text: // Copyright 2018 Ulf Adams
  //
  // The contents of this file may be used under the terms of the Apache License,
  // Version 2.0.
  //
  //    (See accompanying file LICENSE-Apache or copy at
  //     http://www.apache.org/licenses/LICENSE-2.0)
  //
  // Alternatively, the contents of this file may be used under the terms of
  // the Boost Software License, Version 1.0.
  //    (See accompanying file LICENSE-Boost or copy at
  //     https://www.boost.org/LICENSE_1_0.txt)
  //
  // Unless required by applicable law or agreed to in writing, this software
  // is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.
  
  // Runtime compiler options:
  // -DRYU_DEBUG Generate verbose debugging output to stdout.
  //
  // -DRYU_ONLY_64_BIT_OPS Avoid using uint128_t or 64-bit intrinsics. Slower,
  //     depending on your compiler.
  //
  // -DRYU_OPTIMIZE_SIZE Use smaller lookup tables. Instead of storing every
  //     required power of 5, only store every 26th entry, and compute
  //     intermediate values with a multiplication. This reduces the lookup table
  //     size by about 10x (only one case, and only double) at the cost of some
  //     performance. Currently requires MSVC intrinsics.
  
  /*
      This is a derivative work
  */
  
  #ifndef BOOST_JSON_DETAIL_RYU_IMPL_D2S_IPP
  #define BOOST_JSON_DETAIL_RYU_IMPL_D2S_IPP
  
  #include <boost/json/detail/ryu/ryu.hpp>
  #include <cstdlib>
  #include <cstring>
  
  #ifdef RYU_DEBUG
  #include <stdio.h>
  #endif
  
  // ABSL avoids uint128_t on Win32 even if __SIZEOF_INT128__ is defined.
  // Let's do the same for now.
  #if defined(__SIZEOF_INT128__) && !defined(_MSC_VER) && !defined(RYU_ONLY_64_BIT_OPS)
  #define BOOST_JSON_RYU_HAS_UINT128
  #elif defined(_MSC_VER) && !defined(RYU_ONLY_64_BIT_OPS) && defined(_M_X64)
  #define BOOST_JSON_RYU_HAS_64_BIT_INTRINSICS
  #endif
  
  #include <boost/json/detail/ryu/detail/common.hpp>
  #include <boost/json/detail/ryu/detail/digit_table.hpp>
  #include <boost/json/detail/ryu/detail/d2s.hpp>
  #include <boost/json/detail/ryu/detail/d2s_intrinsics.hpp>
  
  BOOST_JSON_NS_BEGIN
  namespace detail {
  
  namespace ryu {
  namespace detail {
  
  // We need a 64x128-bit multiplication and a subsequent 128-bit shift.
  // Multiplication:
  //   The 64-bit factor is variable and passed in, the 128-bit factor comes
  //   from a lookup table. We know that the 64-bit factor only has 55
  //   significant bits (i.e., the 9 topmost bits are zeros). The 128-bit
  //   factor only has 124 significant bits (i.e., the 4 topmost bits are
  //   zeros).
  // Shift:
  //   In principle, the multiplication result requires 55 + 124 = 179 bits to
  //   represent. However, we then shift this value to the right by j, which is
  //   at least j >= 115, so the result is guaranteed to fit into 179 - 115 = 64
  //   bits. This means that we only need the topmost 64 significant bits of
  //   the 64x128-bit multiplication.
  //
  // There are several ways to do this:
  // 1. Best case: the compiler exposes a 128-bit type.
  //    We perform two 64x64-bit multiplications, add the higher 64 bits of the
  //    lower result to the higher result, and shift by j - 64 bits.
  //
  //    We explicitly cast from 64-bit to 128-bit, so the compiler can tell
  //    that these are only 64-bit inputs, and can map these to the best
  //    possible sequence of assembly instructions.
  //    x64 machines happen to have matching assembly instructions for
  //    64x64-bit multiplications and 128-bit shifts.
  //
  // 2. Second best case: the compiler exposes intrinsics for the x64 assembly
  //    instructions mentioned in 1.
  //
  // 3. We only have 64x64 bit instructions that return the lower 64 bits of
  //    the result, i.e., we have to use plain C.
  //    Our inputs are less than the full width, so we have three options:
  //    a. Ignore this fact and just implement the intrinsics manually.
  //    b. Split both into 31-bit pieces, which guarantees no internal overflow,
  //       but requires extra work upfront (unless we change the lookup table).
  //    c. Split only the first factor into 31-bit pieces, which also guarantees
  //       no internal overflow, but requires extra work since the intermediate
  //       results are not perfectly aligned.
  #if defined(BOOST_JSON_RYU_HAS_UINT128)
  
