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Diffstat (limited to 'mfbt/double-conversion/strtod.cc')
| -rw-r--r-- | mfbt/double-conversion/strtod.cc | 555 |
1 files changed, 555 insertions, 0 deletions
diff --git a/mfbt/double-conversion/strtod.cc b/mfbt/double-conversion/strtod.cc new file mode 100644 index 000000000..b097afe0e --- /dev/null +++ b/mfbt/double-conversion/strtod.cc @@ -0,0 +1,555 @@ +// Copyright 2010 the V8 project authors. All rights reserved. +// Redistribution and use in source and binary forms, with or without +// modification, are permitted provided that the following conditions are +// met: +// +// * Redistributions of source code must retain the above copyright +// notice, this list of conditions and the following disclaimer. +// * Redistributions in binary form must reproduce the above +// copyright notice, this list of conditions and the following +// disclaimer in the documentation and/or other materials provided +// with the distribution. +// * Neither the name of Google Inc. nor the names of its +// contributors may be used to endorse or promote products derived +// from this software without specific prior written permission. +// +// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS +// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT +// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR +// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT +// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, +// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT +// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE +// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + +#include <stdarg.h> +#include <limits.h> + +#include "strtod.h" +#include "bignum.h" +#include "cached-powers.h" +#include "ieee.h" + +namespace double_conversion { + +// 2^53 = 9007199254740992. +// Any integer with at most 15 decimal digits will hence fit into a double +// (which has a 53bit significand) without loss of precision. +static const int kMaxExactDoubleIntegerDecimalDigits = 15; +// 2^64 = 18446744073709551616 > 10^19 +static const int kMaxUint64DecimalDigits = 19; + +// Max double: 1.7976931348623157 x 10^308 +// Min non-zero double: 4.9406564584124654 x 10^-324 +// Any x >= 10^309 is interpreted as +infinity. +// Any x <= 10^-324 is interpreted as 0. +// Note that 2.5e-324 (despite being smaller than the min double) will be read +// as non-zero (equal to the min non-zero double). +static const int kMaxDecimalPower = 309; +static const int kMinDecimalPower = -324; + +// 2^64 = 18446744073709551616 +static const uint64_t kMaxUint64 = UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF); + + +static const double exact_powers_of_ten[] = { + 1.0, // 10^0 + 10.0, + 100.0, + 1000.0, + 10000.0, + 100000.0, + 1000000.0, + 10000000.0, + 100000000.0, + 1000000000.0, + 10000000000.0, // 10^10 + 100000000000.0, + 1000000000000.0, + 10000000000000.0, + 100000000000000.0, + 1000000000000000.0, + 10000000000000000.0, + 100000000000000000.0, + 1000000000000000000.0, + 10000000000000000000.0, + 100000000000000000000.0, // 10^20 + 1000000000000000000000.0, + // 10^22 = 0x21e19e0c9bab2400000 = 0x878678326eac9 * 2^22 + 10000000000000000000000.0 +}; +static const int kExactPowersOfTenSize = ARRAY_SIZE(exact_powers_of_ten); + +// Maximum number of significant digits in the decimal representation. +// In fact the value is 772 (see conversions.cc), but to give us some margin +// we round up to 780. +static const int kMaxSignificantDecimalDigits = 780; + +static Vector<const char> TrimLeadingZeros(Vector<const char> buffer) { + for (int i = 0; i < buffer.length(); i++) { + if (buffer[i] != '0') { + return buffer.SubVector(i, buffer.length()); + } + } + return Vector<const char>(buffer.start(), 0); +} + + +static Vector<const char> TrimTrailingZeros(Vector<const char> buffer) { + for (int i = buffer.length() - 1; i >= 0; --i) { + if (buffer[i] != '0') { + return buffer.SubVector(0, i + 1); + } + } + return Vector<const char>(buffer.start(), 0); +} + + +static