// 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.

// This file contains string processing functions related to

// numeric values.

#include "absl/strings/numbers.h"

#include <algorithm>

#include <array>

#include <cassert>

#include <cfloat>  // for DBL_DIG and FLT_DIG

#include <cmath>   // for HUGE_VAL

#include <cstdint>

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <iterator>

#include <limits>

#include <system_error>  // NOLINT(build/c++11)

#include <utility>

#include "absl/base/attributes.h"

#include "absl/base/config.h"

#include "absl/base/internal/endian.h"

#include "absl/base/internal/raw_logging.h"

#include "absl/base/nullability.h"

#include "absl/base/optimization.h"

#include "absl/numeric/bits.h"

#include "absl/numeric/int128.h"

#include "absl/strings/ascii.h"

#include "absl/strings/charconv.h"

#include "absl/strings/match.h"

#include "absl/strings/string_view.h"

namespace absl {

ABSL_NAMESPACE_BEGIN

bool SimpleAtof(absl::string_view str, float* absl_nonnull out) {

  *out = 0.0;

  str = StripAsciiWhitespace(str);

  // std::from_chars doesn't accept an initial +, but SimpleAtof does, so if one

  // is present, skip it, while avoiding accepting "+-0" as valid.

  if (!str.empty() && str[0] == '+') {

    str.remove_prefix(1);

    if (!str.empty() && str[0] == '-') {

      return false;

    }

  }

  auto result = absl::from_chars(str.data(), str.data() + str.size(), *out);

  if (result.ec == std::errc::invalid_argument) {

    return false;

  }

  if (result.ptr != str.data() + str.size()) {

    // not all non-whitespace characters consumed

    return false;

  }

  // from_chars() with DR 3081's current wording will return max() on

  // overflow.  SimpleAtof returns infinity instead.

  if (result.ec == std::errc::result_out_of_range) {

    if (*out > 1.0) {

      *out = std::numeric_limits<float>::infinity();

    } else if (*out < -1.0) {

      *out = -std::numeric_limits<float>::infinity();

    }

  }

  return true;

}

bool SimpleAtod(absl::string_view str, double* absl_nonnull out) {

  *out = 0.0;

  str = StripAsciiWhitespace(str);

  if (str.empty()) {

    // absl::from_chars doesn't accept empty strings.

    return false;

  }

  // std::from_chars doesn't accept an initial +, but SimpleAtod does, so if one

  // is present, skip it, while avoiding accepting "+-0" as valid.

  if (str[0] == '+') {

    str.remove_prefix(1);

    if (str.empty() || str[0] == '-') {

      return false;

    }

  }

  auto result = absl::from_chars(str.data(), str.data() + str.size(), *out);

  if (result.ec == std::errc::invalid_argument) {

    return false;

  }

  if (result.ptr != str.data() + str.size()) {

    // not all non-whitespace characters consumed

    return false;

  }

  // from_chars() with DR 3081's current wording will return max() on

  // overflow.  SimpleAtod returns infinity instead.

  if (result.ec == std::errc::result_out_of_range) {

    if (*out > 1.0) {

      *out = std::numeric_limits<double>::infinity();

    } else if (*out < -1.0) {

      *out = -std::numeric_limits<double>::infinity();

    }

  }

  return true;

}

bool SimpleAtob(absl::string_view str, bool* absl_nonnull out) {

  ABSL_RAW_CHECK(out != nullptr, "Output pointer must not be nullptr.");

  if (EqualsIgnoreCase(str, "true") || EqualsIgnoreCase(str, "t") ||

      EqualsIgnoreCase(str, "yes") || EqualsIgnoreCase(str, "y") ||

      EqualsIgnoreCase(str, "1")) {

    *out = true;

    return true;

  }

  if (EqualsIgnoreCase(str, "false") || EqualsIgnoreCase(str, "f") ||

      EqualsIgnoreCase(str, "no") || EqualsIgnoreCase(str, "n") ||

      EqualsIgnoreCase(str, "0")) {

    *out = false;

    return true;

  }

  return false;

}

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

// FastIntToBuffer() overloads

//

// Like the Fast*ToBuffer() functions above, these are intended for speed.

// Unlike the Fast*ToBuffer() functions, however, these functions write

// their output to the beginning of the buffer.  The caller is responsible

// for ensuring that the buffer has enough space to hold the output.

//

// Returns a pointer to the end of the string (i.e. the null character

// terminating the string).

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

namespace {

// Various routines to encode integers to strings.

// We split data encodings into a group of 2 digits, 4 digits, 8 digits as

// it's easier to combine powers of two into scalar arithmetic.

// Previous implementation used a lookup table of 200 bytes for every 2 bytes

// and it was memory bound, any L1 cache miss would result in a much slower

// result. When benchmarking with a cache eviction rate of several percent,

// this implementation proved to be better.