  // Best case: use 128-bit type.
  inline
  std::uint64_t
      mulShift(
      const std::uint64_t m,
      const std::uint64_t* const mul,
      const std::int32_t j) noexcept
  {
      const uint128_t b0 = ((uint128_t) m) * mul[0];
      const uint128_t b2 = ((uint128_t) m) * mul[1];
      return (std::uint64_t) (((b0 >> 64) + b2) >> (j - 64));
  }
  
  inline
  uint64_t
  mulShiftAll(
      const std::uint64_t m,
      const std::uint64_t* const mul,
      std::int32_t const j,
      std::uint64_t* const vp,
      std::uint64_t* const vm,
      const std::uint32_t mmShift) noexcept
  {
  //  m <<= 2;
  //  uint128_t b0 = ((uint128_t) m) * mul[0]; // 0
  //  uint128_t b2 = ((uint128_t) m) * mul[1]; // 64
  //
  //  uint128_t hi = (b0 >> 64) + b2;
  //  uint128_t lo = b0 & 0xffffffffffffffffull;
  //  uint128_t factor = (((uint128_t) mul[1]) << 64) + mul[0];
  //  uint128_t vpLo = lo + (factor << 1);
  //  *vp = (std::uint64_t) ((hi + (vpLo >> 64)) >> (j - 64));
  //  uint128_t vmLo = lo - (factor << mmShift);
  //  *vm = (std::uint64_t) ((hi + (vmLo >> 64) - (((uint128_t) 1ull) << 64)) >> (j - 64));
  //  return (std::uint64_t) (hi >> (j - 64));
      *vp = mulShift(4 * m + 2, mul, j);
      *vm = mulShift(4 * m - 1 - mmShift, mul, j);
      return mulShift(4 * m, mul, j);
  }
  
  #elif defined(BOOST_JSON_RYU_HAS_64_BIT_INTRINSICS)
  
  inline
  std::uint64_t
  mulShift(
      const std::uint64_t m,
      const std::uint64_t* const mul,
      const std::int32_t j) noexcept
  {
      // m is maximum 55 bits
      std::uint64_t high1;                                   // 128
      std::uint64_t const low1 = umul128(m, mul[1], &high1); // 64
      std::uint64_t high0;                                   // 64
      umul128(m, mul[0], &high0);                            // 0
      std::uint64_t const sum = high0 + low1;
      if (sum < high0)
          ++high1; // overflow into high1
      return shiftright128(sum, high1, j - 64);
  }
  
  inline
  std::uint64_t
  mulShiftAll(
      const std::uint64_t m,
      const std::uint64_t* const mul,
      const std::int32_t j,
      std::uint64_t* const vp,
      std::uint64_t* const vm,
      const std::uint32_t mmShift) noexcept
  {
      *vp = mulShift(4 * m + 2, mul, j);
      *vm = mulShift(4 * m - 1 - mmShift, mul, j);
      return mulShift(4 * m, mul, j);
  }
  
