flutter / third_party / double-conversion / 3833c5ca25dcf026b46e29df8c18efa97ed39a3e / . / src / fast-dtoa.cc

// Copyright 2012 the V8 project authors. All rights reserved. | |

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#include "fast-dtoa.h" | |

#include "cached-powers.h" | |

#include "diy-fp.h" | |

#include "ieee.h" | |

namespace double_conversion { | |

// The minimal and maximal target exponent define the range of w's binary | |

// exponent, where 'w' is the result of multiplying the input by a cached power | |

// of ten. | |

// | |

// A different range might be chosen on a different platform, to optimize digit | |

// generation, but a smaller range requires more powers of ten to be cached. | |

static const int kMinimalTargetExponent = -60; | |

static const int kMaximalTargetExponent = -32; | |

// Adjusts the last digit of the generated number, and screens out generated | |

// solutions that may be inaccurate. A solution may be inaccurate if it is | |

// outside the safe interval, or if we cannot prove that it is closer to the | |

// input than a neighboring representation of the same length. | |

// | |

// Input: * buffer containing the digits of too_high / 10^kappa | |

// * the buffer's length | |

// * distance_too_high_w == (too_high - w).f() * unit | |

// * unsafe_interval == (too_high - too_low).f() * unit | |

// * rest = (too_high - buffer * 10^kappa).f() * unit | |

// * ten_kappa = 10^kappa * unit | |

// * unit = the common multiplier | |

// Output: returns true if the buffer is guaranteed to contain the closest | |

// representable number to the input. | |

// Modifies the generated digits in the buffer to approach (round towards) w. | |

static bool RoundWeed(Vector<char> buffer, | |

int length, | |

uint64_t distance_too_high_w, | |

uint64_t unsafe_interval, | |

uint64_t rest, | |

uint64_t ten_kappa, | |

uint64_t unit) { | |

uint64_t small_distance = distance_too_high_w - unit; | |

uint64_t big_distance = distance_too_high_w + unit; | |

// Let w_low = too_high - big_distance, and | |

// w_high = too_high - small_distance. | |

// Note: w_low < w < w_high | |

// | |

// The real w (* unit) must lie somewhere inside the interval | |

// ]w_low; w_high[ (often written as "(w_low; w_high)") | |

// Basically the buffer currently contains a number in the unsafe interval | |

// ]too_low; too_high[ with too_low < w < too_high | |

// | |

// too_high - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - | |

// ^v 1 unit ^ ^ ^ ^ | |

// boundary_high --------------------- . . . . | |

// ^v 1 unit . . . . | |

// - - - - - - - - - - - - - - - - - - - + - - + - - - - - - . . | |

// . . ^ . . | |

// . big_distance . . . | |

// . . . . rest | |

// small_distance . . . . | |

// v . . . . | |

// w_high - - - - - - - - - - - - - - - - - - . . . . | |

// ^v 1 unit . . . . | |

// w ---------------------------------------- . . . . | |

// ^v 1 unit v . . . | |

// w_low - - - - - - - - - - - - - - - - - - - - - . . . | |

// . . v | |

// buffer --------------------------------------------------+-------+-------- | |

// . . | |

// safe_interval . | |

// v . | |

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - . | |

// ^v 1 unit . | |

// boundary_low ------------------------- unsafe_interval | |

// ^v 1 unit v | |

// too_low - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - | |

// | |

// | |

// Note that the value of buffer could lie anywhere inside the range too_low | |

// to too_high. | |

// | |

// boundary_low, boundary_high and w are approximations of the real boundaries | |

// and v (the input number). They are guaranteed to be precise up to one unit. | |

// In fact the error is guaranteed to be strictly less than one unit. | |

// | |

// Anything that lies outside the unsafe interval is guaranteed not to round | |

// to v when read again. | |

// Anything that lies inside the safe interval is guaranteed to round to v | |

// when read again. | |

// If the number inside the buffer lies inside the unsafe interval but not | |

// inside the safe interval then we simply do not know and bail out (returning | |

// false). | |

// | |

// Similarly we have to take into account the imprecision of 'w' when finding | |

