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branchless .filter(_).count() #39107

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merged 4 commits into from
Feb 5, 2017
Merged

branchless .filter(_).count() #39107

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6c94089
branchless .filter(_).count()
llogiq Jan 16, 2017
27f7615
fix style nits
llogiq Jan 17, 2017
b40432c
add test case for filter+count
llogiq Jan 18, 2017
bfabe81
add explanation, fix test
llogiq Jan 24, 2017
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10 changes: 10 additions & 0 deletions src/libcore/iter/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1099,6 +1099,16 @@ impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool
let (_, upper) = self.iter.size_hint();
(0, upper) // can't know a lower bound, due to the predicate
}

#[inline]
fn count(self) -> usize {
let (mut c, mut predicate, mut iter) = (0, self.predicate, self.iter);
for x in iter.by_ref() {
// branchless count
c += (&mut predicate)(&x) as usize;
}
c
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@bluss bluss Jan 17, 2017 •
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Maybe one doesn't want to argue style for trivial things, but I find the code cramped when simple should work well:

fn count(mut self) -> usize {
    // Attempt to produce branchless count if possible
    let mut count = 0;
    for elt in self.iter {
        count += (self.predicate)(&elt) as usize;
    }
    count
}

The surrounding methods use .by_ref() which seems archaic (just &mut self.iter would be fine if mutable reference only was required)

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@llogiq llogiq Jan 17, 2017 •
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Good idea, will change shortly (perhaps also change the other methods en passant).

}
}

#[stable(feature = "rust1", since = "1.0.0")]
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