bytes),
                       recording_buffer.end());
      }
    }

    bool consume_leading_whitespace()
    {
      return_if_error_or_eof({});

      bool consumed = false;
      while (!is_eof() && is_horizontal_whitespace(*cp))
      {
        if TOML_UNLIKELY(!is_ascii_horizontal_whitespace(*cp))
          set_error_and_return_default("expected space or tab, saw '"sv, escaped_codepoint{ *cp }, "'"sv);

        consumed = true;
        advance_and_return_if_error({});
      }
      return consumed;
    }

    bool consume_line_break()
    {
      return_if_error_or_eof({});

      if TOML_UNLIKELY(is_match(*cp, U'\v', U'\f'))
        set_error_and_return_default(
          R"(vertical tabs '\v' and form-feeds '\f' are not legal line breaks in TOML)"sv);

      if (*cp == U'\r')
      {
        advance_and_return_if_error({}); // skip \r

        if TOML_UNLIKELY(is_eof())
          set_error_and_return_default("expected '\\n' after '\\r', saw EOF"sv);

        if TOML_UNLIKELY(*cp != U'\n')
          set_error_and_return_default("expected '\\n' after '\\r', saw '"sv,
                         escaped_codepoint{ *cp },
                         "'"sv);
      }
      else if (*cp != U'\n')
        return false;

      advance_and_return_if_error({}); // skip \n
      return true;
    }

    bool consume_rest_of_line()
    {
      return_if_error_or_eof({});

      do
      {
        if (is_ascii_vertical_whitespace(*cp))
          return consume_line_break();
        else
          advance();
        return_if_error({});
      }
      while (!is_eof());

      return true;
    }

    bool consume_comment()
    {
      return_if_error_or_eof({});

      if (*cp != U'#')
        return false;

      push_parse_scope("comment"sv);

      advance_and_return_if_error({}); // skip the '#'

      while (!is_eof())
      {
        if (consume_line_break())
          return true;
        return_if_error({});

#if TOML_LANG_AT_LEAST(1, 0, 0)

        // toml/issues/567 (disallow non-TAB control characters in comments)
        if TOML_UNLIKELY(is_nontab_control_character(*cp))
          set_error_and_return_default(
            "control characters other than TAB (U+0009) are explicitly prohibited in comments"sv);

        // toml/pull/720 (disallow surrogates in comments)
        else if TOML_UNLIKELY(is_unicode_surrogate(*cp))
          set_error_and_return_default(
            "unicode surrogates (U+D800 to U+DFFF) are explicitly prohibited in comments"sv);
#endif

        advance_and_return_if_error({});
      }

      return true;
    }

    TOML_NODISCARD
    bool consume_expected_sequence(std::u32string_view seq)
    {
      return_if_error({});
      TOML_ASSERT(!seq.empty());

      for (auto c : seq)
      {
        set_error_and_return_if_eof({});
        if (*cp != c)
          return false;
        advance_and_return_if_error({});
      }
      return true;
    }

    template <typename T>
    TOML_NODISCARD
    bool consume_digit_sequence(T* digits, size_t len)
    {
      return_if_error({});
      TOML_ASSERT_ASSUME(digits);
      TOML_ASSERT_ASSUME(len);

      for (size_t i = 0; i < len; i++)
      {
        set_error_and_return_if_eof({});
        if (!is_decimal_digit(*cp))
          return false;

        digits[i] = static_cast<T>(*cp - U'0');
        advance_and_return_if_error({});
      }
      return true;
    }

    template <typename T>
    TOML_NODISCARD
    size_t consume_variable_length_digit_sequence(T* buffer, size_t max_len)
    {
      return_if_error({});
      TOML_ASSERT_ASSUME(buffer);
      TOML_ASSERT_ASSUME(max_len);

      size_t i = {};
      for (; i < max_len; i++)
      {
        if (is_eof() || !is_decimal_digit(*cp))
          break;

        buffer[i] = static_cast<T>(*cp - U'0');
        advance_and_return_if_error({});
      }
      return i;
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_basic_string(bool multi_line)
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'"');
      push_parse_scope("string"sv);

      // skip the '"'
      advance_and_return_if_error_or_eof({});

      // multi-line strings ignore a single line ending right at the beginning
      if (multi_line)
      {
        consume_line_break();
        return_if_error({});
        set_error_and_return_if_eof({});
      }

      auto& str = string_buffer;
      str.clear();
      bool escaped       = false;
      bool skipping_whitespace = false;
      do
      {
        if (escaped)
        {
          escaped = false;

          // handle 'line ending slashes' in multi-line mode
          if (multi_line && is_whitespace(*cp))
          {
            consume_leading_whitespace();

            if TOML_UNLIKELY(!consume_line_break())
              set_error_and_return_default(
                "line-ending backslashes must be the last non-whitespace character on the line"sv);

            skipping_whitespace = true;
            return_if_error({});
            continue;
          }

          bool skip_escaped_codepoint = true;
          assert_not_eof();
          switch (const auto escaped_codepoint = *cp)
          {
            // 'regular' escape codes
            case U'b': str += '\b'; break;
            case U'f': str += '\f'; break;
            case U'n': str += '\n'; break;
            case U'r': str += '\r'; break;
            case U't': str += '\t'; break;
            case U'"': str += '"'; break;
            case U'\\': str += '\\'; break;

#if TOML_LANG_UNRELEASED // toml/pull/790 (\e shorthand for \x1B)
            case U'e': str += '\x1B'; break;
#else
            case U'e':
              set_error_and_return_default(
                "escape sequence '\\e' is not supported in TOML 1.0.0 and earlier"sv);
#endif

#if TOML_LANG_UNRELEASED // toml/pull/796 (\xHH unicode scalar sequences)
            case U'x': [[fallthrough]];
#else
            case U'x':
              set_error_and_return_default(
                "escape sequence '\\x' is not supported in TOML 1.0.0 and earlier"sv);
#endif

            // unicode scalar sequences
            case U'u': [[fallthrough]];
            case U'U':
            {
              push_parse_scope("unicode scalar sequence"sv);
              advance_and_return_if_error_or_eof({});
              skip_escaped_codepoint = false;

              uint32_t place_value =
                escaped_codepoint == U'U' ? 0x10000000u : (escaped_codepoint == U'u' ? 0x1000u : 0x10u);
              uint32_t sequence_value{};
              while (place_value)
              {
                set_error_and_return_if_eof({});

                if TOML_UNLIKELY(!is_hexadecimal_digit(*cp))
                  set_error_and_return_default("expected hex digit, saw '"sv, to_sv(*cp), "'"sv);

                sequence_value += place_value * hex_to_dec(*cp);
                place_value /= 16u;
                advance_and_return_if_error({});
              }

              if TOML_UNLIKELY(is_unicode_surrogate(sequence_value))
                set_error_and_return_default(
                  "unicode surrogates (U+D800 - U+DFFF) are explicitly prohibited"sv);
              else if TOML_UNLIKELY(sequence_value > 0x10FFFFu)
                set_error_and_return_default("values greater than U+10FFFF are invalid"sv);

              if (sequence_value < 0x80)
              {
                str += static_cast<char>(sequence_value);
              }
              else if (sequence_value < 0x800u)
              {
                str += static_cast<char>((sequence_value >> 6) | 0xC0u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              }
              else if (sequence_value < 0x10000u)
              {
                str += static_cast<char>((sequence_value >> 12) | 0xE0u);
                str += static_cast<char>(((sequence_value >> 6) & 0x3Fu) | 0x80u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              }
              else if (sequence_value < 0x110000u)
              {
                str += static_cast<char>((sequence_value >> 18) | 0xF0u);
                str += static_cast<char>(((sequence_value >> 12) & 0x3Fu) | 0x80u);
                str += static_cast<char>(((sequence_value >> 6) & 0x3Fu) | 0x80u);
                str += static_cast<char>((sequence_value & 0x3Fu) | 0x80u);
              }
              break;
            }

              // ???
              TOML_UNLIKELY_CASE
            default: set_error_and_return_default("unknown escape sequence '\\"sv, to_sv(*cp), "'"sv);
          }

          if (skip_escaped_codepoint)
            advance_and_return_if_error_or_eof({});
        }
        else
        {
          // handle closing delimiters
          if (*cp == U'"')
          {
            if (multi_line)
            {
              size_t lookaheads       = {};
              size_t consecutive_delimiters = 1;
              do
              {
                advance_and_return_if_error({});
                lookaheads++;
                if (!is_eof() && *cp == U'"')
                  consecutive_delimiters++;
                else
                  break;
              }
              while (lookaheads < 4u);

              switch (consecutive_delimiters)
              {
                // """ " (one quote somewhere in a ML string)
                case 1:
                  str += '"';
                  skipping_whitespace = false;
                  continue;

                // """ "" (two quotes somewhere in a ML string)
                case 2:
                  str.append("\"\""sv);
                  skipping_whitespace = false;
                  continue;

                // """ """ (the end of the string)
                case 3: return str;

                // """ """" (one at the end of the string)
                case 4: str += '"'; return str;

                // """ """"" (two quotes at the end of the string)
                case 5:
                  str.append("\"\""sv);
                  advance_and_return_if_error({}); // skip the last '"'
                  return str;

                default: TOML_UNREACHABLE;
              }
            }
            else
            {
              advance_and_return_if_error({}); // skip the closing delimiter
              return str;
            }
          }

          // handle escapes
          else if (*cp == U'\\')
          {
            advance_and_return_if_error_or_eof({}); // skip the '\'
            skipping_whitespace = false;
            escaped       = true;
            continue;
          }

          // handle line endings in multi-line mode
          if (multi_line && is_ascii_vertical_whitespace(*cp))
          {
            consume_line_break();
            return_if_error({});
            if (!skipping_whitespace)
              str += '\n';
            continue;
          }

          // handle control characters
          if TOML_UNLIKELY(is_nontab_control_character(*cp))
            set_error_and_return_default(
              "unescaped control characters other than TAB (U+0009) are explicitly prohibited"sv);

