Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 225 insertions, 0 deletions

diff --git a/third_party/rust/minimal-lexical/src/lemire.rs b/third_party/rust/minimal-lexical/src/lemire.rs
new file mode 100644
index 0000000000..99b1ae7059
--- /dev/null
+++ b/third_party/rust/minimal-lexical/src/lemire.rs
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+//! Implementation of the Eisel-Lemire algorithm.
+//!
+//! This is adapted from [fast-float-rust](https://github.com/aldanor/fast-float-rust),
+//! a port of [fast_float](https://github.com/fastfloat/fast_float) to Rust.
+
+#![cfg(not(feature = "compact"))]
+#![doc(hidden)]
+
+use crate::extended_float::ExtendedFloat;
+use crate::num::Float;
+use crate::number::Number;
+use crate::table::{LARGEST_POWER_OF_FIVE, POWER_OF_FIVE_128, SMALLEST_POWER_OF_FIVE};
+
+/// Ensure truncation of digits doesn't affect our computation, by doing 2 passes.
+#[inline]
+pub fn lemire<F: Float>(num: &Number) -> ExtendedFloat {
+    // If significant digits were truncated, then we can have rounding error
+    // only if `mantissa + 1` produces a different result. We also avoid
+    // redundantly using the Eisel-Lemire algorithm if it was unable to
+    // correctly round on the first pass.
+    let mut fp = compute_float::<F>(num.exponent, num.mantissa);
+    if num.many_digits && fp.exp >= 0 && fp != compute_float::<F>(num.exponent, num.mantissa + 1) {
+        // Need to re-calculate, since the previous values are rounded
+        // when the slow path algorithm expects a normalized extended float.
+        fp = compute_error::<F>(num.exponent, num.mantissa);
+    }
+    fp
+}
+
+/// Compute a float using an extended-precision representation.
+///
+/// Fast conversion of a the significant digits and decimal exponent
+/// a float to a extended representation with a binary float. This
+/// algorithm will accurately parse the vast majority of cases,
+/// and uses a 128-bit representation (with a fallback 192-bit
+/// representation).
+///
+/// This algorithm scales the exponent by the decimal exponent
+/// using pre-computed powers-of-5, and calculates if the
+/// representation can be unambiguously rounded to the nearest
+/// machine float. Near-halfway cases are not handled here,
+/// and are represented by a negative, biased binary exponent.
+///
+/// The algorithm is described in detail in "Daniel Lemire, Number Parsing
+/// at a Gigabyte per Second" in section 5, "Fast Algorithm", and
+/// section 6, "Exact Numbers And Ties", available online:
+/// <https://arxiv.org/abs/2101.11408.pdf>.
+pub fn compute_float<F: Float>(q: i32, mut w: u64) -> ExtendedFloat {
+    let fp_zero = ExtendedFloat {
+        mant: 0,
+        exp: 0,
+    };
+    let fp_inf = ExtendedFloat {
+        mant: 0,
+        exp: F::INFINITE_POWER,
+    };
+
+    // Short-circuit if the value can only be a literal 0 or infinity.
+    if w == 0 || q < F::SMALLEST_POWER_OF_TEN {
+        return fp_zero;
+    } else if q > F::LARGEST_POWER_OF_TEN {
+        return fp_inf;
+    }
+    // Normalize our significant digits, so the most-significant bit is set.
+    let lz = w.leading_zeros() as i32;
+    w <<= lz;
+    let (lo, hi) = compute_product_approx(q, w, F::MANTISSA_SIZE as usize + 3);
+    if lo == 0xFFFF_FFFF_FFFF_FFFF {
+        // If we have failed to approximate w x 5^-q with our 128-bit value.
+        // Since the addition of 1 could lead to an overflow which could then
+        // round up over the half-way point, this can lead to improper rounding
+        // of a float.
+        //
+        // However, this can only occur if q ∈ [-27, 55]. The upper bound of q
+        // is 55 because 5^55 < 2^128, however, this can only happen if 5^q > 2^64,
+        // since otherwise the product can be represented in 64-bits, producing
+        // an exact result. For negative exponents, rounding-to-even can
+        // only occur if 5^-q < 2^64.
+        //
+        // For detailed explanations of rounding for negative exponents, see
+        // <https://arxiv.org/pdf/2101.11408.pdf#section.9.1>. For detailed
+        // explanations of rounding for positive exponents, see
+        // <https://arxiv.org/pdf/2101.11408.pdf#section.8>.
+        let inside_safe_exponent = (q >= -27) && (q <= 55);
+        if !inside_safe_exponent {
+            return compute_error_scaled::<F>(q, hi, lz);
+        }
+    }
+    let upperbit = (hi >> 63) as i32;
+    let mut mantissa = hi >> (upperbit + 64 - F::MANTISSA_SIZE - 3);
+    let mut power2 = power(q) + upperbit - lz - F::MINIMUM_EXPONENT;
+    if power2 <= 0 {
+        if -power2 + 1 >= 64 {
+            // Have more than 64 bits below the minimum exponent, must be 0.
+            return fp_zero;
+        }
+        // Have a subnormal value.
+        mantissa >>= -power2 + 1;
+        mantissa += mantissa & 1;
+        mantissa >>= 1;
+        power2 = (mantissa >= (1_u64 << F::MANTISSA_SIZE)) as i32;
+        return ExtendedFloat {
+            mant: mantissa,
+            exp: power2,
+        };
+    }
+    // Need to handle rounding ties. Normally, we need to round up,
+    // but if we fall right in between and and we have an even basis, we
+    // need to round down.
