core/num/dec2flt/
common.rs

1//! Common utilities, for internal use only.
2
3/// Helper methods to process immutable bytes.
4pub(crate) trait ByteSlice {
5    /// Reads 8 bytes as a 64-bit integer in little-endian order.
6    fn read_u64(&self) -> u64;
7
8    /// Writes a 64-bit integer as 8 bytes in little-endian order.
9    fn write_u64(&mut self, value: u64);
10
11    /// Calculate the difference in length between two slices.
12    fn offset_from(&self, other: &Self) -> isize;
13
14    /// Iteratively parse and consume digits from bytes.
15    ///
16    /// Returns the same bytes with consumed digits being elided. Breaks on invalid digits.
17    fn parse_digits(&self, func: impl FnMut(u8)) -> &Self;
18}
19
20impl ByteSlice for [u8] {
21    #[inline(always)] // inlining this is crucial to remove bound checks
22    fn read_u64(&self) -> u64 {
23        let mut tmp = [0; 8];
24        tmp.copy_from_slice(&self[..8]);
25        u64::from_le_bytes(tmp)
26    }
27
28    #[inline(always)] // inlining this is crucial to remove bound checks
29    fn write_u64(&mut self, value: u64) {
30        self[..8].copy_from_slice(&value.to_le_bytes())
31    }
32
33    #[inline]
34    fn offset_from(&self, other: &Self) -> isize {
35        other.len() as isize - self.len() as isize
36    }
37
38    #[inline]
39    fn parse_digits(&self, mut func: impl FnMut(u8)) -> &Self {
40        let mut s = self;
41
42        while let Some((c, rest)) = s.split_first() {
43            let c = c.wrapping_sub(b'0');
44            if c < 10 {
45                func(c);
46                s = rest;
47            } else {
48                break;
49            }
50        }
51
52        s
53    }
54}
55
56/// Determine if all characters in an 8-byte byte string (represented as a `u64`) are all decimal
57/// digits.
58///
59/// This does not care about the order in which the bytes were loaded.
60pub(crate) fn is_8digits(v: u64) -> bool {
61    let a = v.wrapping_add(0x4646_4646_4646_4646);
62    let b = v.wrapping_sub(0x3030_3030_3030_3030);
63    (a | b) & 0x8080_8080_8080_8080 == 0
64}
65
66/// A custom 64-bit floating point type, representing `m * 2^p`.
67/// p is biased, so it be directly shifted into the exponent bits.
68#[derive(Debug, Copy, Clone, PartialEq, Eq, Default)]
69pub struct BiasedFp {
70    /// The significant digits.
71    pub m: u64,
72    /// The biased, binary exponent.
73    pub p_biased: i32,
74}
75
76impl BiasedFp {
77    /// Represent `0 ^ p`
78    #[inline]
79    pub const fn zero_pow2(p_biased: i32) -> Self {
80        Self { m: 0, p_biased }
81    }
82}
core/num/dec2flt/common.rs

core/num/dec2flt/
common.rs
