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Rollup merge of #146284 - Kivooeo:blazing-fast-division-bignum, r=Mark-Simulacrum

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Remove `div_rem` from `core::num::bignum`

This fixes very old fixme that sounds like this

```
Stupid slow base-2 long division taken from
https://en.wikipedia.org/wiki/Division_algorithm
FIXME use a greater base ($ty) for the long division.
```

By deleting this method since it was never used
This commit is contained in:
Matthias Krüger 2025-09-15 06:03:45 +02:00 • committed by GitHub
commit fa63dbf301
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37
library/core/src/num/bignum.rs
View file

@ -335,43 +335,6 @@ macro_rules! define_bignum {
}
(self, borrow)
}
/// Divide self by another bignum, overwriting `q` with the quotient and `r` with the
/// remainder.
pub fn div_rem(&self, d: &$name, q: &mut $name, r: &mut $name) {
// Stupid slow base-2 long division taken from
// https://en.wikipedia.org/wiki/Division_algorithm
// FIXME use a greater base ($ty) for the long division.
assert!(!d.is_zero());
let digitbits = <$ty>::BITS as usize;
for digit in &mut q.base[..] {
*digit = 0;
}
for digit in &mut r.base[..] {
*digit = 0;
}
r.size = d.size;
q.size = 1;
let mut q_is_zero = true;
let end = self.bit_length();
for i in (0..end).rev() {
r.mul_pow2(1);
r.base[0] |= self.get_bit(i) as $ty;
if &*r >= d {
r.sub(d);
// Set bit `i` of q to 1.
let digit_idx = i / digitbits;
let bit_idx = i % digitbits;
if q_is_zero {
q.size = digit_idx + 1;
q_is_zero = false;
}
q.base[digit_idx] |= 1 << bit_idx;
}
}
debug_assert!(q.base[q.size..].iter().all(|&d| d == 0));
debug_assert!(r.base[r.size..].iter().all(|&d| d == 0));
}
}
impl crate::cmp::PartialEq for $name {

19
library/coretests/tests/num/bignum.rs
View file

@ -167,25 +167,6 @@ fn test_div_rem_small() {
);
}
#[test]
fn test_div_rem() {
fn div_rem(n: u64, d: u64) -> (Big, Big) {
let mut q = Big::from_small(42);
let mut r = Big::from_small(42);
Big::from_u64(n).div_rem(&Big::from_u64(d), &mut q, &mut r);
(q, r)
}
assert_eq!(div_rem(1, 1), (Big::from_small(1), Big::from_small(0)));
assert_eq!(div_rem(4, 3), (Big::from_small(1), Big::from_small(1)));
assert_eq!(div_rem(1, 7), (Big::from_small(0), Big::from_small(1)));
assert_eq!(div_rem(45, 9), (Big::from_small(5), Big::from_small(0)));
assert_eq!(div_rem(103, 9), (Big::from_small(11), Big::from_small(4)));
assert_eq!(div_rem(123456, 77), (Big::from_u64(1603), Big::from_small(25)));
assert_eq!(div_rem(0xffff, 1), (Big::from_u64(0xffff), Big::from_small(0)));
assert_eq!(div_rem(0xeeee, 0xffff), (Big::from_small(0), Big::from_u64(0xeeee)));
assert_eq!(div_rem(2_000_000, 2), (Big::from_u64(1_000_000), Big::from_u64(0)));
}
#[test]
fn test_is_zero() {
assert!(Big::from_small(0).is_zero());