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Auto merge of #25230 - rayglover:patch-bitset, r=Gankro
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Some modest running-time improvements to `std::collections::BitSet` on bit-sets of varying set-membership densities. This is work originally from [here](https://github.com/rayglover/alt_collections). (Benchmarks copied below)
```
std::collections::BitSet / alt_collections::BitSet

copy_dense         ... 3.08x
copy_sparse        ... 4.22x
count_dense        ... 11.01x
count_sparse       ... 8.11x
from_bytes         ... 1.47x
intersect_dense    ... 6.54x
intersect_sparse   ... 4.37x
union_dense        ... 5.53x
union_sparse       ... 5.60x
```

The exception is `from_bytes`, which I've left unaltered since the optimization is rather obscure.

Compiling with the cpu feature `popcnt` gave a further ~10% improvement on my machine, but this wasn't factored in to the benchmarks above.

Similar improvements could be made to `BitVec`, although that would probably require more substantial changes.

criticism welcome!
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@bors
bors committed May 18, 2015
2 parents 2dd5ad0 + 307fab1 commit 4daa62a
Showing 1 changed file with 83 additions and 74 deletions.
157 changes: 83 additions & 74 deletions src/libcollections/bit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1555,7 +1555,7 @@ impl BitSet {
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn iter(&self) -> bit_set::Iter {
SetIter {set: self, next_idx: 0}
SetIter(BlockIter::from_blocks(self.bit_vec.blocks()))
}

/// Iterator over each usize stored in `self` union `other`.
Expand All @@ -1580,13 +1580,11 @@ impl BitSet {
pub fn union<'a>(&'a self, other: &'a BitSet) -> Union<'a> {
fn or(w1: u32, w2: u32) -> u32 { w1 | w2 }

Union(TwoBitPositions {
set: self,
other: other,
Union(BlockIter::from_blocks(TwoBitPositions {
set: self.bit_vec.blocks(),
other: other.bit_vec.blocks(),
merge: or,
current_word: 0,
next_idx: 0
})
}))
}

/// Iterator over each usize stored in `self` intersect `other`.
Expand All @@ -1611,13 +1609,12 @@ impl BitSet {
pub fn intersection<'a>(&'a self, other: &'a BitSet) -> Intersection<'a> {
fn bitand(w1: u32, w2: u32) -> u32 { w1 & w2 }
let min = cmp::min(self.bit_vec.len(), other.bit_vec.len());
Intersection(TwoBitPositions {
set: self,
other: other,

Intersection(BlockIter::from_blocks(TwoBitPositions {
set: self.bit_vec.blocks(),
other: other.bit_vec.blocks(),
merge: bitand,
current_word: 0,
next_idx: 0
}.take(min))
}).take(min))
}

/// Iterator over each usize stored in the `self` setminus `other`.
Expand Down Expand Up @@ -1649,13 +1646,11 @@ impl BitSet {
pub fn difference<'a>(&'a self, other: &'a BitSet) -> Difference<'a> {
fn diff(w1: u32, w2: u32) -> u32 { w1 & !w2 }

Difference(TwoBitPositions {
set: self,
other: other,
Difference(BlockIter::from_blocks(TwoBitPositions {
set: self.bit_vec.blocks(),
other: other.bit_vec.blocks(),
merge: diff,
current_word: 0,
next_idx: 0
})
}))
}

/// Iterator over each usize stored in the symmetric difference of `self` and `other`.
Expand All @@ -1681,13 +1676,11 @@ impl BitSet {
pub fn symmetric_difference<'a>(&'a self, other: &'a BitSet) -> SymmetricDifference<'a> {
fn bitxor(w1: u32, w2: u32) -> u32 { w1 ^ w2 }

SymmetricDifference(TwoBitPositions {
set: self,
other: other,
SymmetricDifference(BlockIter::from_blocks(TwoBitPositions {
set: self.bit_vec.blocks(),
other: other.bit_vec.blocks(),
merge: bitxor,
current_word: 0,
next_idx: 0
})
}))
}

/// Unions in-place with the specified other bit vector.
Expand Down Expand Up @@ -1994,98 +1987,114 @@ impl hash::Hash for BitSet {
}
}

