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

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Timothy Flynn 2022-11-04 11:16:58 -04:00 • committed by Linus Groh
parent d56205f991
commit 5898db8c0f
2 changed files with 4 additions and 5 deletions
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6
Userland/Libraries/LibJS/Runtime/NumberPrototype.cpp
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@ -386,17 +386,13 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_precision)
} }
// 10. Else, // 10. Else,
else { else {
// FIXME: The computations below fall apart for large values of 'p'. A double typically has 52 mantissa bits, which gives us
// up to 2^52 before loss of precision. However, the largest value of 'p' may be 100, resulting in numbers on the order
// of 10^100, thus we lose precision in these computations.
// a. Let e and n be integers such that 10^(p-1) ≤ n < 10^p and for which n × 10^(e-p+1) - x is as close to zero as possible. // a. Let e and n be integers such that 10^(p-1) ≤ n < 10^p and for which n × 10^(e-p+1) - 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-p+1) is larger. // If there are two such sets of e and n, pick the e and n for which n × 10^(e-p+1) is larger.
exponent = static_cast<int>(floor(log10(number))); exponent = static_cast<int>(floor(log10(number)));
number = round(number / pow(10, exponent - precision + 1)); number = round(number / pow(10, exponent - precision + 1));
// b. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes). // b. 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);
// c. If e < -6 or e ≥ p, then // c. If e < -6 or e ≥ p, then
if ((exponent < -6) || (exponent >= precision)) { if ((exponent < -6) || (exponent >= precision)) {

3
Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toPrecision.js
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@ -87,6 +87,9 @@ describe("correct behavior", () => {
[1, 4, "1.000"], [1, 4, "1.000"],
[123, 4, "123.0"], [123, 4, "123.0"],
[123.45, 4, "123.5"], [123.45, 4, "123.5"],
// Disabled for now due to: https://github.com/SerenityOS/serenity/issues/15924
// [3, 100, "3." + "0".repeat(99)],
].forEach(test => { ].forEach(test => {
expect(test[0].toPrecision(test[1])).toBe(test[2]); expect(test[0].toPrecision(test[1])).toBe(test[2]);
}); });