text: // Tencent is pleased to support the open source community by making RapidJSON available.
  //
  // Copyright (C) 2015 THL A29 Limited, a Tencent company, and Milo Yip. All rights reserved.
  //
  // Licensed under the MIT License (the "License"); you may not use this file except
  // in compliance with the License. You may obtain a copy of the License at
  //
  // http://opensource.org/licenses/MIT
  //
  // 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 is a C++ header-only implementation of Grisu2 algorithm from the publication:
  // Loitsch, Florian. "Printing floating-point numbers quickly and accurately with
  // integers." ACM Sigplan Notices 45.6 (2010): 233-243.
  
  #ifndef RAPIDJSON_DTOA_
  #define RAPIDJSON_DTOA_
  
  #include "itoa.h" // GetDigitsLut()
  #include "diyfp.h"
  #include "ieee754.h"
  
  RAPIDJSON_NAMESPACE_BEGIN
  namespace internal {
  
  #ifdef __GNUC__
  RAPIDJSON_DIAG_PUSH
  RAPIDJSON_DIAG_OFF(effc++)
  RAPIDJSON_DIAG_OFF(array - bounds) // some gcc versions generate wrong warnings https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59124
  #endif
  
  inline void GrisuRound(char* buffer, int len, uint64_t delta, uint64_t rest, uint64_t ten_kappa, uint64_t wp_w)
  {
      while (rest < wp_w && delta - rest >= ten_kappa && (rest + ten_kappa < wp_w || /// closer
                                                          wp_w - rest > rest + ten_kappa - wp_w))
      {
          buffer[len - 1]--;
          rest += ten_kappa;
      }
  }
  
  inline int CountDecimalDigit32(uint32_t n)
  {
      // Simple pure C++ implementation was faster than __builtin_clz version in this situation.
      if (n < 10) return 1;
      if (n < 100) return 2;
      if (n < 1000) return 3;
      if (n < 10000) return 4;
      if (n < 100000) return 5;
      if (n < 1000000) return 6;
      if (n < 10000000) return 7;
      if (n < 100000000) return 8;
      // Will not reach 10 digits in DigitGen()
      //if (n < 1000000000) return 9;
      //return 10;
      return 9;
  }
  
  inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buffer, int* len, int* K)
  {
      static const uint32_t kPow10[] = {1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000};
      const DiyFp one(uint64_t(1) << -Mp.e, Mp.e);
      const DiyFp wp_w = Mp - W;
      uint32_t p1 = static_cast<uint32_t>(Mp.f >> -one.e);
      uint64_t p2 = Mp.f & (one.f - 1);
      int kappa = CountDecimalDigit32(p1); // kappa in [0, 9]
      *len = 0;
  
      while (kappa > 0)
      {
          uint32_t d = 0;
          switch (kappa)
          {
          case 9:
              d = p1 / 100000000;
              p1 %= 100000000;
              break;
          case 8:
              d = p1 / 10000000;
              p1 %= 10000000;
              break;
          case 7:
              d = p1 / 1000000;
              p1 %= 1000000;
              break;
          case 6:
              d = p1 / 100000;
              p1 %= 100000;
              break;
          case 5:
              d = p1 / 10000;
              p1 %= 10000;
              break;
          case 4:
              d = p1 / 1000;
              p1 %= 1000;
              break;
          case 3:
              d = p1 / 100;
              p1 %= 100;
              break;
          case 2:
              d = p1 / 10;
              p1 %= 10;
              break;
          case 1:
              d = p1;
              p1 = 0;
              break;
          default:;
          }
          if (d || *len)
              buffer[(*len)++] = static_cast<char>('0' + static_cast<char>(d));
          kappa--;
          uint64_t tmp = (static_cast<uint64_t>(p1) << -one.e) + p2;
          if (tmp <= delta)
          {
              *K += kappa;
              GrisuRound(buffer, *len, delta, tmp, static_cast<uint64_t>(kPow10[kappa]) << -one.e, wp_w.f);
              return;
          }
      }
  
      // kappa = 0
      for (;;)
      {
          p2 *= 10;
          delta *= 10;
          char d = static_cast<char>(p2 >> -one.e);
          if (d || *len)
              buffer[(*len)++] = static_cast<char>('0' + d);
          p2 &= one.f - 1;
          kappa--;
          if (p2 < delta)
          {
              *K += kappa;
              int index = -kappa;
              GrisuRound(buffer, *len, delta, p2, one.f, wp_w.f * (index < 9 ? kPow10[index] : 0));
              return;
          }
      }
  }
  
  inline void Grisu2(double value, char* buffer, int* length, int* K)
  {
      const DiyFp v(value);
      DiyFp w_m, w_p;
      v.NormalizedBoundaries(&w_m, &w_p);
  
