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Merge #232
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232: Branchless array-bitmap operations r=Kerollmops a=RaduBerinde

This commit implements a faster version of `Vec::retain` for array
stores.

Fixes #206.

Co-authored-by: Radu Berinde <[email protected]>
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bors[bot] and RaduBerinde authored Aug 15, 2022
2 parents cbe1650 + 5d8627c commit 365a6b6
Showing 1 changed file with 25 additions and 8 deletions.
33 changes: 25 additions & 8 deletions src/bitmap/store/array_store/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -205,6 +205,23 @@ impl ArrayStore {
pub fn as_slice(&self) -> &[u16] {
&self.vec
}

/// Retains only the elements specified by the predicate.
pub fn retain(&mut self, mut f: impl FnMut(u16) -> bool) {
// Idea to avoid branching from "Engineering Fast Indexes for Big Data
// Applications" talk by Daniel Lemire
// (https://youtu.be/1QMgGxiCFWE?t=1242).
let slice = self.vec.as_mut_slice();
let mut pos = 0;
for i in 0..slice.len() {
let val = slice[i];
// We want to do `slice[pos] = val` but we don't need the bounds check.
// SAFETY: pos is always at most i because `f(val) as usize` is at most 1.
unsafe { *slice.get_unchecked_mut(pos) = val }
pos += f(val) as usize;
}
self.vec.truncate(pos);
}
}

impl Default for ArrayStore {
Expand Down Expand Up @@ -300,17 +317,17 @@ impl BitAndAssign<&Self> for ArrayStore {
#[cfg(not(feature = "simd"))]
{
let mut i = 0;
self.vec.retain(|x| {
i += rhs.iter().skip(i).position(|y| y >= x).unwrap_or(rhs.vec.len());
rhs.vec.get(i).map_or(false, |y| x == y)
self.retain(|x| {
i += rhs.iter().skip(i).position(|y| *y >= x).unwrap_or(rhs.vec.len());
rhs.vec.get(i).map_or(false, |y| x == *y)
});
}
}
}

impl BitAndAssign<&BitmapStore> for ArrayStore {
fn bitand_assign(&mut self, rhs: &BitmapStore) {
self.vec.retain(|x| rhs.contains(*x));
self.retain(|x| rhs.contains(x));
}
}

Expand Down Expand Up @@ -339,17 +356,17 @@ impl SubAssign<&Self> for ArrayStore {
#[cfg(not(feature = "simd"))]
{
let mut i = 0;
self.vec.retain(|x| {
i += rhs.iter().skip(i).position(|y| y >= x).unwrap_or(rhs.vec.len());
rhs.vec.get(i).map_or(true, |y| x != y)
self.retain(|x| {
i += rhs.iter().skip(i).position(|y| *y >= x).unwrap_or(rhs.vec.len());
rhs.vec.get(i).map_or(true, |y| x != *y)
});
}
}
}

impl SubAssign<&BitmapStore> for ArrayStore {
fn sub_assign(&mut self, rhs: &BitmapStore) {
self.vec.retain(|x| !rhs.contains(*x));
self.retain(|x| !rhs.contains(x));
}
}

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