// the closest representation of 'w'. If we have two potential | |

// representations, and one is closer to both w_low and w_high, then we know | |

// it is closer to the actual value v. | |

// | |

// By generating the digits of too_high we got the largest (closest to | |

// too_high) buffer that is still in the unsafe interval. In the case where | |

// w_high < buffer < too_high we try to decrement the buffer. | |

// This way the buffer approaches (rounds towards) w. | |

// There are 3 conditions that stop the decrementation process: | |

// 1) the buffer is already below w_high | |

// 2) decrementing the buffer would make it leave the unsafe interval | |

// 3) decrementing the buffer would yield a number below w_high and farther | |

// away than the current number. In other words: | |

// (buffer{-1} < w_high) && w_high - buffer{-1} > buffer - w_high | |

// Instead of using the buffer directly we use its distance to too_high. | |

// Conceptually rest ~= too_high - buffer | |

// We need to do the following tests in this order to avoid over- and | |

// underflows. | |

ASSERT(rest <= unsafe_interval); | |

while (rest < small_distance && // Negated condition 1 | |

unsafe_interval - rest >= ten_kappa && // Negated condition 2 | |

(rest + ten_kappa < small_distance || // buffer{-1} > w_high | |

small_distance - rest >= rest + ten_kappa - small_distance)) { | |

buffer[length - 1]--; | |

rest += ten_kappa; | |

} | |

// We have approached w+ as much as possible. We now test if approaching w- | |

// would require changing the buffer. If yes, then we have two possible | |

// representations close to w, but we cannot decide which one is closer. | |

if (rest < big_distance && | |

unsafe_interval - rest >= ten_kappa && | |

(rest + ten_kappa < big_distance || | |

big_distance - rest > rest + ten_kappa - big_distance)) { | |

return false; | |

} | |

// Weeding test. | |

// The safe interval is [too_low + 2 ulp; too_high - 2 ulp] | |

// Since too_low = too_high - unsafe_interval this is equivalent to | |

// [too_high - unsafe_interval + 4 ulp; too_high - 2 ulp] | |

// Conceptually we have: rest ~= too_high - buffer | |

return (2 * unit <= rest) && (rest <= unsafe_interval - 4 * unit); | |

} | |

// Rounds the buffer upwards if the result is closer to v by possibly adding | |

// 1 to the buffer. If the precision of the calculation is not sufficient to | |

// round correctly, return false. | |

// The rounding might shift the whole buffer in which case the kappa is | |

// adjusted. For example "99", kappa = 3 might become "10", kappa = 4. | |

// | |

// If 2*rest > ten_kappa then the buffer needs to be round up. | |

// rest can have an error of +/- 1 unit. This function accounts for the | |

// imprecision and returns false, if the rounding direction cannot be | |

// unambiguously determined. | |

// | |

// Precondition: rest < ten_kappa. | |

static bool RoundWeedCounted(Vector<char> buffer, | |

int length, | |

uint64_t rest, | |

uint64_t ten_kappa, | |

uint64_t unit, | |

int* kappa) { | |

ASSERT(rest < ten_kappa); | |

// The following tests are done in a specific order to avoid overflows. They | |

// will work correctly with any uint64 values of rest < ten_kappa and unit. | |

// | |

// If the unit is too big, then we don't know which way to round. For example | |

