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Auto merge of #60772 - timvermeulen:slice_iter_nth_back, r=<try>
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Implement nth_back for slice::{Iter, IterMut}

Part of #54054.

I implemented `nth_back` as straightforwardly as I could, and then slightly changed `nth` to match `nth_back`. I believe I did so correctly, but please double-check 🙂

I also added the helper methods `zst_shrink`, `next_unchecked`, and `next_back_unchecked` to get rid of some duplicated code. These changes hopefully make this code easier to understand for new contributors like me.

I noticed the `is_empty!` and `len!` macros which sole purpose seems to be inlining, according to the comment right above them, but the `is_empty` and `len` methods are already marked with `#[inline(always)]`. Does that mean we could replace these macros with method calls, without affecting anything? I'd love to get rid of them.
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bors committed May 25, 2019
2 parents 315ab95 + 97a6c93 commit 01fe51e
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Showing 2 changed files with 68 additions and 21 deletions.
76 changes: 55 additions & 21 deletions src/libcore/slice/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -3020,6 +3020,28 @@ macro_rules! iterator {
{$( $mut_:tt )*},
{$($extra:tt)*}
) => {
// Returns the first element and moves the start of the iterator forwards by 1.
// Greatly improves performance compared to an inlined function. The iterator
// must not be empty.
macro_rules! next_unchecked {
($self: ident) => {& $( $mut_ )* *$self.post_inc_start(1)}
}

// Returns the last element and moves the end of the iterator backwards by 1.
// Greatly improves performance compared to an inlined function. The iterator
// must not be empty.
macro_rules! next_back_unchecked {
($self: ident) => {& $( $mut_ )* *$self.pre_dec_end(1)}
}

// Shrinks the iterator when T is a ZST, by moving the end of the iterator
// backwards by `n`. `n` must not exceed `self.len()`.
macro_rules! zst_shrink {
($self: ident, $n: ident) => {
$self.end = ($self.end as * $raw_mut u8).wrapping_offset(-$n) as * $raw_mut T;
}
}

impl<'a, T> $name<'a, T> {
// Helper function for creating a slice from the iterator.
#[inline(always)]
Expand All @@ -3029,12 +3051,11 @@ macro_rules! iterator {

// Helper function for moving the start of the iterator forwards by `offset` elements,
// returning the old start.
// Unsafe because the offset must be in-bounds or one-past-the-end.
// Unsafe because the offset must not exceed `self.len()`.
#[inline(always)]
unsafe fn post_inc_start(&mut self, offset: isize) -> * $raw_mut T {
if mem::size_of::<T>() == 0 {
// This is *reducing* the length. `ptr` never changes with ZST.
self.end = (self.end as * $raw_mut u8).wrapping_offset(-offset) as * $raw_mut T;
zst_shrink!(self, offset);
self.ptr
} else {
let old = self.ptr;
Expand All @@ -3045,11 +3066,11 @@ macro_rules! iterator {

// Helper function for moving the end of the iterator backwards by `offset` elements,
// returning the new end.
// Unsafe because the offset must be in-bounds or one-past-the-end.
// Unsafe because the offset must not exceed `self.len()`.
#[inline(always)]
unsafe fn pre_dec_end(&mut self, offset: isize) -> * $raw_mut T {
if mem::size_of::<T>() == 0 {
self.end = (self.end as * $raw_mut u8).wrapping_offset(-offset) as * $raw_mut T;
zst_shrink!(self, offset);
self.ptr
} else {
self.end = self.end.offset(-offset);
Expand Down Expand Up @@ -3086,7 +3107,7 @@ macro_rules! iterator {
if is_empty!(self) {
None
} else {
Some(& $( $mut_ )* *self.post_inc_start(1))
Some(next_unchecked!(self))
}
}
}
Expand Down Expand Up @@ -3115,11 +3136,10 @@ macro_rules! iterator {
}
return None;
}
// We are in bounds. `offset` does the right thing even for ZSTs.
// We are in bounds. `post_inc_start` does the right thing even for ZSTs.
unsafe {
let elem = Some(& $( $mut_ )* *self.ptr.add(n));
self.post_inc_start((n as isize).wrapping_add(1));
elem
self.post_inc_start(n as isize);
Some(next_unchecked!(self))
}
}

Expand All @@ -3136,13 +3156,13 @@ macro_rules! iterator {
let mut accum = init;
unsafe {
while len!(self) >= 4 {
accum = f(accum, & $( $mut_ )* *self.post_inc_start(1))?;
accum = f(accum, & $( $mut_ )* *self.post_inc_start(1))?;
accum = f(accum, & $( $mut_ )* *self.post_inc_start(1))?;
accum = f(accum, & $( $mut_ )* *self.post_inc_start(1))?;
accum = f(accum, next_unchecked!(self))?;
accum = f(accum, next_unchecked!(self))?;
accum = f(accum, next_unchecked!(self))?;
accum = f(accum, next_unchecked!(self))?;
}
while !is_empty!(self) {
accum = f(accum, & $( $mut_ )* *self.post_inc_start(1))?;
accum = f(accum, next_unchecked!(self))?;
}
}
Try::from_ok(accum)
Expand Down Expand Up @@ -3213,11 +3233,25 @@ macro_rules! iterator {
if is_empty!(self) {
None
} else {
Some(& $( $mut_ )* *self.pre_dec_end(1))
Some(next_back_unchecked!(self))
}
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<$elem> {
if n >= len!(self) {
// This iterator is now empty.
self.end = self.ptr;
return None;
}
// We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
unsafe {
self.pre_dec_end(n as isize);
Some(next_back_unchecked!(self))
}
}

#[inline]
fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
Expand All @@ -3226,14 +3260,14 @@ macro_rules! iterator {
let mut accum = init;
unsafe {
while len!(self) >= 4 {
accum = f(accum, & $( $mut_ )* *self.pre_dec_end(1))?;
accum = f(accum, & $( $mut_ )* *self.pre_dec_end(1))?;
accum = f(accum, & $( $mut_ )* *self.pre_dec_end(1))?;
accum = f(accum, & $( $mut_ )* *self.pre_dec_end(1))?;
accum = f(accum, next_back_unchecked!(self))?;
accum = f(accum, next_back_unchecked!(self))?;
accum = f(accum, next_back_unchecked!(self))?;
accum = f(accum, next_back_unchecked!(self))?;
}
// inlining is_empty everywhere makes a huge performance difference
while !is_empty!(self) {
accum = f(accum, & $( $mut_ )* *self.pre_dec_end(1))?;
accum = f(accum, next_back_unchecked!(self))?;
}
}
Try::from_ok(accum)
Expand Down
13 changes: 13 additions & 0 deletions src/libcore/tests/slice.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,19 @@ fn test_iterator_nth() {
assert_eq!(iter.nth(1).unwrap(), &v[4]);
}

#[test]
fn test_iterator_nth_back() {
let v: &[_] = &[0, 1, 2, 3, 4];
for i in 0..v.len() {
assert_eq!(v.iter().nth_back(i).unwrap(), &v[v.len() - i - 1]);
}
assert_eq!(v.iter().nth_back(v.len()), None);

let mut iter = v.iter();
assert_eq!(iter.nth_back(2).unwrap(), &v[2]);
assert_eq!(iter.nth_back(1).unwrap(), &v[0]);
}

#[test]
fn test_iterator_last() {
let v: &[_] = &[0, 1, 2, 3, 4];
Expand Down

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