#if TOML_LANG_AT_LEAST(1, 0, 0)

          // handle surrogates in strings
          if TOML_UNLIKELY(is_unicode_surrogate(*cp))
            set_error_and_return_default(
              "unescaped unicode surrogates (U+D800 to U+DFFF) are explicitly prohibited"sv);
#endif

          if (multi_line)
          {
            if (!skipping_whitespace || !is_horizontal_whitespace(*cp))
            {
              skipping_whitespace = false;
              str.append(cp->bytes, cp->count);
            }
          }
          else
            str.append(cp->bytes, cp->count);

          advance_and_return_if_error({});
        }
      }
      while (!is_eof());

      set_error_and_return_default("encountered end-of-file"sv);
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_literal_string(bool multi_line)
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'\'');
      push_parse_scope("literal string"sv);

      // skip the delimiter
      advance_and_return_if_error_or_eof({});

      // multi-line strings ignore a single line ending right at the beginning
      if (multi_line)
      {
        consume_line_break();
        return_if_error({});
        set_error_and_return_if_eof({});
      }

      auto& str = string_buffer;
      str.clear();
      do
      {
        return_if_error({});

        // handle closing delimiters
        if (*cp == U'\'')
        {
          if (multi_line)
          {
            size_t lookaheads       = {};
            size_t consecutive_delimiters = 1;
            do
            {
              advance_and_return_if_error({});
              lookaheads++;
              if (!is_eof() && *cp == U'\'')
                consecutive_delimiters++;
              else
                break;
            }
            while (lookaheads < 4u);

            switch (consecutive_delimiters)
            {
              // ''' ' (one quote somewhere in a ML string)
              case 1: str += '\''; continue;

              // ''' '' (two quotes somewhere in a ML string)
              case 2: str.append("''"sv); continue;

              // ''' ''' (the end of the string)
              case 3: return str;

              // ''' '''' (one at the end of the string)
              case 4: str += '\''; return str;

              // ''' ''''' (two quotes at the end of the string)
              case 5:
                str.append("''"sv);
                advance_and_return_if_error({}); // skip the last '
                return str;

              default: TOML_UNREACHABLE;
            }
          }
          else
          {
            advance_and_return_if_error({}); // skip the closing delimiter
            return str;
          }
        }

        // handle line endings in multi-line mode
        if (multi_line && is_ascii_vertical_whitespace(*cp))
        {
          consume_line_break();
          return_if_error({});
          str += '\n';
          continue;
        }

        // handle control characters
        if TOML_UNLIKELY(is_nontab_control_character(*cp))
          set_error_and_return_default(
            "control characters other than TAB (U+0009) are explicitly prohibited"sv);

#if TOML_LANG_AT_LEAST(1, 0, 0)

        // handle surrogates in strings
        if TOML_UNLIKELY(is_unicode_surrogate(*cp))
          set_error_and_return_default("unicode surrogates (U+D800 - U+DFFF) are explicitly prohibited"sv);
#endif

        str.append(cp->bytes, cp->count);
        advance_and_return_if_error({});
      }
      while (!is_eof());

      set_error_and_return_default("encountered end-of-file"sv);
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    parsed_string parse_string()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_string_delimiter(*cp));
      push_parse_scope("string"sv);

      // get the first three characters to determine the string type
      const auto first = cp->value;
      advance_and_return_if_error_or_eof({});
      const auto second = cp->value;
      advance_and_return_if_error({});
      const auto third = cp ? cp->value : U'\0';

      // if we were eof at the third character then first and second need to be
      // the same string character (otherwise it's an unterminated string)
      if (is_eof())
      {
        if (second == first)
          return {};

        set_error_and_return_default("encountered end-of-file"sv);
      }

      // if the first three characters are all the same string delimiter then
      // it's a multi-line string.
      else if (first == second && first == third)
      {
        return { first == U'\'' ? parse_literal_string(true) : parse_basic_string(true), true };
      }

      // otherwise it's just a regular string.
      else
      {
        // step back two characters so that the current
        // character is the string delimiter
        go_back(2u);

        return { first == U'\'' ? parse_literal_string(false) : parse_basic_string(false), false };
      }
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    std::string_view parse_bare_key_segment()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_bare_key_character(*cp));

      string_buffer.clear();

      while (!is_eof())
      {
        if (!is_bare_key_character(*cp))
          break;

        string_buffer.append(cp->bytes, cp->count);
        advance_and_return_if_error({});
      }

      return string_buffer;
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    bool parse_boolean()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U't', U'f', U'T', U'F'));
      push_parse_scope("boolean"sv);

      start_recording(true);
      auto result = is_match(*cp, U't', U'T');
      if (!consume_expected_sequence(result ? U"true"sv : U"false"sv))
        set_error_and_return_default("expected '"sv,
                       to_sv(result),
                       "', saw '"sv,
                       to_sv(recording_buffer),
                       "'"sv);
      stop_recording();

      if (cp && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);

      return result;
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_inf_or_nan()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'i', U'n', U'I', U'N', U'+', U'-'));
      push_parse_scope("floating-point"sv);

      start_recording(true);
      const bool negative = *cp == U'-';
      if (negative || *cp == U'+')
        advance_and_return_if_error_or_eof({});

      const bool inf = is_match(*cp, U'i', U'I');
      if (!consume_expected_sequence(inf ? U"inf"sv : U"nan"sv))
        set_error_and_return_default("expected '"sv,
                       inf ? "inf"sv : "nan"sv,
                       "', saw '"sv,
                       to_sv(recording_buffer),
                       "'"sv);
      stop_recording();

      if (cp && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);

      return inf ? (negative ? -std::numeric_limits<double>::infinity() : std::numeric_limits<double>::infinity())
             : std::numeric_limits<double>::quiet_NaN();
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_float()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'+', U'-', U'.') || is_decimal_digit(*cp));
      push_parse_scope("floating-point"sv);

      // sign
      const int sign = *cp == U'-' ? -1 : 1;
      if (is_match(*cp, U'+', U'-'))
        advance_and_return_if_error_or_eof({});

      // consume value chars
      char chars[utf8_buffered_reader::max_history_length];
      size_t length        = {};
      const utf8_codepoint* prev = {};
      bool seen_decimal = false, seen_exponent = false;
      char first_integer_part = '\0';
      while (!is_eof() && !is_value_terminator(*cp))
      {
        if (*cp == U'_')
        {
          if (!prev || !is_decimal_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);

          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        }
        else if TOML_UNLIKELY(prev && *prev == U'_' && !is_decimal_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if TOML_UNLIKELY(length == sizeof(chars))
          set_error_and_return_default("exceeds length limit of "sv,
                         sizeof(chars),
                         " digits"sv,
                         (seen_exponent ? ""sv : " (consider using exponent notation)"sv));
        else if (*cp == U'.')
        {
          // .1
          // -.1
          // +.1 (no integer part)
          if (!first_integer_part)
            set_error_and_return_default("expected decimal digit, saw '.'"sv);

          // 1.0e+.10 (exponent cannot have '.')
          else if (seen_exponent)
            set_error_and_return_default("expected exponent decimal digit or sign, saw '.'"sv);

          // 1.0.e+.10
          // 1..0
          // (multiple '.')
          else if (seen_decimal)
            set_error_and_return_default("expected decimal digit or exponent, saw '.'"sv);

          seen_decimal = true;
        }
        else if (is_match(*cp, U'e', U'E'))
        {
          if (prev && !is_decimal_digit(*prev))
            set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);

          // 1.0ee+10 (multiple 'e')
          else if (seen_exponent)
            set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);

          seen_decimal  = true; // implied
          seen_exponent = true;
        }
        else if (is_match(*cp, U'+', U'-'))
        {
          // 1.-0 (sign in mantissa)
          if (!seen_exponent)
            set_error_and_return_default("expected decimal digit or '.', saw '"sv, to_sv(*cp), "'"sv);

          // 1.0e1-0 (misplaced exponent sign)
          else if (!is_match(*prev, U'e', U'E'))
            set_error_and_return_default("expected exponent digit, saw '"sv, to_sv(*cp), "'"sv);
        }
        else if (is_decimal_digit(*cp))
        {
          if (!seen_decimal)
          {
            if (!first_integer_part)
              first_integer_part = static_cast<char>(cp->bytes[0]);
            else if (first_integer_part == '0')
              set_error_and_return_default("leading zeroes are prohibited"sv);
          }
        }
        else
          set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);

        chars[length++] = static_cast<char>(cp->bytes[0]);
        prev      = cp;
        advance_and_return_if_error({});
      }

      // sanity-check ending state
      if (prev)
      {
        if (*prev == U'_')
        {
          set_error_and_return_if_eof({});
          set_error_and_return_default("underscores must be followed by digits"sv);
        }
        else if (is_match(*prev, U'e', U'E', U'+', U'-', U'.'))
        {
          set_error_and_return_if_eof({});
          set_error_and_return_default("expected decimal digit, saw '"sv, to_sv(*cp), "'"sv);
        }
      }

      // convert to double
      double result;
#if TOML_FLOAT_CHARCONV
      {
        auto fc_result = std::from_chars(chars, chars + length, result);
        switch (fc_result.ec)
        {
          TOML_LIKELY_CASE
          case std::errc{}: // ok
            return result * sign;

          case std::errc::invalid_argument:
            set_error_and_return_default("'"sv,
                           std::string_view{ chars, length },
                           "' could not be interpreted as a value"sv);
            break;

          case std::errc::result_out_of_range:
            set_error_and_return_default("'"sv,
                           std::string_view{ chars, length },
                           "' is not representable in 64 bits"sv);
            break;

          default: //??
            set_error_and_return_default("an unspecified error occurred while trying to interpret '"sv,
                           std::string_view{ chars, length },
                           "' as a value"sv);
        }
      }
#else
      {
        std::stringstream ss;
        ss.imbue(std::locale::classic());
        ss.write(chars, static_cast<std::streamsize>(length));
        if ((ss >> result))
          return result * sign;
        else
          set_error_and_return_default("'"sv,
                         std::string_view{ chars, length },
                         "' could not be interpreted as a value"sv);
      }
#endif
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    double parse_hex_float()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_match(*cp, U'0', U'+', U'-'));
      push_parse_scope("hexadecimal floating-point"sv);

