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LibJS: Implement precise double_to_string
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Dan Klishch
Andrew Kaster
parent
fdc53a5995
commit
56aa21a7dc
1 changed files with 110 additions and 110 deletions
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@ -12,6 +12,7 @@
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#include <AK/FloatingPointStringConversions.h>
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#include <AK/String.h>
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#include <AK/StringBuilder.h>
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#include <AK/StringFloatingPointConversions.h>
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#include <AK/Utf8View.h>
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#include <LibCrypto/BigInt/SignedBigInteger.h>
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#include <LibCrypto/NumberTheory/ModularFunctions.h>
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@ -69,130 +70,129 @@ ALWAYS_INLINE bool both_bigint(Value const& lhs, Value const& rhs)
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}
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// 6.1.6.1.20 Number::toString ( x ), https://tc39.es/ecma262/#sec-numeric-types-number-tostring
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// Implementation for radix = 10
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static String double_to_string(double d)
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{
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auto convert_to_decimal_digits_array = [](auto x, auto& digits, auto& length) {
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for (; x; x /= 10)
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digits[length++] = x % 10 | '0';
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for (i32 i = 0; 2 * i + 1 < length; ++i)
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swap(digits[i], digits[length - i - 1]);
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};
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// 1. If x is NaN, return "NaN".
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if (isnan(d))
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return "NaN";
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// 2. If x is +0𝔽 or -0𝔽, return "0".
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if (d == +0.0 || d == -0.0)
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return "0";
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if (d < +0.0) {
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StringBuilder builder;
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builder.append('-');
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builder.append(double_to_string(-d));
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return builder.to_string();
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// 4. If x is +∞𝔽, return "Infinity".
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if (isinf(d)) {
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if (d > 0)
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return "Infinity";
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else
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return "-Infinity";
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}
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if (d == static_cast<double>(INFINITY))
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return "Infinity";
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StringBuilder number_string_builder;
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size_t start_index = 0;
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size_t end_index = 0;
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size_t int_part_end = 0;
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// generate integer part (reversed)
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double int_part;
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double frac_part;
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frac_part = modf(d, &int_part);
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while (int_part > 0) {
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number_string_builder.append('0' + (int)fmod(int_part, 10));
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end_index++;
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int_part = floor(int_part / 10);
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}
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auto reversed_integer_part = number_string_builder.to_string().reverse();
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number_string_builder.clear();
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number_string_builder.append(reversed_integer_part);
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int_part_end = end_index;
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int exponent = 0;
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// generate fractional part
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while (frac_part > 0) {
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double old_frac_part = frac_part;
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frac_part *= 10;
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frac_part = modf(frac_part, &int_part);
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if (old_frac_part == frac_part)
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break;
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number_string_builder.append('0' + (int)int_part);
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end_index++;
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exponent--;
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}
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auto number_string = number_string_builder.to_string();
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// FIXME: Remove this hack.
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// FIXME: Instead find the shortest round-trippable representation.
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// Remove decimals after the 15th position
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if (end_index > int_part_end + 15) {
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exponent += end_index - int_part_end - 15;
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end_index = int_part_end + 15;
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}
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// remove leading zeroes
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while (start_index < end_index && number_string[start_index] == '0') {
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start_index++;
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}
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// remove trailing zeroes
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while (end_index > 0 && number_string[end_index - 1] == '0') {
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end_index--;
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exponent++;
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}
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if (end_index <= start_index)
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return "0";
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auto digits = number_string.substring_view(start_index, end_index - start_index);
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int number_of_digits = end_index - start_index;
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exponent += number_of_digits;
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StringBuilder builder;
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if (number_of_digits <= exponent && exponent <= 21) {
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builder.append(digits);
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builder.append(String::repeated('0', exponent - number_of_digits));
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return builder.to_string();
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}
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if (0 < exponent && exponent <= 21) {
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builder.append(digits.substring_view(0, exponent));
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builder.append('.');
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builder.append(digits.substring_view(exponent));
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return builder.to_string();
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}
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if (-6 < exponent && exponent <= 0) {
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builder.append("0."sv);
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builder.append(String::repeated('0', -exponent));
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builder.append(digits);
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return builder.to_string();
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}
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if (number_of_digits == 1) {
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builder.append(digits);
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builder.append('e');
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// 5. Let n, k, and s be integers such that k ≥ 1, radix ^ (k - 1) ≤ s < radix ^ k,
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// 𝔽(s × radix ^ (n - k)) is x, and k is as small as possible. Note that k is the number of
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// digits in the representation of s using radix radix, that s is not divisible by radix, and
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// that the least significant digit of s is not necessarily uniquely determined by these criteria.