// These constants represent '00', '0000' and '00000000' as ascii strings in

// integers. We can add these numbers if we encode to bytes from 0 to 9. as

// 'i' = '0' + i for 0 <= i <= 9.

constexpr uint32_t kTwoZeroBytes = 0x0101 * '0';

constexpr uint64_t kFourZeroBytes = 0x01010101 * '0';

constexpr uint64_t kEightZeroBytes = 0x0101010101010101ull * '0';

// * 103 / 1024 is a division by 10 for values from 0 to 99. It's also a

// division of a structure [k takes 2 bytes][m takes 2 bytes], then * 103 / 1024

// will be [k / 10][m / 10]. It allows parallel division.

constexpr uint64_t kDivisionBy10Mul = 103u;

constexpr uint64_t kDivisionBy10Div = 1 << 10;

// * 10486 / 1048576 is a division by 100 for values from 0 to 9999.

constexpr uint64_t kDivisionBy100Mul = 10486u;

constexpr uint64_t kDivisionBy100Div = 1 << 20;

// Encode functions write the ASCII output of input `n` to `out_str`.

inline char* EncodeHundred(uint32_t n, char* absl_nonnull out_str) {

  int num_digits = static_cast<int>(n - 10) >> 8;

  uint32_t div10 = (n * kDivisionBy10Mul) / kDivisionBy10Div;

  uint32_t mod10 = n - 10u * div10;

  uint32_t base = kTwoZeroBytes + div10 + (mod10 << 8);

  base >>= num_digits & 8;

  little_endian::Store16(out_str, static_cast<uint16_t>(base));

  return out_str + 2 + num_digits;

}

inline char* EncodeTenThousand(uint32_t n, char* absl_nonnull out_str) {

  // We split lower 2 digits and upper 2 digits of n into 2 byte consecutive

  // blocks. 123 ->  [\0\1][\0\23]. We divide by 10 both blocks

  // (it's 1 division + zeroing upper bits), and compute modulo 10 as well "in

  // parallel". Then we combine both results to have both ASCII digits,

  // strip trailing zeros, add ASCII '0000' and return.

  uint32_t div100 = (n * kDivisionBy100Mul) / kDivisionBy100Div;

  uint32_t mod100 = n - 100ull * div100;

  uint32_t hundreds = (mod100 << 16) + div100;

  uint32_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;

  tens &= (0xFull << 16) | 0xFull;

  tens += (hundreds - 10ull * tens) << 8;

  ABSL_ASSUME(tens != 0);

  // The result can contain trailing zero bits, we need to strip them to a first

  // significant byte in a final representation. For example, for n = 123, we

  // have tens to have representation \0\1\2\3. We do `& -8` to round

  // to a multiple to 8 to strip zero bytes, not all zero bits.

  // countr_zero to help.

  // 0 minus 8 to make MSVC happy.

  uint32_t zeroes = static_cast<uint32_t>(absl::countr_zero(tens)) & (0 - 8u);

  tens += kFourZeroBytes;

  tens >>= zeroes;

  little_endian::Store32(out_str, tens);

  return out_str + sizeof(tens) - zeroes / 8;

}

// Helper function to produce an ASCII representation of `i`.

//

// Function returns an 8-byte integer which when summed with `kEightZeroBytes`,

// can be treated as a printable buffer with ascii representation of `i`,

// possibly with leading zeros.

//

// Example:

//

//  uint64_t buffer = PrepareEightDigits(102030) + kEightZeroBytes;

//  char* ascii = reinterpret_cast<char*>(&buffer);

//  // Note two leading zeros:

//  EXPECT_EQ(absl::string_view(ascii, 8), "00102030");

//

// Pre-condition: `i` must be less than 100000000.

inline uint64_t PrepareEightDigits(uint32_t i) {

  ABSL_ASSUME(i < 10000'0000);

  // Prepare 2 blocks of 4 digits "in parallel".

  uint32_t hi = i / 10000;

  uint32_t lo = i % 10000;

  uint64_t merged = hi | (uint64_t{lo} << 32);

  uint64_t div100 = ((merged * kDivisionBy100Mul) / kDivisionBy100Div) &

                    ((0x7Full << 32) | 0x7Full);

  uint64_t mod100 = merged - 100ull * div100;

  uint64_t hundreds = (mod100 << 16) + div100;

  uint64_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;

  tens &= (0xFull << 48) | (0xFull << 32) | (0xFull << 16) | 0xFull;

  tens += (hundreds - 10ull * tens) << 8;

  return tens;

}

inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* absl_nonnull EncodeFullU32(

    uint32_t n, char* absl_nonnull out_str) {

  if (n < 10) {

    *out_str = static_cast<char>('0' + n);

    return out_str + 1;

  }

  if (n < 100'000'000) {

    uint64_t bottom = PrepareEightDigits(n);

    ABSL_ASSUME(bottom != 0);

    // 0 minus 8 to make MSVC happy.

    uint32_t zeroes =

        static_cast<uint32_t>(absl::countr_zero(bottom)) & (0 - 8u);

    little_endian::Store64(out_str, (bottom + kEightZeroBytes) >> zeroes);

    return out_str + sizeof(bottom) - zeroes / 8;

