panda3d/dtool/src/dtoolbase/pdtoa.cxx

/*
See pdtoa.h for explanation.

Copyright (C) 2014 Milo Yip
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

#include "pdtoa.h"
#include "cmath.h"

#include <assert.h>
#include <math.h>
#include <stdint.h>

#if defined(_MSC_VER)
#include <intrin.h>
#include <float.h>
#define copysign _copysign

#pragma float_control(precise, on, push)
#endif

#define UINT64_C2(h, l) ((static_cast<uint64_t>(h) << 32) | static_cast<uint64_t>(l))

struct DiyFp {
  DiyFp() {}

  DiyFp(uint64_t f, int e) : f(f), e(e) {}

  DiyFp(double d) {
    union {
      double d;
      uint64_t u64;
    } u = { d };

    int biased_e = (u.u64 & kDpExponentMask) >> kDpSignificandSize;
    uint64_t significand = (u.u64 & kDpSignificandMask);
    if (biased_e != 0) {
      f = significand + kDpHiddenBit;
      e = biased_e - kDpExponentBias;
    }
    else {
      f = significand;
      e = kDpMinExponent + 1;
    }
  }

  DiyFp operator-(const DiyFp& rhs) const {
    assert(e == rhs.e);
    assert(f >= rhs.f);
    return DiyFp(f - rhs.f, e);
  }

  DiyFp operator*(const DiyFp& rhs) const {
#if defined(_MSC_VER) && defined(_M_AMD64)
    uint64_t h;
    uint64_t l = _umul128(f, rhs.f, &h);
    if (l & (uint64_t(1) << 63)) // rounding
      h++;
    return DiyFp(h, e + rhs.e + 64);
#elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)) && defined(__x86_64__)
    unsigned __int128 p = static_cast<unsigned __int128>(f) * static_cast<unsigned __int128>(rhs.f);
    uint64_t h = p >> 64;
    uint64_t l = static_cast<uint64_t>(p);
    if (l & (uint64_t(1) << 63)) // rounding
      h++;
    return DiyFp(h, e + rhs.e + 64);
#else
    const uint64_t M32 = 0xFFFFFFFF;
    const uint64_t a = f >> 32;
    const uint64_t b = f & M32;
    const uint64_t c = rhs.f >> 32;
    const uint64_t d = rhs.f & M32;
    const uint64_t ac = a * c;
    const uint64_t bc = b * c;
    const uint64_t ad = a * d;
    const uint64_t bd = b * d;
    uint64_t tmp = (bd >> 32) + (ad & M32) + (bc & M32);
    tmp += 1U << 31;  /// mult_round
    return DiyFp(ac + (ad >> 32) + (bc >> 32) + (tmp >> 32), e + rhs.e + 64);
#endif
  }

  DiyFp Normalize() const {
#if defined(_MSC_VER) && defined(_M_AMD64)
    unsigned long index;
    _BitScanReverse64(&index, f);
    return DiyFp(f << (63 - index), e - (63 - index));
#elif defined(__GNUC__)
    int s = __builtin_clzll(f);
    return DiyFp(f << s, e - s);
#else
    DiyFp res = *this;
    while (!(res.f & kDpHiddenBit)) {
      res.f <<= 1;
      res.e--;
    }
    res.f <<= (kDiySignificandSize - kDpSignificandSize - 1);
    res.e = res.e - (kDiySignificandSize - kDpSignificandSize - 1);
    return res;
#endif
  }

  DiyFp NormalizeBoundary() const {
#if defined(_MSC_VER) && defined(_M_AMD64)
    unsigned long index;
    _BitScanReverse64(&index, f);
    return DiyFp (f << (63 - index), e - (63 - index));
#else
    DiyFp res = *this;
    while (!(res.f & (kDpHiddenBit << 1))) {
      res.f <<= 1;
      res.e--;
    }
    res.f <<= (kDiySignificandSize - kDpSignificandSize - 2);
    res.e = res.e - (kDiySignificandSize - kDpSignificandSize - 2);
    return res;
#endif
  }

  void NormalizedBoundaries(DiyFp* minus, DiyFp* plus) const {
    DiyFp pl = DiyFp((f << 1) + 1, e - 1).NormalizeBoundary();
    DiyFp mi = (f == kDpHiddenBit) ? DiyFp((f << 2) - 1, e - 2) : DiyFp((f << 1) - 1, e - 1);
    mi.f <<= mi.e - pl.e;
    mi.e = pl.e;
    *plus = pl;
    *minus = mi;
  }

