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Implement advance_by, advance_back_by for slice::{Iter, IterMut} #77633

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Implement advance_by, advance_back_by for slice::{Iter, IterMut} #77633

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5b834eb
Implement slice::{Iter, IterMut}::{advance_by, advance_back_by}
timvermeulen Oct 2, 2020
3f22d0d
Add tests
timvermeulen Oct 2, 2020
dd58782
Fix whitespace
timvermeulen Oct 7, 2020
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41 changes: 10 additions & 31 deletions library/core/src/slice/iter/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -163,26 163,11 @@ macro_rules! iterator {
}

#[inline]
fn nth(&mut self, n: usize) -> Option<$elem> {
if n >= len!(self) {
// This iterator is now empty.
if mem::size_of::<T>() == 0 {
// We have to do it this way as `ptr` may never be 0, but `end`
// could be (due to wrapping).
self.end = self.ptr.as_ptr();
} else {
// SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
unsafe {
self.ptr = NonNull::new_unchecked(self.end as *mut T);
}
}
return None;
}
// SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
unsafe {
self.post_inc_start(n as isize);
Some(next_unchecked!(self))
}
fn advance_by(&mut self, n: usize) -> Result<(), usize> {
let advance = cmp::min(n, len!(self));
// SAFETY: By construction, `advance` does not exceed `self.len()`.
unsafe { self.post_inc_start(advance as isize) };
if advance == n { Ok(()) } else { Err(advance) }
}

#[inline]
Expand Down Expand Up @@ -359,17 344,11 @@ macro_rules! iterator {
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<$elem> {
if n >= len!(self) {
// This iterator is now empty.
self.end = self.ptr.as_ptr();
return None;
}
// SAFETY: 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))
}
fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
let advance = cmp::min(n, len!(self));
// SAFETY: By construction, `advance` does not exceed `self.len()`.
unsafe { self.pre_dec_end(advance as isize) };
if advance == n { Ok(()) } else { Err(advance) }
}
}

Expand Down
42 changes: 42 additions & 0 deletions library/core/tests/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -122,6 122,48 @@ fn test_partition_point() {
assert_eq!(b.partition_point(|&x| x < 8), 5);
}

#[test]
fn test_iterator_advance_by() {
let v = &[0, 1, 2, 3, 4];

for i in 0..=v.len() {
let mut iter = v.iter();
iter.advance_by(i).unwrap();
assert_eq!(iter.as_slice(), &v[i..]);
}

let mut iter = v.iter();
assert_eq!(iter.advance_by(v.len() 1), Err(v.len()));
assert_eq!(iter.as_slice(), &[]);

let mut iter = v.iter();
iter.advance_by(3).unwrap();
assert_eq!(iter.as_slice(), &v[3..]);
iter.advance_by(2).unwrap();
assert_eq!(iter.as_slice(), &[]);
}

#[test]
fn test_iterator_advance_back_by() {
let v = &[0, 1, 2, 3, 4];

for i in 0..=v.len() {
let mut iter = v.iter();
iter.advance_back_by(i).unwrap();
assert_eq!(iter.as_slice(), &v[..v.len() - i]);
}

let mut iter = v.iter();
assert_eq!(iter.advance_back_by(v.len() 1), Err(v.len()));
assert_eq!(iter.as_slice(), &[]);

let mut iter = v.iter();
iter.advance_back_by(3).unwrap();
assert_eq!(iter.as_slice(), &v[..v.len() - 3]);
iter.advance_back_by(2).unwrap();
assert_eq!(iter.as_slice(), &[]);
}

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