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Double ended iteration for iter_set_bits. #81

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Merged
vext01 merged 2 commits into from
Jul 7, 2025
Merged

Double ended iteration for iter_set_bits. #81

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f9372b4
Implement `DoubleEndedIterator` for `iter_set_bits`.
ltratt Jul 7, 2025
d7e9aec
Cut down lots of repetitive, and not very informative, comments.
ltratt Jul 7, 2025
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98 changes: 62 additions & 36 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use std::{
cmp::{min, PartialEq},
fmt::{self, Debug},
hash::{Hash, Hasher},
iter::FromIterator,
iter::{DoubleEndedIterator, FromIterator},
mem::{replace, size_of},
ops::{
Bound::{Excluded, Included, Unbounded},
Expand Down Expand Up @@ -1101,6 +1101,10 @@ impl<'a, T: Debug + PrimInt> IntoIterator for &'a Vob<T> {
}
}

// In all of the iterators below, we try to optimise what we expect to be the common cases (all
// bits set or all bits unset) with the general case (random bit patterns). We assume that
// `leading_zeros` and `trailing_zeros` have efficient implementations on most CPUs.

#[derive(Clone)]
pub struct IterSetBits<'a, T: 'a> {
vob: &'a Vob<T>,
Expand All @@ -1113,24 +1117,15 @@ impl<T: Debug + PrimInt> Iterator for IterSetBits<'_, T> {
fn next(&mut self) -> Option<usize> {
debug_assert!(self.range.end <= self.vob.len);
if let Some(i) = self.range.next() {
// This is a long, but fairly fast, way of finding out what the next set bit is. The
// basic problem is that we have no idea where the next set bit is -- but different
// patterns of set bits are most efficiently handled by different code paths. This code
// is thus a compromise: we prioritise two special cases (all bits set; all bits unset)
// for efficiency, and try and make the other possible cases reasonably fast.
let mut b = block_offset::<T>(i);
let mut v = self.vob.vec[b];
// If all bits are set, we don't need to do any complicated checks.
if v == T::max_value() {
return Some(i);
}
// At this point we've got a block which might or might not have some bits set. We now
// fall back to the general case.
let mut i_off = i % bits_per_block::<T>();
loop {
let tz = (v >> i_off).trailing_zeros() as usize;
if tz < bits_per_block::<T>() {
// There is a bit set after i_off in the block.
let bs = b * bits_per_block::<T>() + i_off + tz;
self.range.start = bs + 1;
if bs >= self.range.end {
Expand All @@ -1140,7 +1135,6 @@ impl<T: Debug + PrimInt> Iterator for IterSetBits<'_, T> {
}
b += 1;
if b == blocks_required::<T>(self.range.end) {
// We've exhausted the iterator.
self.range.start = self.range.end;
break;
}
Expand All @@ -1156,6 +1150,38 @@ impl<T: Debug + PrimInt> Iterator for IterSetBits<'_, T> {
}
}

impl<T: Debug + PrimInt> DoubleEndedIterator for IterSetBits<'_, T> {
fn next_back(&mut self) -> Option<usize> {
if let Some(i) = self.range.next_back() {
let mut b = block_offset::<T>(i);
let mut v = self.vob.vec[b];
if v == T::max_value() {
return Some(i);
}
let mut i_off = i % bits_per_block::<T>();
loop {
let lz = (v << (bits_per_block::<T>() - 1 - i_off)).leading_zeros() as usize;
if lz < bits_per_block::<T>() {
let bs = b * bits_per_block::<T>() + i_off - lz;
self.range.end = bs;
if bs < self.range.start {
break;
}
return Some(bs);
}
if b == block_offset::<T>(self.range.start) {
self.range.start = self.range.end;
break;
}
b -= 1;
v = self.vob.vec[b];
i_off = bits_per_block::<T>() - 1;
}
}
None
}
}

