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Optimize iterator adapters. #25035

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Merged
merged 1 commit into from
May 6, 2015
Merged

Optimize iterator adapters. #25035

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163 changes: 145 additions & 18 deletions src/libcore/iter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1472,6 +1472,32 @@ impl<A, B> Iterator for Chain<A, B> where
}
}

#[inline]
fn count(self) -> usize {
(if !self.flag { self.a.count() } else { 0 }) + self.b.count()
}

#[inline]
fn nth(&mut self, mut n: usize) -> Option<A::Item> {
if !self.flag {
for x in self.a.by_ref() {
if n == 0 {
return Some(x)
}
n -= 1;
}
self.flag = true;
}
self.b.nth(n)
}

#[inline]
fn last(self) -> Option<A::Item> {
let a_last = if self.flag { None } else { self.a.last() };
let b_last = self.b.last();
b_last.or(a_last)
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let (a_lower, a_upper) = self.a.size_hint();
Expand Down Expand Up @@ -1777,6 +1803,20 @@ impl<I> Iterator for Enumerate<I> where I: Iterator {
fn size_hint(&self) -> (usize, Option<usize>) {
self.iter.size_hint()
}

#[inline]
fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
self.iter.nth(n).map(|a| {
let i = self.count + n;
self.count = i + 1;
(i, a)
})
}

#[inline]
fn count(self) -> usize {
self.iter.count()
}
}

#[stable(feature = "rust1", since = "1.0.0")]
Expand Down Expand Up @@ -1834,6 +1874,28 @@ impl<I: Iterator> Iterator for Peekable<I> {
}
}

#[inline]
fn count(self) -> usize {
(if self.peeked.is_some() { 1 } else { 0 }) + self.iter.count()
}

#[inline]
fn nth(&mut self, n: usize) -> Option<I::Item> {
match self.peeked {
Some(_) if n == 0 => self.peeked.take(),
Some(_) => {
self.peeked = None;
self.iter.nth(n-1)
},
None => self.iter.nth(n)
}
}

#[inline]
fn last(self) -> Option<I::Item> {
self.iter.last().or(self.peeked)
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let (lo, hi) = self.iter.size_hint();
Expand Down Expand Up @@ -1960,27 +2022,49 @@ impl<I> Iterator for Skip<I> where I: Iterator {
type Item = <I as Iterator>::Item;

#[inline]
fn next(&mut self) -> Option<<I as Iterator>::Item> {
let mut next = self.iter.next();
fn next(&mut self) -> Option<I::Item> {
if self.n == 0 {
next
self.iter.next()
} else {
let mut n = self.n;
while n > 0 {
n -= 1;
match next {
Some(_) => {
next = self.iter.next();
continue
}
None => {
self.n = 0;
return None
}
}
}
let old_n = self.n;
self.n = 0;
next
self.iter.nth(old_n)
}
}

#[inline]
fn nth(&mut self, n: usize) -> Option<I::Item> {
// Can't just add n + self.n due to overflow.
if self.n == 0 {
self.iter.nth(n)
} else {
let to_skip = self.n;
self.n = 0;
// nth(n) skips n+1
if self.iter.nth(to_skip-1).is_none() {
return None;
}
self.iter.nth(n)
}
}

#[inline]
fn count(self) -> usize {
self.iter.count().saturating_sub(self.n)
}

#[inline]
fn last(mut self) -> Option<I::Item> {
if self.n == 0 {
self.iter.last()
} else {
let next = self.next();
if next.is_some() {
// recurse. n should be 0.
self.last().or(next)
} else {
None
}
}
}

Expand Down Expand Up @@ -2038,6 +2122,20 @@ impl<I> Iterator for Take<I> where I: Iterator{
}
}

#[inline]
fn nth(&mut self, n: usize) -> Option<I::Item> {
if self.n > n {
self.n -= n + 1;
self.iter.nth(n)
} else {
if self.n > 0 {
self.iter.nth(self.n - 1);
self.n = 0;
}
None
}
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let (lower, upper) = self.iter.size_hint();
Expand Down Expand Up @@ -2199,6 +2297,35 @@ impl<I> Iterator for Fuse<I> where I: Iterator {
}
}

#[inline]
fn nth(&mut self, n: usize) -> Option<I::Item> {
if self.done {
None
} else {
let nth = self.iter.nth(n);
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This only retains the semantics of fuse if we assume that nth(n) returns None whenever n+1 calls to next would produce at least one None. That makes me slightly uneasy...

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I'm assuming the implementer of nth is obeying the spec. That is, nth(n) is equivalent to n+1 calls of next(). And shouldn't that be at most n+1 calls to next? nth always stops after the first `None

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Yes, at most. And that's probably reasonable, I just wanted to point out that if the underlying nth did not follow the spec in that sense, then this implementation and the default one could produce different results -- just want to make sure we're comfortable with that.

self.done = nth.is_none();
nth
}
}

#[inline]
fn last(self) -> Option<I::Item> {
if self.done {
None
} else {
self.iter.last()
}
}

#[inline]
fn count(self) -> usize {
if self.done {
0
} else {
self.iter.count()
}
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
if self.done {
Expand Down
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