  static const int kDiySignificandSize = 64;
  static const int kDpSignificandSize = 52;
  static const int kDpExponentBias = 0x3FF + kDpSignificandSize;
  static const int kDpMinExponent = -kDpExponentBias;
  static const uint64_t kDpExponentMask = UINT64_C2(0x7FF00000, 0x00000000);
  static const uint64_t kDpSignificandMask = UINT64_C2(0x000FFFFF, 0xFFFFFFFF);
  static const uint64_t kDpHiddenBit = UINT64_C2(0x00100000, 0x00000000);

  uint64_t f;
  int e;
};

inline static DiyFp GetCachedPower(int e, int* K) {
  // 10^-348, 10^-340, ..., 10^340
  static const uint64_t kCachedPowers_F[] = {
    UINT64_C2(0xfa8fd5a0, 0x081c0288), UINT64_C2(0xbaaee17f, 0xa23ebf76),
    UINT64_C2(0x8b16fb20, 0x3055ac76), UINT64_C2(0xcf42894a, 0x5dce35ea),
    UINT64_C2(0x9a6bb0aa, 0x55653b2d), UINT64_C2(0xe61acf03, 0x3d1a45df),
    UINT64_C2(0xab70fe17, 0xc79ac6ca), UINT64_C2(0xff77b1fc, 0xbebcdc4f),
    UINT64_C2(0xbe5691ef, 0x416bd60c), UINT64_C2(0x8dd01fad, 0x907ffc3c),
    UINT64_C2(0xd3515c28, 0x31559a83), UINT64_C2(0x9d71ac8f, 0xada6c9b5),
    UINT64_C2(0xea9c2277, 0x23ee8bcb), UINT64_C2(0xaecc4991, 0x4078536d),
    UINT64_C2(0x823c1279, 0x5db6ce57), UINT64_C2(0xc2109436, 0x4dfb5637),
    UINT64_C2(0x9096ea6f, 0x3848984f), UINT64_C2(0xd77485cb, 0x25823ac7),
    UINT64_C2(0xa086cfcd, 0x97bf97f4), UINT64_C2(0xef340a98, 0x172aace5),
    UINT64_C2(0xb23867fb, 0x2a35b28e), UINT64_C2(0x84c8d4df, 0xd2c63f3b),
    UINT64_C2(0xc5dd4427, 0x1ad3cdba), UINT64_C2(0x936b9fce, 0xbb25c996),
    UINT64_C2(0xdbac6c24, 0x7d62a584), UINT64_C2(0xa3ab6658, 0x0d5fdaf6),
    UINT64_C2(0xf3e2f893, 0xdec3f126), UINT64_C2(0xb5b5ada8, 0xaaff80b8),
    UINT64_C2(0x87625f05, 0x6c7c4a8b), UINT64_C2(0xc9bcff60, 0x34c13053),
    UINT64_C2(0x964e858c, 0x91ba2655), UINT64_C2(0xdff97724, 0x70297ebd),
    UINT64_C2(0xa6dfbd9f, 0xb8e5b88f), UINT64_C2(0xf8a95fcf, 0x88747d94),
    UINT64_C2(0xb9447093, 0x8fa89bcf), UINT64_C2(0x8a08f0f8, 0xbf0f156b),
    UINT64_C2(0xcdb02555, 0x653131b6), UINT64_C2(0x993fe2c6, 0xd07b7fac),
    UINT64_C2(0xe45c10c4, 0x2a2b3b06), UINT64_C2(0xaa242499, 0x697392d3),
    UINT64_C2(0xfd87b5f2, 0x8300ca0e), UINT64_C2(0xbce50864, 0x92111aeb),
    UINT64_C2(0x8cbccc09, 0x6f5088cc), UINT64_C2(0xd1b71758, 0xe219652c),
    UINT64_C2(0x9c400000, 0x00000000), UINT64_C2(0xe8d4a510, 0x00000000),
    UINT64_C2(0xad78ebc5, 0xac620000), UINT64_C2(0x813f3978, 0xf8940984),
    UINT64_C2(0xc097ce7b, 0xc90715b3), UINT64_C2(0x8f7e32ce, 0x7bea5c70),
    UINT64_C2(0xd5d238a4, 0xabe98068), UINT64_C2(0x9f4f2726, 0x179a2245),
    UINT64_C2(0xed63a231, 0xd4c4fb27), UINT64_C2(0xb0de6538, 0x8cc8ada8),
    UINT64_C2(0x83c7088e, 0x1aab65db), UINT64_C2(0xc45d1df9, 0x42711d9a),
    UINT64_C2(0x924d692c, 0xa61be758), UINT64_C2(0xda01ee64, 0x1a708dea),
    UINT64_C2(0xa26da399, 0x9aef774a), UINT64_C2(0xf209787b, 0xb47d6b85),
    UINT64_C2(0xb454e4a1, 0x79dd1877), UINT64_C2(0x865b8692, 0x5b9bc5c2),
    UINT64_C2(0xc83553c5, 0xc8965d3d), UINT64_C2(0x952ab45c, 0xfa97a0b3),
    UINT64_C2(0xde469fbd, 0x99a05fe3), UINT64_C2(0xa59bc234, 0xdb398c25),
    UINT64_C2(0xf6c69a72, 0xa3989f5c), UINT64_C2(0xb7dcbf53, 0x54e9bece),
    UINT64_C2(0x88fcf317, 0xf22241e2), UINT64_C2(0xcc20ce9b, 0xd35c78a5),
    UINT64_C2(0x98165af3, 0x7b2153df), UINT64_C2(0xe2a0b5dc, 0x971f303a),
    UINT64_C2(0xa8d9d153, 0x5ce3b396), UINT64_C2(0xfb9b7cd9, 0xa4a7443c),
    UINT64_C2(0xbb764c4c, 0xa7a44410), UINT64_C2(0x8bab8eef, 0xb6409c1a),
    UINT64_C2(0xd01fef10, 0xa657842c), UINT64_C2(0x9b10a4e5, 0xe9913129),
    UINT64_C2(0xe7109bfb, 0xa19c0c9d), UINT64_C2(0xac2820d9, 0x623bf429),
    UINT64_C2(0x80444b5e, 0x7aa7cf85), UINT64_C2(0xbf21e440, 0x03acdd2d),
    UINT64_C2(0x8e679c2f, 0x5e44ff8f), UINT64_C2(0xd433179d, 0x9c8cb841),
    UINT64_C2(0x9e19db92, 0xb4e31ba9), UINT64_C2(0xeb96bf6e, 0xbadf77d9),
    UINT64_C2(0xaf87023b, 0x9bf0ee6b)
  };
  static const int16_t kCachedPowers_E[] = {
    -1220, -1193, -1166, -1140, -1113, -1087, -1060, -1034, -1007,  -980,
     -954,  -927,  -901,  -874,  -847,  -821,  -794,  -768,  -741,  -715,
     -688,  -661,  -635,  -608,  -582,  -555,  -529,  -502,  -475,  -449,
     -422,  -396,  -369,  -343,  -316,  -289,  -263,  -236,  -210,  -183,
     -157,  -130,  -103,   -77,   -50,   -24,     3,    30,    56,    83,
      109,   136,   162,   189,   216,   242,   269,   295,   322,   348,
      375,   402,   428,   455,   481,   508,   534,   561,   588,   614,
      641,   667,   694,   720,   747,   774,   800,   827,   853,   880,
      907,   933,   960,   986,  1013,  1039,  1066
  };

