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better implementation of signed div_floor/ceil

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This commit is contained in:
Orson Peters 2024-09-05 22:50:41 +02:00
parent eb33b43bab
commit 6b4ff514d9
1 changed files with 16 additions and 4 deletions
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20
library/core/src/num/int_macros.rs
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@ -3023,8 +3023,16 @@ macro_rules! int_impl {
pub const fn div_floor(self, rhs: Self) -> Self {
let d = self / rhs;
let r = self % rhs;
if (r > 0 && rhs < 0) || (r < 0 && rhs > 0) {
d - 1
// If the remainder is non-zero, we need to subtract one if the
// signs of self and rhs differ, as this means we rounded upwards
// instead of downwards. We do this branchlessly by creating a mask
// which is all-ones iff the signs differ, and 0 otherwise. Then by
// adding this mask (which corresponds to the signed value -1), we
// get our correction.
let correction = (self ^ rhs) >> (Self::BITS - 1);
if r != 0 {
d + correction
} else {
d
}
@ -3059,8 +3067,12 @@ macro_rules! int_impl {
pub const fn div_ceil(self, rhs: Self) -> Self {
let d = self / rhs;
let r = self % rhs;
if (r > 0 && rhs > 0) || (r < 0 && rhs < 0) {
d + 1
// When remainder is non-zero we have a.div_ceil(b) == 1 + a.div_floor(b),
// so we can re-use the algorithm from div_floor, just adding 1.
let correction = 1 + ((self ^ rhs) >> (Self::BITS - 1));
if r != 0 {
d + correction
} else {
d
}