  #else // !defined(BOOST_JSON_RYU_HAS_UINT128) && !defined(BOOST_JSON_RYU_HAS_64_BIT_INTRINSICS)
  
  inline
  std::uint64_t
  mulShiftAll(
      std::uint64_t m,
      const std::uint64_t* const mul,
      const std::int32_t j,
      std::uint64_t* const vp,
      std::uint64_t* const vm,
      const std::uint32_t mmShift)
  {
      m <<= 1;
      // m is maximum 55 bits
      std::uint64_t tmp;
      std::uint64_t const lo = umul128(m, mul[0], &tmp);
      std::uint64_t hi;
      std::uint64_t const mid = tmp + umul128(m, mul[1], &hi);
      hi += mid < tmp; // overflow into hi
  
      const std::uint64_t lo2 = lo + mul[0];
      const std::uint64_t mid2 = mid + mul[1] + (lo2 < lo);
      const std::uint64_t hi2 = hi + (mid2 < mid);
      *vp = shiftright128(mid2, hi2, (std::uint32_t)(j - 64 - 1));
  
      if (mmShift == 1)
      {
          const std::uint64_t lo3 = lo - mul[0];
          const std::uint64_t mid3 = mid - mul[1] - (lo3 > lo);
          const std::uint64_t hi3 = hi - (mid3 > mid);
          *vm = shiftright128(mid3, hi3, (std::uint32_t)(j - 64 - 1));
      }
      else
      {
          const std::uint64_t lo3 = lo + lo;
          const std::uint64_t mid3 = mid + mid + (lo3 < lo);
          const std::uint64_t hi3 = hi + hi + (mid3 < mid);
          const std::uint64_t lo4 = lo3 - mul[0];
          const std::uint64_t mid4 = mid3 - mul[1] - (lo4 > lo3);
          const std::uint64_t hi4 = hi3 - (mid4 > mid3);
          *vm = shiftright128(mid4, hi4, (std::uint32_t)(j - 64));
      }
  
      return shiftright128(mid, hi, (std::uint32_t)(j - 64 - 1));
  }
  
  #endif // BOOST_JSON_RYU_HAS_64_BIT_INTRINSICS
  
  inline
  std::uint32_t
  decimalLength17(
      const std::uint64_t v)
  {
      // This is slightly faster than a loop.
      // The average output length is 16.38 digits, so we check high-to-low.
      // Function precondition: v is not an 18, 19, or 20-digit number.
      // (17 digits are sufficient for round-tripping.)
      BOOST_ASSERT(v < 100000000000000000L);
      if (v >= 10000000000000000L) { return 17; }
      if (v >= 1000000000000000L) { return 16; }
      if (v >= 100000000000000L) { return 15; }
      if (v >= 10000000000000L) { return 14; }
      if (v >= 1000000000000L) { return 13; }
      if (v >= 100000000000L) { return 12; }
      if (v >= 10000000000L) { return 11; }
      if (v >= 1000000000L) { return 10; }
      if (v >= 100000000L) { return 9; }
      if (v >= 10000000L) { return 8; }
      if (v >= 1000000L) { return 7; }
      if (v >= 100000L) { return 6; }
      if (v >= 10000L) { return 5; }
      if (v >= 1000L) { return 4; }
      if (v >= 100L) { return 3; }
      if (v >= 10L) { return 2; }
      return 1;
  }
  
  // A floating decimal representing m * 10^e.
  struct floating_decimal_64
  {
      std::uint64_t mantissa;
      // Decimal exponent's range is -324 to 308
      // inclusive, and can fit in a short if needed.
      std::int32_t exponent;
  };
  
  inline
  floating_decimal_64
  d2d(
      const std::uint64_t ieeeMantissa,
      const std::uint32_t ieeeExponent)
  {
      std::int32_t e2;
      std::uint64_t m2;
      if (ieeeExponent == 0)
      {
          // We subtract 2 so that the bounds computation has 2 additional bits.
          e2 = 1 - DOUBLE_BIAS - DOUBLE_MANTISSA_BITS - 2;
          m2 = ieeeMantissa;
      }
      else
      {
          e2 = (std::int32_t)ieeeExponent - DOUBLE_BIAS - DOUBLE_MANTISSA_BITS - 2;
          m2 = (1ull << DOUBLE_MANTISSA_BITS) | ieeeMantissa;
      }
      const bool even = (m2 & 1) == 0;
      const bool acceptBounds = even;
  