  //int k = static_cast<int>(ceil((-61 - e) * 0.30102999566398114)) + 374;
  double dk = (-61 - e) * 0.30102999566398114 + 347;  // dk must be positive, so can do ceiling in positive
  int k = static_cast<int>(dk);
  if (k != dk)
    k++;

  unsigned index = static_cast<unsigned>((k >> 3) + 1);
  *K = -(-348 + static_cast<int>(index << 3));  // decimal exponent no need lookup table

  assert(index < sizeof(kCachedPowers_F) / sizeof(kCachedPowers_F[0]));
  return DiyFp(kCachedPowers_F[index], kCachedPowers_E[index]);
}

inline static 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 static unsigned 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;
  if (n < 1000000000) return 9;
  return 10;
}

inline static 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, 0, 0, 0, 0, 0 };
  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 = static_cast<int>(CountDecimalDigit32(p1));
  *len = 0;

  while (kappa > 0) {
    uint32_t d = 0;
    switch (kappa) {
      case 10: d = p1 / 1000000000; p1 %= 1000000000; break;
      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;
      NODEFAULT
    }
    if (d || *len)
      buffer[(*len)++] = '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)++] = '0' + d;
    p2 &= one.f - 1;
    kappa--;
    if (p2 < delta) {
      *K += kappa;
      GrisuRound(buffer, *len, delta, p2, one.f, wp_w.f * kPow10[-kappa]);
      return;
    }
  }
}