// a unit of 50 means that the real number lies within rest +/- 50. If | |

// 10^kappa == 40 then there is no way to tell which way to round. | |

if (unit >= ten_kappa) return false; | |

// Even if unit is just half the size of 10^kappa we are already completely | |

// lost. (And after the previous test we know that the expression will not | |

// over/underflow.) | |

if (ten_kappa - unit <= unit) return false; | |

// If 2 * (rest + unit) <= 10^kappa we can safely round down. | |

if ((ten_kappa - rest > rest) && (ten_kappa - 2 * rest >= 2 * unit)) { | |

return true; | |

} | |

// If 2 * (rest - unit) >= 10^kappa, then we can safely round up. | |

if ((rest > unit) && (ten_kappa - (rest - unit) <= (rest - unit))) { | |

// Increment the last digit recursively until we find a non '9' digit. | |

buffer[length - 1]++; | |

for (int i = length - 1; i > 0; --i) { | |

if (buffer[i] != '0' + 10) break; | |

buffer[i] = '0'; | |

buffer[i - 1]++; | |

} | |

// If the first digit is now '0'+ 10 we had a buffer with all '9's. With the | |

// exception of the first digit all digits are now '0'. Simply switch the | |

// first digit to '1' and adjust the kappa. Example: "99" becomes "10" and | |

// the power (the kappa) is increased. | |

if (buffer[0] == '0' + 10) { | |

buffer[0] = '1'; | |

(*kappa) += 1; | |

} | |

return true; | |

} | |

return false; | |

} | |

// Returns the biggest power of ten that is less than or equal to the given | |

// number. We furthermore receive the maximum number of bits 'number' has. | |

// | |

// Returns power == 10^(exponent_plus_one-1) such that | |

// power <= number < power * 10. | |

// If number_bits == 0 then 0^(0-1) is returned. | |

// The number of bits must be <= 32. | |

// Precondition: number < (1 << (number_bits + 1)). | |

// Inspired by the method for finding an integer log base 10 from here: | |

// http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog10 | |

static unsigned int const kSmallPowersOfTen[] = | |

{0, 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, | |

1000000000}; | |

static void BiggestPowerTen(uint32_t number, | |

int number_bits, | |

uint32_t* power, | |

int* exponent_plus_one) { | |

ASSERT(number < (1u << (number_bits + 1))); | |

// 1233/4096 is approximately 1/lg(10). | |

int exponent_plus_one_guess = ((number_bits + 1) * 1233 >> 12); | |

// We increment to skip over the first entry in the kPowersOf10 table. | |

// Note: kPowersOf10[i] == 10^(i-1). | |

exponent_plus_one_guess++; | |

// We don't have any guarantees that 2^number_bits <= number. | |

// TODO(floitsch): can we change the 'while' into an 'if'? We definitely see | |

// number < (2^number_bits - 1), but I haven't encountered | |

// number < (2^number_bits - 2) yet. | |

while (number < kSmallPowersOfTen[exponent_plus_one_guess]) { | |

exponent_plus_one_guess--; | |

} | |

*power = kSmallPowersOfTen[exponent_plus_one_guess]; | |

*exponent_plus_one = exponent_plus_one_guess; | |

} | |

// Generates the digits of input number w. | |

// w is a floating-point number (DiyFp), consisting of a significand and an | |

// exponent. Its exponent is bounded by kMinimalTargetExponent and | |

// kMaximalTargetExponent. | |

// Hence -60 <= w.e() <= -32. | |

// | |

// Returns false if it fails, in which case the generated digits in the buffer | |

// should not be used. | |

// Preconditions: | |

// * low, w and high are correct up to 1 ulp (unit in the last place). That | |

// is, their error must be less than a unit of their last digits. | |

// * low.e() == w.e() == high.e() | |

// * low < w < high, and taking into account their error: low~ <= high~ | |

// * kMinimalTargetExponent <= w.e() <= kMaximalTargetExponent | |

// Postconditions: returns false if procedure fails. | |

// otherwise: | |

// * buffer is not null-terminated, but len contains the number of digits. | |

// * buffer contains the shortest possible decimal digit-sequence | |

// such that LOW < buffer * 10^kappa < HIGH, where LOW and HIGH are the | |

// correct values of low and high (without their error). | |

// * if more than one decimal representation gives the minimal number of | |

// decimal digits then the one closest to W (where W is the correct value | |

// of w) is chosen. | |

// Remark: this procedure takes into account the imprecision of its input | |

// numbers. If the precision is not enough to guarantee all the postconditions | |

// then false is returned. This usually happens rarely (~0.5%). | |

// | |

// Say, for the sake of example, that | |

// w.e() == -48, and w.f() == 0x1234567890abcdef | |

// w's value can be computed by w.f() * 2^w.e() | |

// We can obtain w's integral digits by simply shifting w.f() by -w.e(). | |

// -> w's integral part is 0x1234 | |

// w's fractional part is therefore 0x567890abcdef. | |

// Printing w's integral part is easy (simply print 0x1234 in decimal). | |

// In order to print its fraction we repeatedly multiply the fraction by 10 and | |

// get each digit. Example the first digit after the point would be computed by | |

