//

// Copyright 2017 The Abseil Authors.

//

// 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

//

//      https://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.

//

// -----------------------------------------------------------------------------

// File: str_cat.h

// -----------------------------------------------------------------------------

//

// This package contains functions for efficiently concatenating and appending

// strings: `StrCat()` and `StrAppend()`. Most of the work within these routines

// is actually handled through use of a special AlphaNum type, which was

// designed to be used as a parameter type that efficiently manages conversion

// to strings and avoids copies in the above operations.

//

// Any routine accepting either a string or a number may accept `AlphaNum`.

// The basic idea is that by accepting a `const AlphaNum &` as an argument

// to your function, your callers will automagically convert bools, integers,

// and floating point values to strings for you.

//

// NOTE: Use of `AlphaNum` outside of the //absl/strings package is unsupported

// except for the specific case of function parameters of type `AlphaNum` or

// `const AlphaNum &`. In particular, instantiating `AlphaNum` directly as a

// stack variable is not supported.

//

// Conversion from 8-bit values is not accepted because, if it were, then an

// attempt to pass ':' instead of ":" might result in a 58 ending up in your

// result.

//

// Bools convert to "0" or "1". Pointers to types other than `char *` are not

// valid inputs. No output is generated for null `char *` pointers.

//

// Floating point numbers are formatted with six-digit precision, which is

// the default for "std::cout <<" or printf "%g" (the same as "%.6g").

//

// You can convert to hexadecimal output rather than decimal output using the

// `Hex` type contained here. To do so, pass `Hex(my_int)` as a parameter to

// `StrCat()` or `StrAppend()`. You may specify a minimum hex field width using

// a `PadSpec` enum.

//

// User-defined types can be formatted with the `AbslStringify()` customization

// point. The API relies on detecting an overload in the user-defined type's

// namespace of a free (non-member) `AbslStringify()` function as a definition

// (typically declared as a friend and implemented in-line.

// with the following signature:

//

// class MyClass { ... };

//

// template <typename Sink>

// void AbslStringify(Sink& sink, const MyClass& value);

//

// An `AbslStringify()` overload for a type should only be declared in the same

// file and namespace as said type.

//

// Note that `AbslStringify()` also supports use with `absl::StrFormat()` and

// `absl::Substitute()`.

//

// Example:

//

// struct Point {

//   // To add formatting support to `Point`, we simply need to add a free

//   // (non-member) function `AbslStringify()`. This method specifies how

//   // Point should be printed when absl::StrCat() is called on it. You can add

//   // such a free function using a friend declaration within the body of the

//   // class. The sink parameter is a templated type to avoid requiring

//   // dependencies.

//   template <typename Sink> friend void AbslStringify(Sink&

//   sink, const Point& p) {

//     absl::Format(&sink, "(%v, %v)", p.x, p.y);

//   }

//

//   int x;

//   int y;

// };

// -----------------------------------------------------------------------------

#ifndef ABSL_STRINGS_STR_CAT_H_

#define ABSL_STRINGS_STR_CAT_H_

#include <algorithm>

#include <array>

#include <cassert>

#include <cstddef>

#include <cstdint>

#include <cstring>

#include <string>

#include <type_traits>

#include <utility>

#include <vector>

#include "absl/base/attributes.h"

#include "absl/base/nullability.h"

#include "absl/base/port.h"

#include "absl/meta/type_traits.h"

#include "absl/strings/has_absl_stringify.h"

#include "absl/strings/internal/resize_uninitialized.h"

#include "absl/strings/internal/stringify_sink.h"

#include "absl/strings/numbers.h"

#include "absl/strings/string_view.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

namespace strings_internal {

// AlphaNumBuffer allows a way to pass a string to StrCat without having to do

// memory allocation.  It is simply a pair of a fixed-size character array, and

// a size.  Please don't use outside of absl, yet.

template <size_t max_size>

struct AlphaNumBuffer {

  std::array<char, max_size> data;

  size_t size;