#if TOML_LANG_UNRELEASED // toml/issues/562 (hexfloats)

      // sign
      const int sign = *cp == U'-' ? -1 : 1;
      if (is_match(*cp, U'+', U'-'))
        advance_and_return_if_error_or_eof({});

      // '0'
      if (*cp != U'0')
        set_error_and_return_default(" expected '0', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // 'x' or 'X'
      if (!is_match(*cp, U'x', U'X'))
        set_error_and_return_default("expected 'x' or 'X', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // <HEX DIGITS> ([.]<HEX DIGITS>)? [pP] [+-]? <DEC DIGITS>

      // consume value fragments
      struct fragment
      {
        char chars[24];
        size_t length;
        double value;
      };
      fragment fragments[] = {
        {}, // mantissa, whole part
        {}, // mantissa, fractional part
        {}  // exponent
      };
      fragment* current_fragment = fragments;
      const utf8_codepoint* prev = {};
      int exponent_sign      = 1;
      while (!is_eof() && !is_value_terminator(*cp))
      {
        if (*cp == U'_')
        {
          if (!prev || !is_hexadecimal_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);

          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        }
        else if (prev && *prev == U'_' && !is_hexadecimal_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if (*cp == U'.')
        {
          // 0x10.0p-.0 (exponent cannot have '.')
          if (current_fragment == fragments + 2)
            set_error_and_return_default("expected exponent digit or sign, saw '.'"sv);

          // 0x10.0.p-0 (multiple '.')
          else if (current_fragment == fragments + 1)
            set_error_and_return_default("expected hexadecimal digit or exponent, saw '.'"sv);

          else
            current_fragment++;
        }
        else if (is_match(*cp, U'p', U'P'))
        {
          // 0x10.0pp-0 (multiple 'p')
          if (current_fragment == fragments + 2)
            set_error_and_return_default("expected exponent digit or sign, saw '"sv, to_sv(*cp), "'"sv);

          // 0x.p-0 (mantissa is just '.')
          else if (fragments[0].length == 0u && fragments[1].length == 0u)
            set_error_and_return_default("expected hexadecimal digit, saw '"sv, to_sv(*cp), "'"sv);

          else
            current_fragment = fragments + 2;
        }
        else if (is_match(*cp, U'+', U'-'))
        {
          // 0x-10.0p-0 (sign in mantissa)
          if (current_fragment != fragments + 2)
            set_error_and_return_default("expected hexadecimal digit or '.', saw '"sv, to_sv(*cp), "'"sv);

          // 0x10.0p0- (misplaced exponent sign)
          else if (!is_match(*prev, U'p', U'P'))
            set_error_and_return_default("expected exponent digit, saw '"sv, to_sv(*cp), "'"sv);

          else
            exponent_sign = *cp == U'-' ? -1 : 1;
        }
        else if (current_fragment < fragments + 2 && !is_hexadecimal_digit(*cp))
          set_error_and_return_default("expected hexadecimal digit or '.', saw '"sv, to_sv(*cp), "'"sv);
        else if (current_fragment == fragments + 2 && !is_decimal_digit(*cp))
          set_error_and_return_default("expected exponent digit or sign, saw '"sv, to_sv(*cp), "'"sv);
        else if (current_fragment->length == sizeof(fragment::chars))
          set_error_and_return_default("fragment exceeeds maximum length of "sv,
                         sizeof(fragment::chars),
                         " characters"sv);
        else
          current_fragment->chars[current_fragment->length++] = static_cast<char>(cp->bytes[0]);

        prev = cp;
        advance_and_return_if_error({});
      }

      // sanity-check ending state
      if (current_fragment != fragments + 2 || current_fragment->length == 0u)
      {
        set_error_and_return_if_eof({});
        set_error_and_return_default("missing exponent"sv);
      }
      else if (prev && *prev == U'_')
      {
        set_error_and_return_if_eof({});
        set_error_and_return_default("underscores must be followed by digits"sv);
      }

      // calculate values for the three fragments
      for (int fragment_idx = 0; fragment_idx < 3; fragment_idx++)
      {
        auto& f       = fragments[fragment_idx];
        const uint32_t base = fragment_idx == 2 ? 10u : 16u;

        // left-trim zeroes
        const char* c = f.chars;
        size_t sig    = {};
        while (f.length && *c == '0')
        {
          f.length--;
          c++;
          sig++;
        }
        if (!f.length)
          continue;

        // calculate value
        auto place = 1u;
        for (size_t i = 0; i < f.length - 1u; i++)
          place *= base;
        uint32_t val{};
        while (place)
        {
          if (base == 16)
            val += place * hex_to_dec(*c);
          else
            val += place * static_cast<uint32_t>(*c - '0');
          if (fragment_idx == 1)
            sig++;
          c++;
          place /= base;
        }
        f.value = static_cast<double>(val);

        // shift the fractional part
        if (fragment_idx == 1)
        {
          while (sig--)
            f.value /= base;
        }
      }

      return (fragments[0].value + fragments[1].value) * pow(2.0, fragments[2].value * exponent_sign) * sign;

#else // !TOML_LANG_UNRELEASED

      set_error_and_return_default("hexadecimal floating-point values are not supported "
                     "in TOML 1.0.0 and earlier"sv);

#endif // !TOML_LANG_UNRELEASED
    }

    template <uint64_t base>
    TOML_NODISCARD
    TOML_NEVER_INLINE
    int64_t parse_integer()
    {
      return_if_error({});
      assert_not_eof();
      using traits = parse_integer_traits<base>;
      push_parse_scope(traits::scope_qualifier);

      [[maybe_unused]] int64_t sign = 1;
      if constexpr (traits::is_signed)
      {
        sign = *cp == U'-' ? -1 : 1;
        if (is_match(*cp, U'+', U'-'))
          advance_and_return_if_error_or_eof({});
      }

      if constexpr (base == 10)
      {
        if (!traits::is_digit(*cp))
          set_error_and_return_default("expected expected digit or sign, saw '"sv, to_sv(*cp), "'"sv);
      }
      else
      {
        // '0'
        if (*cp != U'0')
          set_error_and_return_default("expected '0', saw '"sv, to_sv(*cp), "'"sv);
        advance_and_return_if_error_or_eof({});

        // 'b', 'o', 'x'
        if (*cp != traits::prefix_codepoint)
          set_error_and_return_default("expected '"sv, traits::prefix, "', saw '"sv, to_sv(*cp), "'"sv);
        advance_and_return_if_error_or_eof({});

        if (!traits::is_digit(*cp))
          set_error_and_return_default("expected digit, saw '"sv, to_sv(*cp), "'"sv);
      }

      // consume digits
      char digits[utf8_buffered_reader::max_history_length];
      size_t length        = {};
      const utf8_codepoint* prev = {};
      while (!is_eof() && !is_value_terminator(*cp))
      {
        if (*cp == U'_')
        {
          if (!prev || !traits::is_digit(*prev))
            set_error_and_return_default("underscores may only follow digits"sv);

          prev = cp;
          advance_and_return_if_error_or_eof({});
          continue;
        }
        else if TOML_UNLIKELY(prev && *prev == U'_' && !traits::is_digit(*cp))
          set_error_and_return_default("underscores must be followed by digits"sv);
        else if TOML_UNLIKELY(!traits::is_digit(*cp))
          set_error_and_return_default("expected digit, saw '"sv, to_sv(*cp), "'"sv);
        else if TOML_UNLIKELY(length == sizeof(digits))
          set_error_and_return_default("exceeds length limit of "sv, sizeof(digits), " digits"sv);
        else
          digits[length++] = static_cast<char>(cp->bytes[0]);

        prev = cp;
        advance_and_return_if_error({});
      }

      // sanity check ending state
      if (prev && *prev == U'_')
      {
        set_error_and_return_if_eof({});
        set_error_and_return_default("underscores must be followed by digits"sv);
      }

      // single digits can be converted trivially
      if (length == 1u)
      {
        int64_t result;

        if constexpr (base == 16)
          result = static_cast<int64_t>(hex_to_dec(digits[0]));
        else
          result = static_cast<int64_t>(digits[0] - '0');

        if constexpr (traits::is_signed)
          result *= sign;

        return result;
      }

      // bin, oct and hex allow leading zeroes so trim them first
      const char* end = digits + length;
      const char* msd = digits;
      if constexpr (base != 10)
      {
        while (msd < end && *msd == '0')
          msd++;
        if (msd == end)
          return 0ll;
      }

      // decimal integers do not allow leading zeroes
      else
      {
        if TOML_UNLIKELY(digits[0] == '0')
          set_error_and_return_default("leading zeroes are prohibited"sv);
      }

      // range check
      if TOML_UNLIKELY(static_cast<size_t>(end - msd) > traits::max_digits)
        set_error_and_return_default("'"sv,
                       traits::full_prefix,
                       std::string_view{ digits, length },
                       "' is not representable as a signed 64-bit integer"sv);

      // do the thing
      {
        uint64_t result = {};
        {
          uint64_t power = 1;
          while (--end >= msd)
          {
            if constexpr (base == 16)
              result += power * hex_to_dec(*end);
            else
              result += power * static_cast<uint64_t>(*end - '0');

            power *= base;
          }
        }

        // range check
        static constexpr auto i64_max = static_cast<uint64_t>((std::numeric_limits<int64_t>::max)());
        if TOML_UNLIKELY(result > i64_max + (sign < 0 ? 1u : 0u))
          set_error_and_return_default("'"sv,
                         traits::full_prefix,
                         std::string_view{ digits, length },
                         "' is not representable as a signed 64-bit integer"sv);

        if constexpr (traits::is_signed)
        {
          // avoid signed multiply UB when parsing INT64_MIN
          if TOML_UNLIKELY(sign < 0 && result == i64_max + 1u)
            return (std::numeric_limits<int64_t>::min)();

          return static_cast<int64_t>(result) * sign;
        }
        else
          return static_cast<int64_t>(result);
      }
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    date parse_date(bool part_of_datetime = false)
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("date"sv);

