build(ariadne): vendor serde crates

third_party/crates/serde_json-1.0.151/src/lexical/algorithm.rs

@@ -0,0 +1,195 @@
+// 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 Some(value) = mantissa.checked_mul(power) else {
+            return None;
+        };
+        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)
+    }
+}