src/libcore/num/flt2dec/decoder.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Decodes a floating-point value into individual parts and error ranges.
  12
  13 use {f32, f64};
  14 use num::FpCategory;
  15 use num::dec2flt::rawfp::RawFloat;
  16
  17 /// Decoded unsigned finite value, such that:
  18 ///
  19 /// - The original value equals to `mant * 2^exp`.
  20 ///
  21 /// - Any number from `(mant - minus) * 2^exp` to `(mant + plus) * 2^exp` will
  22 ///   round to the original value. The range is inclusive only when
  23 ///   `inclusive` is true.
  24 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
  25 pub struct Decoded {
  26     /// The scaled mantissa.
  27     pub mant: u64,
  28     /// The lower error range.
  29     pub minus: u64,
  30     /// The upper error range.
  31     pub plus: u64,
  32     /// The shared exponent in base 2.
  33     pub exp: i16,
  34     /// True when the error range is inclusive.
  35     ///
  36     /// In IEEE 754, this is true when the original mantissa was even.
  37     pub inclusive: bool,
  38 }
  39
  40 /// Decoded unsigned value.
  41 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
  42 pub enum FullDecoded {
  43     /// Not-a-number.
  44     Nan,
  45     /// Infinities, either positive or negative.
  46     Infinite,
  47     /// Zero, either positive or negative.
  48     Zero,
  49     /// Finite numbers with further decoded fields.
  50     Finite(Decoded),
  51 }
  52
  53 /// A floating point type which can be `decode`d.
  54 pub trait DecodableFloat: RawFloat + Copy {
  55     /// The minimum positive normalized value.
  56     fn min_pos_norm_value() -> Self;
  57 }
  58
  59 impl DecodableFloat for f32 {
  60     fn min_pos_norm_value() -> Self { f32::MIN_POSITIVE }
  61 }
  62
  63 impl DecodableFloat for f64 {
  64     fn min_pos_norm_value() -> Self { f64::MIN_POSITIVE }
  65 }
  66
  67 /// Returns a sign (true when negative) and `FullDecoded` value
  68 /// from given floating point number.
  69 pub fn decode<T: DecodableFloat>(v: T) -> (/*negative?*/ bool, FullDecoded) {
  70     let (mant, exp, sign) = v.integer_decode();
  71     let even = (mant & 1) == 0;
  72     let decoded = match v.classify() {
  73         FpCategory::Nan => FullDecoded::Nan,
  74         FpCategory::Infinite => FullDecoded::Infinite,
  75         FpCategory::Zero => FullDecoded::Zero,
  76         FpCategory::Subnormal => {
  77             // neighbors: (mant - 2, exp) -- (mant, exp) -- (mant + 2, exp)
  78             // Float::integer_decode always preserves the exponent,
  79             // so the mantissa is scaled for subnormals.
  80             FullDecoded::Finite(Decoded { mant, minus: 1, plus: 1,
  81                                           exp, inclusive: even })
  82         }
  83         FpCategory::Normal => {
  84             let minnorm = <T as DecodableFloat>::min_pos_norm_value().integer_decode();
  85             if mant == minnorm.0 {
  86                 // neighbors: (maxmant, exp - 1) -- (minnormmant, exp) -- (minnormmant + 1, exp)
  87                 // where maxmant = minnormmant * 2 - 1
  88                 FullDecoded::Finite(Decoded { mant: mant << 2, minus: 1, plus: 2,
  89                                               exp: exp - 2, inclusive: even })
  90             } else {
  91                 // neighbors: (mant - 1, exp) -- (mant, exp) -- (mant + 1, exp)
  92                 FullDecoded::Finite(Decoded { mant: mant << 1, minus: 1, plus: 1,
  93                                               exp: exp - 1, inclusive: even })
  94             }
  95         }
  96     };
  97     (sign < 0, decoded)
  98 }
  99