  #ifdef RYU_DEBUG
      printf("-> %" PRIu64 " * 2^%d\n", m2, e2 + 2);
  #endif
  
      // Step 2: Determine the interval of valid decimal representations.
      const std::uint64_t mv = 4 * m2;
      // Implicit bool -> int conversion. True is 1, false is 0.
      const std::uint32_t mmShift = ieeeMantissa != 0 || ieeeExponent <= 1;
      // We would compute mp and mm like this:
      // uint64_t mp = 4 * m2 + 2;
      // uint64_t mm = mv - 1 - mmShift;
  
      // Step 3: Convert to a decimal power base using 128-bit arithmetic.
      std::uint64_t vr, vp, vm;
      std::int32_t e10;
      bool vmIsTrailingZeros = false;
      bool vrIsTrailingZeros = false;
      if (e2 >= 0) {
          // I tried special-casing q == 0, but there was no effect on performance.
          // This expression is slightly faster than max(0, log10Pow2(e2) - 1).
          const std::uint32_t q = log10Pow2(e2) - (e2 > 3);
          e10 = (std::int32_t)q;
          const std::int32_t k = DOUBLE_POW5_INV_BITCOUNT + pow5bits((int32_t)q) - 1;
          const std::int32_t i = -e2 + (std::int32_t)q + k;
  #if defined(BOOST_JSON_RYU_OPTIMIZE_SIZE)
          uint64_t pow5[2];
          double_computeInvPow5(q, pow5);
          vr = mulShiftAll(m2, pow5, i, &vp, &vm, mmShift);
  #else
          vr = mulShiftAll(m2, DOUBLE_POW5_INV_SPLIT()[q], i, &vp, &vm, mmShift);
  #endif
  #ifdef RYU_DEBUG
          printf("%" PRIu64 " * 2^%d / 10^%u\n", mv, e2, q);
          printf("V+=%" PRIu64 "\nV =%" PRIu64 "\nV-=%" PRIu64 "\n", vp, vr, vm);
  #endif
          if (q <= 21)
          {
              // This should use q <= 22, but I think 21 is also safe. Smaller values
              // may still be safe, but it's more difficult to reason about them.
              // Only one of mp, mv, and mm can be a multiple of 5, if any.
              const std::uint32_t mvMod5 = ((std::uint32_t)mv) - 5 * ((std::uint32_t)div5(mv));
              if (mvMod5 == 0)
              {
                  vrIsTrailingZeros = multipleOfPowerOf5(mv, q);
              }
              else if (acceptBounds)
              {
                  // Same as min(e2 + (~mm & 1), pow5Factor(mm)) >= q
                  // <=> e2 + (~mm & 1) >= q && pow5Factor(mm) >= q
                  // <=> true && pow5Factor(mm) >= q, since e2 >= q.
                  vmIsTrailingZeros = multipleOfPowerOf5(mv - 1 - mmShift, q);
              }
              else
              {
