// 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.

#ifndef RAPIDJSON_STRTOD_
#define RAPIDJSON_STRTOD_

#include "ieee754.h"
#include "biginteger.h"
#include "diyfp.h"
#include "pow10.h"

RAPIDJSON_NAMESPACE_BEGIN
namespace internal
{

inline double FastPath(double significand, int exp)
{ if (exp < -308)
    return 0.0;
  else if (exp >= 0)
    return significand * internal::Pow10(exp);
  else
    return significand / internal::Pow10(-exp);
}

inline double StrtodNormalPrecision(double d, int p)
{ if (p < -308)
  { // Prevent expSum < -308, making Pow10(p) = 0
    d = FastPath(d, -308);
    d = FastPath(d, p + 308);
  }
  else
    d = FastPath(d, p);
  return d;
}

template <typename T>
inline T Min3(T a, T b, T c)
{ T m = a;
  if (m > b) m = b;
  if (m > c) m = c;
  return m;
}

inline int CheckWithinHalfULP(double b, const BigInteger& d, int dExp)
{ const Double db(b);
  const uint64_t bInt = db.IntegerSignificand();
  const int bExp = db.IntegerExponent();
  const int hExp = bExp - 1;

  int dS_Exp2 = 0, dS_Exp5 = 0, bS_Exp2 = 0, bS_Exp5 = 0, hS_Exp2 = 0, hS_Exp5 = 0;

  // Adjust for decimal exponent
  if (dExp >= 0)
  { dS_Exp2 += dExp;
    dS_Exp5 += dExp;
  }
  else
  { bS_Exp2 -= dExp;
    bS_Exp5 -= dExp;
    hS_Exp2 -= dExp;
    hS_Exp5 -= dExp;
  }

  // Adjust for binary exponent
  if (bExp >= 0)
    bS_Exp2 += bExp;
  else
  { dS_Exp2 -= bExp;
    hS_Exp2 -= bExp;
  }

  // Adjust for half ulp exponent
  if (hExp >= 0)
    hS_Exp2 += hExp;
  else
  { dS_Exp2 -= hExp;
    bS_Exp2 -= hExp;
  }

  // Remove common power of two factor from all three scaled values
  int common_Exp2 = Min3(dS_Exp2, bS_Exp2, hS_Exp2);
  dS_Exp2 -= common_Exp2;
  bS_Exp2 -= common_Exp2;
  hS_Exp2 -= common_Exp2;

  BigInteger dS = d;
  dS.MultiplyPow5(static_cast<unsigned>(dS_Exp5)) <<= static_cast<unsigned>(dS_Exp2);

  BigInteger bS(bInt);
  bS.MultiplyPow5(static_cast<unsigned>(bS_Exp5)) <<= static_cast<unsigned>(bS_Exp2);

  BigInteger hS(1);
  hS.MultiplyPow5(static_cast<unsigned>(hS_Exp5)) <<= static_cast<unsigned>(hS_Exp2);

  BigInteger delta(0);
  dS.Difference(bS, &delta);

  return delta.Compare(hS);
}

inline bool StrtodFast(double d, int p, double* result)
{ // Use fast path for string-to-double conversion if possible
  // see http://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/
  if (p > 22  && p < 22 + 16)
  { // Fast Path Cases In Disguise
    d *= internal::Pow10(p - 22);
    p = 22;
  }

  if (p >= -22 && p <= 22 && d <= 9007199254740991.0)   // 2^53 - 1
  { *result = FastPath(d, p);
    return true;
  }
  else
    return false;
}

// Compute an approximation and see if it is within 1/2 ULP
inline bool StrtodDiyFp(const char* decimals, size_t length, size_t decimalPosition, int exp, double* result)
{ uint64_t significand = 0;
  size_t i = 0;   // 2^64 - 1 = 18446744073709551615, 1844674407370955161 = 0x1999999999999999
  for (; i < length; i++)
  { if (significand  >  RAPIDJSON_UINT64_C2(0x19999999, 0x99999999) ||
        (significand == RAPIDJSON_UINT64_C2(0x19999999, 0x99999999) && decimals[i] > '5'))
      break;
    significand = significand * 10u + static_cast<unsigned>(decimals[i] - '0');
  }

  if (i < length && decimals[i] >= '5') // Rounding
    significand++;

  size_t remaining = length - i;
  const unsigned kUlpShift = 3;
  const unsigned kUlp = 1 << kUlpShift;
  int64_t error = (remaining == 0) ? 0 : kUlp / 2;