  }

  uint32_t div08 = n / 100'000'000;

  uint32_t mod08 = n % 100'000'000;

  uint64_t bottom = PrepareEightDigits(mod08) + kEightZeroBytes;

  out_str = EncodeHundred(div08, out_str);

  little_endian::Store64(out_str, bottom);

  return out_str + sizeof(bottom);

}

inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* EncodeFullU64(uint64_t i,

                                                        char* buffer) {

  if (i <= std::numeric_limits<uint32_t>::max()) {

    return EncodeFullU32(static_cast<uint32_t>(i), buffer);

  }

  uint32_t mod08;

  if (i < 1'0000'0000'0000'0000ull) {

    uint32_t div08 = static_cast<uint32_t>(i / 100'000'000ull);

    mod08 =  static_cast<uint32_t>(i % 100'000'000ull);

    buffer = EncodeFullU32(div08, buffer);

  } else {

    uint64_t div08 = i / 100'000'000ull;

    mod08 =  static_cast<uint32_t>(i % 100'000'000ull);

    uint32_t div016 = static_cast<uint32_t>(div08 / 100'000'000ull);

    uint32_t div08mod08 = static_cast<uint32_t>(div08 % 100'000'000ull);

    uint64_t mid_result = PrepareEightDigits(div08mod08) + kEightZeroBytes;

    buffer = EncodeTenThousand(div016, buffer);

    little_endian::Store64(buffer, mid_result);

    buffer += sizeof(mid_result);

  }

  uint64_t mod_result = PrepareEightDigits(mod08) + kEightZeroBytes;

  little_endian::Store64(buffer, mod_result);

  return buffer + sizeof(mod_result);

}

}  // namespace

void numbers_internal::PutTwoDigits(uint32_t i, char* absl_nonnull buf) {

  assert(i < 100);

  uint32_t base = kTwoZeroBytes;

  uint32_t div10 = (i * kDivisionBy10Mul) / kDivisionBy10Div;

  uint32_t mod10 = i - 10u * div10;

  base += div10 + (mod10 << 8);

  little_endian::Store16(buf, static_cast<uint16_t>(base));

}

char* absl_nonnull numbers_internal::FastIntToBuffer(

    uint32_t n, char* absl_nonnull out_str) {

  out_str = EncodeFullU32(n, out_str);

  *out_str = '\0';

  return out_str;

}

char* absl_nonnull numbers_internal::FastIntToBuffer(

    int32_t i, char* absl_nonnull buffer) {

  uint32_t u = static_cast<uint32_t>(i);

  if (i < 0) {

    *buffer++ = '-';

    // We need to do the negation in modular (i.e., "unsigned")

    // arithmetic; MSVC++ apparently warns for plain "-u", so

    // we write the equivalent expression "0 - u" instead.

    u = 0 - u;

  }

  buffer = EncodeFullU32(u, buffer);

  *buffer = '\0';

  return buffer;

}

char* absl_nonnull numbers_internal::FastIntToBuffer(

    uint64_t i, char* absl_nonnull buffer) {

  buffer = EncodeFullU64(i, buffer);

  *buffer = '\0';

  return buffer;

}

char* absl_nonnull numbers_internal::FastIntToBuffer(

    int64_t i, char* absl_nonnull buffer) {

  uint64_t u = static_cast<uint64_t>(i);

  if (i < 0) {

    *buffer++ = '-';

    // We need to do the negation in modular (i.e., "unsigned")

    // arithmetic; MSVC++ apparently warns for plain "-u", so

    // we write the equivalent expression "0 - u" instead.

    u = 0 - u;

  }

  buffer = EncodeFullU64(u, buffer);

  *buffer = '\0';

  return buffer;

}

// Given a 128-bit number expressed as a pair of uint64_t, high half first,

// return that number multiplied by the given 32-bit value.  If the result is

// too large to fit in a 128-bit number, divide it by 2 until it fits.

static std::pair<uint64_t, uint64_t> Mul32(std::pair<uint64_t, uint64_t> num,

                                           uint32_t mul) {

  uint64_t bits0_31 = num.second & 0xFFFFFFFF;

  uint64_t bits32_63 = num.second >> 32;

  uint64_t bits64_95 = num.first & 0xFFFFFFFF;

  uint64_t bits96_127 = num.first >> 32;

  // The picture so far: each of these 64-bit values has only the lower 32 bits

  // filled in.

  // bits96_127:          [ 00000000 xxxxxxxx ]

  // bits64_95:                    [ 00000000 xxxxxxxx ]

  // bits32_63:                             [ 00000000 xxxxxxxx ]

  // bits0_31:                                       [ 00000000 xxxxxxxx ]

  bits0_31 *= mul;

  bits32_63 *= mul;

  bits64_95 *= mul;

  bits96_127 *= mul;

  // Now the top halves may also have value, though all 64 of their bits will

  // never be set at the same time, since they are a result of a 32x32 bit

  // multiply.  This makes the carry calculation slightly easier.

