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LibJS: Use more accurate number-to-string method in Number toExponential

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Timothy Flynn 2022-11-04 11:18:09 -04:00 • committed by Linus Groh
parent 5898db8c0f
commit 8eec25b7ae
2 changed files with 16 additions and 54 deletions
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65
Userland/Libraries/LibJS/Runtime/NumberPrototype.cpp
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@ -8,6 +8,7 @@
#include <AK/Array.h>
#include <AK/Function.h>
#include <AK/StringFloatingPointConversions.h>
#include <AK/TypeCasts.h>
#include <LibJS/Runtime/AbstractOperations.h>
#include <LibJS/Runtime/Completion.h>
@ -36,53 +37,6 @@ static constexpr AK::Array<char, 36> digits = {
'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
};
static String decimal_digits_to_string(double number)
{
StringBuilder builder;
double integral_part = 0;
(void)modf(number, &integral_part);
while (integral_part > 0) {
auto index = static_cast<size_t>(fmod(integral_part, 10));
builder.append(digits[index]);
integral_part = floor(integral_part / 10.0);
}
return builder.build().reverse();
}
static size_t compute_fraction_digits(double number, int exponent)
{
double integral_part = 0;
double fraction_part = modf(number, &integral_part);
auto fraction = String::number(fraction_part);
size_t fraction_digits = 0;
if (integral_part != 0)
fraction_digits = exponent;
if (auto decimal_index = fraction.find('.'); decimal_index.has_value()) {
fraction_digits += fraction.length() - *decimal_index - 1;
if (integral_part == 0) {
--fraction_digits;
for (size_t i = *decimal_index + 1; (i < fraction.length()) && (fraction[i] == '0'); ++i)
--fraction_digits;
}
} else if (integral_part != 0) {
auto integral = decimal_digits_to_string(integral_part);
for (size_t i = integral.length(); (i > 0) && (integral[i - 1] == '0'); --i)
--fraction_digits;
}
return fraction_digits;
}
NumberPrototype::NumberPrototype(Realm& realm)
: NumberObject(0, *realm.intrinsics().object_prototype())
{
@ -170,10 +124,6 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_exponential)
}
// 10. Else,
else {
// FIXME: The computations below fall apart for large values of 'f'. A double typically has 52 mantissa bits, which gives us
// up to 2^52 before loss of precision. However, the largest value of 'f' may be 100, resulting in numbers on the order
// of 10^100, thus we lose precision in these computations.
// a. If fractionDigits is not undefined, then
// i. Let e and n be integers such that 10^f ≤ n < 10^(f+1) and for which n × 10^(e-f) - x is as close to zero as possible.
// If there are two such sets of e and n, pick the e and n for which n × 10^(e-f) is larger.
@ -182,13 +132,20 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_exponential)
// Note that the decimal representation of n has f + 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
exponent = static_cast<int>(floor(log10(number)));
if (fraction_digits_value.is_undefined())
fraction_digits = compute_fraction_digits(number, exponent);
if (fraction_digits_value.is_undefined()) {
auto mantissa = convert_floating_point_to_decimal_exponential_form(number).fraction;
auto mantissa_length = 0;
for (; mantissa; mantissa /= 10)
++mantissa_length;
fraction_digits = mantissa_length - 1;
}
number = round(number / pow(10, exponent - fraction_digits));
// c. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
number_string = decimal_digits_to_string(number);
number_string = number_to_string(number, NumberToStringMode::WithoutExponent);
}
// 11. If f ≠ 0, then