};

}  // namespace strings_internal

// Enum that specifies the number of significant digits to return in a `Hex` or

// `Dec` conversion and fill character to use. A `kZeroPad2` value, for example,

// would produce hexadecimal strings such as "0a","0f" and a 'kSpacePad5' value

// would produce hexadecimal strings such as "    a","    f".

enum PadSpec : uint8_t {

  kNoPad = 1,

  kZeroPad2,

  kZeroPad3,

  kZeroPad4,

  kZeroPad5,

  kZeroPad6,

  kZeroPad7,

  kZeroPad8,

  kZeroPad9,

  kZeroPad10,

  kZeroPad11,

  kZeroPad12,

  kZeroPad13,

  kZeroPad14,

  kZeroPad15,

  kZeroPad16,

  kZeroPad17,

  kZeroPad18,

  kZeroPad19,

  kZeroPad20,

  kSpacePad2 = kZeroPad2 + 64,

  kSpacePad3,

  kSpacePad4,

  kSpacePad5,

  kSpacePad6,

  kSpacePad7,

  kSpacePad8,

  kSpacePad9,

  kSpacePad10,

  kSpacePad11,

  kSpacePad12,

  kSpacePad13,

  kSpacePad14,

  kSpacePad15,

  kSpacePad16,

  kSpacePad17,

  kSpacePad18,

  kSpacePad19,

  kSpacePad20,

};

// -----------------------------------------------------------------------------

// Hex

// -----------------------------------------------------------------------------

//

// `Hex` stores a set of hexadecimal string conversion parameters for use

// within `AlphaNum` string conversions.

struct Hex {

  uint64_t value;

  uint8_t width;

  char fill;

  template <typename Int>

  explicit Hex(

      Int v, PadSpec spec = absl::kNoPad,

      typename std::enable_if<sizeof(Int) == 1 &&

                              !std::is_pointer<Int>::value>::type* = nullptr)

      : Hex(spec, static_cast<uint8_t>(v)) {}

  template <typename Int>

  explicit Hex(

      Int v, PadSpec spec = absl::kNoPad,

      typename std::enable_if<sizeof(Int) == 2 &&

                              !std::is_pointer<Int>::value>::type* = nullptr)

      : Hex(spec, static_cast<uint16_t>(v)) {}

  template <typename Int>

  explicit Hex(

      Int v, PadSpec spec = absl::kNoPad,

      typename std::enable_if<sizeof(Int) == 4 &&

                              !std::is_pointer<Int>::value>::type* = nullptr)

      : Hex(spec, static_cast<uint32_t>(v)) {}

  template <typename Int>

  explicit Hex(

      Int v, PadSpec spec = absl::kNoPad,

      typename std::enable_if<sizeof(Int) == 8 &&

                              !std::is_pointer<Int>::value>::type* = nullptr)

      : Hex(spec, static_cast<uint64_t>(v)) {}

  template <typename Pointee>

  explicit Hex(absl::Nullable<Pointee*> v, PadSpec spec = absl::kNoPad)

      : Hex(spec, reinterpret_cast<uintptr_t>(v)) {}

  template <typename S>

  friend void AbslStringify(S& sink, Hex hex) {

    static_assert(

        numbers_internal::kFastToBufferSize >= 32,

        "This function only works when output buffer >= 32 bytes long");

    char buffer[numbers_internal::kFastToBufferSize];

    char* const end = &buffer[numbers_internal::kFastToBufferSize];

    auto real_width =

        absl::numbers_internal::FastHexToBufferZeroPad16(hex.value, end - 16);

    if (real_width >= hex.width) {

      sink.Append(absl::string_view(end - real_width, real_width));

    } else {

      // Pad first 16 chars because FastHexToBufferZeroPad16 pads only to 16 and

      // max pad width can be up to 20.

      std::memset(end - 32, hex.fill, 16);

      // Patch up everything else up to the real_width.