#[derive(Clone)]
pub struct IterUnsetBits<'a, T: 'a> {
vob: &'a Vob<T>,
Expand All @@ -1168,19 +1194,11 @@ impl<T: Debug + PrimInt> Iterator for IterUnsetBits<'_, T> {
fn next(&mut self) -> Option<usize> {
debug_assert!(self.range.end <= self.vob.len);
if let Some(i) = self.range.next() {
// This is a long, but fairly fast, way of finding out what the next usset bit is. The
// basic problem is that we have no idea where the next set bit is -- but different
// patterns of set bits are most efficiently handled by different code paths. This code
// is thus a compromise: we prioritise two special cases (all bits set; all bits unset)
// for efficiency, and try and make the other possible cases reasonably fast.
let mut b = block_offset::<T>(i);
let mut v = self.vob.vec[b];
// If no bits are set, we don't need to do any complicated checks.
if v == T::zero() {
return Some(i);
}
// At this point we've got a block which might or might not have some bits unset. We
// now fall back to the general case.
let mut i_off = i % bits_per_block::<T>();
loop {
let tz = (!v >> i_off).trailing_zeros() as usize;
Expand All @@ -1189,15 +1207,12 @@ impl<T: Debug + PrimInt> Iterator for IterUnsetBits<'_, T> {
let bs = b * bits_per_block::<T>() + i_off + tz;
self.range.start = bs + 1;
if bs >= self.range.end {
// The unset bit is after the range we're looking for, so we've reached
// the end of the iterator.
break;
}
return Some(bs);
}
b += 1;
if b == blocks_required::<T>(self.range.end) {
// We've exhausted the iterator.
self.range.start = self.range.end;
break;
}
Expand Down Expand Up @@ -1542,6 +1557,7 @@ mod tests {
hash::{Hash, Hasher},
iter::FromIterator,
mem::size_of,
ops::RangeBounds,
};

#[test]
Expand Down Expand Up @@ -1791,28 +1807,38 @@ mod tests {

#[test]
fn test_iter_set_bits() {
fn t<R>(v: &Vob, range: R, expected: Vec<usize>)
where
R: Clone + RangeBounds<usize>,
{
let rev = expected.iter().cloned().rev().collect::<Vec<usize>>();
assert_eq!(
v.iter_set_bits(range.clone()).collect::<Vec<usize>>(),
expected
);
assert_eq!(v.iter_set_bits(range).rev().collect::<Vec<usize>>(), rev);
}

t(&vob![], .., vec![]);
t(&Vob::from_elem(true, 131), .., (0..131).collect::<Vec<_>>());

let mut v1 = vob![false, true, false, true];
assert_eq!(v1.iter_set_bits(..).collect::<Vec<usize>>(), vec![1, 3]);
t(&v1, .., vec![1, 3]);
v1.resize(127, false);
v1.push(true);
v1.push(false);
v1.push(true);
v1.push(true);
v1.resize(256, false);
v1.push(true);
assert_eq!(
v1.iter_set_bits(..).collect::<Vec<usize>>(),
vec![1, 3, 127, 129, 130, 256]
);
assert_eq!(
v1.iter_set_bits(2..256).collect::<Vec<usize>>(),
vec![3, 127, 129, 130]
);
assert_eq!(
v1.iter_set_bits(2..).collect::<Vec<usize>>(),
vec![3, 127, 129, 130, 256]
);
assert_eq!(v1.iter_set_bits(..3).collect::<Vec<usize>>(), vec![1]);
assert_eq!(v1.len(), 257);
t(&v1, .., vec![1, 3, 127, 129, 130, 256]);
t(&v1, 2..257, vec![3, 127, 129, 130, 256]);
t(&v1, 2..256, vec![3, 127, 129, 130]);
t(&v1, 2.., vec![3, 127, 129, 130, 256]);
t(&v1, 1..255, vec![1, 3, 127, 129, 130]);
t(&v1, ..3, vec![1]);
t(&v1, 128.., vec![129, 130, 256]);
}

#[test]
Expand Down