// (0x567890abcdef * 10) >> 48. -> 3 | |

// The whole thing becomes slightly more complicated because we want to stop | |

// once we have enough digits. That is, once the digits inside the buffer | |

// represent 'w' we can stop. Everything inside the interval low - high | |

// represents w. However we have to pay attention to low, high and w's | |

// imprecision. | |

static bool DigitGen(DiyFp low, | |

DiyFp w, | |

DiyFp high, | |

Vector<char> buffer, | |

int* length, | |

int* kappa) { | |

ASSERT(low.e() == w.e() && w.e() == high.e()); | |

ASSERT(low.f() + 1 <= high.f() - 1); | |

ASSERT(kMinimalTargetExponent <= w.e() && w.e() <= kMaximalTargetExponent); | |

// low, w and high are imprecise, but by less than one ulp (unit in the last | |

// place). | |

// If we remove (resp. add) 1 ulp from low (resp. high) we are certain that | |

// the new numbers are outside of the interval we want the final | |

// representation to lie in. | |

// Inversely adding (resp. removing) 1 ulp from low (resp. high) would yield | |

// numbers that are certain to lie in the interval. We will use this fact | |

// later on. | |

// We will now start by generating the digits within the uncertain | |

// interval. Later we will weed out representations that lie outside the safe | |

// interval and thus _might_ lie outside the correct interval. | |

uint64_t unit = 1; | |

DiyFp too_low = DiyFp(low.f() - unit, low.e()); | |

DiyFp too_high = DiyFp(high.f() + unit, high.e()); | |

// too_low and too_high are guaranteed to lie outside the interval we want the | |

// generated number in. | |

DiyFp unsafe_interval = DiyFp::Minus(too_high, too_low); | |

// We now cut the input number into two parts: the integral digits and the | |

// fractionals. We will not write any decimal separator though, but adapt | |

// kappa instead. | |

// Reminder: we are currently computing the digits (stored inside the buffer) | |

// such that: too_low < buffer * 10^kappa < too_high | |

// We use too_high for the digit_generation and stop as soon as possible. | |

// If we stop early we effectively round down. | |

DiyFp one = DiyFp(static_cast<uint64_t>(1) << -w.e(), w.e()); | |

// Division by one is a shift. | |

uint32_t integrals = static_cast<uint32_t>(too_high.f() >> -one.e()); | |

// Modulo by one is an and. | |

uint64_t fractionals = too_high.f() & (one.f() - 1); | |

uint32_t divisor; | |

int divisor_exponent_plus_one; | |

BiggestPowerTen(integrals, DiyFp::kSignificandSize - (-one.e()), | |

&divisor, &divisor_exponent_plus_one); | |

*kappa = divisor_exponent_plus_one; | |

*length = 0; | |

// Loop invariant: buffer = too_high / 10^kappa (integer division) | |

// The invariant holds for the first iteration: kappa has been initialized | |

// with the divisor exponent + 1. And the divisor is the biggest power of ten | |

// that is smaller than integrals. | |

while (*kappa > 0) { | |

int digit = integrals / divisor; | |

buffer[*length] = '0' + digit; | |

(*length)++; | |

integrals %= divisor; | |

(*kappa)--; | |

// Note that kappa now equals the exponent of the divisor and that the | |

// invariant thus holds again. | |

uint64_t rest = | |

(static_cast<uint64_t>(integrals) << -one.e()) + fractionals; | |

// Invariant: too_high = buffer * 10^kappa + DiyFp(rest, one.e()) | |

// Reminder: unsafe_interval.e() == one.e() | |

if (rest < unsafe_interval.f()) { | |

// Rounding down (by not emitting the remaining digits) yields a number | |

// that lies within the unsafe interval. | |

return RoundWeed(buffer, *length, DiyFp::Minus(too_high, w).f(), | |

unsafe_interval.f(), rest, | |

static_cast<uint64_t>(divisor) << -one.e(), unit); | |

} | |

divisor /= 10; | |

} | |

// The integrals have been generated. We are at the point of the decimal | |

// separator. In the following loop we simply multiply the remaining digits by | |

// 10 and divide by one. We just need to pay attention to multiply associated | |

// data (like the interval or 'unit'), too. | |

// Note that the multiplication by 10 does not overflow, because w.e >= -60 | |

// and thus one.e >= -60. | |

ASSERT(one.e() >= -60); | |

ASSERT(fractionals < one.f()); | |

ASSERT(UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF) / 10 >= one.f()); | |

while (true) { | |

fractionals *= 10; | |

unit *= 10; | |

unsafe_interval.set_f(unsafe_interval.f() * 10); | |

// Integer division by one. | |

int digit = static_cast<int>(fractionals >> -one.e()); | |

buffer[*length] = '0' + digit; | |

(*length)++; | |

fractionals &= one.f() - 1; // Modulo by one. | |

(*kappa)--; | |

if (fractionals < unsafe_interval.f()) { | |

return RoundWeed(buffer, *length, DiyFp::Minus(too_high, w).f() * unit, | |

unsafe_interval.f(), fractionals, one.f(), unit); | |

} | |

} | |

} | |

// Generates (at most) requested_digits digits of input number w. | |

// w is a floating-point number (DiyFp), consisting of a significand and an | |

// exponent. Its exponent is bounded by kMinimalTargetExponent and | |

// kMaximalTargetExponent. | |

// Hence -60 <= w.e() <= -32. | |

// | |

// Returns false if it fails, in which case the generated digits in the buffer | |