  // bits96_127:          [ mmmmmmmm | mmmmmmmm ]

  // bits64_95:                    [ | mmmmmmmm mmmmmmmm | ]

  // bits32_63:                      |        [ mmmmmmmm | mmmmmmmm ]

  // bits0_31:                       |                 [ | mmmmmmmm mmmmmmmm ]

  // eventually:        [ bits128_up | ...bits64_127.... | ..bits0_63... ]

  uint64_t bits0_63 = bits0_31 + (bits32_63 << 32);

  uint64_t bits64_127 = bits64_95 + (bits96_127 << 32) + (bits32_63 >> 32) +

                        (bits0_63 < bits0_31);

  uint64_t bits128_up = (bits96_127 >> 32) + (bits64_127 < bits64_95);

  if (bits128_up == 0) return {bits64_127, bits0_63};

  auto shift = static_cast<unsigned>(bit_width(bits128_up));

  uint64_t lo = (bits0_63 >> shift) + (bits64_127 << (64 - shift));

  uint64_t hi = (bits64_127 >> shift) + (bits128_up << (64 - shift));

  return {hi, lo};

}

// Compute num * 5 ^ expfive, and return the first 128 bits of the result,

// where the first bit is always a one.  So PowFive(1, 0) starts 0b100000,

// PowFive(1, 1) starts 0b101000, PowFive(1, 2) starts 0b110010, etc.

static std::pair<uint64_t, uint64_t> PowFive(uint64_t num, int expfive) {

  std::pair<uint64_t, uint64_t> result = {num, 0};

  while (expfive >= 13) {

    // 5^13 is the highest power of five that will fit in a 32-bit integer.

    result = Mul32(result, 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5);

    expfive -= 13;

  }

  constexpr uint32_t powers_of_five[13] = {

      1,

      5,

      5 * 5,

      5 * 5 * 5,

      5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5,

      5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5 * 5};

  result = Mul32(result, powers_of_five[expfive & 15]);

  int shift = countl_zero(result.first);

  if (shift != 0) {

    result.first = (result.first << shift) + (result.second >> (64 - shift));

    result.second = (result.second << shift);

  }

  return result;

}

struct ExpDigits {

  int32_t exponent;

  char digits[6];

};

// SplitToSix converts value, a positive double-precision floating-point number,

// into a base-10 exponent and 6 ASCII digits, where the first digit is never

// zero.  For example, SplitToSix(1) returns an exponent of zero and a digits

// array of {'1', '0', '0', '0', '0', '0'}.  If value is exactly halfway between

// two possible representations, e.g. value = 100000.5, then "round to even" is

// performed.

static ExpDigits SplitToSix(const double value) {

  ExpDigits exp_dig;

  int exp = 5;

  double d = value;

  // First step: calculate a close approximation of the output, where the

  // value d will be between 100,000 and 999,999, representing the digits

  // in the output ASCII array, and exp is the base-10 exponent.  It would be

  // faster to use a table here, and to look up the base-2 exponent of value,

  // however value is an IEEE-754 64-bit number, so the table would have 2,000

  // entries, which is not cache-friendly.

  if (d >= 999999.5) {

    if (d >= 1e+261) exp += 256, d *= 1e-256;

    if (d >= 1e+133) exp += 128, d *= 1e-128;

    if (d >= 1e+69) exp += 64, d *= 1e-64;

    if (d >= 1e+37) exp += 32, d *= 1e-32;

    if (d >= 1e+21) exp += 16, d *= 1e-16;

    if (d >= 1e+13) exp += 8, d *= 1e-8;

    if (d >= 1e+9) exp += 4, d *= 1e-4;

    if (d >= 1e+7) exp += 2, d *= 1e-2;

    if (d >= 1e+6) exp += 1, d *= 1e-1;

  } else {

    if (d < 1e-250) exp -= 256, d *= 1e256;

    if (d < 1e-122) exp -= 128, d *= 1e128;

    if (d < 1e-58) exp -= 64, d *= 1e64;

    if (d < 1e-26) exp -= 32, d *= 1e32;

    if (d < 1e-10) exp -= 16, d *= 1e16;

    if (d < 1e-2) exp -= 8, d *= 1e8;

    if (d < 1e+2) exp -= 4, d *= 1e4;

    if (d < 1e+4) exp -= 2, d *= 1e2;

    if (d < 1e+5) exp -= 1, d *= 1e1;

  }

  // At this point, d is in the range [99999.5..999999.5) and exp is in the

  // range [-324..308]. Since we need to round d up, we want to add a half

  // and truncate.

  // However, the technique above may have lost some precision, due to its

  // repeated multiplication by constants that each may be off by half a bit

  // of precision.  This only matters if we're close to the edge though.

  // Since we'd like to know if the fractional part of d is close to a half,

  // we multiply it by 65536 and see if the fractional part is close to 32768.

