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libcore: Simplify Enumerate, Zip::next_back #16728

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libcore: Simplify Enumerate, Zip::next_back #16728

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34 changes: 18 additions & 16 deletions src/libcore/iter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -728,6 +728,10 @@ pub trait ExactSize<A> : DoubleEndedIterator<A> {
/// Return the exact length of the iterator.
fn len(&self) -> uint {
let (lower, upper) = self.size_hint();
// Note: This assertion is overly defensive, but it checks the invariant
// guaranteed by the trait. If this trait were rust-internal,
// we could use debug_assert!; assert_eq! will check all Rust user
// implementations too.
assert_eq!(upper, Some(lower));
lower
}
Expand Down Expand Up @@ -1195,21 +1199,20 @@ impl<A, B, T: ExactSize<A>, U: ExactSize<B>> DoubleEndedIterator<(A, B)>
for Zip<T, U> {
#[inline]
fn next_back(&mut self) -> Option<(A, B)> {
let (a_sz, a_upper) = self.a.size_hint();
let (b_sz, b_upper) = self.b.size_hint();
assert!(a_upper == Some(a_sz));
assert!(b_upper == Some(b_sz));
if a_sz < b_sz {
for _ in range(0, b_sz - a_sz) { self.b.next_back(); }
} else if a_sz > b_sz {
for _ in range(0, a_sz - b_sz) { self.a.next_back(); }
}
let (a_sz, _) = self.a.size_hint();
let (b_sz, _) = self.b.size_hint();
assert!(a_sz == b_sz);
let a_sz = self.a.len();
let b_sz = self.b.len();
if a_sz != b_sz {
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Rather than two checks here, couldn't you just do one cmp?

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Actually, strictly speaking you can just have the a_sz > b_sz check, because in the else branch if a_sz = b_sz, then the range is empty and the loop doesn't run.

Not sure what's the most efficient/readable here.

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Thanks for the input, I don't know exactly either. But the evening out step should only happen once, and that's why I wanted a single comparison to exclude it.

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Ah, good call!

// Adjust a, b to equal length
if a_sz > b_sz {
for _ in range(0, a_sz - b_sz) { self.a.next_back(); }
} else {
for _ in range(0, b_sz - a_sz) { self.b.next_back(); }
}
}
match (self.a.next_back(), self.b.next_back()) {
(Some(x), Some(y)) => Some((x, y)),
_ => None
(None, None) => None,
_ => unreachable!(),
}
}
}
Expand Down Expand Up @@ -1395,9 +1398,8 @@ impl<A, T: ExactSize<A>> DoubleEndedIterator<(uint, A)> for Enumerate<T> {
fn next_back(&mut self) -> Option<(uint, A)> {
match self.iter.next_back() {
Some(a) => {
let (lower, upper) = self.iter.size_hint();
assert!(upper == Some(lower));
Some((self.count + lower, a))
let len = self.iter.len();
Some((self.count + len, a))
}
_ => None
}
Expand Down