void CutToMaxSignificantDigits(Vector<const char> buffer, + int exponent, + char* significant_buffer, + int* significant_exponent) { + for (int i = 0; i < kMaxSignificantDecimalDigits - 1; ++i) { + significant_buffer[i] = buffer[i]; + } + // The input buffer has been trimmed. Therefore the last digit must be + // different from '0'. + ASSERT(buffer[buffer.length() - 1] != '0'); + // Set the last digit to be non-zero. This is sufficient to guarantee + // correct rounding. + significant_buffer[kMaxSignificantDecimalDigits - 1] = '1'; + *significant_exponent = + exponent + (buffer.length() - kMaxSignificantDecimalDigits); +} + + +// Trims the buffer and cuts it to at most kMaxSignificantDecimalDigits. +// If possible the input-buffer is reused, but if the buffer needs to be +// modified (due to cutting), then the input needs to be copied into the +// buffer_copy_space. +static void TrimAndCut(Vector<const char> buffer, int exponent, + char* buffer_copy_space, int space_size, + Vector<const char>* trimmed, int* updated_exponent) { + Vector<const char> left_trimmed = TrimLeadingZeros(buffer); + Vector<const char> right_trimmed = TrimTrailingZeros(left_trimmed); + exponent += left_trimmed.length() - right_trimmed.length(); + if (right_trimmed.length() > kMaxSignificantDecimalDigits) { + ASSERT(space_size >= kMaxSignificantDecimalDigits); + CutToMaxSignificantDigits(right_trimmed, exponent, + buffer_copy_space, updated_exponent); + *trimmed = Vector<const char>(buffer_copy_space, + kMaxSignificantDecimalDigits); + } else { + *trimmed = right_trimmed; + *updated_exponent = exponent; + } +} + + +// Reads digits from the buffer and converts them to a uint64. +// Reads in as many digits as fit into a uint64. +// When the string starts with "1844674407370955161" no further digit is read. +// Since 2^64 = 18446744073709551616 it would still be possible read another +// digit if it was less or equal than 6, but this would complicate the code. +static uint64_t ReadUint64(Vector<const char> buffer, + int* number_of_read_digits) { + uint64_t result = 0; + int i = 0; + while (i < buffer.length() && result <= (kMaxUint64 / 10 - 1)) { + int digit = buffer[i++] - '0'; + ASSERT(0 <= digit && digit <= 9); + result = 10 * result + digit; + } + *number_of_read_digits = i; + return result; +} + + +// Reads a DiyFp from the buffer. +// The returned DiyFp is not necessarily normalized. +// If remaining_decimals is zero then the returned DiyFp is accurate. +// Otherwise it has been rounded and has error of at most 1/2 ulp. +static void ReadDiyFp(Vector<const char> buffer, + DiyFp* result, + int* remaining_decimals) { + int read_digits; + uint64_t significand = ReadUint64(buffer, &read_digits); + if (buffer.length() == read_digits) { + *result = DiyFp(significand, 0); + *remaining_decimals = 0; + } else { + // Round the significand. + if (buffer[read_digits] >= '5') { + significand++; + } + // Compute the binary exponent. + int exponent = 0; + *result = DiyFp(significand, exponent); + *remaining_decimals = buffer.length() - read_digits; + } +} + + +static bool DoubleStrtod(Vector<const char> trimmed, + int exponent, + double* result) { +#if !defined(DOUBLE_CONVERSION_CORRECT_DOUBLE_OPERATIONS) + // On x86 the floating-point stack can be 64 or 80 bits wide. If it is + // 80 bits wide (as is the case on Linux) then double-rounding occurs and the + // result is not accurate. + // We know that Windows32 uses 64 bits and is therefore accurate. + // Note that the ARM simulator is compiled for 32bits. It therefore exhibits + // the same problem. + return false; +#endif + if (trimmed.length() <= kMaxExactDoubleIntegerDecimalDigits) { + int read_digits; + // The trimmed input fits into a double. + // If the 10^exponent (resp. 10^-exponent) fits into a double too then we + // can compute the result-double simply by multiplying (resp. dividing) the + // two numbers. + // This is possible because IEEE guarantees that floating-point operations + // return the best possible approximation. + if (exponent < 0 && -exponent < kExactPowersOfTenSize) { + // 10^-exponent fits into a double. + *result = static_cast<double>(ReadUint64(trimmed, &read_digits)); + ASSERT(read_digits == trimmed.length()); + *result /= exact_powers_of_ten[-exponent]; + return true; + } + if (0 <= exponent && exponent < kExactPowersOfTenSize) { + // 10^exponent fits into a double. + *result = static_cast<double>(ReadUint64(trimmed, &read_digits)); + ASSERT(read_digits == trimmed.length()); + *result *= exact_powers_of_ten[exponent]; + return true; + } + int remaining_digits = + kMaxExactDoubleIntegerDecimalDigits - trimmed.length(); + if ((0 <= exponent) && + (exponent - remaining_digits < kExactPowersOfTenSize)) { + // The trimmed string was short and we can multiply it with + // 10^remaining_digits. As a result the remaining exponent now fits + // into a double too. + *result = static_cast<double>(ReadUint64(trimmed, &read_digits)); + ASSERT(read_digits == trimmed.length()); + *result *= exact_powers_of_ten[remaining_digits]; + *result *= exact_powers_of_ten[exponent - remaining_digits]; + return true; + } + } + return false; +} + + +// Returns 10^exponent as an exact DiyFp. +// The given exponent must be in the range [1; kDecimalExponentDistance[. +static DiyFp AdjustmentPowerOfTen(int exponent) { + ASSERT(0 < exponent); + ASSERT(exponent < PowersOfTenCache::kDecimalExponentDistance); + // Simply hardcode the remaining powers for the given decimal exponent + // distance. + ASSERT(PowersOfTenCache::kDecimalExponentDistance == 8); + switch (exponent) { + case 1: return DiyFp(UINT64_2PART_C(0xa0000000, 00000000), -60); + case 2: return DiyFp(UINT64_2PART_C(0xc8000000, 00000000), -57); + case 3: return DiyFp(UINT64_2PART_C(0xfa000000, 00000000), -54); + case 4: return DiyFp(UINT64_2PART_C(0x9c400000, 00000000), -50); + case 5: return DiyFp(UINT64_2PART_C(0xc3500000, 00000000), -47); + case 6: return DiyFp(UINT64_2PART_C(0xf4240000, 00000000), -44); + case 7: return DiyFp(UINT64_2PART_C(0x98968000, 00000000), -40); + default: + UNREACHABLE(); + return DiyFp(0, 0); + } +} + + +// If the function returns true then the result is the correct double. +// Otherwise it is either the correct double or the double that is just below +// the correct double. +static bool DiyFpStrtod(Vector<const char> buffer, + int exponent, + double* result) { + DiyFp input; + int remaining_decimals; + ReadDiyFp(buffer, &input, &remaining_decimals); + // Since we may have dropped some digits the input is not accurate. + // If remaining_decimals is different than 0 than the error is at most + // .5 ulp (unit in the last place). + // We don't want to deal with fractions and therefore keep a common + // denominator. + const int kDenominatorLog = 3; + const int kDenominator = 1 << kDenominatorLog; + // Move the remaining decimals into the exponent. + exponent += remaining_decimals; + int error = (remaining_decimals == 0 ? 0 : kDenominator / 2); + + int old_e = input.e(); + input.Normalize(); + error <<= old_e - input.e(); + + ASSERT(exponent <= PowersOfTenCache::kMaxDecimalExponent); + if (exponent < PowersOfTenCache::kMinDecimalExponent) { + *result = 0.0; + return true; + } + DiyFp cached_power; + int cached_decimal_exponent; + PowersOfTenCache::GetCachedPowerForDecimalExponent(exponent, + &cached_power, + &cached_decimal_exponent); + + if (cached_decimal_exponent != exponent) { + int adjustment_exponent = exponent - cached_decimal_exponent; + DiyFp adjustment_power = AdjustmentPowerOfTen(adjustment_exponent); + input.Multiply(adjustment_power); + if (kMaxUint64DecimalDigits - buffer.length() >= adjustment_exponent) { + // The product of input with the adjustment power fits into a 64 bit + // integer. + ASSERT(DiyFp::kSignificandSize == 64); + } else { + // The adjustment power is exact. There is hence only an error of 0.5. + error += kDenominator / 2; + } + } + + input.Multiply(cached_power); + // The error introduced by a multiplication of a*b equals + // error_a + error_b + error_a*error_b/2^64 + 0.5 + // Substituting a with 'input' and b with 'cached_power' we have + // error_b = 0.5 (all cached powers have an error of less than 0.5 ulp), + // error_ab = 0 or 1 / kDenominator > error_a*error_b/ 2^64 + int error_b = kDenominator / 2; + int error_ab = (error == 0 ? 