                  // Same as min(e2 + 1, pow5Factor(mp)) >= q.
                  vp -= multipleOfPowerOf5(mv + 2, q);
              }
          }
      }
      else
      {
          // This expression is slightly faster than max(0, log10Pow5(-e2) - 1).
          const std::uint32_t q = log10Pow5(-e2) - (-e2 > 1);
          e10 = (std::int32_t)q + e2;
          const std::int32_t i = -e2 - (std::int32_t)q;
          const std::int32_t k = pow5bits(i) - DOUBLE_POW5_BITCOUNT;
          const std::int32_t j = (std::int32_t)q - k;
  #if defined(BOOST_JSON_RYU_OPTIMIZE_SIZE)
          std::uint64_t pow5[2];
          double_computePow5(i, pow5);
          vr = mulShiftAll(m2, pow5, j, &vp, &vm, mmShift);
  #else
          vr = mulShiftAll(m2, DOUBLE_POW5_SPLIT()[i], j, &vp, &vm, mmShift);
  #endif
  #ifdef RYU_DEBUG
          printf("%" PRIu64 " * 5^%d / 10^%u\n", mv, -e2, q);
          printf("%u %d %d %d\n", q, i, k, j);
          printf("V+=%" PRIu64 "\nV =%" PRIu64 "\nV-=%" PRIu64 "\n", vp, vr, vm);
  #endif
          if (q <= 1)
          {
              // {vr,vp,vm} is trailing zeros if {mv,mp,mm} has at least q trailing 0 bits.
              // mv = 4 * m2, so it always has at least two trailing 0 bits.
              vrIsTrailingZeros = true;
              if (acceptBounds)
              {
                  // mm = mv - 1 - mmShift, so it has 1 trailing 0 bit iff mmShift == 1.
                  vmIsTrailingZeros = mmShift == 1;
              }
              else
              {
                  // mp = mv + 2, so it always has at least one trailing 0 bit.
                  --vp;
              }
          }
          else if (q < 63)
          {
              // TODO(ulfjack): Use a tighter bound here.
              // We want to know if the full product has at least q trailing zeros.
              // We need to compute min(p2(mv), p5(mv) - e2) >= q
              // <=> p2(mv) >= q && p5(mv) - e2 >= q
              // <=> p2(mv) >= q (because -e2 >= q)
              vrIsTrailingZeros = multipleOfPowerOf2(mv, q);
  #ifdef RYU_DEBUG
              printf("vr is trailing zeros=%s\n", vrIsTrailingZeros ? "true" : "false");
  #endif
          }
      }
  #ifdef RYU_DEBUG
      printf("e10=%d\n", e10);
      printf("V+=%" PRIu64 "\nV =%" PRIu64 "\nV-=%" PRIu64 "\n", vp, vr, vm);
      printf("vm is trailing zeros=%s\n", vmIsTrailingZeros ? "true" : "false");
      printf("vr is trailing zeros=%s\n", vrIsTrailingZeros ? "true" : "false");
  #endif
  
