c82da7a54b
This is a fork of the flt2dec portion of rust-strconv [1] with a necessary relicensing (the original code was licensed CC0-1.0). Each module is accompanied with large unit tests, integrated in this commit as coretest::num::flt2dec. This module is added in order to replace the existing core::fmt::float method. The forked revision of rust-strconv is from 2015-04-20, with a commit ID 9adf6d3571c6764a6f240a740c823024f70dc1c7. [1] https://github.com/lifthrasiir/rust-strconv/
100 lines
3.5 KiB
Rust
100 lines
3.5 KiB
Rust
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Decodes a floating-point value into individual parts and error ranges.
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use prelude::*;
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use {f32, f64};
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use num::{Float, FpCategory};
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/// Decoded unsigned finite value, such that:
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///
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/// - The original value equals to `mant * 2^exp`.
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///
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/// - Any number from `(mant - minus) * 2^exp` to `(mant + plus) * 2^exp` will
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/// round to the original value. The range is inclusive only when
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/// `inclusive` is true.
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#[derive(Copy, Clone, Debug, PartialEq)]
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pub struct Decoded {
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/// The scaled mantissa.
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pub mant: u64,
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/// The lower error range.
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pub minus: u64,
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/// The upper error range.
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pub plus: u64,
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/// The shared exponent in base 2.
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pub exp: i16,
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/// True when the error range is inclusive.
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///
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/// In IEEE 754, this is true when the original mantissa was even.
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pub inclusive: bool,
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}
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/// Decoded unsigned value.
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#[derive(Copy, Clone, Debug, PartialEq)]
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pub enum FullDecoded {
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/// Not-a-number.
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Nan,
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/// Infinities, either positive or negative.
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Infinite,
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/// Zero, either positive or negative.
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Zero,
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/// Finite numbers with further decoded fields.
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Finite(Decoded),
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}
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/// A floating point type which can be `decode`d.
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pub trait DecodableFloat: Float + Copy {
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/// The minimum positive normalized value.
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fn min_pos_norm_value() -> Self;
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}
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impl DecodableFloat for f32 {
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fn min_pos_norm_value() -> Self { f32::MIN_POSITIVE }
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}
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impl DecodableFloat for f64 {
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fn min_pos_norm_value() -> Self { f64::MIN_POSITIVE }
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}
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/// Returns a sign (true when negative) and `FullDecoded` value
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/// from given floating point number.
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pub fn decode<T: DecodableFloat>(v: T) -> (/*negative?*/ bool, FullDecoded) {
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let (mant, exp, sign) = v.integer_decode();
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let even = (mant & 1) == 0;
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let decoded = match v.classify() {
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FpCategory::Nan => FullDecoded::Nan,
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FpCategory::Infinite => FullDecoded::Infinite,
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FpCategory::Zero => FullDecoded::Zero,
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FpCategory::Subnormal => {
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// (mant - 2, exp) -- (mant, exp) -- (mant + 2, exp)
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// Float::integer_decode always preserves the exponent,
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// so the mantissa is scaled for subnormals.
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FullDecoded::Finite(Decoded { mant: mant, minus: 1, plus: 1,
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exp: exp, inclusive: even })
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}
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FpCategory::Normal => {
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let minnorm = <T as DecodableFloat>::min_pos_norm_value().integer_decode();
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if mant == minnorm.0 && exp == minnorm.1 {
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// (maxmant, exp - 1) -- (minnormmant, exp) -- (minnormmant + 1, exp)
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// where maxmant = minnormmant * 2 - 1
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FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 2,
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exp: exp - 1, inclusive: even })
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} else {
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// (mant - 1, exp) -- (mant, exp) -- (mant + 1, exp)
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FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 1,
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exp: exp - 1, inclusive: even })
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}
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}
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};
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(sign < 0, decoded)
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}
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