      // "YYYY"
      uint32_t digits[4];
      if (!consume_digit_sequence(digits, 4u))
        set_error_and_return_default("expected 4-digit year, saw '"sv, to_sv(cp), "'"sv);
      const auto year     = digits[3] + digits[2] * 10u + digits[1] * 100u + digits[0] * 1000u;
      const auto is_leap_year = (year % 4u == 0u) && ((year % 100u != 0u) || (year % 400u == 0u));
      set_error_and_return_if_eof({});

      // '-'
      if (*cp != U'-')
        set_error_and_return_default("expected '-', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "MM"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit month, saw '"sv, to_sv(cp), "'"sv);
      const auto month = digits[1] + digits[0] * 10u;
      if (month == 0u || month > 12u)
        set_error_and_return_default("expected month between 1 and 12 (inclusive), saw "sv, month);
      const auto max_days_in_month = month == 2u
                       ? (is_leap_year ? 29u : 28u)
                       : (month == 4u || month == 6u || month == 9u || month == 11u ? 30u : 31u);
      set_error_and_return_if_eof({});

      // '-'
      if (*cp != U'-')
        set_error_and_return_default("expected '-', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "DD"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit day, saw '"sv, to_sv(cp), "'"sv);
      const auto day = digits[1] + digits[0] * 10u;
      if (day == 0u || day > max_days_in_month)
        set_error_and_return_default("expected day between 1 and "sv,
                       max_days_in_month,
                       " (inclusive), saw "sv,
                       day);

      if (!part_of_datetime && !is_eof() && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);

      return { year, month, day };
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    time parse_time(bool part_of_datetime = false)
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("time"sv);

      static constexpr size_t max_digits = 64; // far more than necessary but needed to allow fractional
                           // millisecond truncation per the spec
      uint32_t digits[max_digits];

      // "HH"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit hour, saw '"sv, to_sv(cp), "'"sv);
      const auto hour = digits[1] + digits[0] * 10u;
      if (hour > 23u)
        set_error_and_return_default("expected hour between 0 to 59 (inclusive), saw "sv, hour);
      set_error_and_return_if_eof({});

      // ':'
      if (*cp != U':')
        set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "MM"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit minute, saw '"sv, to_sv(cp), "'"sv);
      const auto minute = digits[1] + digits[0] * 10u;
      if (minute > 59u)
        set_error_and_return_default("expected minute between 0 and 59 (inclusive), saw "sv, minute);
      auto time = toml::time{ hour, minute };

      // ':'
      if constexpr (TOML_LANG_UNRELEASED) // toml/issues/671 (allow omission of seconds)
      {
        if (is_eof() || is_value_terminator(*cp) || (part_of_datetime && is_match(*cp, U'+', U'-', U'Z', U'z')))
          return time;
      }
      else
        set_error_and_return_if_eof({});
      if (*cp != U':')
        set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "SS"
      if (!consume_digit_sequence(digits, 2u))
        set_error_and_return_default("expected 2-digit second, saw '"sv, to_sv(cp), "'"sv);
      const auto second = digits[1] + digits[0] * 10u;
      if (second > 59u)
        set_error_and_return_default("expected second between 0 and 59 (inclusive), saw "sv, second);
      time.second = static_cast<decltype(time.second)>(second);

      // '.' (early-exiting is allowed; fractional is optional)
      if (is_eof() || is_value_terminator(*cp) || (part_of_datetime && is_match(*cp, U'+', U'-', U'Z', U'z')))
        return time;
      if (*cp != U'.')
        set_error_and_return_default("expected '.', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "FFFFFFFFF"
      size_t digit_count = consume_variable_length_digit_sequence(digits, max_digits);
      if (!digit_count)
      {
        set_error_and_return_if_eof({});
        set_error_and_return_default("expected fractional digits, saw '"sv, to_sv(*cp), "'"sv);
      }
      else if (!is_eof())
      {
        if (digit_count == max_digits && is_decimal_digit(*cp))
          set_error_and_return_default("fractional component exceeds maximum precision of "sv, max_digits);
        else if (!part_of_datetime && !is_value_terminator(*cp))
          set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);
      }
      uint32_t value = 0u;
      uint32_t place = 1u;
      for (auto i = impl::min<size_t>(digit_count, 9u); i-- > 0u;)
      {
        value += digits[i] * place;
        place *= 10u;
      }
      for (auto i = digit_count; i < 9u; i++) // implicit zeros
        value *= 10u;
      time.nanosecond = value;
      return time;
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    date_time parse_date_time()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_decimal_digit(*cp));
      push_parse_scope("date-time"sv);

      // "YYYY-MM-DD"
      auto date = parse_date(true);
      set_error_and_return_if_eof({});

      // ' ', 'T' or 't'
      if (!is_match(*cp, U' ', U'T', U't'))
        set_error_and_return_default("expected space, 'T' or 't', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // "HH:MM:SS.FFFFFFFFF"
      auto time = parse_time(true);
      return_if_error({});

      // no offset
      if (is_eof() || is_value_terminator(*cp))
        return { date, time };

      // zero offset ('Z' or 'z')
      time_offset offset{};
      if (is_match(*cp, U'Z', U'z'))
        advance_and_return_if_error({});

      // explicit offset ("+/-HH:MM")
      else if (is_match(*cp, U'+', U'-'))
      {
        push_parse_scope("date-time offset"sv);

        // sign
        int sign = *cp == U'-' ? -1 : 1;
        advance_and_return_if_error_or_eof({});

        // "HH"
        int digits[2];
        if (!consume_digit_sequence(digits, 2u))
          set_error_and_return_default("expected 2-digit hour, saw '"sv, to_sv(cp), "'"sv);
        const auto hour = digits[1] + digits[0] * 10;
        if (hour > 23)
          set_error_and_return_default("expected hour between 0 and 23 (inclusive), saw "sv, hour);
        set_error_and_return_if_eof({});

        // ':'
        if (*cp != U':')
          set_error_and_return_default("expected ':', saw '"sv, to_sv(*cp), "'"sv);
        advance_and_return_if_error_or_eof({});

        // "MM"
        if (!consume_digit_sequence(digits, 2u))
          set_error_and_return_default("expected 2-digit minute, saw '"sv, to_sv(cp), "'"sv);
        const auto minute = digits[1] + digits[0] * 10;
        if (minute > 59)
          set_error_and_return_default("expected minute between 0 and 59 (inclusive), saw "sv, minute);
        offset.minutes = static_cast<decltype(offset.minutes)>((hour * 60 + minute) * sign);
      }

      if (!is_eof() && !is_value_terminator(*cp))
        set_error_and_return_default("expected value-terminator, saw '"sv, to_sv(*cp), "'"sv);

      return { date, time, offset };
    }

    TOML_NODISCARD
    node_ptr parse_array();

    TOML_NODISCARD
    node_ptr parse_inline_table();

    TOML_NODISCARD
    node_ptr parse_value_known_prefixes()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(!is_control_character(*cp));
      TOML_ASSERT_ASSUME(*cp != U'_');

      switch (cp->value)
      {
        // arrays
        case U'[': return parse_array();

        // inline tables
        case U'{': return parse_inline_table();

        // floats beginning with '.'
        case U'.': return node_ptr{ new value{ parse_float() } };

        // strings
        case U'"': [[fallthrough]];
        case U'\'': return node_ptr{ new value{ parse_string().value } };

        default:
        {
          const auto cp_upper = static_cast<uint_least32_t>(cp->value) & ~0x20u;

          // bools
          if (cp_upper == 70u || cp_upper == 84u) // F or T
            return node_ptr{ new value{ parse_boolean() } };

          // inf/nan
          else if (cp_upper == 73u || cp_upper == 78u) // I or N
            return node_ptr{ new value{ parse_inf_or_nan() } };

          else
            return nullptr;
        }
      }
      TOML_UNREACHABLE;
    }

    TOML_NODISCARD
    node_ptr parse_value()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(!is_value_terminator(*cp));
      push_parse_scope("value"sv);

      const depth_counter_scope depth_counter{ nested_values };
      if TOML_UNLIKELY(nested_values > max_nested_values)
        set_error_and_return_default("exceeded maximum nested value depth of "sv,
                       max_nested_values,
                       " (TOML_MAX_NESTED_VALUES)"sv);

      // check if it begins with some control character
      // (note that this will also fail for whitespace but we're assuming we've
      // called consume_leading_whitespace() before calling parse_value())
      if TOML_UNLIKELY(is_control_character(*cp))
        set_error_and_return_default("unexpected control character"sv);

      // underscores at the beginning
      else if (*cp == U'_')
        set_error_and_return_default("values may not begin with underscores"sv);

      const auto begin_pos = cp->position;
      node_ptr val;

      do
      {
        TOML_ASSERT_ASSUME(!is_control_character(*cp));
        TOML_ASSERT_ASSUME(*cp != U'_');

        // detect the value type and parse accordingly,
        // starting with value types that can be detected
        // unambiguously from just one character.

        val = parse_value_known_prefixes();
        return_if_error({});
        if (val)
          break;

        // value types from here down require more than one character to unambiguously identify
        // so scan ahead and collect a set of value 'traits'.
        enum TOML_CLOSED_FLAGS_ENUM value_traits : int
        {
          has_nothing  = 0,
          has_digits   = 1,
          has_b    = 1 << 1, // as second char only (0b)
          has_e    = 1 << 2, // only float exponents
          has_o    = 1 << 3, // as second char only (0o)
          has_p    = 1 << 4, // only hexfloat exponents
          has_t    = 1 << 5,
          has_x    = 1 << 6, // as second or third char only (0x, -0x, +0x)
          has_z    = 1 << 7,
          has_colon  = 1 << 8,
          has_plus   = 1 << 9,
          has_minus  = 1 << 10,
          has_dot    = 1 << 11,
          begins_sign  = 1 << 12,
          begins_digit = 1 << 13,
          begins_zero  = 1 << 14,

          signs_msk  = has_plus | has_minus,
          bdigit_msk = has_digits | begins_digit,
          bzero_msk  = bdigit_msk | begins_zero,
        };
        value_traits traits  = has_nothing;
        const auto has_any   = [&](auto t) noexcept { return (traits & t) != has_nothing; };
        const auto has_none  = [&](auto t) noexcept { return (traits & t) == has_nothing; };
        const auto add_trait = [&](auto t) noexcept { traits = static_cast<value_traits>(traits | t); };