      // Step 4: Find the shortest decimal representation in the interval of valid representations.
      std::int32_t removed = 0;
      std::uint8_t lastRemovedDigit = 0;
      std::uint64_t output;
      // On average, we remove ~2 digits.
      if (vmIsTrailingZeros || vrIsTrailingZeros)
      {
          // General case, which happens rarely (~0.7%).
          for (;;)
          {
              const std::uint64_t vpDiv10 = div10(vp);
              const std::uint64_t vmDiv10 = div10(vm);
              if (vpDiv10 <= vmDiv10)
                  break;
              const std::uint32_t vmMod10 = ((std::uint32_t)vm) - 10 * ((std::uint32_t)vmDiv10);
              const std::uint64_t vrDiv10 = div10(vr);
              const std::uint32_t vrMod10 = ((std::uint32_t)vr) - 10 * ((std::uint32_t)vrDiv10);
              vmIsTrailingZeros &= vmMod10 == 0;
              vrIsTrailingZeros &= lastRemovedDigit == 0;
              lastRemovedDigit = (uint8_t)vrMod10;
              vr = vrDiv10;
              vp = vpDiv10;
              vm = vmDiv10;
              ++removed;
          }
  #ifdef RYU_DEBUG
          printf("V+=%" PRIu64 "\nV =%" PRIu64 "\nV-=%" PRIu64 "\n", vp, vr, vm);
          printf("d-10=%s\n", vmIsTrailingZeros ? "true" : "false");
  #endif
          if (vmIsTrailingZeros)
          {
              for (;;)
              {
                  const std::uint64_t vmDiv10 = div10(vm);
                  const std::uint32_t vmMod10 = ((std::uint32_t)vm) - 10 * ((std::uint32_t)vmDiv10);
                  if (vmMod10 != 0)
                      break;
                  const std::uint64_t vpDiv10 = div10(vp);
                  const std::uint64_t vrDiv10 = div10(vr);
                  const std::uint32_t vrMod10 = ((std::uint32_t)vr) - 10 * ((std::uint32_t)vrDiv10);
                  vrIsTrailingZeros &= lastRemovedDigit == 0;
                  lastRemovedDigit = (uint8_t)vrMod10;
                  vr = vrDiv10;
                  vp = vpDiv10;
                  vm = vmDiv10;
                  ++removed;
              }
          }
  #ifdef RYU_DEBUG
          printf("%" PRIu64 " %d\n", vr, lastRemovedDigit);
          printf("vr is trailing zeros=%s\n", vrIsTrailingZeros ? "true" : "false");
  #endif
          if (vrIsTrailingZeros && lastRemovedDigit == 5 && vr % 2 == 0)
          {
              // Round even if the exact number is .....50..0.
              lastRemovedDigit = 4;
          }
          // We need to take vr + 1 if vr is outside bounds or we need to round up.
          output = vr + ((vr == vm && (!acceptBounds || !vmIsTrailingZeros)) || lastRemovedDigit >= 5);
      }
      else
      {
          // Specialized for the common case (~99.3%). Percentages below are relative to this.
          bool roundUp = false;
          const std::uint64_t vpDiv100 = div100(vp);
          const std::uint64_t vmDiv100 = div100(vm);
          if (vpDiv100 > vmDiv100)
          {
              // Optimization: remove two digits at a time (~86.2%).
              const std::uint64_t vrDiv100 = div100(vr);
              const std::uint32_t vrMod100 = ((std::uint32_t)vr) - 100 * ((std::uint32_t)vrDiv100);
              roundUp = vrMod100 >= 50;
              vr = vrDiv100;
              vp = vpDiv100;
              vm = vmDiv100;
              removed += 2;
          }
          // Loop iterations below (approximately), without optimization above:
          // 0: 0.03%, 1: 13.8%, 2: 70.6%, 3: 14.0%, 4: 1.40%, 5: 0.14%, 6+: 0.02%
          // Loop iterations below (approximately), with optimization above:
          // 0: 70.6%, 1: 27.8%, 2: 1.40%, 3: 0.14%, 4+: 0.02%
          for (;;)
          {
              const std::uint64_t vpDiv10 = div10(vp);
              const std::uint64_t vmDiv10 = div10(vm);
              if (vpDiv10 <= vmDiv10)
                  break;
              const std::uint64_t vrDiv10 = div10(vr);
              const std::uint32_t vrMod10 = ((std::uint32_t)vr) - 10 * ((std::uint32_t)vrDiv10);
              roundUp = vrMod10 >= 5;
              vr = vrDiv10;
              vp = vpDiv10;
              vm = vmDiv10;
              ++removed;
          }
  #ifdef RYU_DEBUG
          printf("%" PRIu64 " roundUp=%s\n", vr, roundUp ? "true" : "false");
          printf("vr is trailing zeros=%s\n", vrIsTrailingZeros ? "true" : "false");
  #endif
          // We need to take vr + 1 if vr is outside bounds or we need to round up.
          output = vr + (vr == vm || roundUp);
      }
      const std::int32_t exp = e10 + removed;
  