      std::memset(end - real_width - 16, hex.fill, 16);

      sink.Append(absl::string_view(end - hex.width, hex.width));

    }

  }

 private:

  Hex(PadSpec spec, uint64_t v)

      : value(v),

        width(spec == absl::kNoPad

                  ? 1

                  : spec >= absl::kSpacePad2 ? spec - absl::kSpacePad2 + 2

                                             : spec - absl::kZeroPad2 + 2),

        fill(spec >= absl::kSpacePad2 ? ' ' : '0') {}

};

// -----------------------------------------------------------------------------

// Dec

// -----------------------------------------------------------------------------

//

// `Dec` stores a set of decimal string conversion parameters for use

// within `AlphaNum` string conversions.  Dec is slower than the default

// integer conversion, so use it only if you need padding.

struct Dec {

  uint64_t value;

  uint8_t width;

  char fill;

  bool neg;

  template <typename Int>

  explicit Dec(Int v, PadSpec spec = absl::kNoPad,

               typename std::enable_if<(sizeof(Int) <= 8)>::type* = nullptr)

      : value(v >= 0 ? static_cast<uint64_t>(v)

                     : uint64_t{0} - static_cast<uint64_t>(v)),

        width(spec == absl::kNoPad       ? 1

              : spec >= absl::kSpacePad2 ? spec - absl::kSpacePad2 + 2

                                         : spec - absl::kZeroPad2 + 2),

        fill(spec >= absl::kSpacePad2 ? ' ' : '0'),

        neg(v < 0) {}

  template <typename S>

  friend void AbslStringify(S& sink, Dec dec) {

    assert(dec.width <= numbers_internal::kFastToBufferSize);

    char buffer[numbers_internal::kFastToBufferSize];

    char* const end = &buffer[numbers_internal::kFastToBufferSize];

    char* const minfill = end - dec.width;

    char* writer = end;

    uint64_t val = dec.value;

    while (val > 9) {

      *--writer = '0' + (val % 10);

      val /= 10;

    }

    *--writer = '0' + static_cast<char>(val);

    if (dec.neg) *--writer = '-';

    ptrdiff_t fillers = writer - minfill;

    if (fillers > 0) {

      // Tricky: if the fill character is ' ', then it's <fill><+/-><digits>

      // But...: if the fill character is '0', then it's <+/-><fill><digits>

      bool add_sign_again = false;

      if (dec.neg && dec.fill == '0') {  // If filling with '0',

        ++writer;                    // ignore the sign we just added

        add_sign_again = true;       // and re-add the sign later.

      }

      writer -= fillers;

      std::fill_n(writer, fillers, dec.fill);

      if (add_sign_again) *--writer = '-';

    }

    sink.Append(absl::string_view(writer, static_cast<size_t>(end - writer)));

  }

};

// -----------------------------------------------------------------------------

// AlphaNum

// -----------------------------------------------------------------------------

//

// The `AlphaNum` class acts as the main parameter type for `StrCat()` and

// `StrAppend()`, providing efficient conversion of numeric, boolean, decimal,

// and hexadecimal values (through the `Dec` and `Hex` types) into strings.

// `AlphaNum` should only be used as a function parameter. Do not instantiate

//  `AlphaNum` directly as a stack variable.

class AlphaNum {

 public:

  // No bool ctor -- bools convert to an integral type.

  // A bool ctor would also convert incoming pointers (bletch).