        // examine the first character to get the 'begins with' traits
        // (good fail-fast opportunity; all the remaining types begin with numeric digits or signs)
        if (is_decimal_digit(*cp))
        {
          add_trait(begins_digit);
          if (*cp == U'0')
            add_trait(begins_zero);
        }
        else if (is_match(*cp, U'+', U'-'))
          add_trait(begins_sign);
        else
          break;

        // scan the rest of the value to determine the remaining traits
        char32_t chars[utf8_buffered_reader::max_history_length];
        size_t char_count = {}, advance_count = {};
        bool eof_while_scanning = false;
        const auto scan     = [&]() noexcept(!TOML_COMPILER_HAS_EXCEPTIONS)
        {
          if (is_eof())
            return;
          TOML_ASSERT_ASSUME(!is_value_terminator(*cp));

          do
          {
            if (const auto c = **cp; c != U'_')
            {
              chars[char_count++] = c;

              if (is_decimal_digit(c))
                add_trait(has_digits);
              else if (is_ascii_letter(c))
              {
                TOML_ASSERT_ASSUME((c >= U'a' && c <= U'z') || (c >= U'A' && c <= U'Z'));
                switch (static_cast<char32_t>(c | 32u))
                {
                  case U'b':
                    if (char_count == 2u && has_any(begins_zero))
                      add_trait(has_b);
                    break;

                  case U'e':
                    if (char_count > 1u
                      && has_none(has_b | has_o | has_p | has_t | has_x | has_z | has_colon)
                      && (has_none(has_plus | has_minus) || has_any(begins_sign)))
                      add_trait(has_e);
                    break;

                  case U'o':
                    if (char_count == 2u && has_any(begins_zero))
                      add_trait(has_o);
                    break;

                  case U'p':
                    if (has_any(has_x))
                      add_trait(has_p);
                    break;

                  case U'x':
                    if ((char_count == 2u && has_any(begins_zero))
                      || (char_count == 3u && has_any(begins_sign) && chars[1] == U'0'))
                      add_trait(has_x);
                    break;

                  case U't': add_trait(has_t); break;
                  case U'z': add_trait(has_z); break;
                }
              }
              else if (c <= U':')
              {
                TOML_ASSERT_ASSUME(c < U'0' || c > U'9');
                switch (c)
                {
                  case U'+': add_trait(has_plus); break;
                  case U'-': add_trait(has_minus); break;
                  case U'.': add_trait(has_dot); break;
                  case U':': add_trait(has_colon); break;
                }
              }
            }

            advance_and_return_if_error();
            advance_count++;
            eof_while_scanning = is_eof();
          }
          while (advance_count < (utf8_buffered_reader::max_history_length - 1u) && !is_eof()
               && !is_value_terminator(*cp));
        };
        scan();
        return_if_error({});

        // force further scanning if this could have been a date-time with a space instead of a T
        if (char_count == 10u                   //
          && (traits | begins_zero) == (bzero_msk | has_minus) //
          && chars[4] == U'-'                   //
          && chars[7] == U'-'                   //
          && !is_eof()                     //
          && *cp == U' ')
        {
          const auto pre_advance_count = advance_count;
          const auto pre_scan_traits   = traits;
          chars[char_count++]      = *cp;
          add_trait(has_t);

          const auto backpedal = [&]() noexcept
          {
            go_back(advance_count - pre_advance_count);
            advance_count = pre_advance_count;
            traits      = pre_scan_traits;
            char_count    = 10u;
          };

          advance_and_return_if_error({});
          advance_count++;

          if (is_eof() || !is_decimal_digit(*cp))
            backpedal();
          else
          {
            chars[char_count++] = *cp;

            advance_and_return_if_error({});
            advance_count++;

            scan();
            return_if_error({});

            if (char_count == 12u)
              backpedal();
          }
        }

        // set the reader back to where we started
        go_back(advance_count);

        // if after scanning ahead we still only have one value character,
        // the only valid value type is an integer.
        if (char_count == 1u)
        {
          if (has_any(begins_digit))
          {
            val.reset(new value{ static_cast<int64_t>(chars[0] - U'0') });
            advance(); // skip the digit
            break;
          }

          // anything else would be ambiguous.
          else
            set_error_and_return_default(eof_while_scanning ? "encountered end-of-file"sv
                                    : "could not determine value type"sv);
        }

        // now things that can be identified from two or more characters
        return_if_error({});
        TOML_ASSERT_ASSUME(char_count >= 2u);

        // do some 'fuzzy matching' where there's no ambiguity, since that allows the specific
        // typed parse functions to take over and show better diagnostics if there's an issue
        // (as opposed to the fallback "could not determine type" message)
        if (has_any(has_p))
          val.reset(new value{ parse_hex_float() });
        else if (has_any(has_x | has_o | has_b))
        {
          int64_t i;
          value_flags flags;
          if (has_any(has_x))
          {
            i   = parse_integer<16>();
            flags = value_flags::format_as_hexadecimal;
          }
          else if (has_any(has_o))
          {
            i   = parse_integer<8>();
            flags = value_flags::format_as_octal;
          }
          else // has_b
          {
            i   = parse_integer<2>();
            flags = value_flags::format_as_binary;
          }
          return_if_error({});

          val.reset(new value{ i });
          val->ref_cast<int64_t>().flags(flags);
        }
        else if (has_any(has_e) || (has_any(begins_digit) && chars[1] == U'.'))
          val.reset(new value{ parse_float() });
        else if (has_any(begins_sign))
        {
          // single-digit signed integers
          if (char_count == 2u && has_any(has_digits))
          {
            val.reset(new value{ static_cast<int64_t>(chars[1] - U'0') * (chars[0] == U'-' ? -1LL : 1LL) });
            advance(); // skip the sign
            advance(); // skip the digit
            break;
          }

          // simple signed floats (e.g. +1.0)
          if (is_decimal_digit(chars[1]) && chars[2] == U'.')
            val.reset(new value{ parse_float() });

          // signed infinity or nan
          else if (is_match(chars[1], U'i', U'n', U'I', U'N'))
            val.reset(new value{ parse_inf_or_nan() });
        }

        return_if_error({});
        if (val)
          break;

        // match trait masks against what they can match exclusively.
        // all correct value parses will come out of this list, so doing this as a switch is likely to
        // be a better friend to the optimizer on the success path (failure path can be slow but that
        // doesn't matter much).
        switch (unwrap_enum(traits))
        {
          // binary integers
          // 0b10
          case bzero_msk | has_b:
            val.reset(new value{ parse_integer<2>() });
            val->ref_cast<int64_t>().flags(value_flags::format_as_binary);
            break;

          // octal integers
          // 0o10
          case bzero_msk | has_o:
            val.reset(new value{ parse_integer<8>() });
            val->ref_cast<int64_t>().flags(value_flags::format_as_octal);
            break;

          // decimal integers
          // 00
          // 10
          // +10
          // -10
          case bzero_msk: [[fallthrough]];
          case bdigit_msk: [[fallthrough]];
          case begins_sign | has_digits | has_minus: [[fallthrough]];
          case begins_sign | has_digits | has_plus:
          {
            // if the value was so long we exhausted the history buffer it's reasonable to assume
            // there was more and the value's actual type is impossible to identify without making the
            // buffer bigger (since it could have actually been a float), so emit an error.
            //
            // (this will likely only come up during fuzzing and similar scenarios)
            static constexpr size_t max_numeric_value_length =
              utf8_buffered_reader::max_history_length - 2u;
            if TOML_UNLIKELY(!eof_while_scanning && advance_count > max_numeric_value_length)
              set_error_and_return_default("numeric value too long to identify type - cannot exceed "sv,
                             max_numeric_value_length,
                             " characters"sv);

            val.reset(new value{ parse_integer<10>() });
            break;
          }

          // hexadecimal integers
          // 0x10
          case bzero_msk | has_x:
            val.reset(new value{ parse_integer<16>() });
            val->ref_cast<int64_t>().flags(value_flags::format_as_hexadecimal);
            break;

          // decimal floats
          // 0e1
          // 0e-1
          // 0e+1
          // 0.0
          // 0.0e1
          // 0.0e-1
          // 0.0e+1
          case bzero_msk | has_e: [[fallthrough]];
          case bzero_msk | has_e | has_minus: [[fallthrough]];
          case bzero_msk | has_e | has_plus: [[fallthrough]];
          case bzero_msk | has_dot: [[fallthrough]];
          case bzero_msk | has_dot | has_e: [[fallthrough]];
          case bzero_msk | has_dot | has_e | has_minus: [[fallthrough]];
          case bzero_msk | has_dot | has_e | has_plus: [[fallthrough]];
          // 1e1
          // 1e-1
          // 1e+1
          // 1.0
          // 1.0e1
          // 1.0e-1
          // 1.0e+1
          case bdigit_msk | has_e: [[fallthrough]];
          case bdigit_msk | has_e | has_minus: [[fallthrough]];
          case bdigit_msk | has_e | has_plus: [[fallthrough]];
          case bdigit_msk | has_dot: [[fallthrough]];
          case bdigit_msk | has_dot | has_e: [[fallthrough]];
          case bdigit_msk | has_dot | has_e | has_minus: [[fallthrough]];
          case bdigit_msk | has_dot | has_e | has_plus: [[fallthrough]];
          // +1e1
          // +1.0
          // +1.0e1
          // +1.0e+1
          // +1.0e-1
          // -1.0e+1
          case begins_sign | has_digits | has_e | has_plus: [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_plus: [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | has_plus: [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | signs_msk: [[fallthrough]];
          // -1e1
          // -1e+1
          // +1e-1
          // -1.0
          // -1.0e1
          // -1.0e-1
          case begins_sign | has_digits | has_e | has_minus: [[fallthrough]];
          case begins_sign | has_digits | has_e | signs_msk: [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_minus: [[fallthrough]];
          case begins_sign | has_digits | has_dot | has_e | has_minus:
            val.reset(new value{ parse_float() });
            break;

