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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 00:47:55 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 00:47:55 +0000 |
commit | 26a029d407be480d791972afb5975cf62c9360a6 (patch) | |
tree | f435a8308119effd964b339f76abb83a57c29483 /third_party/rust/serde_json/src/lexical/bhcomp.rs | |
parent | Initial commit. (diff) | |
download | firefox-26a029d407be480d791972afb5975cf62c9360a6.tar.xz firefox-26a029d407be480d791972afb5975cf62c9360a6.zip |
Adding upstream version 124.0.1.upstream/124.0.1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'third_party/rust/serde_json/src/lexical/bhcomp.rs')
-rw-r--r-- | third_party/rust/serde_json/src/lexical/bhcomp.rs | 218 |
1 files changed, 218 insertions, 0 deletions
diff --git a/third_party/rust/serde_json/src/lexical/bhcomp.rs b/third_party/rust/serde_json/src/lexical/bhcomp.rs new file mode 100644 index 0000000000..1f2a7bbdee --- /dev/null +++ b/third_party/rust/serde_json/src/lexical/bhcomp.rs @@ -0,0 +1,218 @@ +// Adapted from https://github.com/Alexhuszagh/rust-lexical. + +//! Compare the mantissa to the halfway representation of the float. +//! +//! Compares the actual significant digits of the mantissa to the +//! theoretical digits from `b+h`, scaled into the proper range. + +use super::bignum::*; +use super::digit::*; +use super::exponent::*; +use super::float::*; +use super::math::*; +use super::num::*; +use super::rounding::*; +use core::{cmp, mem}; + +// MANTISSA + +/// Parse the full mantissa into a big integer. +/// +/// Max digits is the maximum number of digits plus one. +fn parse_mantissa<F>(integer: &[u8], fraction: &[u8]) -> Bigint +where + F: Float, +{ + // Main loop + let small_powers = POW10_LIMB; + let step = small_powers.len() - 2; + let max_digits = F::MAX_DIGITS - 1; + let mut counter = 0; + let mut value: Limb = 0; + let mut i: usize = 0; + let mut result = Bigint::default(); + + // Iteratively process all the data in the mantissa. + for &digit in integer.iter().chain(fraction) { + // We've parsed the max digits using small values, add to bignum + if counter == step { + result.imul_small(small_powers[counter]); + result.iadd_small(value); + counter = 0; + value = 0; + } + + value *= 10; + value += as_limb(to_digit(digit).unwrap()); + + i += 1; + counter += 1; + if i == max_digits { + break; + } + } + + // We will always have a remainder, as long as we entered the loop + // once, or counter % step is 0. + if counter != 0 { + result.imul_small(small_powers[counter]); + result.iadd_small(value); + } + + // If we have any remaining digits after the last value, we need + // to add a 1 after the rest of the array, it doesn't matter where, + // just move it up. This is good for the worst-possible float + // representation. We also need to return an index. + // Since we already trimmed trailing zeros, we know there has + // to be a non-zero digit if there are any left. + if i < integer.len() + fraction.len() { + result.imul_small(10); + result.iadd_small(1); + } + + result +} + +// FLOAT OPS + +/// Calculate `b` from a a representation of `b` as a float. +#[inline] +pub(super) fn b_extended<F: Float>(f: F) -> ExtendedFloat { + ExtendedFloat::from_float(f) +} + +/// Calculate `b+h` from a a representation of `b` as a float. +#[inline] +pub(super) fn bh_extended<F: Float>(f: F) -> ExtendedFloat { + // None of these can overflow. + let b = b_extended(f); + ExtendedFloat { + mant: (b.mant << 1) + 1, + exp: b.exp - 1, + } +} + +// ROUNDING + +/// Custom round-nearest, tie-event algorithm for bhcomp. +#[inline] +fn round_nearest_tie_even(fp: &mut ExtendedFloat, shift: i32, is_truncated: bool) { + let (mut is_above, mut is_halfway) = round_nearest(fp, shift); + if is_halfway && is_truncated { + is_above = true; + is_halfway = false; + } + tie_even(fp, is_above, is_halfway); +} + +// BHCOMP + +/// Calculate the mantissa for a big integer with a positive exponent. +fn large_atof<F>(mantissa: Bigint, exponent: i32) -> F +where + F: Float, +{ + let bits = mem::size_of::<u64>() * 8; + + // Simple, we just need to multiply by the power of the radix. + // Now, we can calculate the mantissa and the exponent from this. + // The binary exponent is the binary exponent for the mantissa + // shifted to the hidden bit. + let mut bigmant = mantissa; + bigmant.imul_pow10(exponent as u32); + + // Get the exact representation of the float from the big integer. + let (mant, is_truncated) = bigmant.hi64(); + let exp = bigmant.bit_length() as i32 - bits as i32; + let mut fp = ExtendedFloat { mant, exp }; + fp.round_to_native::<F, _>(|fp, shift| round_nearest_tie_even(fp, shift, is_truncated)); + into_float(fp) +} + +/// Calculate the mantissa for a big integer with a negative exponent. +/// +/// This invokes the comparison with `b+h`. +fn small_atof<F>(mantissa: Bigint, exponent: i32, f: F) -> F +where + F: Float, +{ + // Get the significant digits and radix exponent for the real digits. + let mut real_digits = mantissa; + let real_exp = exponent; + debug_assert!(real_exp < 0); + + // Get the significant digits and the binary exponent for `b+h`. + let theor = bh_extended(f); + let mut theor_digits = Bigint::from_u64(theor.mant); + let theor_exp = theor.exp; + + // We need to scale the real digits and `b+h` digits to be the same + // order. We currently have `real_exp`, in `radix`, that needs to be + // shifted to `theor_digits` (since it is negative), and `theor_exp` + // to either `theor_digits` or `real_digits` as a power of 2 (since it + // may be positive or negative). Try to remove as many powers of 2 + // as possible. All values are relative to `theor_digits`, that is, + // reflect the power you need to multiply `theor_digits` by. + + // Can remove a power-of-two, since the radix is 10. + // Both are on opposite-sides of equation, can factor out a + // power of two. + // + // Example: 10^-10, 2^-10 -> ( 0, 10, 0) + // Example: 10^-10, 2^-15 -> (-5, 10, 0) + // Example: 10^-10, 2^-5 -> ( 5, 10, 0) + // Example: 10^-10, 2^5 -> (15, 10, 0) + let binary_exp = theor_exp - real_exp; + let halfradix_exp = -real_exp; + let radix_exp = 0; + + // Carry out our multiplication. + if halfradix_exp != 0 { + theor_digits.imul_pow5(halfradix_exp as u32); + } + if radix_exp != 0 { + theor_digits.imul_pow10(radix_exp as u32); + } + if binary_exp > 0 { + theor_digits.imul_pow2(binary_exp as u32); + } else if binary_exp < 0 { + real_digits.imul_pow2(-binary_exp as u32); + } + + // Compare real digits to theoretical digits and round the float. + match real_digits.compare(&theor_digits) { + cmp::Ordering::Greater => f.next_positive(), + cmp::Ordering::Less => f, + cmp::Ordering::Equal => f.round_positive_even(), + } +} + +/// Calculate the exact value of the float. +/// +/// Note: fraction must not have trailing zeros. +pub(crate) fn bhcomp<F>(b: F, integer: &[u8], mut fraction: &[u8], exponent: i32) -> F +where + F: Float, +{ + // Calculate the number of integer digits and use that to determine + // where the significant digits start in the fraction. + let integer_digits = integer.len(); + let fraction_digits = fraction.len(); + let digits_start = if integer_digits == 0 { + let start = fraction.iter().take_while(|&x| *x == b'0').count(); + fraction = &fraction[start..]; + start + } else { + 0 + }; + let sci_exp = scientific_exponent(exponent, integer_digits, digits_start); + let count = F::MAX_DIGITS.min(integer_digits + fraction_digits - digits_start); + let scaled_exponent = sci_exp + 1 - count as i32; + + let mantissa = parse_mantissa::<F>(integer, fraction); + if scaled_exponent >= 0 { + large_atof(mantissa, scaled_exponent) + } else { + small_atof(mantissa, scaled_exponent, b) + } +} |