  // (The number doesn't have to be a power of two,but powers of two are faster)

  uint64_t d64k = static_cast<uint64_t>(d * 65536);

  uint32_t dddddd;  // A 6-digit decimal integer.

  if ((d64k % 65536) == 32767 || (d64k % 65536) == 32768) {

    // OK, it's fairly likely that precision was lost above, which is

    // not a surprise given only 52 mantissa bits are available.  Therefore

    // redo the calculation using 128-bit numbers.  (64 bits are not enough).

    // Start out with digits rounded down; maybe add one below.

    dddddd = static_cast<uint32_t>(d64k / 65536);

    // mantissa is a 64-bit integer representing M.mmm... * 2^63.  The actual

    // value we're representing, of course, is M.mmm... * 2^exp2.

    int exp2;

    double m = std::frexp(value, &exp2);

    uint64_t mantissa =

        static_cast<uint64_t>(m * (32768.0 * 65536.0 * 65536.0 * 65536.0));

    // std::frexp returns an m value in the range [0.5, 1.0), however we

    // can't multiply it by 2^64 and convert to an integer because some FPUs

    // throw an exception when converting an number higher than 2^63 into an

    // integer - even an unsigned 64-bit integer!  Fortunately it doesn't matter

    // since m only has 52 significant bits anyway.

    mantissa <<= 1;

    exp2 -= 64;  // not needed, but nice for debugging

    // OK, we are here to compare:

    //     (dddddd + 0.5) * 10^(exp-5)  vs.  mantissa * 2^exp2

    // so we can round up dddddd if appropriate.  Those values span the full

    // range of 600 orders of magnitude of IEE 64-bit floating-point.

    // Fortunately, we already know they are very close, so we don't need to

    // track the base-2 exponent of both sides.  This greatly simplifies the

    // the math since the 2^exp2 calculation is unnecessary and the power-of-10

    // calculation can become a power-of-5 instead.

    std::pair<uint64_t, uint64_t> edge, val;

    if (exp >= 6) {

      // Compare (dddddd + 0.5) * 5 ^ (exp - 5) to mantissa

      // Since we're tossing powers of two, 2 * dddddd + 1 is the

      // same as dddddd + 0.5

      edge = PowFive(2 * dddddd + 1, exp - 5);

      val.first = mantissa;

      val.second = 0;

    } else {

      // We can't compare (dddddd + 0.5) * 5 ^ (exp - 5) to mantissa as we did

      // above because (exp - 5) is negative.  So we compare (dddddd + 0.5) to

      // mantissa * 5 ^ (5 - exp)

      edge = PowFive(2 * dddddd + 1, 0);

      val = PowFive(mantissa, 5 - exp);

    }

    // printf("exp=%d %016lx %016lx vs %016lx %016lx\n", exp, val.first,

    //        val.second, edge.first, edge.second);

    if (val > edge) {

      dddddd++;

    } else if (val == edge) {

      dddddd += (dddddd & 1);

    }

  } else {

    // Here, we are not close to the edge.

    dddddd = static_cast<uint32_t>((d64k + 32768) / 65536);

  }

  if (dddddd == 1000000) {

    dddddd = 100000;

    exp += 1;

  }

  exp_dig.exponent = exp;

  uint32_t two_digits = dddddd / 10000;

  dddddd -= two_digits * 10000;

  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[0]);

  two_digits = dddddd / 100;

  dddddd -= two_digits * 100;

  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[2]);

  numbers_internal::PutTwoDigits(dddddd, &exp_dig.digits[4]);

  return exp_dig;

}

// Helper function for fast formatting of floating-point.

// The result is the same as "%g", a.k.a. "%.6g".

size_t numbers_internal::SixDigitsToBuffer(double d,

                                           char* absl_nonnull const buffer) {

  static_assert(std::numeric_limits<float>::is_iec559,

                "IEEE-754/IEC-559 support only");

  char* out = buffer;  // we write data to out, incrementing as we go, but

                       // FloatToBuffer always returns the address of the buffer

                       // passed in.

  if (std::isnan(d)) {

    strcpy(out, "nan");  // NOLINT(runtime/printf)

    return 3;

  }

  if (d == 0) {  // +0 and -0 are handled here

    if (std::signbit(d)) *out++ = '-';

    *out++ = '0';

    *out = 0;

    return static_cast<size_t>(out - buffer);

  }

  if (d < 0) {

    *out++ = '-';

    d = -d;

  }

  if (d > std::numeric_limits<double>::max()) {

    strcpy(out, "inf");  // NOLINT(runtime/printf)

    return static_cast<size_t>(out + 3 - buffer);

  }

  auto exp_dig = SplitToSix(d);

  int exp = exp_dig.exponent;

  const char* digits = exp_dig.digits;

  out[0] = '0';

  out[1] = '.';

  switch (exp) {

    case 5:

      memcpy(out, &digits[0], 6), out += 6;