          // hexadecimal floats
          // 0x10p0
          // 0x10p-0
          // 0x10p+0
          case bzero_msk | has_x | has_p: [[fallthrough]];
          case bzero_msk | has_x | has_p | has_minus: [[fallthrough]];
          case bzero_msk | has_x | has_p | has_plus: [[fallthrough]];
          // -0x10p0
          // -0x10p-0
          // +0x10p0
          // +0x10p+0
          // -0x10p+0
          // +0x10p-0
          case begins_sign | has_digits | has_x | has_p | has_minus: [[fallthrough]];
          case begins_sign | has_digits | has_x | has_p | has_plus: [[fallthrough]];
          case begins_sign | has_digits | has_x | has_p | signs_msk: [[fallthrough]];
          // 0x10.1p0
          // 0x10.1p-0
          // 0x10.1p+0
          case bzero_msk | has_x | has_dot | has_p: [[fallthrough]];
          case bzero_msk | has_x | has_dot | has_p | has_minus: [[fallthrough]];
          case bzero_msk | has_x | has_dot | has_p | has_plus: [[fallthrough]];
          // -0x10.1p0
          // -0x10.1p-0
          // +0x10.1p0
          // +0x10.1p+0
          // -0x10.1p+0
          // +0x10.1p-0
          case begins_sign | has_digits | has_x | has_dot | has_p | has_minus: [[fallthrough]];
          case begins_sign | has_digits | has_x | has_dot | has_p | has_plus: [[fallthrough]];
          case begins_sign | has_digits | has_x | has_dot | has_p | signs_msk:
            val.reset(new value{ parse_hex_float() });
            break;

          // times
          // HH:MM
          // HH:MM:SS
          // HH:MM:SS.FFFFFF
          case bzero_msk | has_colon: [[fallthrough]];
          case bzero_msk | has_colon | has_dot: [[fallthrough]];
          case bdigit_msk | has_colon: [[fallthrough]];
          case bdigit_msk | has_colon | has_dot: val.reset(new value{ parse_time() }); break;

          // local dates
          // YYYY-MM-DD
          case bzero_msk | has_minus: [[fallthrough]];
          case bdigit_msk | has_minus: val.reset(new value{ parse_date() }); break;

          // date-times
          // YYYY-MM-DDTHH:MM
          // YYYY-MM-DDTHH:MM-HH:MM
          // YYYY-MM-DDTHH:MM+HH:MM
          // YYYY-MM-DD HH:MM
          // YYYY-MM-DD HH:MM-HH:MM
          // YYYY-MM-DD HH:MM+HH:MM
          // YYYY-MM-DDTHH:MM:SS
          // YYYY-MM-DDTHH:MM:SS-HH:MM
          // YYYY-MM-DDTHH:MM:SS+HH:MM
          // YYYY-MM-DD HH:MM:SS
          // YYYY-MM-DD HH:MM:SS-HH:MM
          // YYYY-MM-DD HH:MM:SS+HH:MM
          case bzero_msk | has_minus | has_colon | has_t: [[fallthrough]];
          case bzero_msk | signs_msk | has_colon | has_t: [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_t: [[fallthrough]];
          case bdigit_msk | signs_msk | has_colon | has_t: [[fallthrough]];
          // YYYY-MM-DDTHH:MM:SS.FFFFFF
          // YYYY-MM-DDTHH:MM:SS.FFFFFF-HH:MM
          // YYYY-MM-DDTHH:MM:SS.FFFFFF+HH:MM
          // YYYY-MM-DD HH:MM:SS.FFFFFF
          // YYYY-MM-DD HH:MM:SS.FFFFFF-HH:MM
          // YYYY-MM-DD HH:MM:SS.FFFFFF+HH:MM
          case bzero_msk | has_minus | has_colon | has_dot | has_t: [[fallthrough]];
          case bzero_msk | signs_msk | has_colon | has_dot | has_t: [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_dot | has_t: [[fallthrough]];
          case bdigit_msk | signs_msk | has_colon | has_dot | has_t: [[fallthrough]];
          // YYYY-MM-DDTHH:MMZ
          // YYYY-MM-DD HH:MMZ
          // YYYY-MM-DDTHH:MM:SSZ
          // YYYY-MM-DD HH:MM:SSZ
          // YYYY-MM-DDTHH:MM:SS.FFFFFFZ
          // YYYY-MM-DD HH:MM:SS.FFFFFFZ
          case bzero_msk | has_minus | has_colon | has_z | has_t: [[fallthrough]];
          case bzero_msk | has_minus | has_colon | has_dot | has_z | has_t: [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_z | has_t: [[fallthrough]];
          case bdigit_msk | has_minus | has_colon | has_dot | has_z | has_t:
            val.reset(new value{ parse_date_time() });
            break;
        }
      }
      while (false);

      if (!val)
      {
        set_error_at(begin_pos, "could not determine value type"sv);
        return_after_error({});
      }

      val->source_ = { begin_pos, current_position(1), reader.source_path() };
      return val;
    }

    TOML_NEVER_INLINE
    bool parse_key()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_bare_key_character(*cp) || is_string_delimiter(*cp));
      push_parse_scope("key"sv);

      key_buffer.clear();
      recording_whitespace = false;

      while (!is_error())
      {
        std::string_view key_segment;
        const auto key_begin = current_position();

        // bare_key_segment
        if (is_bare_key_character(*cp))
          key_segment = parse_bare_key_segment();

        // "quoted key segment"
        else if (is_string_delimiter(*cp))
        {
          const auto begin_pos = cp->position;

          recording_whitespace = true;
          parsed_string str  = parse_string();
          recording_whitespace = false;
          return_if_error({});

          if (str.was_multi_line)
          {
            set_error_at(begin_pos,
                   "multi-line strings are prohibited in "sv,
                   key_buffer.empty() ? ""sv : "dotted "sv,
                   "keys"sv);
            return_after_error({});
          }
          else
            key_segment = str.value;
        }

        // ???
        else
          set_error_and_return_default("expected bare key starting character or string delimiter, saw '"sv,
                         to_sv(*cp),
                         "'"sv);

        const auto key_end = current_position();

        // whitespace following the key segment
        consume_leading_whitespace();

        // store segment
        key_buffer.push_back(key_segment, key_begin, key_end);

        if TOML_UNLIKELY(key_buffer.size() > max_dotted_keys_depth)
          set_error_and_return_default("exceeded maximum dotted keys depth of "sv,
                         max_dotted_keys_depth,
                         " (TOML_MAX_DOTTED_KEYS_DEPTH)"sv);

        // eof or no more key to come
        if (is_eof() || *cp != U'.')
          break;

        // was a dotted key - go around again
        advance_and_return_if_error_or_eof({});
        consume_leading_whitespace();
        set_error_and_return_if_eof({});
      }
      return_if_error({});

      return true;
    }

    TOML_NODISCARD
    key make_key(size_t segment_index) const
    {
      TOML_ASSERT(key_buffer.size() > segment_index);

      return key{
        key_buffer[segment_index],
        source_region{ key_buffer.starts[segment_index], key_buffer.ends[segment_index], root.source().path }
      };
    }

    TOML_NODISCARD
    TOML_NEVER_INLINE
    table* parse_table_header()
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(*cp == U'[');
      push_parse_scope("table header"sv);

      const source_position header_begin_pos = cp->position;
      source_position header_end_pos;
      bool is_arr = false;

      // parse header
      {
        // skip first '['
        advance_and_return_if_error_or_eof({});

        // skip past any whitespace that followed the '['
        const bool had_leading_whitespace = consume_leading_whitespace();
        set_error_and_return_if_eof({});

        // skip second '[' (if present)
        if (*cp == U'[')
        {
          if (had_leading_whitespace)
            set_error_and_return_default(
              "[[array-of-table]] brackets must be contiguous (i.e. [ [ this ] ] is prohibited)"sv);

          is_arr = true;
          advance_and_return_if_error_or_eof({});

          // skip past any whitespace that followed the '['
          consume_leading_whitespace();
          set_error_and_return_if_eof({});
        }

        // check for a premature closing ']'
        if (*cp == U']')
          set_error_and_return_default("tables with blank bare keys are explicitly prohibited"sv);

        // get the actual key
        start_recording();
        parse_key();
        stop_recording(1u);
        return_if_error({});

        // skip past any whitespace that followed the key
        consume_leading_whitespace();
        return_if_error({});
        set_error_and_return_if_eof({});

        // consume the closing ']'
        if (*cp != U']')
          set_error_and_return_default("expected ']', saw '"sv, to_sv(*cp), "'"sv);
        if (is_arr)
        {
          advance_and_return_if_error_or_eof({});
          if (*cp != U']')
            set_error_and_return_default("expected ']', saw '"sv, to_sv(*cp), "'"sv);
        }
        advance_and_return_if_error({});
        header_end_pos = current_position(1);

        // handle the rest of the line after the header
        consume_leading_whitespace();
        if (!is_eof() && !consume_comment() && !consume_line_break())
          set_error_and_return_default("expected a comment or whitespace, saw '"sv, to_sv(cp), "'"sv);
      }
      TOML_ASSERT(!key_buffer.empty());

      // check if each parent is a table/table array, or can be created implicitly as a table.
      table* parent = &root;
      for (size_t i = 0, e = key_buffer.size() - 1u; i < e; i++)
      {
        const std::string_view segment = key_buffer[i];
        auto pit             = parent->lower_bound(segment);

