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BlockRng optimizations #40

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BlockRng optimizations #40

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49c3d62
Optimize BlockRng::fill_bytes
dhardy Dec 23, 2025
6473121
Optimize BlockRng::next_u64_from_u32
dhardy Dec 23, 2025
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32 changes: 17 additions & 15 deletions src/block.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -260,22 +260,24 @@ impl<const N: usize, G: Generator<Output = [u32; N]>> BlockRng<G> {
#[inline]
pub fn next_u64_from_u32(&mut self) -> u64 {
let index = self.index();
let (lo, hi);
let mut new_index;
let (mut lo, mut hi);
if index < N - 1 {
lo = self.results[index];
hi = self.results[index + 1];
self.set_index(index + 2);
} else if index >= N {
self.core.generate(&mut self.results);
lo = self.results[0];
hi = self.results[1];
self.set_index(2);
new_index = index + 2;
} else {
lo = self.results[N - 1];
self.core.generate(&mut self.results);
hi = self.results[0];
self.set_index(1);
new_index = 1;
if index >= N {
lo = hi;
hi = self.results[1];
new_index = 2;
}
}
self.set_index(new_index);
(u64::from(hi) << 32) | u64::from(lo)
}
}
Expand All @@ -285,8 +287,8 @@ impl<W: Word, const N: usize, G: Generator<Output = [W; N]>> BlockRng<G> {
#[inline]
pub fn fill_bytes(&mut self, dest: &mut [u8]) {
let mut read_len = 0;
let mut index = self.index();
while read_len < dest.len() {
let mut index = self.index();
if index >= N {
self.core.generate(&mut self.results);
index = 0;
Expand All @@ -304,17 +306,17 @@ impl<W: Word, const N: usize, G: Generator<Output = [W; N]>> BlockRng<G> {

if let Some(src) = src.next() {
// We have consumed all full chunks of dest, but not src.
let dest = chunks.into_remainder();
let n = dest.len();
let dest_rem = chunks.into_remainder();
let n = dest_rem.len();
if n > 0 {
dest.copy_from_slice(&src.to_le_bytes().as_ref()[..n]);
dest_rem.copy_from_slice(&src.to_le_bytes().as_ref()[..n]);
index += 1;
read_len += n;
debug_assert_eq!(read_len + n, dest.len());
}
break;
}

self.set_index(index);
}
self.set_index(index);
}
}

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