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case 4:

      memcpy(out, &digits[0], 5), out += 5;

      if (digits[5] != '0') {

        *out++ = '.';

        *out++ = digits[5];

      }

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case 3:

      memcpy(out, &digits[0], 4), out += 4;

      if ((digits[5] | digits[4]) != '0') {

        *out++ = '.';

        *out++ = digits[4];

        if (digits[5] != '0') *out++ = digits[5];

      }

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case 2:

      memcpy(out, &digits[0], 3), out += 3;

      *out++ = '.';

      memcpy(out, &digits[3], 3);

      out += 3;

      while (out[-1] == '0') --out;

      if (out[-1] == '.') --out;

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case 1:

      memcpy(out, &digits[0], 2), out += 2;

      *out++ = '.';

      memcpy(out, &digits[2], 4);

      out += 4;

      while (out[-1] == '0') --out;

      if (out[-1] == '.') --out;

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case 0:

      memcpy(out, &digits[0], 1), out += 1;

      *out++ = '.';

      memcpy(out, &digits[1], 5);

      out += 5;

      while (out[-1] == '0') --out;

      if (out[-1] == '.') --out;

      *out = 0;

      return static_cast<size_t>(out - buffer);

    case -4:

      out[2] = '0';

      ++out;

      ABSL_FALLTHROUGH_INTENDED;

    case -3:

      out[2] = '0';

      ++out;

      ABSL_FALLTHROUGH_INTENDED;

    case -2:

      out[2] = '0';

      ++out;

      ABSL_FALLTHROUGH_INTENDED;

    case -1:

      out += 2;

      memcpy(out, &digits[0], 6);

      out += 6;

      while (out[-1] == '0') --out;

      *out = 0;

      return static_cast<size_t>(out - buffer);

  }

  assert(exp < -4 || exp >= 6);

  out[0] = digits[0];

  assert(out[1] == '.');

  out += 2;

  memcpy(out, &digits[1], 5), out += 5;

  while (out[-1] == '0') --out;

  if (out[-1] == '.') --out;

  *out++ = 'e';

  if (exp > 0) {

    *out++ = '+';

  } else {

    *out++ = '-';

    exp = -exp;

  }

  if (exp > 99) {

    int dig1 = exp / 100;

    exp -= dig1 * 100;

    *out++ = '0' + static_cast<char>(dig1);

  }

  PutTwoDigits(static_cast<uint32_t>(exp), out);

  out += 2;

  *out = 0;

  return static_cast<size_t>(out - buffer);

}

namespace {

// Represents integer values of digits.

// Uses 36 to indicate an invalid character since we support

// bases up to 36.

static constexpr std::array<int8_t, 256> kAsciiToInt = {

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,  // 16 36s.

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 0,  1,  2,  3,  4,  5,

    6,  7,  8,  9,  36, 36, 36, 36, 36, 36, 36, 10, 11, 12, 13, 14, 15, 16, 17,

    18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,

    36, 36, 36, 36, 36, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,

    24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36,

    36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36};

// Parse the sign and optional hex or oct prefix in text.

inline bool safe_parse_sign_and_base(

    absl::string_view* absl_nonnull text /*inout*/,

    int* absl_nonnull base_ptr /*inout*/,

    bool* absl_nonnull negative_ptr /*output*/) {

  if (text->data() == nullptr) {

    return false;

  }

  const char* start = text->data();

  const char* end = start + text->size();

  int base = *base_ptr;

  // Consume whitespace.

  while (start < end &&

         absl::ascii_isspace(static_cast<unsigned char>(start[0]))) {

    ++start;

  }

  while (start < end &&

         absl::ascii_isspace(static_cast<unsigned char>(end[-1]))) {

    --end;

  }

  if (start >= end) {

    return false;

  }

  // Consume sign.

  *negative_ptr = (start[0] == '-');

  if (*negative_ptr || start[0] == '+') {

    ++start;

    if (start >= end) {

      return false;

    }

  }

  // Consume base-dependent prefix.

  //  base 0: "0x" -> base 16, "0" -> base 8, default -> base 10

  //  base 16: "0x" -> base 16

  // Also validate the base.

  if (base == 0) {

    if (end - start >= 2 && start[0] == '0' &&

        (start[1] == 'x' || start[1] == 'X')) {

      base = 16;

      start += 2;

      if (start >= end) {

        // "0x" with no digits after is invalid.

        return false;

      }

    } else if (end - start >= 1 && start[0] == '0') {

      base = 8;

      start += 1;

    } else {

      base = 10;

    }

  } else if (base == 16) {

    if (end - start >= 2 && start[0] == '0' &&

        (start[1] == 'x' || start[1] == 'X')) {

      start += 2;

      if (start >= end) {

        // "0x" with no digits after is invalid.

        return false;

      }

    }

  } else if (base >= 2 && base <= 36) {

    // okay

  } else {

    return false;

  }

  *text = absl::string_view(start, static_cast<size_t>(end - start));

  *base_ptr = base;

  return true;

}

// Consume digits.

