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Diffstat (limited to 'vendor/serde_json/src/lexical/algorithm.rs')
| -rw-r--r-- | vendor/serde_json/src/lexical/algorithm.rs | 193 |
1 files changed, 193 insertions, 0 deletions
diff --git a/vendor/serde_json/src/lexical/algorithm.rs b/vendor/serde_json/src/lexical/algorithm.rs new file mode 100644 index 000000000..a2cbf18af --- /dev/null +++ b/vendor/serde_json/src/lexical/algorithm.rs @@ -0,0 +1,193 @@ +// Adapted from https://github.com/Alexhuszagh/rust-lexical. + +//! Algorithms to efficiently convert strings to floats. + +use super::bhcomp::*; +use super::cached::*; +use super::errors::*; +use super::float::ExtendedFloat; +use super::num::*; +use super::small_powers::*; + +// FAST +// ---- + +/// Convert mantissa to exact value for a non-base2 power. +/// +/// Returns the resulting float and if the value can be represented exactly. +pub(crate) fn fast_path<F>(mantissa: u64, exponent: i32) -> Option<F> +where + F: Float, +{ + // `mantissa >> (F::MANTISSA_SIZE+1) != 0` effectively checks if the + // value has a no bits above the hidden bit, which is what we want. + let (min_exp, max_exp) = F::exponent_limit(); + let shift_exp = F::mantissa_limit(); + let mantissa_size = F::MANTISSA_SIZE + 1; + if mantissa == 0 { + Some(F::ZERO) + } else if mantissa >> mantissa_size != 0 { + // Would require truncation of the mantissa. + None + } else if exponent == 0 { + // 0 exponent, same as value, exact representation. + let float = F::as_cast(mantissa); + Some(float) + } else if exponent >= min_exp && exponent <= max_exp { + // Value can be exactly represented, return the value. + // Do not use powi, since powi can incrementally introduce + // error. + let float = F::as_cast(mantissa); + Some(float.pow10(exponent)) + } else if exponent >= 0 && exponent <= max_exp + shift_exp { + // Check to see if we have a disguised fast-path, where the + // number of digits in the mantissa is very small, but and + // so digits can be shifted from the exponent to the mantissa. + // https://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/ + let small_powers = POW10_64; + let shift = exponent - max_exp; + let power = small_powers[shift as usize]; + + // Compute the product of the power, if it overflows, + // prematurely return early, otherwise, if we didn't overshoot, + // we can get an exact value. + let value = mantissa.checked_mul(power)?; + if value >> mantissa_size != 0 { + None + } else { + // Use powi, since it's correct, and faster on + // the fast-path. + let float = F::as_cast(value); + Some(float.pow10(max_exp)) + } + } else { + // Cannot be exactly represented, exponent too small or too big, + // would require truncation. + None + } +} + +// MODERATE +// -------- + +/// Multiply the floating-point by the exponent. +/// +/// Multiply by pre-calculated powers of the base, modify the extended- +/// float, and return if new value and if the value can be represented +/// accurately. +fn multiply_exponent_extended<F>(fp: &mut ExtendedFloat, exponent: i32, truncated: bool) -> bool +where + F: Float, +{ + let powers = ExtendedFloat::get_powers(); + let exponent = exponent.saturating_add(powers.bias); + let small_index = exponent % powers.step; + let large_index = exponent / powers.step; + if exponent < 0 { + // Guaranteed underflow (assign 0). + fp.mant = 0; + true + } else if large_index as usize >= powers.large.len() { + // Overflow (assign infinity) + fp.mant = 1 << 63; + fp.exp = 0x7FF; + true + } else { + // Within the valid exponent range, multiply by the large and small + // exponents and return the resulting value. + + // Track errors to as a factor of unit in last-precision. + let mut errors: u32 = 0; + if truncated { + errors += u64::error_halfscale(); + } + + // Multiply by the small power. + // Check if we can directly multiply by an integer, if not, + // use extended-precision multiplication. + match fp + .mant + .overflowing_mul(powers.get_small_int(small_index as usize)) + { + // Overflow, multiplication unsuccessful, go slow path. + (_, true) => { + fp.normalize(); + fp.imul(&powers.get_small(small_index as usize)); + errors += u64::error_halfscale(); + } + // No overflow, multiplication successful. + (mant, false) => { + fp.mant = mant; + fp.normalize(); + } + } + + // Multiply by the large power + fp.imul(&powers.get_large(large_index as usize)); + if errors > 0 { + errors += 1; + } + errors += u64::error_halfscale(); + + // Normalize the floating point (and the errors). + let shift = fp.normalize(); + errors <<= shift; + + u64::error_is_accurate::<F>(errors, fp) + } +} + +/// Create a precise native float using an intermediate extended-precision float. +/// +/// Return the float approximation and if the value can be accurately +/// represented with mantissa bits of precision. +#[inline] +pub(crate) fn moderate_path<F>( + mantissa: u64, + exponent: i32, + truncated: bool, +) -> (ExtendedFloat, bool) +where + F: Float, +{ + let mut fp = ExtendedFloat { + mant: mantissa, + exp: 0, + }; + let valid = multiply_exponent_extended::<F>(&mut fp, exponent, truncated); + (fp, valid) +} + +// FALLBACK +// -------- + +/// Fallback path when the fast path does not work. +/// +/// Uses the moderate path, if applicable, otherwise, uses the slow path +/// as required. +pub(crate) fn fallback_path<F>( + integer: &[u8], + fraction: &[u8], + mantissa: u64, + exponent: i32, + mantissa_exponent: i32, + truncated: bool, +) -> F +where + F: Float, +{ + // Moderate path (use an extended 80-bit representation). + let (fp, valid) = moderate_path::<F>(mantissa, mantissa_exponent, truncated); + if valid { + return fp.into_float::<F>(); + } + + // Slow path, fast path didn't work. + let b = fp.into_downward_float::<F>(); + if b.is_special() { + // We have a non-finite number, we get to leave early. + b + } else { + bhcomp(b, integer, fraction, exponent) + } +} |