/src/spotify-json/include/spotify/json/detail/decode_helpers.hpp

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/*
 * Copyright (c) 2014-2019 Spotify AB
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

#pragma once

#include <cstddef>
#include <cstdint>
#include <cstring>
#include <string>
#include <unordered_set>
#include <vector>

#include <spotify/json/decode_context.hpp>
#include <spotify/json/detail/macros.hpp>
#include <spotify/json/detail/skip_chars.hpp>

#if _MSC_VER
#pragma intrinsic (memcmp)
#endif

namespace spotify {
namespace json {
namespace detail {

json_noreturn void fail(const decode_context &context, const char *error, ptrdiff_t d = 0);

json_force_inline void fail_if(
    const decode_context &context,
    const bool condition,
    const char *error,
    const ptrdiff_t d = 0) {
  if (json_unlikely(condition)) {
    fail(context, error, d);
  }
}

template <size_t num_required_bytes, typename string_type>
json_force_inline void require_bytes(const decode_context &context, const string_type &error) {
  fail_if(context, context.remaining() < num_required_bytes, error);
}

template <size_t num_required_bytes>
json_force_inline void require_bytes(const decode_context &context) {
  require_bytes<num_required_bytes>(context, "Unexpected end of input");
}

json_force_inline char peek_unchecked(const decode_context &context) {
  return *context.position;
}

/**
 * Peek at the current character that a decode_context refers to. If the
 * decode_context has ended, '\0' is returned. This is a useful helper when
 * while decoding it is necessary to ensure that the current character is a
 * specific one, for example '['.
 */
json_force_inline char peek(const decode_context &context) {
  return (context.remaining() ? peek_unchecked(context) : 0);
}

/**
 * Returns true if the next two characters are `first` and `second`.
 * Returns false if the characters do not match, or if there is less than 2
 * characters remaining.
 */
json_force_inline bool peek_2(const decode_context &context, const char first, const char second) {
  if (context.remaining() < 2) {
    return false;
  }
  return first == *context.position && second == *(context.position + 1);
}

json_force_inline char next_unchecked(decode_context &context) {
  return *(context.position++);
}

template <typename string_type>
json_force_inline char next(decode_context &context, const string_type &error) {
  require_bytes<1>(context, error);
  return next_unchecked(context);
}

json_force_inline char next(decode_context &context) {
  return next(context, "Unexpected end of input");
}

json_force_inline void skip_unchecked_1(decode_context &context) {
  context.position++;
}

json_force_inline void skip_unchecked_n(decode_context &context, const size_t num_bytes) {
  context.position += num_bytes;
}

json_force_inline void skip_any_n(decode_context &context, const size_t num_bytes) {
  fail_if(context, context.remaining() < num_bytes, "Unexpected end of input");
  skip_unchecked_n(context, num_bytes);
}

json_force_inline void skip_any_1(decode_context &context) {
  require_bytes<1>(context, "Unexpected end of input");
  context.position++;
}

/**
 * Skip past a specific character. If the context position does not point to a
 * matching character, a decode_exception is thrown.
 */
json_force_inline void skip_1(decode_context &context, char character) {
  fail_if(context, next(context) != character, "Unexpected input", -1);
}

/**
 * Skip past four specific characters. If the context position does not point to
 * matching characters, a decode_exception is thrown. 'characters' must be a C
 * string of at least length 4. Only the first four characters will be read.
 */
json_force_inline void skip_4(decode_context &context, const char characters[4]) {
  require_bytes<4>(context);
  fail_if(context, memcmp(characters, context.position, 4) != 0, "Unexpected input");
  context.position += 4;
}

/**
 * Helper function for parsing the comma separated entities in JSON: objects
 * and arrays. intro and outro are the characters before and after the entity:
 * {} and [], respectively. parse is a callback that is called for each
 * element in the comma separated list. It should advance the parse context to
 * after that element or mark the context as failed.
 *
 * The parse callback must mark the context as failed if it sees a premature end
 * of input, otherwise this function might enter an infinite loop!
 *
 * context.has_failed() must be false when this function is called.
 */
template <typename parse_function>
json_never_inline void decode_comma_separated(decode_context &context, char intro, char outro, parse_function parse) {
  skip_1(context, intro);
  skip_any_whitespace(context);

  if (json_likely(peek(context) != outro)) {
    parse();
    skip_any_whitespace(context);

    while (json_likely(peek(context) != outro)) {
      skip_1(context, ',');
      skip_any_whitespace(context);
      parse();
      skip_any_whitespace(context);
    }
  }

  context.position++;
}

/**
 * Helper for parsing JSON objects. callback is called once for each key/value
 * pair. It is given the already parsed key and is expected to parse the value
 * and store it away as needed. The callback may be invoked a few times even if
 * parsing fails later on.
 */
template <typename key_codec_type, typename callback_function>
json_force_inline void decode_object(decode_context &context, const callback_function &callback) {
  auto codec = key_codec_type();
  decode_comma_separated(context, '{', '}', [&]{
    auto key = codec.decode(context);
    skip_any_whitespace(context);
    skip_1(context, ':');
    skip_any_whitespace(context);
    callback(std::move(key));
  });
}

json_force_inline void skip_true(decode_context &context) {
  skip_4(context, "true");
}

json_force_inline void skip_false(decode_context &context) {
  context.position++;  // skip past the 'f' in 'false', we know it is there
  skip_4(context, "alse");
}

json_force_inline void skip_null(decode_context &context) {
  skip_4(context, "null");
}

}  // namespace detail
}  // namespace json
}  // namespace spotify

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Created: 2025-08-03 06:22