        // parent already existed
        if (pit != parent->end() && pit->first == segment)
        {
          node& p = pit->second;

          if (auto tbl = p.as_table())
          {
            // adding to closed inline tables is illegal
            if (tbl->is_inline() && !impl::find(open_inline_tables.begin(), open_inline_tables.end(), tbl))
              set_error_and_return_default("cannot insert '"sv,
                             to_sv(recording_buffer),
                             "' into existing inline table"sv);

            parent = tbl;
          }
          else if (auto arr = p.as_array(); arr && impl::find(table_arrays.begin(), table_arrays.end(), arr))
          {
            // table arrays are a special case;
            // the spec dictates we select the most recently declared element in the array.
            TOML_ASSERT(!arr->empty());
            TOML_ASSERT(arr->back().is_table());
            parent = &arr->back().ref_cast<table>();
          }
          else
          {
            if (!is_arr && p.type() == node_type::table)
              set_error_and_return_default("cannot redefine existing table '"sv,
                             to_sv(recording_buffer),
                             "'"sv);
            else
              set_error_and_return_default("cannot redefine existing "sv,
                             to_sv(p.type()),
                             " '"sv,
                             to_sv(recording_buffer),
                             "' as "sv,
                             is_arr ? "array-of-tables"sv : "table"sv);
          }
        }

        // need to create a new implicit table
        else
        {
          pit     = parent->emplace_hint<table>(pit, make_key(i));
          table& p  = pit->second.ref_cast<table>();
          p.source_ = { header_begin_pos, header_end_pos, reader.source_path() };

          implicit_tables.push_back(&p);
          parent = &p;
        }
      }

      const auto last_segment = key_buffer.back();
      auto it         = parent->lower_bound(last_segment);

      // if there was already a matching node some sanity checking is necessary;
      // this is ok if we're making an array and the existing element is already an array (new element)
      // or if we're making a table and the existing element is an implicitly-created table (promote it),
      // otherwise this is a redefinition error.
      if (it != parent->end() && it->first == last_segment)
      {
        node& matching_node = it->second;
        if (auto arr = matching_node.as_array();
          is_arr && arr && impl::find(table_arrays.begin(), table_arrays.end(), arr))
        {
          table& tbl  = arr->emplace_back<table>();
          tbl.source_ = { header_begin_pos, header_end_pos, reader.source_path() };
          return &tbl;
        }

        else if (auto tbl = matching_node.as_table(); !is_arr && tbl && !implicit_tables.empty())
        {
          if (auto found = impl::find(implicit_tables.begin(), implicit_tables.end(), tbl); found)
          {
            bool ok = true;
            if (!tbl->empty())
            {
              for (auto& [_, child] : *tbl)
              {
                if (!child.is_table() && !child.is_array_of_tables())
                {
                  ok = false;
                  break;
                }
              }
            }

            if (ok)
            {
              implicit_tables.erase(implicit_tables.cbegin() + (found - implicit_tables.data()));
              tbl->source_.begin = header_begin_pos;
              tbl->source_.end   = header_end_pos;
              return tbl;
            }
          }
        }

        // if we get here it's a redefinition error.
        if (!is_arr && matching_node.type() == node_type::table)
        {
          set_error_at(header_begin_pos,
                 "cannot redefine existing table '"sv,
                 to_sv(recording_buffer),
                 "'"sv);
          return_after_error({});
        }
        else
        {
          set_error_at(header_begin_pos,
                 "cannot redefine existing "sv,
                 to_sv(matching_node.type()),
                 " '"sv,
                 to_sv(recording_buffer),
                 "' as "sv,
                 is_arr ? "array-of-tables"sv : "table"sv);
          return_after_error({});
        }
      }

      // there was no matching node, sweet - we can freely instantiate a new table/table array.
      else
      {
        auto last_key = make_key(key_buffer.size() - 1u);

        // if it's an array we need to make the array and it's first table element,
        // set the starting regions, and return the table element
        if (is_arr)
        {
          it         = parent->emplace_hint<array>(it, std::move(last_key));
          array& tbl_arr = it->second.ref_cast<array>();
          table_arrays.push_back(&tbl_arr);
          tbl_arr.source_ = { header_begin_pos, header_end_pos, reader.source_path() };

          table& tbl  = tbl_arr.emplace_back<table>();
          tbl.source_ = { header_begin_pos, header_end_pos, reader.source_path() };
          return &tbl;
        }

        // otherwise we're just making a table
        else
        {
          it      = parent->emplace_hint<table>(it, std::move(last_key));
          table& tbl  = it->second.ref_cast<table>();
          tbl.source_ = { header_begin_pos, header_end_pos, reader.source_path() };
          return &tbl;
        }
      }
    }

    TOML_NEVER_INLINE
    bool parse_key_value_pair_and_insert(table* tbl)
    {
      return_if_error({});
      assert_not_eof();
      TOML_ASSERT_ASSUME(is_string_delimiter(*cp) || is_bare_key_character(*cp));
      push_parse_scope("key-value pair"sv);

      // read the key into the key buffer
      start_recording();
      parse_key();
      stop_recording(1u);
      return_if_error({});
      TOML_ASSERT(key_buffer.size() >= 1u);

      // skip past any whitespace that followed the key
      consume_leading_whitespace();
      set_error_and_return_if_eof({});

      // '='
      if (*cp != U'=')
        set_error_and_return_default("expected '=', saw '"sv, to_sv(*cp), "'"sv);
      advance_and_return_if_error_or_eof({});

      // skip past any whitespace that followed the '='
      consume_leading_whitespace();
      return_if_error({});
      set_error_and_return_if_eof({});

      // check that the next character could actually be a value
      if (is_value_terminator(*cp))
        set_error_and_return_default("expected value, saw '"sv, to_sv(*cp), "'"sv);

      // if it's a dotted kvp we need to spawn the parent sub-tables if necessary,
      // and set the target table to the second-to-last one in the chain
      if (key_buffer.size() > 1u)
      {
        for (size_t i = 0; i < key_buffer.size() - 1u; i++)
        {
          const std::string_view segment = key_buffer[i];
          auto pit             = tbl->lower_bound(segment);

          // parent already existed
          if (pit != tbl->end() && pit->first == segment)
          {
            table* p = pit->second.as_table();

            // redefinition
            if TOML_UNLIKELY(!p
              || !(impl::find(dotted_key_tables.begin(), dotted_key_tables.end(), p)
                 || impl::find(implicit_tables.begin(), implicit_tables.end(), p)))
            {
              set_error_at(key_buffer.starts[i],
                     "cannot redefine existing "sv,
                     to_sv(pit->second.type()),
                     " as dotted key-value pair"sv);
              return_after_error({});
            }

            tbl = p;
          }

          // need to create a new implicit table
          else
          {
            pit     = tbl->emplace_hint<table>(pit, make_key(i));
            table& p  = pit->second.ref_cast<table>();
            p.source_ = pit->first.source();

            dotted_key_tables.push_back(&p);
            tbl = &p;
          }
        }
      }

      // ensure this isn't a redefinition
      const std::string_view last_segment = key_buffer.back();
      auto it               = tbl->lower_bound(last_segment);
      if (it != tbl->end() && it->first == last_segment)
      {
        set_error("cannot redefine existing "sv,
              to_sv(it->second.type()),
              " '"sv,
              to_sv(recording_buffer),
              "'"sv);
        return_after_error({});
      }

      // create the key first since the key buffer will likely get overwritten during value parsing (inline
      // tables)
      auto last_key = make_key(key_buffer.size() - 1u);

      // now we can actually parse the value
      node_ptr val = parse_value();
      return_if_error({});

      tbl->emplace_hint<node_ptr>(it, std::move(last_key), std::move(val));
      return true;
    }

    void parse_document()
    {
      assert_not_error();
      assert_not_eof();
      push_parse_scope("root table"sv);

      table* current_table = &root;

      do
      {
        return_if_error();

        // leading whitespace, line endings, comments
        if (consume_leading_whitespace() || consume_line_break() || consume_comment())
          continue;
        return_if_error();

        // [tables]
        // [[table array]]
        if (*cp == U'[')
          current_table = parse_table_header();

        // bare_keys
        // dotted.keys
        // "quoted keys"
        else if (is_bare_key_character(*cp) || is_string_delimiter(*cp))
        {
          push_parse_scope("key-value pair"sv);

          parse_key_value_pair_and_insert(current_table);

          // handle the rest of the line after the kvp
          // (this is not done in parse_key_value_pair() because that is also used for inline tables)
          consume_leading_whitespace();
          return_if_error();
          if (!is_eof() && !consume_comment() && !consume_line_break())
            set_error("expected a comment or whitespace, saw '"sv, to_sv(cp), "'"sv);
        }

        else // ??
          set_error("expected keys, tables, whitespace or comments, saw '"sv, to_sv(cp), "'"sv);
      }
      while (!is_eof());

      auto eof_pos   = current_position(1);
      root.source_.end = eof_pos;
      if (current_table && current_table != &root && current_table->source_.end <= current_table->source_.begin)
        current_table->source_.end = eof_pos;
    }

    static void update_region_ends(node& nde) noexcept
    {
      const auto type = nde.type();
      if (type > node_type::array)
        return;

      if (type == node_type::table)
      {
        auto& tbl = nde.ref_cast<table>();
        if (tbl.is_inline()) // inline tables (and all their inline descendants) are already correctly
                   // terminated
          return;

        auto end = nde.source_.end;
        for (auto&& [k, v] : tbl)
        {
          TOML_UNUSED(k);
          update_region_ends(v);
          if (end < v.source_.end)
            end = v.source_.end;
        }
      }
      else // arrays
      {
        auto& arr = nde.ref_cast<array>();
        auto end  = nde.source_.end;
        for (auto&& v : arr)
        {
          update_region_ends(v);
          if (end < v.source_.end)
            end = v.source_.end;
        }
        nde.source_.end = end;
      }
    }

    public:
    parser(utf8_reader_interface&& reader_) //
      : reader{ reader_ }
    {
      root.source_ = { prev_pos, prev_pos, reader.source_path() };

      if (!reader.peek_eof())
      {
        cp = reader.read_next();