      const DiyFp c_mk = GetCachedPower(w_p.e, K);
      const DiyFp W = v.Normalize() * c_mk;
      DiyFp Wp = w_p * c_mk;
      DiyFp Wm = w_m * c_mk;
      Wm.f++;
      Wp.f--;
      DigitGen(W, Wp, Wp.f - Wm.f, buffer, length, K);
  }
  
  inline char* WriteExponent(int K, char* buffer)
  {
      if (K < 0)
      {
          *buffer++ = '-';
          K = -K;
      }
  
      if (K >= 100)
      {
          *buffer++ = static_cast<char>('0' + static_cast<char>(K / 100));
          K %= 100;
          const char* d = GetDigitsLut() + K * 2;
          *buffer++ = d[0];
          *buffer++ = d[1];
      }
      else if (K >= 10)
      {
          const char* d = GetDigitsLut() + K * 2;
          *buffer++ = d[0];
          *buffer++ = d[1];
      }
      else
          *buffer++ = static_cast<char>('0' + static_cast<char>(K));
  
      return buffer;
  }
  
  inline char* Prettify(char* buffer, int length, int k, int maxDecimalPlaces)
  {
      const int kk = length + k; // 10^(kk-1) <= v < 10^kk
  
      if (0 <= k && kk <= 21)
      {
          // 1234e7 -> 12340000000
          for (int i = length; i < kk; i++)
              buffer[i] = '0';
          buffer[kk] = '.';
          buffer[kk + 1] = '0';
          return &buffer[kk + 2];
      }
      else if (0 < kk && kk <= 21)
      {
          // 1234e-2 -> 12.34
          std::memmove(&buffer[kk + 1], &buffer[kk], static_cast<size_t>(length - kk));
          buffer[kk] = '.';
          if (0 > k + maxDecimalPlaces)
          {
              // When maxDecimalPlaces = 2, 1.2345 -> 1.23, 1.102 -> 1.1
              // Remove extra trailing zeros (at least one) after truncation.
              for (int i = kk + maxDecimalPlaces; i > kk + 1; i--)
                  if (buffer[i] != '0')
                      return &buffer[i + 1];
              return &buffer[kk + 2]; // Reserve one zero
          }
          else
              return &buffer[length + 1];
      }
      else if (-6 < kk && kk <= 0)
      {
          // 1234e-6 -> 0.001234
          const int offset = 2 - kk;
          std::memmove(&buffer[offset], &buffer[0], static_cast<size_t>(length));
          buffer[0] = '0';
          buffer[1] = '.';
          for (int i = 2; i < offset; i++)
              buffer[i] = '0';
          if (length - kk > maxDecimalPlaces)
          {
              // When maxDecimalPlaces = 2, 0.123 -> 0.12, 0.102 -> 0.1
              // Remove extra trailing zeros (at least one) after truncation.
              for (int i = maxDecimalPlaces + 1; i > 2; i--)
                  if (buffer[i] != '0')
                      return &buffer[i + 1];
              return &buffer[3]; // Reserve one zero
          }
          else
              return &buffer[length + offset];
      }
      else if (kk < -maxDecimalPlaces)
      {
          // Truncate to zero
          buffer[0] = '0';
          buffer[1] = '.';
          buffer[2] = '0';
          return &buffer[3];
      }
      else if (length == 1)
      {
          // 1e30
          buffer[1] = 'e';
          return WriteExponent(kk - 1, &buffer[2]);
      }
      else
      {
          // 1234e30 -> 1.234e33
          std::memmove(&buffer[2], &buffer[1], static_cast<size_t>(length - 1));
          buffer[1] = '.';
          buffer[length + 1] = 'e';
          return WriteExponent(kk - 1, &buffer[0 + length + 2]);
      }
  }
  
  inline char* dtoa(double value, char* buffer, int maxDecimalPlaces = 324)
  {
      RAPIDJSON_ASSERT(maxDecimalPlaces >= 1);
      Double d(value);
      if (d.IsZero())
      {
          if (d.Sign())
              *buffer++ = '-'; // -0.0, Issue #289
          buffer[0] = '0';
          buffer[1] = '.';
          buffer[2] = '0';
          return &buffer[3];
      }
      else
      {
          if (value < 0)
          {
              *buffer++ = '-';
              value = -value;
          }
          int length, K;
          Grisu2(value, buffer, &length, &K);
          return Prettify(buffer, length, K, maxDecimalPlaces);
      }
  }
  
  #ifdef __GNUC__
  RAPIDJSON_DIAG_POP
  #endif
  
  } // namespace internal
  RAPIDJSON_NAMESPACE_END
  
  #endif // RAPIDJSON_DTOA_
  