  DiyFp v(significand, 0);
  v = v.Normalize();
  error <<= -v.e;

  const int dExp = static_cast<int>(decimalPosition) - static_cast<int>(i) + exp;

  int actualExp;
  DiyFp cachedPower = GetCachedPower10(dExp, &actualExp);
  if (actualExp != dExp)
  { static const DiyFp kPow10[] =
    { DiyFp(RAPIDJSON_UINT64_C2(0xa0000000, 00000000), -60),  // 10^1
      DiyFp(RAPIDJSON_UINT64_C2(0xc8000000, 00000000), -57),  // 10^2
      DiyFp(RAPIDJSON_UINT64_C2(0xfa000000, 00000000), -54),  // 10^3
      DiyFp(RAPIDJSON_UINT64_C2(0x9c400000, 00000000), -50),  // 10^4
      DiyFp(RAPIDJSON_UINT64_C2(0xc3500000, 00000000), -47),  // 10^5
      DiyFp(RAPIDJSON_UINT64_C2(0xf4240000, 00000000), -44),  // 10^6
      DiyFp(RAPIDJSON_UINT64_C2(0x98968000, 00000000), -40)   // 10^7
    };
    int  adjustment = dExp - actualExp - 1;
    RAPIDJSON_ASSERT(adjustment >= 0 && adjustment < 7);
    v = v * kPow10[adjustment];
    if (length + static_cast<unsigned>(adjustment)> 19u) // has more digits than decimal digits in 64-bit
      error += kUlp / 2;
  }

  v = v * cachedPower;

  error += kUlp + (error == 0 ? 0 : 1);

  const int oldExp = v.e;
  v = v.Normalize();
  error <<= oldExp - v.e;

  const unsigned effectiveSignificandSize = Double::EffectiveSignificandSize(64 + v.e);
  unsigned precisionSize = 64 - effectiveSignificandSize;
  if (precisionSize + kUlpShift >= 64)
  { unsigned scaleExp = (precisionSize + kUlpShift) - 63;
    v.f >>= scaleExp;
    v.e += scaleExp;
    error = (error >> scaleExp) + 1 + static_cast<int>(kUlp);
    precisionSize -= scaleExp;
  }

  DiyFp rounded(v.f >> precisionSize, v.e + static_cast<int>(precisionSize));
  const uint64_t precisionBits = (v.f & ((uint64_t(1) << precisionSize) - 1)) * kUlp;
  const uint64_t halfWay = (uint64_t(1) << (precisionSize - 1)) * kUlp;
  if (precisionBits >= halfWay + static_cast<unsigned>(error))
  { rounded.f++;
    if (rounded.f & (DiyFp::kDpHiddenBit << 1))   // rounding overflows mantissa (issue #340)
    { rounded.f >>= 1;
      rounded.e++;
    }
  }

  *result = rounded.ToDouble();

  return halfWay - static_cast<unsigned>(error) >= precisionBits || precisionBits >= halfWay + static_cast<unsigned>(error);
}

inline double StrtodBigInteger(double approx, const char* decimals, size_t length, size_t decimalPosition, int exp)
{ const BigInteger dInt(decimals, length);
  const int dExp = static_cast<int>(decimalPosition) - static_cast<int>(length) + exp;
  Double a(approx);
  int cmp = CheckWithinHalfULP(a.Value(), dInt, dExp);
  if (cmp < 0)
    return a.Value();  // within half ULP
  else if (cmp == 0)
  { // Round towards even
    if (a.Significand() & 1)
      return a.NextPositiveDouble();
    else
      return a.Value();
  }
  else // adjustment
    return a.NextPositiveDouble();
}

inline double StrtodFullPrecision(double d, int p, const char* decimals, size_t length, size_t decimalPosition, int exp)
{ RAPIDJSON_ASSERT(d >= 0.0);
  RAPIDJSON_ASSERT(length >= 1);

  double result;
  if (StrtodFast(d, p, &result))
    return result;

  // Trim leading zeros
  while (*decimals == '0' && length > 1)
  { length--;
    decimals++;
    decimalPosition--;
  }

  // Trim trailing zeros
  while (decimals[length - 1] == '0' && length > 1)
  { length--;
    decimalPosition--;
    exp++;
  }

  // Trim right-most digits
  const int kMaxDecimalDigit = 780;
  if (static_cast<int>(length) > kMaxDecimalDigit)
  { int delta = (static_cast<int>(length) - kMaxDecimalDigit);
    exp += delta;
    decimalPosition -= static_cast<unsigned>(delta);
    length = kMaxDecimalDigit;
  }

  // If too small, underflow to zero
  if (int(length) + exp < -324)
    return 0.0;

  if (StrtodDiyFp(decimals, length, decimalPosition, exp, &result))
    return result;

  // Use approximation from StrtodDiyFp and make adjustment with BigInteger comparison
  return StrtodBigInteger(result, decimals, length, decimalPosition, exp);
}

} // namespace internal
RAPIDJSON_NAMESPACE_END

#endif // RAPIDJSON_STRTOD_
