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Specialize StepBy::nth #47552

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Merged
merged 6 commits into from
Jan 31, 2018
Merged

Specialize StepBy::nth #47552

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37771d4
Specialize StepBy::nth
oberien Jan 18, 2018
5850f0b
Fix off-by-ones
oberien Jan 19, 2018
d33cc12
Unit Tests
oberien Jan 19, 2018
f08dec1
Handle Overflow
oberien Jan 19, 2018
f72b7f7
Optimize StepBy::nth overflow handling
oberien Jan 19, 2018
4a0da4c
Spacing
oberien Jan 19, 2018
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12 changes: 12 additions & 0 deletions src/libcore/iter/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -694,6 +694,18 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
(f(inner_hint.0), inner_hint.1.map(f))
}
}

#[inline]
fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
if self.first_take {
if n == 0 {
self.first_take = false;
return self.iter.next()
}
n -= 1;
}
self.iter.nth(n * self.step)
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Good change! ❤️

I think that this needs to handle overflow in the multiplication, though, since I could write (0u64..).step_by(0x10000).nth(0x10000) on a 32-bit machine, for example.

Also, tests?

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Tests are a good call, I found 2 off-by-ones while implementing them.

I added overflow behaviour, but it is pretty weighty. While I think that it shouldn't influence normal execution due to intrinsics::likely and branch prediction, I do think that it is pretty complicated code, which doesn't really add too much value. Anyway, now that it exists, I don't mind keeping it :)

}
}

// StepBy can only make the iterator shorter, so the len will still fit.
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