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Improve performance characteristics of Vector iterators to better match Vec iterators #104

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Improve performance characteristics of Vector iterators to better match Vec iterators #104

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586eda8
Optimized versions of len, count, and nth for Vector iterators
jessegrosjean Sep 24, 2019
f750850
More work optimizing Vector Iterators to for len, count, nth, and nth…
jessegrosjean Sep 24, 2019
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155 changes: 149 additions & 6 deletions src/vector/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1881,8 +1881,21 @@ impl<'a, A: Clone> Iterator for Iter<'a, A> {
}

fn size_hint(&self) -> (usize, Option<usize>) {
let remaining = self.back_index - self.front_index;
(remaining, Some(remaining))
let len = self.len();
(len, Some(len))
}

fn count(self) -> usize {
self.len()
}

fn nth(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
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I don't believe this shortcut is correct. It introduces a bug.

Before this change, this works:

let vec = vector![1, 2, 3];
let mut iter = vec.iter();
assert_eq!(iter.nth(3), None);
assert_eq!(iter.next(), None);

Afterwards, it panics because the call to next() returns Some(1).

Since the implementation of next() already has a check for exhaustion, I would remove this if altogether and replace it with its else case.

} else {
self.front_index += n;
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What if this overflows?

self.next()
}
}
}

Expand All @@ -1899,9 +1912,22 @@ impl<'a, A: Clone> DoubleEndedIterator for Iter<'a, A> {
let focus: &'a mut Focus<'a, A> = unsafe { &mut *(&mut self.focus as *mut _) };
focus.get(self.back_index)
}

fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.back_index -= n;
self.next_back()
}
}
}

impl<'a, A: Clone> ExactSizeIterator for Iter<'a, A> {}
impl<'a, A: Clone> ExactSizeIterator for Iter<'a, A> {
fn len(&self) -> usize {
self.back_index - self.front_index
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What if this underflows?

Suggested change
self.back_index - self.front_index
self.back_index.saturating_sub(self.front_index)

}
}

impl<'a, A: Clone> FusedIterator for Iter<'a, A> {}

Expand Down Expand Up @@ -1963,8 +1989,21 @@ where
}

fn size_hint(&self) -> (usize, Option<usize>) {
let remaining = self.back_index - self.front_index;
(remaining, Some(remaining))
let len = self.len();
(len, Some(len))
}

fn count(self) -> usize {
self.len()
}

fn nth(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.front_index += n;
self.next()
}
}
}

Expand All @@ -1984,9 +2023,22 @@ where
let focus: &'a mut FocusMut<'a, A> = unsafe { &mut *(&mut self.focus as *mut _) };
focus.get_mut(self.back_index)
}

fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.back_index -= n;
self.next_back()
}
}
}

impl<'a, A: Clone> ExactSizeIterator for IterMut<'a, A> {}
impl<'a, A: Clone> ExactSizeIterator for IterMut<'a, A> {
fn len(&self) -> usize {
self.back_index - self.front_index
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Suggested change
self.back_index - self.front_index
self.back_index.saturing_sub(self.front_index)

}
}

impl<'a, A: Clone> FusedIterator for IterMut<'a, A> {}

Expand Down Expand Up @@ -2089,6 +2141,24 @@ impl<'a, A: Clone> Iterator for Chunks<'a, A> {
self.front_index = range.end;
Some(value)
}

fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.len();
(len, Some(len))
}

fn count(self) -> usize {
self.len()
}

fn nth(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.front_index += n;
self.next()
}
}
}

impl<'a, A: Clone> DoubleEndedIterator for Chunks<'a, A> {
Expand All @@ -2106,6 +2176,21 @@ impl<'a, A: Clone> DoubleEndedIterator for Chunks<'a, A> {
self.back_index = range.start;
Some(value)
}

fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.back_index -= n;
self.next_back()
}
}
}

impl<'a, A: Clone> ExactSizeIterator for Chunks<'a, A> {
fn len(&self) -> usize {
self.back_index - self.front_index
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Suggested change
self.back_index - self.front_index
self.back_index.saturing_sub(self.front_index)

}
}

impl<'a, A: Clone> FusedIterator for Chunks<'a, A> {}
Expand Down Expand Up @@ -2148,6 +2233,24 @@ impl<'a, A: Clone> Iterator for ChunksMut<'a, A> {
self.front_index = range.end;
Some(value)
}

fn size_hint(&self) -> (usize, Option<usize>) {
let len = self.len();
(len, Some(len))
}

fn count(self) -> usize {
self.len()
}

fn nth(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.front_index += n;
self.next()
}
}
}

impl<'a, A: Clone> DoubleEndedIterator for ChunksMut<'a, A> {
Expand All @@ -2165,6 +2268,21 @@ impl<'a, A: Clone> DoubleEndedIterator for ChunksMut<'a, A> {
self.back_index = range.start;
Some(value)
}

fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
if n >= self.len() {
None
} else {
self.back_index -= n;
self.next_back()
}
}
}

impl<'a, A: Clone> ExactSizeIterator for ChunksMut<'a, A> {
fn len(&self) -> usize {
self.back_index - self.front_index
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Suggested change
self.back_index - self.front_index
self.back_index.saturing_sub(self.front_index)

}
}

impl<'a, A: Clone> FusedIterator for ChunksMut<'a, A> {}
Expand Down Expand Up @@ -2635,6 +2753,31 @@ mod test {
assert_eq!(&vec[index], item);
}
assert_eq!(vec.len(), seq.len());
assert_eq!(vec.iter().count(), seq.iter().count());
}

#[test]
#[allow(clippy::iter_nth)]
fn iter_nth(ref vec in vec(i32::ANY, 0..1000)) {
let seq: Vector<i32> = Vector::from_iter(vec.iter().cloned());
assert_eq!(vec.iter().nth(0), seq.iter().nth(0));
assert_eq!(vec.iter().nth(1), seq.iter().nth(1));
let last_index = seq.len().checked_sub(1).unwrap_or(0);
assert_eq!(vec.iter().nth(last_index), seq.iter().nth(last_index));
let last_index_plus_one = seq.len();
assert_eq!(vec.iter().nth(last_index_plus_one), seq.iter().nth(last_index_plus_one));
}

#[test]
#[allow(clippy::iter_nth)]
fn iter_nth_back(ref vec in vec(i32::ANY, 0..1000)) {
let seq: Vector<i32> = Vector::from_iter(vec.iter().cloned());
assert_eq!(vec.iter().nth_back(0), seq.iter().nth_back(0));
assert_eq!(vec.iter().nth_back(1), seq.iter().nth_back(1));
let last_index = seq.len().checked_sub(1).unwrap_or(0);
assert_eq!(vec.iter().nth_back(last_index), seq.iter().nth_back(last_index));
let last_index_plus_one = seq.len();
assert_eq!(vec.iter().nth_back(last_index_plus_one), seq.iter().nth_back(last_index_plus_one));
}

#[test]
Expand Down