0 : 1); // We round up to 1. + int fixed_error = kDenominator / 2; + error += error_b + error_ab + fixed_error; + + old_e = input.e(); + input.Normalize(); + error <<= old_e - input.e(); + + // See if the double's significand changes if we add/subtract the error. + int order_of_magnitude = DiyFp::kSignificandSize + input.e(); + int effective_significand_size = + Double::SignificandSizeForOrderOfMagnitude(order_of_magnitude); + int precision_digits_count = + DiyFp::kSignificandSize - effective_significand_size; + if (precision_digits_count + kDenominatorLog >= DiyFp::kSignificandSize) { + // This can only happen for very small denormals. In this case the + // half-way multiplied by the denominator exceeds the range of an uint64. + // Simply shift everything to the right. + int shift_amount = (precision_digits_count + kDenominatorLog) - + DiyFp::kSignificandSize + 1; + input.set_f(input.f() >> shift_amount); + input.set_e(input.e() + shift_amount); + // We add 1 for the lost precision of error, and kDenominator for + // the lost precision of input.f(). + error = (error >> shift_amount) + 1 + kDenominator; + precision_digits_count -= shift_amount; + } + // We use uint64_ts now. This only works if the DiyFp uses uint64_ts too. + ASSERT(DiyFp::kSignificandSize == 64); + ASSERT(precision_digits_count < 64); + uint64_t one64 = 1; + uint64_t precision_bits_mask = (one64 << precision_digits_count) - 1; + uint64_t precision_bits = input.f() & precision_bits_mask; + uint64_t half_way = one64 << (precision_digits_count - 1); + precision_bits *= kDenominator; + half_way *= kDenominator; + DiyFp rounded_input(input.f() >> precision_digits_count, + input.e() + precision_digits_count); + if (precision_bits >= half_way + error) { + rounded_input.set_f(rounded_input.f() + 1); + } + // If the last_bits are too close to the half-way case than we are too + // inaccurate and round down. In this case we return false so that we can + // fall back to a more precise algorithm. + + *result = Double(rounded_input).value(); + if (half_way - error < precision_bits && precision_bits < half_way + error) { + // Too imprecise. The caller will have to fall back to a slower version. + // However the returned number is guaranteed to be either the correct + // double, or the next-lower double. + return false; + } else { + return true; + } +} + + +// Returns +// - -1 if buffer*10^exponent < diy_fp. +// - 0 if buffer*10^exponent == diy_fp. +// - +1 if buffer*10^exponent > diy_fp. +// Preconditions: +// buffer.length() + exponent <= kMaxDecimalPower + 1 +// buffer.length() + exponent > kMinDecimalPower +// buffer.length() <= kMaxDecimalSignificantDigits +static int CompareBufferWithDiyFp(Vector<const char> buffer, + int exponent, + DiyFp diy_fp) { + ASSERT(buffer.length() + exponent <= kMaxDecimalPower + 1); + ASSERT(buffer.length() + exponent > kMinDecimalPower); + ASSERT(buffer.length() <= kMaxSignificantDecimalDigits); + // Make sure that the Bignum will be able to hold all our numbers. + // Our Bignum implementation has a separate field for exponents. Shifts will + // consume at most one bigit (< 64 bits). + // ln(10) == 3.3219... + ASSERT(((kMaxDecimalPower + 1) * 333 / 100) < Bignum::kMaxSignificantBits); + Bignum buffer_bignum; + Bignum diy_fp_bignum; + buffer_bignum.AssignDecimalString(buffer); + diy_fp_bignum.AssignUInt64(diy_fp.f()); + if (exponent >= 0) { + buffer_bignum.MultiplyByPowerOfTen(exponent); + } else { + diy_fp_bignum.MultiplyByPowerOfTen(-exponent); + } + if (diy_fp.e() > 0) { + diy_fp_bignum.ShiftLeft(diy_fp.e()); + } else { + buffer_bignum.ShiftLeft(-diy_fp.e()); + } + return Bignum::Compare(buffer_bignum, diy_fp_bignum); +} + + +// Returns true if the guess is the correct double. +// Returns false, when guess is either correct or the next-lower