// should not be used. | |

// Preconditions: | |

// * w is correct up to 1 ulp (unit in the last place). That | |

// is, its error must be strictly less than a unit of its last digit. | |

// * kMinimalTargetExponent <= w.e() <= kMaximalTargetExponent | |

// | |

// Postconditions: returns false if procedure fails. | |

// otherwise: | |

// * buffer is not null-terminated, but length contains the number of | |

// digits. | |

// * the representation in buffer is the most precise representation of | |

// requested_digits digits. | |

// * buffer contains at most requested_digits digits of w. If there are less | |

// than requested_digits digits then some trailing '0's have been removed. | |

// * kappa is such that | |

// w = buffer * 10^kappa + eps with |eps| < 10^kappa / 2. | |

// | |

// Remark: This procedure takes into account the imprecision of its input | |

// numbers. If the precision is not enough to guarantee all the postconditions | |

// then false is returned. This usually happens rarely, but the failure-rate | |

// increases with higher requested_digits. | |

static bool DigitGenCounted(DiyFp w, | |

int requested_digits, | |

Vector<char> buffer, | |

int* length, | |

int* kappa) { | |

ASSERT(kMinimalTargetExponent <= w.e() && w.e() <= kMaximalTargetExponent); | |

ASSERT(kMinimalTargetExponent >= -60); | |

ASSERT(kMaximalTargetExponent <= -32); | |

// w is assumed to have an error less than 1 unit. Whenever w is scaled we | |

// also scale its error. | |

uint64_t w_error = 1; | |

// We cut the input number into two parts: the integral digits and the | |

// fractional digits. We don't emit any decimal separator, but adapt kappa | |

// instead. Example: instead of writing "1.2" we put "12" into the buffer and | |

// increase kappa by 1. | |

DiyFp one = DiyFp(static_cast<uint64_t>(1) << -w.e(), w.e()); | |

// Division by one is a shift. | |

uint32_t integrals = static_cast<uint32_t>(w.f() >> -one.e()); | |

// Modulo by one is an and. | |

uint64_t fractionals = w.f() & (one.f() - 1); | |

uint32_t divisor; | |

int divisor_exponent_plus_one; | |

BiggestPowerTen(integrals, DiyFp::kSignificandSize - (-one.e()), | |

&divisor, &divisor_exponent_plus_one); | |

*kappa = divisor_exponent_plus_one; | |

*length = 0; | |

// Loop invariant: buffer = w / 10^kappa (integer division) | |

// The invariant holds for the first iteration: kappa has been initialized | |

// with the divisor exponent + 1. And the divisor is the biggest power of ten | |

// that is smaller than 'integrals'. | |

while (*kappa > 0) { | |

int digit = integrals / divisor; | |

buffer[*length] = '0' + digit; | |

(*length)++; | |

requested_digits--; | |

integrals %= divisor; | |

(*kappa)--; | |

// Note that kappa now equals the exponent of the divisor and that the | |

// invariant thus holds again. | |

if (requested_digits == 0) break; | |

divisor /= 10; | |

} | |

if (requested_digits == 0) { | |

uint64_t rest = | |

(static_cast<uint64_t>(integrals) << -one.e()) + fractionals; | |

return RoundWeedCounted(buffer, *length, rest, | |

static_cast<uint64_t>(divisor) << -one.e(), w_error, | |

kappa); | |

} | |

// The integrals have been generated. We are at the point of the decimal | |

// separator. In the following loop we simply multiply the remaining digits by | |

// 10 and divide by one. We just need to pay attention to multiply associated | |