inline static 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);
}

static const char cDigitsLut[200] = {
  '0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '0', '6', '0', '7', '0', '8', '0', '9',
  '1', '0', '1', '1', '1', '2', '1', '3', '1', '4', '1', '5', '1', '6', '1', '7', '1', '8', '1', '9',
  '2', '0', '2', '1', '2', '2', '2', '3', '2', '4', '2', '5', '2', '6', '2', '7', '2', '8', '2', '9',
  '3', '0', '3', '1', '3', '2', '3', '3', '3', '4', '3', '5', '3', '6', '3', '7', '3', '8', '3', '9',
  '4', '0', '4', '1', '4', '2', '4', '3', '4', '4', '4', '5', '4', '6', '4', '7', '4', '8', '4', '9',
  '5', '0', '5', '1', '5', '2', '5', '3', '5', '4', '5', '5', '5', '6', '5', '7', '5', '8', '5', '9',
  '6', '0', '6', '1', '6', '2', '6', '3', '6', '4', '6', '5', '6', '6', '6', '7', '6', '8', '6', '9',
  '7', '0', '7', '1', '7', '2', '7', '3', '7', '4', '7', '5', '7', '6', '7', '7', '7', '8', '7', '9',
  '8', '0', '8', '1', '8', '2', '8', '3', '8', '4', '8', '5', '8', '6', '8', '7', '8', '8', '8', '9',
  '9', '0', '9', '1', '9', '2', '9', '3', '9', '4', '9', '5', '9', '6', '9', '7', '9', '8', '9', '9'
};

inline void WriteExponent(int K, char* buffer) {
  if (K < 0) {
    *buffer++ = '-';
    K = -K;
  }

  if (K >= 100) {
    *buffer++ = '0' + static_cast<char>(K / 100);
    K %= 100;
    const char* d = cDigitsLut + K * 2;
    *buffer++ = d[0];
    *buffer++ = d[1];
  }
  else if (K >= 10) {
    const char* d = cDigitsLut + K * 2;
    *buffer++ = d[0];
    *buffer++ = d[1];
  }
  else
    *buffer++ = '0' + static_cast<char>(K);

  *buffer = '\0';
}

inline static void Prettify(char* buffer, int length, int k) {
  const int kk = length + k;  // 10^(kk-1) <= v < 10^kk

  if (length <= kk && kk <= 21) {
    // 1234e7 -> 12340000000
    for (int i = length; i < kk; i++)
      buffer[i] = '0';
    buffer[kk] = '.';
    buffer[kk + 1] = '0';
    buffer[kk + 2] = '\0';
  }
  else if (0 < kk && kk <= 21) {
    // 1234e-2 -> 12.34
    memmove(&buffer[kk + 1], &buffer[kk], length - kk);
    buffer[kk] = '.';
    buffer[length + 1] = '\0';
  }
  else if (-6 < kk && kk <= 0) {
    // 1234e-6 -> 0.001234
    const int offset = 2 - kk;
    memmove(&buffer[offset], &buffer[0], length);
    buffer[0] = '0';
    buffer[1] = '.';
    for (int i = 2; i < offset; i++)
      buffer[i] = '0';
    buffer[length + offset] = '\0';
  }
  else if (length == 1) {
    // 1e30
    buffer[1] = 'e';
    WriteExponent(kk - 1, &buffer[2]);
  }
  else {
    // 1234e30 -> 1.234e33
    memmove(&buffer[2], &buffer[1], length - 1);
    buffer[1] = '.';
    buffer[length + 1] = 'e';
    WriteExponent(kk - 1, &buffer[0 + length + 2]);
  }
}

void pdtoa(double value, char *buffer) {
#ifdef _MSC_VER
  if (copysign(1.0, value) < 0) {
#else
  if (std::signbit(value)) {
#endif
    *buffer++ = '-';
    value = -value;
  }
  if (cinf(value)) {
    buffer[0] = 'i';
    buffer[1] = 'n';
    buffer[2] = 'f';
    buffer[3] = '\0';
  } else if (cnan(value)) {
    buffer[0] = 'n';
    buffer[1] = 'a';
    buffer[2] = 'n';
    buffer[3] = '\0';
  } else if (value == 0.0) {
    buffer[0] = '0';
    buffer[1] = '.';
    buffer[2] = '0';
    buffer[3] = '\0';
  } else if (value == 1.0) {
    buffer[0] = '1';
    buffer[1] = '.';
    buffer[2] = '0';
    buffer[3] = '\0';
  } else {
    int length, K;
    Grisu2(value, buffer, &length, &K);
    Prettify(buffer, length, K);
  }
}

