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(div_ceil_pow2(nbits, 3))
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(to_limbs_u32(nbits))
34}
35
36/// Calculate the number of limbs required to store the given number of bits as a `u32`.
37#[inline(always)]
38pub(crate) const fn to_limbs_u32(nbits: u32) -> u32 {
39    div_ceil_pow2(nbits, Limb::LOG2_BITS)
40}
41
42/// Divide by a power of two, rounding up, without a remainder-dependent branch.
43#[inline(always)]
44const fn div_ceil_pow2(n: u32, log2: u32) -> u32 {
45    let mask = (1 << log2) - 1;
46    (n >> log2).wrapping_add(((n & mask).wrapping_add(mask)) >> log2)
47}
48
49#[inline(always)]
50const fn u32_to_usize(n: u32) -> usize {
51    const {
52        // Ensure cast from `u32` to `usize` won't overflow
53        assert!(usize::BITS >= u32::BITS, "usize too small");
54    }
55
56    n as usize
57}
58
59#[allow(
60    clippy::cast_possible_truncation,
61    reason = "assertion ensures panic instead of truncation"
62)]
63#[inline(always)]
64const fn usize_to_u32(n: usize) -> u32 {
65    debug_assert!(n < u32_to_usize(u32::MAX), "u32 overflow");
66    n as u32
67}
68
69#[inline(always)]
70const fn mul(size: usize, n: u32) -> u32 {
71    if cfg!(debug_assertions) {
72        usize_to_u32(size).checked_mul(n).expect("u32 overflow")
73    } else {
74        usize_to_u32(size) * n
75    }
76}
77
78#[cfg(test)]
79mod tests {
80    #[test]
81    fn from_bytes() {
82        assert_eq!(super::from_bytes(0), 0);
83        assert_eq!(super::from_bytes(1), 8);
84        assert_eq!(super::from_bytes(2), 16);
85    }
86
87    #[test]
88    fn to_bytes() {
89        assert_eq!(super::to_bytes(0), 0);
90        assert_eq!(super::to_bytes(1), 1);
91        assert_eq!(super::to_bytes(8), 1);
92        assert_eq!(super::to_bytes(9), 2);
93        assert_eq!(super::to_bytes(16), 2);
94        assert_eq!(super::to_bytes(17), 3);
95    }
96
97    #[test]
98    fn to_limbs() {
99        use crate::Limb;
100
101        assert_eq!(super::to_limbs(0), 0);
102        assert_eq!(super::to_limbs(1), 1);
103        assert_eq!(super::to_limbs(Limb::BITS), 1);
104        assert_eq!(super::to_limbs(Limb::BITS + 1), 2);
105        assert_eq!(
106            super::to_limbs(u32::MAX),
107            u32::MAX.div_ceil(Limb::BITS) as usize
108        );
109    }
110}
crypto_bigint/bitlen.rs

crypto_bigint/
bitlen.rs