  #ifdef RYU_DEBUG
      printf("V+=%" PRIu64 "\nV =%" PRIu64 "\nV-=%" PRIu64 "\n", vp, vr, vm);
      printf("O=%" PRIu64 "\n", output);
      printf("EXP=%d\n", exp);
  #endif
  
      floating_decimal_64 fd;
      fd.exponent = exp;
      fd.mantissa = output;
      return fd;
  }
  
  inline
  int
  to_chars(
      const floating_decimal_64 v,
      const bool sign,
      char* const result)
  {
      // Step 5: Print the decimal representation.
      int index = 0;
      if (sign)
          result[index++] = '-';
  
      std::uint64_t output = v.mantissa;
      std::uint32_t const olength = decimalLength17(output);
  
  #ifdef RYU_DEBUG
      printf("DIGITS=%" PRIu64 "\n", v.mantissa);
      printf("OLEN=%u\n", olength);
      printf("EXP=%u\n", v.exponent + olength);
  #endif
  
      // Print the decimal digits.
      // The following code is equivalent to:
      // for (uint32_t i = 0; i < olength - 1; ++i) {
      //   const uint32_t c = output % 10; output /= 10;
      //   result[index + olength - i] = (char) ('0' + c);
      // }
      // result[index] = '0' + output % 10;
  
      std::uint32_t i = 0;
      // We prefer 32-bit operations, even on 64-bit platforms.
      // We have at most 17 digits, and uint32_t can store 9 digits.
      // If output doesn't fit into uint32_t, we cut off 8 digits,
      // so the rest will fit into uint32_t.
      if ((output >> 32) != 0)
      {
          // Expensive 64-bit division.
          std::uint64_t const q = div1e8(output);
          std::uint32_t output2 = ((std::uint32_t)output) - 100000000 * ((std::uint32_t)q);
          output = q;
  
          const std::uint32_t c = output2 % 10000;
          output2 /= 10000;
          const std::uint32_t d = output2 % 10000;
          const std::uint32_t c0 = (c % 100) << 1;
          const std::uint32_t c1 = (c / 100) << 1;
          const std::uint32_t d0 = (d % 100) << 1;
          const std::uint32_t d1 = (d / 100) << 1;
          std::memcpy(result + index + olength - i - 1, DIGIT_TABLE() + c0, 2);
          std::memcpy(result + index + olength - i - 3, DIGIT_TABLE() + c1, 2);
          std::memcpy(result + index + olength - i - 5, DIGIT_TABLE() + d0, 2);
          std::memcpy(result + index + olength - i - 7, DIGIT_TABLE() + d1, 2);
          i += 8;
      }
      uint32_t output2 = (std::uint32_t)output;
      while (output2 >= 10000)
      {
  #ifdef __clang__ // https://bugs.llvm.org/show_bug.cgi?id=38217
          const uint32_t c = output2 - 10000 * (output2 / 10000);
  #else
          const uint32_t c = output2 % 10000;
  #endif
          output2 /= 10000;
          const uint32_t c0 = (c % 100) << 1;
          const uint32_t c1 = (c / 100) << 1;
          memcpy(result + index + olength - i - 1, DIGIT_TABLE() + c0, 2);
          memcpy(result + index + olength - i - 3, DIGIT_TABLE() + c1, 2);
          i += 4;
      }
      if (output2 >= 100) {
          const uint32_t c = (output2 % 100) << 1;
          output2 /= 100;
          memcpy(result + index + olength - i - 1, DIGIT_TABLE() + c, 2);
          i += 2;
      }
      if (output2 >= 10) {
          const uint32_t c = output2 << 1;
          // We can't use memcpy here: the decimal dot goes between these two digits.
          result[index + olength - i] = DIGIT_TABLE()[c + 1];
          result[index] = DIGIT_TABLE()[c];
      }
      else {
          result[index] = (char)('0' + output2);
      }
  
      // Print decimal point if needed.
      if (olength > 1) {
          result[index + 1] = '.';
          index += olength + 1;
      }
      else {
          ++index;
      }
  
      // Print the exponent.
      result[index++] = 'E';
      int32_t exp = v.exponent + (int32_t)olength - 1;
      if (exp < 0) {
          result[index++] = '-';
          exp = -exp;
      }
  
      if (exp >= 100) {
          const int32_t c = exp % 10;
          memcpy(result + index, DIGIT_TABLE() + 2 * (exp / 10), 2);
          result[index + 2] = (char)('0' + c);
          index += 3;
      }
      else if (exp >= 10) {
          memcpy(result + index, DIGIT_TABLE() + 2 * exp, 2);
          index += 2;
      }
      else {
          result[index++] = (char)('0' + exp);
      }
  