//

// The classic loop:

//

//   for each digit

//     value = value * base + digit

//   value *= sign

//

// The classic loop needs overflow checking.  It also fails on the most

// negative integer, -2147483648 in 32-bit two's complement representation.

//

// My improved loop:

//

//  if (!negative)

//    for each digit

//      value = value * base

//      value = value + digit

//  else

//    for each digit

//      value = value * base

//      value = value - digit

//

// Overflow checking becomes simple.

// Lookup tables per IntType:

// vmax/base and vmin/base are precomputed because division costs at least 8ns.

// TODO(junyer): Doing this per base instead (i.e. an array of structs, not a

// struct of arrays) would probably be better in terms of d-cache for the most

// commonly used bases.

template <typename IntType>

struct LookupTables {

  ABSL_CONST_INIT static const IntType kVmaxOverBase[];

  ABSL_CONST_INIT static const IntType kVminOverBase[];

};

// An array initializer macro for X/base where base in [0, 36].

// However, note that lookups for base in [0, 1] should never happen because

// base has been validated to be in [2, 36] by safe_parse_sign_and_base().

#define X_OVER_BASE_INITIALIZER(X)                                        \

  {                                                                       \

    0, 0, X / 2, X / 3, X / 4, X / 5, X / 6, X / 7, X / 8, X / 9, X / 10, \

        X / 11, X / 12, X / 13, X / 14, X / 15, X / 16, X / 17, X / 18,   \

        X / 19, X / 20, X / 21, X / 22, X / 23, X / 24, X / 25, X / 26,   \

        X / 27, X / 28, X / 29, X / 30, X / 31, X / 32, X / 33, X / 34,   \

        X / 35, X / 36,                                                   \

  }

// This kVmaxOverBase is generated with

//  for (int base = 2; base < 37; ++base) {

//    absl::uint128 max = std::numeric_limits<absl::uint128>::max();

//    auto result = max / base;

//    std::cout << "    MakeUint128(" << absl::Uint128High64(result) << "u, "

//              << absl::Uint128Low64(result) << "u),\n";

//  }

// See https://godbolt.org/z/aneYsb

//

// uint128& operator/=(uint128) is not constexpr, so hardcode the resulting

// array to avoid a static initializer.

template <>

ABSL_CONST_INIT const uint128 LookupTables<uint128>::kVmaxOverBase[] = {

    0,

    0,

    MakeUint128(9223372036854775807u, 18446744073709551615u),

    MakeUint128(6148914691236517205u, 6148914691236517205u),

    MakeUint128(4611686018427387903u, 18446744073709551615u),

    MakeUint128(3689348814741910323u, 3689348814741910323u),

    MakeUint128(3074457345618258602u, 12297829382473034410u),

    MakeUint128(2635249153387078802u, 5270498306774157604u),

    MakeUint128(2305843009213693951u, 18446744073709551615u),

    MakeUint128(2049638230412172401u, 14347467612885206812u),

    MakeUint128(1844674407370955161u, 11068046444225730969u),

    MakeUint128(1676976733973595601u, 8384883669867978007u),

    MakeUint128(1537228672809129301u, 6148914691236517205u),

    MakeUint128(1418980313362273201u, 4256940940086819603u),

    MakeUint128(1317624576693539401u, 2635249153387078802u),

    MakeUint128(1229782938247303441u, 1229782938247303441u),

    MakeUint128(1152921504606846975u, 18446744073709551615u),

    MakeUint128(1085102592571150095u, 1085102592571150095u),

    MakeUint128(1024819115206086200u, 16397105843297379214u),

    MakeUint128(970881267037344821u, 16504981539634861972u),

    MakeUint128(922337203685477580u, 14757395258967641292u),

    MakeUint128(878416384462359600u, 14054662151397753612u),

    MakeUint128(838488366986797800u, 13415813871788764811u),

    MakeUint128(802032351030850070u, 4812194106185100421u),

    MakeUint128(768614336404564650u, 12297829382473034410u),

    MakeUint128(737869762948382064u, 11805916207174113034u),

    MakeUint128(709490156681136600u, 11351842506898185609u),

    MakeUint128(683212743470724133u, 17080318586768103348u),

    MakeUint128(658812288346769700u, 10540996613548315209u),

    MakeUint128(636094623231363848u, 15266270957552732371u),

    MakeUint128(614891469123651720u, 9838263505978427528u),

    MakeUint128(595056260442243600u, 9520900167075897608u),

    MakeUint128(576460752303423487u, 18446744073709551615u),

    MakeUint128(558992244657865200u, 8943875914525843207u),

    MakeUint128(542551296285575047u, 9765923333140350855u),

    MakeUint128(527049830677415760u, 8432797290838652167u),

    MakeUint128(512409557603043100u, 8198552921648689607u),

};