// data (the 'unit'), too. | |

// Note that the multiplication by 10 does not overflow, because w.e >= -60 | |

// and thus one.e >= -60. | |

ASSERT(one.e() >= -60); | |

ASSERT(fractionals < one.f()); | |

ASSERT(UINT64_2PART_C(0xFFFFFFFF, FFFFFFFF) / 10 >= one.f()); | |

while (requested_digits > 0 && fractionals > w_error) { | |

fractionals *= 10; | |

w_error *= 10; | |

// Integer division by one. | |

int digit = static_cast<int>(fractionals >> -one.e()); | |

buffer[*length] = '0' + digit; | |

(*length)++; | |

requested_digits--; | |

fractionals &= one.f() - 1; // Modulo by one. | |

(*kappa)--; | |

} | |

if (requested_digits != 0) return false; | |

return RoundWeedCounted(buffer, *length, fractionals, one.f(), w_error, | |

kappa); | |

} | |

// Provides a decimal representation of v. | |

// Returns true if it succeeds, otherwise the result cannot be trusted. | |

// There will be *length digits inside the buffer (not null-terminated). | |

// If the function returns true then | |

// v == (double) (buffer * 10^decimal_exponent). | |

// The digits in the buffer are the shortest representation possible: no | |

// 0.09999999999999999 instead of 0.1. The shorter representation will even be | |

// chosen even if the longer one would be closer to v. | |

// The last digit will be closest to the actual v. That is, even if several | |

// digits might correctly yield 'v' when read again, the closest will be | |

// computed. | |

static bool Grisu3(double v, | |

FastDtoaMode mode, | |

Vector<char> buffer, | |

int* length, | |

int* decimal_exponent) { | |

DiyFp w = Double(v).AsNormalizedDiyFp(); | |

// boundary_minus and boundary_plus are the boundaries between v and its | |

// closest floating-point neighbors. Any number strictly between | |

// boundary_minus and boundary_plus will round to v when convert to a double. | |

// Grisu3 will never output representations that lie exactly on a boundary. | |

DiyFp boundary_minus, boundary_plus; | |

if (mode == FAST_DTOA_SHORTEST) { | |

Double(v).NormalizedBoundaries(&boundary_minus, &boundary_plus); | |

} else { | |

ASSERT(mode == FAST_DTOA_SHORTEST_SINGLE); | |

float single_v = static_cast<float>(v); | |

Single(single_v).NormalizedBoundaries(&boundary_minus, &boundary_plus); | |

} | |

ASSERT(boundary_plus.e() == w.e()); | |

DiyFp ten_mk; // Cached power of ten: 10^-k | |

int mk; // -k | |

int ten_mk_minimal_binary_exponent = | |

kMinimalTargetExponent - (w.e() + DiyFp::kSignificandSize); | |

int ten_mk_maximal_binary_exponent = | |

kMaximalTargetExponent - (w.e() + DiyFp::kSignificandSize); | |

PowersOfTenCache::GetCachedPowerForBinaryExponentRange( | |

ten_mk_minimal_binary_exponent, | |

ten_mk_maximal_binary_exponent, | |

&ten_mk, &mk); | |

ASSERT((kMinimalTargetExponent <= w.e() + ten_mk.e() + | |

DiyFp::kSignificandSize) && | |

(kMaximalTargetExponent >= w.e() + ten_mk.e() + | |

DiyFp::kSignificandSize)); | |

// Note that ten_mk is only an approximation of 10^-k. A DiyFp only contains a | |

// 64 bit significand and ten_mk is thus only precise up to 64 bits. | |

// The DiyFp::Times procedure rounds its result, and ten_mk is approximated | |

// too. The variable scaled_w (as well as scaled_boundary_minus/plus) are now | |

// off by a small amount. | |

// In fact: scaled_w - w*10^k < 1ulp (unit in the last place) of scaled_w. | |

// In other words: let f = scaled_w.f() and e = scaled_w.e(), then | |

// (f-1) * 2^e < w*10^k < (f+1) * 2^e | |

DiyFp scaled_w = DiyFp::Times(w, ten_mk); | |

ASSERT(scaled_w.e() == | |

boundary_plus.e() + ten_mk.e() + DiyFp::kSignificandSize); | |

// In theory it would be possible to avoid some recomputations by computing | |

// the difference between w and boundary_minus/plus (a power of 2) and to | |

// compute scaled_boundary_minus/plus by subtracting/adding from | |

// scaled_w. However the code becomes much less readable and the speed | |

// enhancements are not terriffic. | |

DiyFp scaled_boundary_minus = DiyFp::Times(boundary_minus, ten_mk); | |

DiyFp scaled_boundary_plus = DiyFp::Times(boundary_plus, ten_mk); | |

// DigitGen will generate the digits of scaled_w. Therefore we have | |

// v == (double) (scaled_w * 10^-mk). | |

// Set decimal_exponent == -mk and pass it to DigitGen. If scaled_w is not an | |

