crypto_bigint/
bitlen.rs

1//! Bit calculations.
2//!
3//! These provide a single place to handle bits-to-bytes conversions and the `u32`/`usize`
4//! discrepancy that can only be solved by panics or casting.
5
6use crate::Limb;
7
8/// Calculate a `u32` representing a number of bits from the given number of bytes as a `usize`.
9#[cfg_attr(not(feature = "alloc"), allow(dead_code))]
10#[inline(always)]
11pub(crate) const fn from_bytes(nbytes: usize) -> u32 {
12    mul(nbytes, 8)
13}
14
15/// Calculate a `u32` representing a number of bits in an integer with the given number of limbs.
16#[inline(always)]
17pub(crate) const fn from_limbs(nlimbs: usize) -> u32 {
18    mul(nlimbs, Limb::BITS)
19}
20
21/// Calculate a `usize` representing the number of bytes required to store the given number of bits
22/// represented as a `u32` (i.e. rounding up).
23#[cfg_attr(not(feature = "alloc"), allow(dead_code))]
24#[inline(always)]
25pub(crate) const fn to_bytes(nbits: u32) -> usize {
26    u32_to_usize(nbits.div_ceil(8))
27}
28
29/// Calculate a `usize` representing the number of limbs required to store the given number of bits
30/// represented as a `u32` (i.e. rounding up).
31#[inline(always)]
32pub(crate) const fn to_limbs(nbits: u32) -> usize {
33    u32_to_usize(nbits.div_ceil(Limb::BITS))
34}
35
36#[inline(always)]
37const fn u32_to_usize(n: u32) -> usize {
38    const {
39        // Ensure cast from `u32` to `usize` won't overflow
40        assert!(usize::BITS >= u32::BITS, "usize too small");
41    }
42
43    n as usize
44}
45
46#[allow(
47    clippy::cast_possible_truncation,
48    reason = "assertion ensures panic instead of truncation"
49)]
50#[inline(always)]
51const fn usize_to_u32(n: usize) -> u32 {
52    debug_assert!(n < u32_to_usize(u32::MAX), "u32 overflow");
53    n as u32
54}
55
56#[inline(always)]
57const fn mul(size: usize, n: u32) -> u32 {
58    if cfg!(debug_assertions) {
59        usize_to_u32(size).checked_mul(n).expect("u32 overflow")
60    } else {
61        usize_to_u32(size) * n
62    }
63}
64
65#[cfg(test)]
66mod tests {
67    #[test]
68    fn from_bytes() {
69        assert_eq!(super::from_bytes(0), 0);
70        assert_eq!(super::from_bytes(1), 8);
71        assert_eq!(super::from_bytes(2), 16);
72    }
73
74    #[test]
75    fn to_bytes() {
76        assert_eq!(super::to_bytes(0), 0);
77        assert_eq!(super::to_bytes(1), 1);
78        assert_eq!(super::to_bytes(8), 1);
79        assert_eq!(super::to_bytes(9), 2);
80        assert_eq!(super::to_bytes(16), 2);
81        assert_eq!(super::to_bytes(17), 3);
82    }
83}
crypto_bigint/bitlen.rs

crypto_bigint/
bitlen.rs