  AlphaNum(int x)  // NOLINT(runtime/explicit)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(unsigned int x)  // NOLINT(runtime/explicit)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(long x)  // NOLINT(*)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(unsigned long x)  // NOLINT(*)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(long long x)  // NOLINT(*)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(unsigned long long x)  // NOLINT(*)

      : piece_(digits_, static_cast<size_t>(

                            numbers_internal::FastIntToBuffer(x, digits_) -

                            &digits_[0])) {}

  AlphaNum(float f)  // NOLINT(runtime/explicit)

      : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}

  AlphaNum(double f)  // NOLINT(runtime/explicit)

      : piece_(digits_, numbers_internal::SixDigitsToBuffer(f, digits_)) {}

  template <size_t size>

  AlphaNum(  // NOLINT(runtime/explicit)

      const strings_internal::AlphaNumBuffer<size>& buf

          ABSL_ATTRIBUTE_LIFETIME_BOUND)

      : piece_(&buf.data[0], buf.size) {}

  AlphaNum(absl::Nullable<const char*> c_str  // NOLINT(runtime/explicit)

               ABSL_ATTRIBUTE_LIFETIME_BOUND)

      : piece_(NullSafeStringView(c_str)) {}

  AlphaNum(absl::string_view pc  // NOLINT(runtime/explicit)

               ABSL_ATTRIBUTE_LIFETIME_BOUND)

      : piece_(pc) {}

  template <typename T, typename = typename std::enable_if<

                            HasAbslStringify<T>::value>::type>

  AlphaNum(  // NOLINT(runtime/explicit)

      const T& v ABSL_ATTRIBUTE_LIFETIME_BOUND,

      strings_internal::StringifySink&& sink ABSL_ATTRIBUTE_LIFETIME_BOUND = {})

      : piece_(strings_internal::ExtractStringification(sink, v)) {}

  template <typename Allocator>

  AlphaNum(  // NOLINT(runtime/explicit)

      const std::basic_string<char, std::char_traits<char>, Allocator>& str

          ABSL_ATTRIBUTE_LIFETIME_BOUND)

      : piece_(str) {}

  // Use string literals ":" instead of character literals ':'.

  AlphaNum(char c) = delete;  // NOLINT(runtime/explicit)

  AlphaNum(const AlphaNum&) = delete;

  AlphaNum& operator=(const AlphaNum&) = delete;

  absl::string_view::size_type size() const { return piece_.size(); }

  absl::Nullable<const char*> data() const { return piece_.data(); }

  absl::string_view Piece() const { return piece_; }

  // Match unscoped enums.  Use integral promotion so that a `char`-backed

  // enum becomes a wider integral type AlphaNum will accept.

  template <typename T,

            typename = typename std::enable_if<

                std::is_enum<T>{} && std::is_convertible<T, int>{} &&

                !HasAbslStringify<T>::value>::type>

  AlphaNum(T e)  // NOLINT(runtime/explicit)

      : AlphaNum(+e) {}

  // This overload matches scoped enums.  We must explicitly cast to the

  // underlying type, but use integral promotion for the same reason as above.

  template <typename T,

            typename std::enable_if<std::is_enum<T>{} &&

                                        !std::is_convertible<T, int>{} &&

                                        !HasAbslStringify<T>::value,

                                    char*>::type = nullptr>

  AlphaNum(T e)  // NOLINT(runtime/explicit)

      : AlphaNum(+static_cast<typename std::underlying_type<T>::type>(e)) {}

  // vector<bool>::reference and const_reference require special help to

  // convert to `AlphaNum` because it requires two user defined conversions.

  template <

      typename T,

      typename std::enable_if<

          std::is_class<T>::value &&

          (std::is_same<T, std::vector<bool>::reference>::value ||

           std::is_same<T, std::vector<bool>::const_reference>::value)>::type* =

          nullptr>

  AlphaNum(T e) : AlphaNum(static_cast<bool>(e)) {}  // NOLINT(runtime/explicit)

 private:

  absl::string_view piece_;

  char digits_[numbers_internal::kFastToBufferSize];

};

// -----------------------------------------------------------------------------

// StrCat()

// -----------------------------------------------------------------------------

//

// Merges given strings or numbers, using no delimiter(s), returning the merged

// result as a string.

//

// `StrCat()` is designed to be the fastest possible way to construct a string

// out of a mix of raw C strings, string_views, strings, bool values,

// and numeric values.