      return index;
  }
  
  static inline bool d2d_small_int(const uint64_t ieeeMantissa, const uint32_t ieeeExponent,
    floating_decimal_64* const v) {
    const uint64_t m2 = (1ull << DOUBLE_MANTISSA_BITS) | ieeeMantissa;
    const int32_t e2 = (int32_t) ieeeExponent - DOUBLE_BIAS - DOUBLE_MANTISSA_BITS;
  
    if (e2 > 0) {
      // f = m2 * 2^e2 >= 2^53 is an integer.
      // Ignore this case for now.
      return false;
    }
  
    if (e2 < -52) {
      // f < 1.
      return false;
    }
  
    // Since 2^52 <= m2 < 2^53 and 0 <= -e2 <= 52: 1 <= f = m2 / 2^-e2 < 2^53.
    // Test if the lower -e2 bits of the significand are 0, i.e. whether the fraction is 0.
    const uint64_t mask = (1ull << -e2) - 1;
    const uint64_t fraction = m2 & mask;
    if (fraction != 0) {
      return false;
    }
  
    // f is an integer in the range [1, 2^53).
    // Note: mantissa might contain trailing (decimal) 0's.
    // Note: since 2^53 < 10^16, there is no need to adjust decimalLength17().
    v->mantissa = m2 >> -e2;
    v->exponent = 0;
    return true;
  }
  
  } // detail
  
  int
  d2s_buffered_n(
      double f,
      char* result) noexcept
  {
      using namespace detail;
      // Step 1: Decode the floating-point number, and unify normalized and subnormal cases.
      std::uint64_t const bits = double_to_bits(f);
  
  #ifdef RYU_DEBUG
      printf("IN=");
      for (std::int32_t bit = 63; bit >= 0; --bit) {
          printf("%d", (int)((bits >> bit) & 1));
      }
      printf("\n");
  #endif
  
      // Decode bits into sign, mantissa, and exponent.
      const bool ieeeSign = ((bits >> (DOUBLE_MANTISSA_BITS + DOUBLE_EXPONENT_BITS)) & 1) != 0;
      const std::uint64_t ieeeMantissa = bits & ((1ull << DOUBLE_MANTISSA_BITS) - 1);
      const std::uint32_t ieeeExponent = (std::uint32_t)((bits >> DOUBLE_MANTISSA_BITS) & ((1u << DOUBLE_EXPONENT_BITS) - 1));
      // Case distinction; exit early for the easy cases.
      if (ieeeExponent == ((1u << DOUBLE_EXPONENT_BITS) - 1u) || (ieeeExponent == 0 && ieeeMantissa == 0)) {
          return copy_special_str(result, ieeeSign, ieeeExponent != 0, ieeeMantissa != 0);
      }
  
      floating_decimal_64 v;
      const bool isSmallInt = d2d_small_int(ieeeMantissa, ieeeExponent, &v);
      if (isSmallInt) {
          // For small integers in the range [1, 2^53), v.mantissa might contain trailing (decimal) zeros.
          // For scientific notation we need to move these zeros into the exponent.
          // (This is not needed for fixed-point notation, so it might be beneficial to trim
          // trailing zeros in to_chars only if needed - once fixed-point notation output is implemented.)
          for (;;) {
              std::uint64_t const q = div10(v.mantissa);
              std::uint32_t const r = ((std::uint32_t) v.mantissa) - 10 * ((std::uint32_t) q);
              if (r != 0)
                  break;
              v.mantissa = q;
              ++v.exponent;
          }
      }
      else {
          v = d2d(ieeeMantissa, ieeeExponent);
      }
  
      return to_chars(v, ieeeSign, result);
  }
  
  void
  d2s_buffered(
      double f,
      char* result) noexcept
  {
      const int index = d2s_buffered_n(f, result);
  
      // Terminate the string.
      result[index] = '\0';
  }
  
  char*
  d2s(double f) noexcept
  {
      static thread_local char result[25];
      d2s_buffered(f, result);
      return result;
  }
  
  } // ryu
  
  } // detail
  BOOST_JSON_NS_END
  
  #endif
  