// This kVmaxOverBase generated with

//   for (int base = 2; base < 37; ++base) {

//    absl::int128 max = std::numeric_limits<absl::int128>::max();

//    auto result = max / base;

//    std::cout << "\tMakeInt128(" << absl::Int128High64(result) << ", "

//              << absl::Int128Low64(result) << "u),\n";

//  }

// See https://godbolt.org/z/7djYWz

//

// int128& operator/=(int128) is not constexpr, so hardcode the resulting array

// to avoid a static initializer.

template <>

ABSL_CONST_INIT const int128 LookupTables<int128>::kVmaxOverBase[] = {

    0,

    0,

    MakeInt128(4611686018427387903, 18446744073709551615u),

    MakeInt128(3074457345618258602, 12297829382473034410u),

    MakeInt128(2305843009213693951, 18446744073709551615u),

    MakeInt128(1844674407370955161, 11068046444225730969u),

    MakeInt128(1537228672809129301, 6148914691236517205u),

    MakeInt128(1317624576693539401, 2635249153387078802u),

    MakeInt128(1152921504606846975, 18446744073709551615u),

    MakeInt128(1024819115206086200, 16397105843297379214u),

    MakeInt128(922337203685477580, 14757395258967641292u),

    MakeInt128(838488366986797800, 13415813871788764811u),

    MakeInt128(768614336404564650, 12297829382473034410u),

    MakeInt128(709490156681136600, 11351842506898185609u),

    MakeInt128(658812288346769700, 10540996613548315209u),

    MakeInt128(614891469123651720, 9838263505978427528u),

    MakeInt128(576460752303423487, 18446744073709551615u),

    MakeInt128(542551296285575047, 9765923333140350855u),

    MakeInt128(512409557603043100, 8198552921648689607u),

    MakeInt128(485440633518672410, 17475862806672206794u),

    MakeInt128(461168601842738790, 7378697629483820646u),

    MakeInt128(439208192231179800, 7027331075698876806u),

    MakeInt128(419244183493398900, 6707906935894382405u),

    MakeInt128(401016175515425035, 2406097053092550210u),

    MakeInt128(384307168202282325, 6148914691236517205u),

    MakeInt128(368934881474191032, 5902958103587056517u),

    MakeInt128(354745078340568300, 5675921253449092804u),

    MakeInt128(341606371735362066, 17763531330238827482u),

    MakeInt128(329406144173384850, 5270498306774157604u),

    MakeInt128(318047311615681924, 7633135478776366185u),

    MakeInt128(307445734561825860, 4919131752989213764u),

    MakeInt128(297528130221121800, 4760450083537948804u),

    MakeInt128(288230376151711743, 18446744073709551615u),

    MakeInt128(279496122328932600, 4471937957262921603u),

    MakeInt128(271275648142787523, 14106333703424951235u),

    MakeInt128(263524915338707880, 4216398645419326083u),

    MakeInt128(256204778801521550, 4099276460824344803u),

};

// This kVminOverBase generated with

//  for (int base = 2; base < 37; ++base) {

//    absl::int128 min = std::numeric_limits<absl::int128>::min();

//    auto result = min / base;

//    std::cout << "\tMakeInt128(" << absl::Int128High64(result) << ", "

//              << absl::Int128Low64(result) << "u),\n";

//  }

//

// See https://godbolt.org/z/7djYWz

//

// int128& operator/=(int128) is not constexpr, so hardcode the resulting array

// to avoid a static initializer.

template <>

ABSL_CONST_INIT const int128 LookupTables<int128>::kVminOverBase[] = {

    0,

    0,

    MakeInt128(-4611686018427387904, 0u),

    MakeInt128(-3074457345618258603, 6148914691236517206u),

    MakeInt128(-2305843009213693952, 0u),

    MakeInt128(-1844674407370955162, 7378697629483820647u),

    MakeInt128(-1537228672809129302, 12297829382473034411u),

    MakeInt128(-1317624576693539402, 15811494920322472814u),

    MakeInt128(-1152921504606846976, 0u),

    MakeInt128(-1024819115206086201, 2049638230412172402u),

    MakeInt128(-922337203685477581, 3689348814741910324u),

    MakeInt128(-838488366986797801, 5030930201920786805u),

    MakeInt128(-768614336404564651, 6148914691236517206u),

    MakeInt128(-709490156681136601, 7094901566811366007u),

    MakeInt128(-658812288346769701, 7905747460161236407u),

    MakeInt128(-614891469123651721, 8608480567731124088u),

    MakeInt128(-576460752303423488, 0u),

    MakeInt128(-542551296285575048, 8680820740569200761u),

    MakeInt128(-512409557603043101, 10248191152060862009u),

    MakeInt128(-485440633518672411, 970881267037344822u),

    MakeInt128(-461168601842738791, 11068046444225730970u),

    MakeInt128(-439208192231179801, 11419412998010674810u),

    MakeInt128(-419244183493398901, 11738837137815169211u),

    MakeInt128(-401016175515425036, 16040647020617001406u),