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//
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// Note: guarantees provided by convert_floating_point_to_decimal_exponential_form satisfy
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// requirements of NOTE 2.
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auto [sign, mantissa, exponent] = convert_floating_point_to_decimal_exponential_form(d);
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i32 k = 0;
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AK::Array<char, 20> mantissa_digits;
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convert_to_decimal_digits_array(mantissa, mantissa_digits, k);
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if (exponent - 1 > 0)
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builder.append('+');
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else
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builder.append('-');
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i32 n = exponent + k; // s = mantissa
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builder.append(String::number(AK::abs(exponent - 1)));
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return builder.to_string();
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}
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builder.append(digits[0]);
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builder.append('.');
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builder.append(digits.substring_view(1));
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builder.append('e');
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if (exponent - 1 > 0)
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builder.append('+');
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else
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// 3. If x < -0𝔽, return the string-concatenation of "-" and Number::toString(-x, radix).
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if (sign)
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builder.append('-');
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builder.append(String::number(AK::abs(exponent - 1)));
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// 6. If radix ≠ 10 or n is in the inclusive interval from -5 to 21, then
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if (n >= -5 && n <= 21) {
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// a. If n ≥ k, then
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if (n >= k) {
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// i. Return the string-concatenation of:
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// the code units of the k digits of the representation of s using radix radix
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builder.append(mantissa_digits.data(), k);
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// n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
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builder.append_repeated('0', n - k);
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// b. Else if n > 0, then
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} else if (n > 0) {
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// i. Return the string-concatenation of:
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// the code units of the most significant n digits of the representation of s using radix radix
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builder.append(mantissa_digits.data(), n);
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// the code unit 0x002E (FULL STOP)
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builder.append('.');
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// the code units of the remaining k - n digits of the representation of s using radix radix
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builder.append(mantissa_digits.data() + n, k - n);
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// c. Else,
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} else {
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// i. Assert: n ≤ 0.
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VERIFY(n <= 0);
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// ii. Return the string-concatenation of:
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// the code unit 0x0030 (DIGIT ZERO)
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builder.append('0');
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// the code unit 0x002E (FULL STOP)
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builder.append('.');
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// -n occurrences of the code unit 0x0030 (DIGIT ZERO)
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builder.append_repeated('0', -n);
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// the code units of the k digits of the representation of s using radix radix
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builder.append(mantissa_digits.data(), k);
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}
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return builder.to_string();
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}
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// 7. NOTE: In this case, the input will be represented using scientific E notation, such as 1.2e+3.
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// 9. If n < 0, then
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// a. Let exponentSign be the code unit 0x002D (HYPHEN-MINUS).
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// 10. Else,
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// a. Let exponentSign be the code unit 0x002B (PLUS SIGN).
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char exponent_sign = n < 0 ? '-' : '+';
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AK::Array<char, 5> exponent_digits;
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i32 exponent_length = 0;
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convert_to_decimal_digits_array(abs(n - 1), exponent_digits, exponent_length);
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// 11. If k is 1, then
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if (k == 1) {
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// a. Return the string-concatenation of:
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// the code unit of the single digit of s
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builder.append(mantissa_digits[0]);
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// the code unit 0x0065 (LATIN SMALL LETTER E)
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builder.append('e');
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// exponentSign
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builder.append(exponent_sign);
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// the code units of the decimal representation of abs(n - 1)
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builder.append(exponent_digits.data(), exponent_length);
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return builder.to_string();
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}
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// 12. Return the string-concatenation of:
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// the code unit of the most significant digit of the decimal representation of s
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builder.append(mantissa_digits[0]);
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// the code unit 0x002E (FULL STOP)
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builder.append('.');
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// the code units of the remaining k - 1 digits of the decimal representation of s
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builder.append(mantissa_digits.data() + 1, k - 1);
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// the code unit 0x0065 (LATIN SMALL LETTER E)
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builder.append('e');
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// exponentSign
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builder.append(exponent_sign);
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// the code units of the decimal representation of abs(n - 1)
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builder.append(exponent_digits.data(), exponent_length);
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return builder.to_string();
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}
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