#if !TOML_EXCEPTIONS
        if (reader.error())
        {
          err = std::move(reader.error());
          return;
        }
#endif

        if (cp)
          parse_document();
      }

      update_region_ends(root);
    }

    TOML_NODISCARD
    operator parse_result() && noexcept
    {
#if TOML_EXCEPTIONS

      return { std::move(root) };

#else

      if (err)
        return parse_result{ *std::move(err) };
      else
        return parse_result{ std::move(root) };

#endif
    }
  };

  TOML_EXTERNAL_LINKAGE
  node_ptr parser::parse_array()
  {
    return_if_error({});
    assert_not_eof();
    TOML_ASSERT_ASSUME(*cp == U'[');
    push_parse_scope("array"sv);

    // skip opening '['
    advance_and_return_if_error_or_eof({});

    node_ptr arr_ptr{ new array{} };
    array& arr = arr_ptr->ref_cast<array>();
    enum class TOML_CLOSED_ENUM parse_type : int
    {
      none,
      comma,
      val
    };
    parse_type prev = parse_type::none;

    while (!is_error())
    {
      while (consume_leading_whitespace() || consume_line_break() || consume_comment())
        continue;
      set_error_and_return_if_eof({});

      // commas - only legal after a value
      if (*cp == U',')
      {
        if (prev == parse_type::val)
        {
          prev = parse_type::comma;
          advance_and_return_if_error_or_eof({});
          continue;
        }
        set_error_and_return_default("expected value or closing ']', saw comma"sv);
      }

      // closing ']'
      else if (*cp == U']')
      {
        advance_and_return_if_error({});
        break;
      }

      // must be a value
      else
      {
        if (prev == parse_type::val)
        {
          set_error_and_return_default("expected comma or closing ']', saw '"sv, to_sv(*cp), "'"sv);
          continue;
        }
        prev = parse_type::val;

        auto val = parse_value();
        return_if_error({});

        if (!arr.capacity())
          arr.reserve(4u);
        arr.emplace_back<node_ptr>(std::move(val));
      }
    }

    return_if_error({});
    return arr_ptr;
  }

  TOML_EXTERNAL_LINKAGE
  node_ptr parser::parse_inline_table()
  {
    return_if_error({});
    assert_not_eof();
    TOML_ASSERT_ASSUME(*cp == U'{');
    push_parse_scope("inline table"sv);

    // skip opening '{'
    advance_and_return_if_error_or_eof({});

    node_ptr tbl_ptr{ new table{} };
    table& tbl = tbl_ptr->ref_cast<table>();
    tbl.is_inline(true);
    table_vector_scope table_scope{ open_inline_tables, tbl };

    enum class TOML_CLOSED_ENUM parse_type : int
    {
      none,
      comma,
      kvp
    };
    parse_type prev = parse_type::none;
    while (!is_error())
    {
      if constexpr (TOML_LANG_UNRELEASED) // toml/issues/516 (newlines/trailing commas in inline tables)
      {
        while (consume_leading_whitespace() || consume_line_break() || consume_comment())
          continue;
      }
      else
      {
        while (consume_leading_whitespace())
          continue;
      }
      return_if_error({});
      set_error_and_return_if_eof({});

      // commas - only legal after a key-value pair
      if (*cp == U',')
      {
        if (prev == parse_type::kvp)
        {
          prev = parse_type::comma;
          advance_and_return_if_error_or_eof({});
        }
        else
          set_error_and_return_default("expected key-value pair or closing '}', saw comma"sv);
      }

      // closing '}'
      else if (*cp == U'}')
      {
        if constexpr (!TOML_LANG_UNRELEASED) // toml/issues/516 (newlines/trailing commas in inline tables)
        {
          if (prev == parse_type::comma)
          {
            set_error_and_return_default("expected key-value pair, saw closing '}' (dangling comma)"sv);
            continue;
          }
        }
        advance_and_return_if_error({});
        break;
      }

      // key-value pair
      else if (is_string_delimiter(*cp) || is_bare_key_character(*cp))
      {
        if (prev == parse_type::kvp)
          set_error_and_return_default("expected comma or closing '}', saw '"sv, to_sv(*cp), "'"sv);
        else
        {
          prev = parse_type::kvp;
          parse_key_value_pair_and_insert(&tbl);
        }
      }

      /// ???
      else
        set_error_and_return_default("expected key or closing '}', saw '"sv, to_sv(*cp), "'"sv);
    }

    return_if_error({});
    return tbl_ptr;
  }

  TOML_ABI_NAMESPACE_END; // TOML_EXCEPTIONS
}
TOML_IMPL_NAMESPACE_END;

#undef TOML_RETURNS_BY_THROWING
#undef advance_and_return_if_error
#undef advance_and_return_if_error_or_eof
#undef assert_not_eof
#undef assert_not_error
#undef is_eof
#undef is_error
#undef parse_error_break
#undef push_parse_scope
#undef push_parse_scope_1
#undef push_parse_scope_2
#undef return_after_error
#undef return_if_eof
#undef return_if_error
#undef return_if_error_or_eof
#undef set_error_and_return
#undef set_error_and_return_default
#undef set_error_and_return_if_eof
#undef utf8_buffered_reader_error_check
#undef utf8_reader_error
#undef utf8_reader_error_check
#undef utf8_reader_return_after_error

//#---------------------------------------------------------------------------------------------------------------------
//# PARSER PUBLIC IMPLEMENTATION
//#---------------------------------------------------------------------------------------------------------------------

TOML_ANON_NAMESPACE_START
{
  TOML_NODISCARD
  TOML_INTERNAL_LINKAGE
  parse_result do_parse(utf8_reader_interface && reader)
  {
    return impl::parser{ std::move(reader) };
  }

  TOML_NODISCARD
  TOML_INTERNAL_LINKAGE
  parse_result do_parse_file(std::string_view file_path)
  {
#if TOML_EXCEPTIONS
#define TOML_PARSE_FILE_ERROR(msg, path)                                                                               \
  throw parse_error(msg, source_position{}, std::make_shared<const std::string>(std::move(path)))
#else
#define TOML_PARSE_FILE_ERROR(msg, path)                                                                               \
  return parse_result(parse_error(msg, source_position{}, std::make_shared<const std::string>(std::move(path))))
#endif

    std::string file_path_str(file_path);

    // open file with a custom-sized stack buffer
    std::ifstream file;
    TOML_OVERALIGNED char file_buffer[sizeof(void*) * 1024u];
    file.rdbuf()->pubsetbuf(file_buffer, sizeof(file_buffer));
#if TOML_WINDOWS && !(defined(__MINGW32__) || defined(__MINGW64__))
    file.open(impl::widen(file_path_str).c_str(), std::ifstream::in | std::ifstream::binary | std::ifstream::ate);
#else
    file.open(file_path_str, std::ifstream::in | std::ifstream::binary | std::ifstream::ate);
#endif
    if (!file.is_open())
      TOML_PARSE_FILE_ERROR("File could not be opened for reading", file_path_str);

    // get size
    const auto file_size = file.tellg();
    if (file_size == -1)
      TOML_PARSE_FILE_ERROR("Could not determine file size", file_path_str);
    file.seekg(0, std::ifstream::beg);

    // read the whole file into memory first if the file isn't too large
    constexpr auto large_file_threshold = 1024 * 1024 * 2; // 2 MB
    if (file_size <= large_file_threshold)
    {
      std::vector<char> file_data;
      file_data.resize(static_cast<size_t>(file_size));
      file.read(file_data.data(), static_cast<std::streamsize>(file_size));
      return parse(std::string_view{ file_data.data(), file_data.size() }, std::move(file_path_str));
    }

    // otherwise parse it using the streams
    else
      return parse(file, std::move(file_path_str));

#undef TOML_PARSE_FILE_ERROR
  }
}
TOML_ANON_NAMESPACE_END;

TOML_NAMESPACE_START
{
  TOML_ABI_NAMESPACE_BOOL(TOML_EXCEPTIONS, ex, noex);

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::string_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, source_path });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::string && source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, std::move(source_path) });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::string_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, source_path });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::string && source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, std::move(source_path) });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::string_view file_path)
  {
    return TOML_ANON_NAMESPACE::do_parse_file(file_path);
  }

#if TOML_HAS_CHAR8

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::string_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, source_path });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::string && source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, std::move(source_path) });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::u8string_view file_path)
  {
    std::string file_path_str;
    file_path_str.resize(file_path.length());
    memcpy(file_path_str.data(), file_path.data(), file_path.length());
    return TOML_ANON_NAMESPACE::do_parse_file(file_path_str);
  }

#endif // TOML_HAS_CHAR8

#if TOML_ENABLE_WINDOWS_COMPAT

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::string_view doc, std::wstring_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, impl::narrow(source_path) });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::istream & doc, std::wstring_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, impl::narrow(source_path) });
  }

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse_file(std::wstring_view file_path)
  {
    return TOML_ANON_NAMESPACE::do_parse_file(impl::narrow(file_path));
  }

#endif // TOML_ENABLE_WINDOWS_COMPAT

#if TOML_HAS_CHAR8 && TOML_ENABLE_WINDOWS_COMPAT

  TOML_EXTERNAL_LINKAGE
  parse_result TOML_CALLCONV parse(std::u8string_view doc, std::wstring_view source_path)
  {
    return TOML_ANON_NAMESPACE::do_parse(TOML_ANON_NAMESPACE::utf8_reader{ doc, impl::narrow(source_path) });
  }

#endif // TOML_HAS_CHAR8 && TOML_ENABLE_WINDOWS_COMPAT

  TOML_ABI_NAMESPACE_END; // TOML_EXCEPTIONS
}
TOML_NAMESPACE_END;

#undef TOML_OVERALIGNED
#include "header_end.hpp"
#endif // TOML_ENABLE_PARSER

Coverage Report

Created: 2026-04-10 06:49