double. +static bool ComputeGuess(Vector<const char> trimmed, int exponent, + double* guess) { + if (trimmed.length() == 0) { + *guess = 0.0; + return true; + } + if (exponent + trimmed.length() - 1 >= kMaxDecimalPower) { + *guess = Double::Infinity(); + return true; + } + if (exponent + trimmed.length() <= kMinDecimalPower) { + *guess = 0.0; + return true; + } + + if (DoubleStrtod(trimmed, exponent, guess) || + DiyFpStrtod(trimmed, exponent, guess)) { + return true; + } + if (*guess == Double::Infinity()) { + return true; + } + return false; +} + +double Strtod(Vector<const char> buffer, int exponent) { + char copy_buffer[kMaxSignificantDecimalDigits]; + Vector<const char> trimmed; + int updated_exponent; + TrimAndCut(buffer, exponent, copy_buffer, kMaxSignificantDecimalDigits, + &trimmed, &updated_exponent); + exponent = updated_exponent; + + double guess; + bool is_correct = ComputeGuess(trimmed, exponent, &guess); + if (is_correct) return guess; + + DiyFp upper_boundary = Double(guess).UpperBoundary(); + int comparison = CompareBufferWithDiyFp(trimmed, exponent, upper_boundary); + if (comparison < 0) { + return guess; + } else if (comparison > 0) { + return Double(guess).NextDouble(); + } else if ((Double(guess).Significand() & 1) == 0) { + // Round towards even. + return guess; + } else { + return Double(guess).NextDouble(); + } +} + +float Strtof(Vector<const char> buffer, int exponent) { + char copy_buffer[kMaxSignificantDecimalDigits]; + Vector<const char> trimmed; + int updated_exponent; + TrimAndCut(buffer, exponent, copy_buffer, kMaxSignificantDecimalDigits, + &trimmed, &updated_exponent); + exponent = updated_exponent; + + double double_guess; + bool is_correct = ComputeGuess(trimmed, exponent, &double_guess); + + float float_guess = static_cast<float>(double_guess); + if (float_guess == double_guess) { + // This shortcut triggers for integer values. + return float_guess; + } + + // We must catch double-rounding. Say the double has been rounded up, and is + // now a boundary of a float, and rounds up again. This is why we have to + // look at previous too. + // Example (in decimal numbers): + // input: 12349 + // high-precision (4 digits): 1235 + // low-precision (3 digits): + // when read from input: 123 + // when rounded from high precision: 124. + // To do this we simply look at the neigbors of the correct result and see + // if they would round to the same float. If the guess is not correct we have + // to look at four values (since two different doubles could be the correct + // double). + + double double_next = Double(double_guess).NextDouble(); + double double_previous = Double(double_guess).PreviousDouble(); + + float f1 = static_cast<float>(double_previous); + float f2 = float_guess; + float f3 = static_cast<float>(double_next); + float f4; + if (is_correct) { + f4 = f3; + } else { + double double_next2 = Double(double_next).NextDouble(); + f4 = static_cast<float>(double_next2); + } + (void) f2; // Mark variable as used. + ASSERT(f1 <= f2 && f2 <= f3 && f3 <= f4); + + // If the guess doesn't lie near a single-precision boundary we can simply + // return its float-value. + if (f1 == f4) { + return float_guess; + } + + ASSERT((f1 != f2 && f2 == f3 && f3 == f4) || + (f1 == f2 && f2 != f3 && f3 == f4) || + (f1 == f2 && f2 == f3 && f3 != f4)); + + // guess and next are the two possible canditates (in the same way that + // double_guess was the lower candidate for a double-precision guess). + float guess = f1; + float next = f4; + DiyFp upper_boundary; + if (guess == 0.0f) { + float min_float = 1e-45f; + upper_boundary = Double(static_cast<double>(min_float) / 2).AsDiyFp(); + } else { + upper_boundary = Single(guess).UpperBoundary(); + } + int comparison = CompareBufferWithDiyFp(trimmed, exponent, upper_boundary); + if (comparison < 0) { + return guess; + } else if (comparison > 0) { + return next; + } else if ((Single(guess).Significand() & 1) == 0) { + // Round towards even. + return guess; + } else { + return next; + } +} + +} // namespace double_conversion |