//

// Don't use `StrCat()` for user-visible strings. The localization process

// works poorly on strings built up out of fragments.

//

// For clarity and performance, don't use `StrCat()` when appending to a

// string. Use `StrAppend()` instead. In particular, avoid using any of these

// (anti-)patterns:

//

//   str.append(StrCat(...))

//   str += StrCat(...)

//   str = StrCat(str, ...)

//

// The last case is the worst, with a potential to change a loop

// from a linear time operation with O(1) dynamic allocations into a

// quadratic time operation with O(n) dynamic allocations.

//

// See `StrAppend()` below for more information.

namespace strings_internal {

// Do not call directly - this is not part of the public API.

std::string CatPieces(std::initializer_list<absl::string_view> pieces);

void AppendPieces(absl::Nonnull<std::string*> dest,

                  std::initializer_list<absl::string_view> pieces);

void STLStringAppendUninitializedAmortized(std::string* dest, size_t to_append);

// `SingleArgStrCat` overloads take built-in `int`, `long` and `long long` types

// (signed / unsigned) to avoid ambiguity on the call side. If we used int32_t

// and int64_t, then at least one of the three (`int` / `long` / `long long`)

// would have been ambiguous when passed to `SingleArgStrCat`.

std::string SingleArgStrCat(int x);

std::string SingleArgStrCat(unsigned int x);

std::string SingleArgStrCat(long x);                // NOLINT

std::string SingleArgStrCat(unsigned long x);       // NOLINT

std::string SingleArgStrCat(long long x);           // NOLINT

std::string SingleArgStrCat(unsigned long long x);  // NOLINT

std::string SingleArgStrCat(float x);

std::string SingleArgStrCat(double x);

// `SingleArgStrAppend` overloads are defined here for the same reasons as with

// `SingleArgStrCat` above.

void SingleArgStrAppend(std::string& str, int x);

void SingleArgStrAppend(std::string& str, unsigned int x);

void SingleArgStrAppend(std::string& str, long x);                // NOLINT

void SingleArgStrAppend(std::string& str, unsigned long x);       // NOLINT

void SingleArgStrAppend(std::string& str, long long x);           // NOLINT

void SingleArgStrAppend(std::string& str, unsigned long long x);  // NOLINT

template <typename T,

          typename = std::enable_if_t<std::is_arithmetic<T>::value &&

                                      !std::is_same<T, char>::value &&

                                      !std::is_same<T, bool>::value>>

using EnableIfFastCase = T;

}  // namespace strings_internal

ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }

template <typename T>

ABSL_MUST_USE_RESULT inline std::string StrCat(

    strings_internal::EnableIfFastCase<T> a) {

  return strings_internal::SingleArgStrCat(a);

}

ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) {

  return std::string(a.data(), a.size());

}

ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b);

ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,

                                        const AlphaNum& c);

ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,

                                        const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments

template <typename... AV>

ABSL_MUST_USE_RESULT inline std::string StrCat(

    const AlphaNum& a, const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,

    const AlphaNum& e, const AV&... args) {

  return strings_internal::CatPieces(

      {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),

       static_cast<const AlphaNum&>(args).Piece()...});

}

// -----------------------------------------------------------------------------

// StrAppend()

// -----------------------------------------------------------------------------

//

// Appends a string or set of strings to an existing string, in a similar

// fashion to `StrCat()`.

//

// WARNING: `StrAppend(&str, a, b, c, ...)` requires that none of the

// a, b, c, parameters be a reference into str. For speed, `StrAppend()` does

// not try to check each of its input arguments to be sure that they are not

// a subset of the string being appended to. That is, while this will work:

//

//   std::string s = "foo";

//   s += s;

//

// This output is undefined:

//

//   std::string s = "foo";

//   StrAppend(&s, s);

//

// This output is undefined as well, since `absl::string_view` does not own its

// data:

//

//   std::string s = "foobar";

//   absl::string_view p = s;

