build(ariadne): vendor serde crates

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

@@ -0,0 +1,132 @@
+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+//! Estimate the error in an 80-bit approximation of a float.
+//!
+//! This estimates the error in a floating-point representation.
+//!
+//! This implementation is loosely based off the Golang implementation,
+//! found here: <https://golang.org/src/strconv/atof.go>
+
+use super::float::*;
+use super::num::*;
+use super::rounding::*;
+
+pub(crate) trait FloatErrors {
+    /// Get the full error scale.
+    fn error_scale() -> u32;
+    /// Get the half error scale.
+    fn error_halfscale() -> u32;
+    /// Determine if the number of errors is tolerable for float precision.
+    fn error_is_accurate<F: Float>(count: u32, fp: &ExtendedFloat) -> bool;
+}
+
+/// Check if the error is accurate with a round-nearest rounding scheme.
+#[inline]
+fn nearest_error_is_accurate(errors: u64, fp: &ExtendedFloat, extrabits: u64) -> bool {
+    // Round-to-nearest, need to use the halfway point.
+    if extrabits == 65 {
+        // Underflow, we have a shift larger than the mantissa.
+        // Representation is valid **only** if the value is close enough
+        // overflow to the next bit within errors. If it overflows,
+        // the representation is **not** valid.
+        !fp.mant.overflowing_add(errors).1
+    } else {
+        let mask: u64 = lower_n_mask(extrabits);
+        let extra: u64 = fp.mant & mask;
+
+        // Round-to-nearest, need to check if we're close to halfway.
+        // IE, b10100 | 100000, where `|` signifies the truncation point.
+        let halfway: u64 = lower_n_halfway(extrabits);
+        let cmp1 = halfway.wrapping_sub(errors) < extra;
+        let cmp2 = extra < halfway.wrapping_add(errors);
+
+        // If both comparisons are true, we have significant rounding error,
+        // and the value cannot be exactly represented. Otherwise, the
+        // representation is valid.
+        !(cmp1 && cmp2)
+    }
+}
+
+impl FloatErrors for u64 {
+    #[inline]
+    fn error_scale() -> u32 {
+        8
+    }
+
+    #[inline]
+    fn error_halfscale() -> u32 {
+        u64::error_scale() / 2
+    }
+
+    #[inline]
+    fn error_is_accurate<F: Float>(count: u32, fp: &ExtendedFloat) -> bool {
+        // Determine if extended-precision float is a good approximation.
+        // If the error has affected too many units, the float will be
+        // inaccurate, or if the representation is too close to halfway
+        // that any operations could affect this halfway representation.
+        // See the documentation for dtoa for more information.
+        let bias = -(F::EXPONENT_BIAS - F::MANTISSA_SIZE);
+        let denormal_exp = bias - 63;
+        // This is always a valid u32, since (denormal_exp - fp.exp)
+        // will always be positive and the significand size is {23, 52}.
+        let extrabits = if fp.exp <= denormal_exp {
+            64 - F::MANTISSA_SIZE + denormal_exp - fp.exp
+        } else {
+            63 - F::MANTISSA_SIZE
+        };
+
+        // Our logic is as follows: we want to determine if the actual
+        // mantissa and the errors during calculation differ significantly
+        // from the rounding point. The rounding point for round-nearest
+        // is the halfway point, IE, this when the truncated bits start
+        // with b1000..., while the rounding point for the round-toward
+        // is when the truncated bits are equal to 0.
+        // To do so, we can check whether the rounding point +/- the error
+        // are >/< the actual lower n bits.
+        //
+        // For whether we need to use signed or unsigned types for this
+        // analysis, see this example, using u8 rather than u64 to simplify
+        // things.
+        //
+        // # Comparisons
+        //      cmp1 = (halfway - errors) < extra
+        //      cmp1 = extra < (halfway + errors)
+        //
+        // # Large Extrabits, Low Errors
+        //
+        //      extrabits = 8
+        //      halfway          =  0b10000000
+        //      extra            =  0b10000010
+        //      errors           =  0b00000100
+        //      halfway - errors =  0b01111100
+        //      halfway + errors =  0b10000100
+        //
+        //      Unsigned:
+        //          halfway - errors = 124
+        //          halfway + errors = 132
+        //          extra            = 130
+        //          cmp1             = true
+        //          cmp2             = true
+        //      Signed:
+        //          halfway - errors = 124
+        //          halfway + errors = -124
+        //          extra            = -126
+        //          cmp1             = false
+        //          cmp2             = true
+        //
+        // # Conclusion
+        //
+        // Since errors will always be small, and since we want to detect
+        // if the representation is accurate, we need to use an **unsigned**
+        // type for comparisons.
+
+        let extrabits = extrabits as u64;
+        let errors = count as u64;
+        if extrabits > 65 {
+            // Underflow, we have a literal 0.
+            return true;
+        }
+
+        nearest_error_is_accurate(errors, fp, extrabits)
+    }
+}