+    //
+    // This will only occur if:
+    //  1. The lower 64 bits of the 128-bit representation is 0.
+    //      IE, 5^q fits in single 64-bit word.
+    //  2. The least-significant bit prior to truncated mantissa is odd.
+    //  3. All the bits truncated when shifting to mantissa bits + 1 are 0.
+    //
+    // Or, we may fall between two floats: we are exactly halfway.
+    if lo <= 1
+        && q >= F::MIN_EXPONENT_ROUND_TO_EVEN
+        && q <= F::MAX_EXPONENT_ROUND_TO_EVEN
+        && mantissa & 3 == 1
+        && (mantissa << (upperbit + 64 - F::MANTISSA_SIZE - 3)) == hi
+    {
+        // Zero the lowest bit, so we don't round up.
+        mantissa &= !1_u64;
+    }
+    // Round-to-even, then shift the significant digits into place.
+    mantissa += mantissa & 1;
+    mantissa >>= 1;
+    if mantissa >= (2_u64 << F::MANTISSA_SIZE) {
+        // Rounding up overflowed, so the carry bit is set. Set the
+        // mantissa to 1 (only the implicit, hidden bit is set) and
+        // increase the exponent.
+        mantissa = 1_u64 << F::MANTISSA_SIZE;
+        power2 += 1;
+    }
+    // Zero out the hidden bit.
+    mantissa &= !(1_u64 << F::MANTISSA_SIZE);
+    if power2 >= F::INFINITE_POWER {
+        // Exponent is above largest normal value, must be infinite.
+        return fp_inf;
+    }
+    ExtendedFloat {
+        mant: mantissa,
+        exp: power2,
+    }
+}
+
+/// Fallback algorithm to calculate the non-rounded representation.
+/// This calculates the extended representation, and then normalizes
+/// the resulting representation, so the high bit is set.
+#[inline]
+pub fn compute_error<F: Float>(q: i32, mut w: u64) -> ExtendedFloat {
+    let lz = w.leading_zeros() as i32;
+    w <<= lz;
+    let hi = compute_product_approx(q, w, F::MANTISSA_SIZE as usize + 3).1;
+    compute_error_scaled::<F>(q, hi, lz)
+}
+
+/// Compute the error from a mantissa scaled to the exponent.
+#[inline]
+pub fn compute_error_scaled<F: Float>(q: i32, mut w: u64, lz: i32) -> ExtendedFloat {
+    // Want to normalize the float, but this is faster than ctlz on most architectures.
+    let hilz = (w >> 63) as i32 ^ 1;
+    w <<= hilz;
+    let power2 = power(q as i32) + F::EXPONENT_BIAS - hilz - lz - 62;
+
+    ExtendedFloat {
+        mant: w,
+        exp: power2 + F::INVALID_FP,
+    }
+}
+
+/// Calculate a base 2 exponent from a decimal exponent.
+/// This uses a pre-computed integer approximation for
+/// log2(10), where 217706 / 2^16 is accurate for the
+/// entire range of non-finite decimal exponents.
+#[inline]
+fn power(q: i32) -> i32 {
+    (q.wrapping_mul(152_170 + 65536) >> 16) + 63
+}
+
+#[inline]
+fn full_multiplication(a: u64, b: u64) -> (u64, u64) {
+    let r = (a as u128) * (b as u128);
+    (r as u64, (r >> 64) as u64)
+}
+
+// This will compute or rather approximate w * 5**q and return a pair of 64-bit words
+// approximating the result, with the "high" part corresponding to the most significant
+// bits and the low part corresponding to the least significant bits.
+fn compute_product_approx(q: i32, w: u64, precision: usize) -> (u64, u64) {
+    debug_assert!(q >= SMALLEST_POWER_OF_FIVE);
+    debug_assert!(q <= LARGEST_POWER_OF_FIVE);
+    debug_assert!(precision <= 64);
+
+    let mask = if precision < 64 {
+        0xFFFF_FFFF_FFFF_FFFF_u64 >> precision
+    } else {
+        0xFFFF_FFFF_FFFF_FFFF_u64
+    };
+
+    // 5^q < 2^64, then the multiplication always provides an exact value.
+    // That means whenever we need to round ties to even, we always have
+    // an exact value.
+    let index = (q - SMALLEST_POWER_OF_FIVE) as usize;
+    let (lo5, hi5) = POWER_OF_FIVE_128[index];
+    // Only need one multiplication as long as there is 1 zero but
+    // in the explicit mantissa bits, +1 for the hidden bit, +1 to
+    // determine the rounding direction, +1 for if the computed
+    // product has a leading zero.
+    let (mut first_lo, mut first_hi) = full_multiplication(w, lo5);
+    if first_hi & mask == mask {
+        // Need to do a second multiplication to get better precision
+        // for the lower product. This will always be exact
+        // where q is < 55, since 5^55 < 2^128. If this wraps,
+        // then we need to need to round up the hi product.
+        let (_, second_hi) = full_multiplication(w, hi5);
+        first_lo = first_lo.wrapping_add(second_hi);
+        if second_hi > first_lo {
+            first_hi += 1;
+        }
+    }
+    (first_lo, first_hi)
+}