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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/rounding.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/rounding.rs')
-rw-r--r-- | third_party/rust/serde_json/src/lexical/rounding.rs | 231 |
1 files changed, 231 insertions, 0 deletions
diff --git a/third_party/rust/serde_json/src/lexical/rounding.rs b/third_party/rust/serde_json/src/lexical/rounding.rs new file mode 100644 index 0000000000..6ec1292aa5 --- /dev/null +++ b/third_party/rust/serde_json/src/lexical/rounding.rs @@ -0,0 +1,231 @@ +// Adapted from https://github.com/Alexhuszagh/rust-lexical. + +//! Defines rounding schemes for floating-point numbers. + +use super::float::ExtendedFloat; +use super::num::*; +use super::shift::*; +use core::mem; + +// MASKS + +/// Calculate a scalar factor of 2 above the halfway point. +#[inline] +pub(crate) fn nth_bit(n: u64) -> u64 { + let bits: u64 = mem::size_of::<u64>() as u64 * 8; + debug_assert!(n < bits, "nth_bit() overflow in shl."); + + 1 << n +} + +/// Generate a bitwise mask for the lower `n` bits. +#[inline] +pub(crate) fn lower_n_mask(n: u64) -> u64 { + let bits: u64 = mem::size_of::<u64>() as u64 * 8; + debug_assert!(n <= bits, "lower_n_mask() overflow in shl."); + + if n == bits { + u64::max_value() + } else { + (1 << n) - 1 + } +} + +/// Calculate the halfway point for the lower `n` bits. +#[inline] +pub(crate) fn lower_n_halfway(n: u64) -> u64 { + let bits: u64 = mem::size_of::<u64>() as u64 * 8; + debug_assert!(n <= bits, "lower_n_halfway() overflow in shl."); + + if n == 0 { + 0 + } else { + nth_bit(n - 1) + } +} + +/// Calculate a bitwise mask with `n` 1 bits starting at the `bit` position. +#[inline] +pub(crate) fn internal_n_mask(bit: u64, n: u64) -> u64 { + let bits: u64 = mem::size_of::<u64>() as u64 * 8; + debug_assert!(bit <= bits, "internal_n_halfway() overflow in shl."); + debug_assert!(n <= bits, "internal_n_halfway() overflow in shl."); + debug_assert!(bit >= n, "internal_n_halfway() overflow in sub."); + + lower_n_mask(bit) ^ lower_n_mask(bit - n) +} + +// NEAREST ROUNDING + +// Shift right N-bytes and round to the nearest. +// +// Return if we are above halfway and if we are halfway. +#[inline] +pub(crate) fn round_nearest(fp: &mut ExtendedFloat, shift: i32) -> (bool, bool) { + // Extract the truncated bits using mask. + // Calculate if the value of the truncated bits are either above + // the mid-way point, or equal to it. + // + // For example, for 4 truncated bytes, the mask would be b1111 + // and the midway point would be b1000. + let mask: u64 = lower_n_mask(shift as u64); + let halfway: u64 = lower_n_halfway(shift as u64); + + let truncated_bits = fp.mant & mask; + let is_above = truncated_bits > halfway; + let is_halfway = truncated_bits == halfway; + + // Bit shift so the leading bit is in the hidden bit. + overflowing_shr(fp, shift); + + (is_above, is_halfway) +} + +// Tie rounded floating point to event. +#[inline] +pub(crate) fn tie_even(fp: &mut ExtendedFloat, is_above: bool, is_halfway: bool) { + // Extract the last bit after shifting (and determine if it is odd). + let is_odd = fp.mant & 1 == 1; + + // Calculate if we need to roundup. + // We need to roundup if we are above halfway, or if we are odd + // and at half-way (need to tie-to-even). + if is_above || (is_odd && is_halfway) { + fp.mant += 1; + } +} + +// Shift right N-bytes and round nearest, tie-to-even. +// +// Floating-point arithmetic uses round to nearest, ties to even, +// which rounds to the nearest value, if the value is halfway in between, +// round to an even value. +#[inline] +pub(crate) fn round_nearest_tie_even(fp: &mut ExtendedFloat, shift: i32) { + let (is_above, is_halfway) = round_nearest(fp, shift); + tie_even(fp, is_above, is_halfway); +} + +// DIRECTED ROUNDING + +// Shift right N-bytes and round towards a direction. +// +// Return if we have any truncated bytes. +#[inline] +fn round_toward(fp: &mut ExtendedFloat, shift: i32) -> bool { + let mask: u64 = lower_n_mask(shift as u64); + let truncated_bits = fp.mant & mask; + + // Bit shift so the leading bit is in the hidden bit. + overflowing_shr(fp, shift); + + truncated_bits != 0 +} + +// Round down. +#[inline] +fn downard(_: &mut ExtendedFloat, _: bool) {} + +// Shift right N-bytes and round toward zero. +// +// Floating-point arithmetic defines round toward zero, which rounds +// towards positive zero. +#[inline] +pub(crate) fn round_downward(fp: &mut ExtendedFloat, shift: i32) { + // Bit shift so the leading bit is in the hidden bit. + // No rounding schemes, so we just ignore everything else. + let is_truncated = round_toward(fp, shift); + downard(fp, is_truncated); +} + +// ROUND TO FLOAT + +// Shift the ExtendedFloat fraction to the fraction bits in a native float. +// +// Floating-point arithmetic uses round to nearest, ties to even, +// which rounds to the nearest value, if the value is halfway in between, +// round to an even value. +#[inline] +pub(crate) fn round_to_float<F, Algorithm>(fp: &mut ExtendedFloat, algorithm: Algorithm) +where + F: Float, + Algorithm: FnOnce(&mut ExtendedFloat, i32), +{ + // Calculate the difference to allow a single calculation + // rather than a loop, to minimize the number of ops required. + // This does underflow detection. + let final_exp = fp.exp + F::DEFAULT_SHIFT; + if final_exp < F::DENORMAL_EXPONENT { + // We would end up with a denormal exponent, try to round to more + // digits. Only shift right if we can avoid zeroing out the value, + // which requires the exponent diff to be < M::BITS. The value + // is already normalized, so we shouldn't have any issue zeroing + // out the value. + let diff = F::DENORMAL_EXPONENT - fp.exp; + if diff <= u64::FULL { + // We can avoid underflow, can get a valid representation. + algorithm(fp, diff); + } else { + // Certain underflow, assign literal 0s. + fp.mant = 0; + fp.exp = 0; + } + } else { + algorithm(fp, F::DEFAULT_SHIFT); + } + + if fp.mant & F::CARRY_MASK == F::CARRY_MASK { + // Roundup carried over to 1 past the hidden bit. + shr(fp, 1); + } +} + +// AVOID OVERFLOW/UNDERFLOW + +// Avoid overflow for large values, shift left as needed. +// +// Shift until a 1-bit is in the hidden bit, if the mantissa is not 0. +#[inline] +pub(crate) fn avoid_overflow<F>(fp: &mut ExtendedFloat) +where + F: Float, +{ + // Calculate the difference to allow a single calculation + // rather than a loop, minimizing the number of ops required. + if fp.exp >= F::MAX_EXPONENT { + let diff = fp.exp - F::MAX_EXPONENT; + if diff <= F::MANTISSA_SIZE { + // Our overflow mask needs to start at the hidden bit, or at + // `F::MANTISSA_SIZE+1`, and needs to have `diff+1` bits set, + // to see if our value overflows. + let bit = (F::MANTISSA_SIZE + 1) as u64; + let n = (diff + 1) as u64; + let mask = internal_n_mask(bit, n); + if (fp.mant & mask) == 0 { + // If we have no 1-bit in the hidden-bit position, + // which is index 0, we need to shift 1. + let shift = diff + 1; + shl(fp, shift); + } + } + } +} + +// ROUND TO NATIVE + +// Round an extended-precision float to a native float representation. +#[inline] +pub(crate) fn round_to_native<F, Algorithm>(fp: &mut ExtendedFloat, algorithm: Algorithm) +where + F: Float, + Algorithm: FnOnce(&mut ExtendedFloat, i32), +{ + // Shift all the way left, to ensure a consistent representation. + // The following right-shifts do not work for a non-normalized number. + fp.normalize(); + + // Round so the fraction is in a native mantissa representation, + // and avoid overflow/underflow. + round_to_float::<F, _>(fp, algorithm); + avoid_overflow::<F>(fp); +} |