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Implement ones using trailing_zeros #18

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merged 2 commits into from
Jan 23, 2021
Merged

Implement ones using trailing_zeros #18

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1ffc9cb
Remove trailing whitespace
vks Feb 22, 2018
6c79c7c
Implement `ones()` using `trailing_zeros()`
vks Feb 22, 2018
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58 changes: 18 additions & 40 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -70,13 +70,14 @@ impl FixedBitSet
length: bits,
}
}

/// Create a new **FixedBitSet** with a specific number of bits,
/// initialized from provided blocks.
///
/// If the blocks are not the exact size needed for the capacity
/// they will be padded with zeros (if shorter) or truncated to
/// the capacity (if longer).
///
///
/// For example:
/// ```
/// let data = vec![4];
Expand Down Expand Up @@ -302,17 +303,15 @@ impl FixedBitSet
match self.as_slice().split_first() {
Some((&block, rem)) => {
Ones {
current_bit_idx: 0,
current_block_idx: 0,
current_block: block,
bitset: block,
block_idx: 0,
remaining_blocks: rem
}
}
None => {
Ones {
current_bit_idx: 0,
current_block_idx: 0,
current_block: 0,
bitset: 0,
block_idx: 0,
remaining_blocks: &[]
}
}
Expand Down Expand Up @@ -593,49 +592,28 @@ impl Iterator for Masks {
///
/// This struct is created by the [`FixedBitSet::ones`] method.
pub struct Ones<'a> {
current_bit_idx: usize,
current_block_idx: usize,
bitset: Block,
block_idx: usize,
remaining_blocks: &'a [Block],
current_block: Block
}

impl<'a> Iterator for Ones<'a> {
type Item = usize; // the bit position of the '1'

#[inline]
fn next(&mut self) -> Option<Self::Item> {
let mut block = self.current_block;
let mut idx = self.current_bit_idx;

loop {
loop {
if (block & 1) == 1 {
self.current_block = block >> 1;
self.current_bit_idx = idx + 1;
return Some(idx);
}
// reordering the two lines below makes a huge (2x) difference in performance!
block = block >> 1;
idx += 1;
if block == 0 {
break;
}
}

// go to next block
match self.remaining_blocks.split_first() {
Some((&next_block, rest)) => {
self.remaining_blocks = rest;
self.current_block_idx += 1;
idx = self.current_block_idx * BITS;
block = next_block;
}
None => {
// last block => done
return None;
}
while self.bitset == 0 {
if self.remaining_blocks.is_empty() {
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We can accomplish all of is_empty, [0] and [1..] in one method call - self.remaining_blocks.split_first() for a presumed minor win in terms of bounds checking overhead 🙂

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Or for that matter, reading split_first's implmentation with slice patterns - but I think that's a much more recent feature if let [first, tail @ ..] = self { Some((first, tail)) } else { None } but it's good to read and use elsewhere if not here.

return None;
}
self.bitset = self.remaining_blocks[0];
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@akraines akraines Jan 11, 2021 •
edited
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Instead of immediately setting bitset to remaining_blocks[0] and updating remaining blocks, why don't you search for the first none zero block and only then update remaining_blocks? (just to save some assignments)

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@akraines akraines Jan 11, 2021 •
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(the PR has been opened for almost 3 years, so I guess nothing needs be done until the maintainer shows interest, but I figured I'd add the comment in case something comes of it)

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I'm not sure that this is faster, because it might mess up vectorization. Could be interesting to benchmark.

self.remaining_blocks = &self.remaining_blocks[1..];
self.block_idx += 1;
}
let t = self.bitset & (0 as Block).wrapping_sub(self.bitset);
let r = self.bitset.trailing_zeros() as usize;
self.bitset ^= t;
Some(self.block_idx * BITS + r)
}
}

Expand Down