//   StrAppend(&s, p);

inline void StrAppend(absl::Nonnull<std::string*>) {}

void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a);

void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,

               const AlphaNum& b);

void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,

               const AlphaNum& b, const AlphaNum& c);

void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,

               const AlphaNum& b, const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments

template <typename... AV>

inline void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,

                      const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,

                      const AlphaNum& e, const AV&... args) {

  strings_internal::AppendPieces(

      dest, {a.Piece(), b.Piece(), c.Piece(), d.Piece(), e.Piece(),

             static_cast<const AlphaNum&>(args).Piece()...});

}

template <class String, class T>

std::enable_if_t<

    std::is_integral<absl::strings_internal::EnableIfFastCase<T>>::value, void>

StrAppend(absl::Nonnull<String*> result, T i) {

  return absl::strings_internal::SingleArgStrAppend(*result, i);

}

// This overload is only selected if all the parameters are numbers that can be

// handled quickly.

// Later we can look into how we can extend this to more general argument

// mixtures without bloating codegen too much, or copying unnecessarily.

template <typename String, typename... T>

std::enable_if_t<

    (sizeof...(T) > 1),

    std::common_type_t<std::conditional_t<

        true, void, absl::strings_internal::EnableIfFastCase<T>>...>>

StrAppend(absl::Nonnull<String*> str, T... args) {

  // Do not add unnecessary variables, logic, or even "free" lambdas here.

  // They can add overhead for the compiler and/or at run time.

  // Furthermore, assume this function will be inlined.

  // This function is carefully tailored to be able to be largely optimized away

  // so that it becomes near-equivalent to the caller handling each argument

  // individually while minimizing register pressure, so that the compiler

  // can inline it with minimal overhead.

  // First, calculate the total length, so we can perform just a single resize.

  // Save all the lengths for later.

  size_t total_length = 0;

  const ptrdiff_t lengths[] = {

      absl::numbers_internal::GetNumDigitsOrNegativeIfNegative(args)...};

  for (const ptrdiff_t possibly_negative_length : lengths) {

    // Lengths are negative for negative numbers. Keep them for later use, but

    // take their absolute values for calculating total lengths;

    total_length += possibly_negative_length < 0

                        ? static_cast<size_t>(-possibly_negative_length)

                        : static_cast<size_t>(possibly_negative_length);

  }

  // Now reserve space for all the arguments.

  const size_t old_size = str->size();

  absl::strings_internal::STLStringAppendUninitializedAmortized(str,

                                                                total_length);

  // Finally, output each argument one-by-one, from left to right.

  size_t i = 0;  // The current argument we're processing

  ptrdiff_t n;   // The length of the current argument

  typename String::pointer pos = &(*str)[old_size];

  using SomeTrivialEmptyType = std::false_type;

  // Ugly code due to the lack of C++14 fold expression makes us.

  const SomeTrivialEmptyType dummy1;

  for (const SomeTrivialEmptyType& dummy2 :

       {(/* Comma expressions are poor man's C++17 fold expression for C++14 */

         (void)(n = lengths[i]),

         (void)(n < 0 ? (void)(*pos++ = '-'), (n = ~n) : 0),

         (void)absl::numbers_internal::FastIntToBufferBackward(

             absl::numbers_internal::UnsignedAbsoluteValue(std::move(args)),

             pos += n, static_cast<uint32_t>(n)),

         (void)++i, dummy1)...}) {

    (void)dummy2;  // Remove & migrate to fold expressions in C++17

  }

}

// Helper function for the future StrCat default floating-point format, %.6g

// This is fast.

inline strings_internal::AlphaNumBuffer<

    numbers_internal::kSixDigitsToBufferSize>

SixDigits(double d) {

  strings_internal::AlphaNumBuffer<numbers_internal::kSixDigitsToBufferSize>

      result;

  result.size = numbers_internal::SixDigitsToBuffer(d, &result.data[0]);

  return result;

}

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_STRINGS_STR_CAT_H_