/// An iterator for `BitSet`.
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SetIter<'a> {
set: &'a BitSet,
next_idx: usize
struct BlockIter<T> where T: Iterator<Item=u32> {
head: u32,
head_offset: usize,
tail: T,
}

impl<'a, T> BlockIter<T> where T: Iterator<Item=u32> {
fn from_blocks(mut blocks: T) -> BlockIter<T> {
let h = blocks.next().unwrap_or(0);
BlockIter {tail: blocks, head: h, head_offset: 0}
}
}

/// An iterator combining two `BitSet` iterators.
#[derive(Clone)]
struct TwoBitPositions<'a> {
set: &'a BitSet,
other: &'a BitSet,
set: Blocks<'a>,
other: Blocks<'a>,
merge: fn(u32, u32) -> u32,
current_word: u32,
next_idx: usize
}

/// An iterator for `BitSet`.
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SetIter<'a>(BlockIter<Blocks<'a>>);
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Union<'a>(TwoBitPositions<'a>);
pub struct Union<'a>(BlockIter<TwoBitPositions<'a>>);
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Intersection<'a>(Take<TwoBitPositions<'a>>);
pub struct Intersection<'a>(Take<BlockIter<TwoBitPositions<'a>>>);
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Difference<'a>(TwoBitPositions<'a>);
pub struct Difference<'a>(BlockIter<TwoBitPositions<'a>>);
#[derive(Clone)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SymmetricDifference<'a>(TwoBitPositions<'a>);
pub struct SymmetricDifference<'a>(BlockIter<TwoBitPositions<'a>>);

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for SetIter<'a> {
impl<'a, T> Iterator for BlockIter<T> where T: Iterator<Item=u32> {
type Item = usize;

fn next(&mut self) -> Option<usize> {
while self.next_idx < self.set.bit_vec.len() {
let idx = self.next_idx;
self.next_idx += 1;

if self.set.contains(&idx) {
return Some(idx);
while self.head == 0 {
match self.tail.next() {
Some(w) => self.head = w,
None => return None
}
self.head_offset += u32::BITS;
}

return None;
// from the current block, isolate the
// LSB and subtract 1, producing k:
// a block with a number of set bits
// equal to the index of the LSB
let k = (self.head & (!self.head + 1)) - 1;
// update block, removing the LSB
self.head &= self.head - 1;
// return offset + (index of LSB)
Some(self.head_offset + (u32::count_ones(k) as usize))
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
(0, Some(self.set.bit_vec.len() - self.next_idx))
match self.tail.size_hint() {
(_, Some(h)) => (0, Some(1 + h * (u32::BITS as usize))),
_ => (0, None)
}
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for TwoBitPositions<'a> {
type Item = usize;

fn next(&mut self) -> Option<usize> {
while self.next_idx < self.set.bit_vec.len() ||
self.next_idx < self.other.bit_vec.len() {
let bit_idx = self.next_idx % u32::BITS;
if bit_idx == 0 {
let s_bit_vec = &self.set.bit_vec;
let o_bit_vec = &self.other.bit_vec;
// Merging the two words is a bit of an awkward dance since
// one BitVec might be longer than the other
let word_idx = self.next_idx / u32::BITS;
let w1 = if word_idx < s_bit_vec.storage.len() {
s_bit_vec.storage[word_idx]
} else { 0 };
let w2 = if word_idx < o_bit_vec.storage.len() {
o_bit_vec.storage[word_idx]
} else { 0 };
self.current_word = (self.merge)(w1, w2);
}

self.next_idx += 1;
if self.current_word & (1 << bit_idx) != 0 {
return Some(self.next_idx - 1);
}
type Item = u32;

fn next(&mut self) -> Option<u32> {
match (self.set.next(), self.other.next()) {
(Some(a), Some(b)) => Some((self.merge)(a, b)),
(Some(a), None) => Some((self.merge)(a, 0)),
(None, Some(b)) => Some((self.merge)(0, b)),
_ => return None
}
return None;
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let cap = cmp::max(self.set.bit_vec.len(), self.other.bit_vec.len());
(0, Some(cap - self.next_idx))
let (a, au) = self.set.size_hint();
let (b, bu) = self.other.size_hint();

let upper = match (au, bu) {
(Some(au), Some(bu)) => Some(cmp::max(au, bu)),
_ => None
};

(cmp::max(a, b), upper)
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for SetIter<'a> {
type Item = usize;

#[inline] fn next(&mut self) -> Option<usize> { self.0.next() }
#[inline] fn size_hint(&self) -> (usize, Option<usize>) { self.0.size_hint() }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Iterator for Union<'a> {
type Item = usize;
Expand Down

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