// integer than it will be updated. For instance if scaled_w == 1.23 then | |

// the buffer will be filled with "123" und the decimal_exponent will be | |

// decreased by 2. | |

int kappa; | |

bool result = DigitGen(scaled_boundary_minus, scaled_w, scaled_boundary_plus, | |

buffer, length, &kappa); | |

*decimal_exponent = -mk + kappa; | |

return result; | |

} | |

// The "counted" version of grisu3 (see above) only generates requested_digits | |

// number of digits. This version does not generate the shortest representation, | |

// and with enough requested digits 0.1 will at some point print as 0.9999999... | |

// Grisu3 is too imprecise for real halfway cases (1.5 will not work) and | |

// therefore the rounding strategy for halfway cases is irrelevant. | |

static bool Grisu3Counted(double v, | |

int requested_digits, | |

Vector<char> buffer, | |

int* length, | |

int* decimal_exponent) { | |

DiyFp w = Double(v).AsNormalizedDiyFp(); | |

DiyFp ten_mk; // Cached power of ten: 10^-k | |

int mk; // -k | |

int ten_mk_minimal_binary_exponent = | |

kMinimalTargetExponent - (w.e() + DiyFp::kSignificandSize); | |

int ten_mk_maximal_binary_exponent = | |

kMaximalTargetExponent - (w.e() + DiyFp::kSignificandSize); | |

PowersOfTenCache::GetCachedPowerForBinaryExponentRange( | |

ten_mk_minimal_binary_exponent, | |

ten_mk_maximal_binary_exponent, | |

&ten_mk, &mk); | |

ASSERT((kMinimalTargetExponent <= w.e() + ten_mk.e() + | |

DiyFp::kSignificandSize) && | |

(kMaximalTargetExponent >= w.e() + ten_mk.e() + | |

DiyFp::kSignificandSize)); | |

// Note that ten_mk is only an approximation of 10^-k. A DiyFp only contains a | |

// 64 bit significand and ten_mk is thus only precise up to 64 bits. | |

// The DiyFp::Times procedure rounds its result, and ten_mk is approximated | |

// too. The variable scaled_w (as well as scaled_boundary_minus/plus) are now | |

// off by a small amount. | |

// In fact: scaled_w - w*10^k < 1ulp (unit in the last place) of scaled_w. | |

// In other words: let f = scaled_w.f() and e = scaled_w.e(), then | |

// (f-1) * 2^e < w*10^k < (f+1) * 2^e | |

DiyFp scaled_w = DiyFp::Times(w, ten_mk); | |

// We now have (double) (scaled_w * 10^-mk). | |

// DigitGen will generate the first requested_digits digits of scaled_w and | |

// return together with a kappa such that scaled_w ~= buffer * 10^kappa. (It | |

// will not always be exactly the same since DigitGenCounted only produces a | |

// limited number of digits.) | |

int kappa; | |

bool result = DigitGenCounted(scaled_w, requested_digits, | |

buffer, length, &kappa); | |

*decimal_exponent = -mk + kappa; | |

return result; | |

} | |

bool FastDtoa(double v, | |

FastDtoaMode mode, | |

int requested_digits, | |

Vector<char> buffer, | |

int* length, | |

int* decimal_point) { | |

ASSERT(v > 0); | |

ASSERT(!Double(v).IsSpecial()); | |

bool result = false; | |

int decimal_exponent = 0; | |

switch (mode) { | |

case FAST_DTOA_SHORTEST: | |

case FAST_DTOA_SHORTEST_SINGLE: | |

result = Grisu3(v, mode, buffer, length, &decimal_exponent); | |

break; | |

case FAST_DTOA_PRECISION: | |

result = Grisu3Counted(v, requested_digits, | |

buffer, length, &decimal_exponent); | |

break; | |

default: | |

UNREACHABLE(); | |

} | |

if (result) { | |

*decimal_point = *length + decimal_exponent; | |

buffer[*length] = '\0'; | |

} | |

return result; | |

} | |

} // namespace double_conversion |