/**
 * Version of pdtoa that tries hard to find the minimal string representation
 * for a single-precision floating-point number.
 */
void pftoa(float value, char *buffer) {
#ifdef _MSC_VER
  if (copysign(1.0f, value) < 0) {
#else
  if (std::signbit(value)) {
#endif
    *buffer++ = '-';
    value = -value;
  }
  if (cinf(value)) {
    buffer[0] = 'i';
    buffer[1] = 'n';
    buffer[2] = 'f';
    buffer[3] = '\0';
  } else if (cnan(value)) {
    buffer[0] = 'n';
    buffer[1] = 'a';
    buffer[2] = 'n';
    buffer[3] = '\0';
  } else if (value == 0.0f) {
    buffer[0] = '0';
    buffer[1] = '.';
    buffer[2] = '0';
    buffer[3] = '\0';
  } else if (value == 1.0f) {
    buffer[0] = '1';
    buffer[1] = '.';
    buffer[2] = '0';
    buffer[3] = '\0';
  } else {
    int length, k;
    Grisu2(value, buffer, &length, &k);

    const int kk = length + k;  // 10^(kk-1) <= v < 10^kk

    if (length <= kk && kk <= 21) {
      // 1234e7 -> 12340000000
      for (int i = length; i < kk; i++)
        buffer[i] = '0';
      buffer[kk] = '.';
      buffer[kk + 1] = '0';
      buffer[kk + 2] = '\0';
    }
    else if (0 < kk && kk <= 21) {
      // 1234e-2 -> 12.34
      memmove(&buffer[kk + 1], &buffer[kk], length - kk);

      // We want the shortest possible representation, so keep reading digits
      // until strtod would give the correct float value.
      buffer[kk] = '\0';
      double v = (double)atoi(buffer);
      buffer[kk] = '.';

      double multiplicand = 0.1;
      for (int i = kk + 1; i <= length; ++i) {
        double vplus = v + (buffer[i] - '0' + 1) * multiplicand;
        v += (buffer[i] - '0') * multiplicand;
        multiplicand *= 0.1;

        if ((float)v == value) {
          length = i;
          break;
        }
        if (buffer[i] < '9' && (float)vplus == value) {
          ++buffer[i];
          length = i;
          break;
        }
      }

      buffer[length + 1] = '\0';
    }
    else if (-6 < kk && kk <= 0) {
      // 1234e-6 -> 0.001234
      const int offset = 2 - kk;
      memmove(&buffer[offset], &buffer[0], length);
      buffer[0] = '0';
      buffer[1] = '.';

      // We want the shortest possible representation, so keep reading digits
      // until strtod would give the correct float value.
      double multiplicand = 1.0;
      for (int i = 2; i < offset; i++) {
        buffer[i] = '0';
        multiplicand *= 0.1;
      }
      if ((float)multiplicand == value) {
        length = 0;
        buffer[offset - 1] = '1';
      } else {
        multiplicand *= 0.1;
        double v = 0.0;
        for (int i = offset; i < length + offset; ++i) {
          double vplus = v + (buffer[i] - '0' + 1) * multiplicand;
          v += (buffer[i] - '0') * multiplicand;
          multiplicand *= 0.1;

          if ((float)v == value) {
            buffer[i + 1] = '\0';
            break;
          }
          if (buffer[i] < '9' && (float)vplus == value) {
            buffer[i]++;
            buffer[i + 1] = '\0';
            break;
          }
        }
      }
      buffer[length + offset] = '\0';
    }
    else if (length == 1) {
      // 1e30
      buffer[1] = 'e';
      WriteExponent(kk - 1, &buffer[2]);
    }
    else {
      // 1234e30 -> 1.234e33
      memmove(&buffer[2], &buffer[1], length - 1);
      buffer[1] = '.';
      buffer[length + 1] = 'e';

      double e_mult = pow(10.0, kk - 1);
      if ((float)(10.0 * e_mult) == value) {
        buffer[0] = '1';
        buffer[1] = 'e';
        WriteExponent(kk, &buffer[2]);
      } else {
        // We want the shortest possible representation, so keep reading
        // digits until strtod would give the correct float value.
        double v = buffer[0] - '0';
        double multiplicand = 0.1;
        for (int i = 2; i < length + 2; ++i) {
          double vplus = v + (buffer[i] - '0' + 1) * multiplicand;
          v += (buffer[i] - '0') * multiplicand;
          multiplicand *= 0.1;

          if ((float)(v * e_mult) == value) {
            length = i;
            buffer[i + 1] = 'e';
            break;
          }
          if (buffer[i] < '9' && (float)(vplus * e_mult) == value) {
            buffer[i]++;
            length = i;
            buffer[i + 1] = 'e';
            break;
          }
        }
        WriteExponent(kk - 1, &buffer[0 + length + 2]);
      }
    }
  }
}

#ifdef _MSC_VER
#pragma float_control(pop)
#endif