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auto merge of #16728 : bluss/rust/zip-next-back, r=alexcrichton

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Use ExactSize::len() and defer to its decisions about overly defensive
assertions. Remove the length double-check and simply put a failure
case if the Zip finds an uneven end in .next_back().

Fixing this up since I think I wrote this, and it's been known to
confuse rusties (PR #15886).
This commit is contained in:
bors 2014-08-24 18:46:01 +00:00
commit 17f79af31c
1 changed files with 17 additions and 15 deletions
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32
src/libcore/iter.rs
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@ -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
}
@ -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.len();
let b_sz = self.b.len();
if a_sz != b_sz {
// 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(); }
}
}
let (a_sz, _) = self.a.size_hint();
let (b_sz, _) = self.b.size_hint();
assert!(a_sz == b_sz);
match (self.a.next_back(), self.b.next_back()) {
(Some(x), Some(y)) => Some((x, y)),
_ => None
(None, None) => None,
_ => unreachable!(),
}
}
}
@ -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
}