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Diffstat (limited to 'vendor/serde_json/src/lexical/errors.rs')
| -rw-r--r-- | vendor/serde_json/src/lexical/errors.rs | 132 |
1 files changed, 0 insertions, 132 deletions
diff --git a/vendor/serde_json/src/lexical/errors.rs b/vendor/serde_json/src/lexical/errors.rs deleted file mode 100644 index f4f41cd..0000000 --- a/vendor/serde_json/src/lexical/errors.rs +++ /dev/null @@ -1,132 +0,0 @@ -// 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) - } -} |