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Implement advance_by, advance_back_by for slice::{Iter, IterMut} #87387

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Merged
bors merged 3 commits into from
Jul 31, 2021
Merged

Implement advance_by, advance_back_by for slice::{Iter, IterMut} #87387

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2b89914
Implement slice::{Iter, IterMut}::{advance_by, advance_back_by}
timvermeulen Oct 2, 2020
5e90909
Add tests
timvermeulen Oct 2, 2020
5485f8a
Fix whitespace
timvermeulen Oct 7, 2020
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15 changes: 15 additions & 0 deletions library/core/src/slice/iter/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -185,6 +185,13 @@ macro_rules! iterator {
}
}

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]
fn last(mut self) -> Option<$elem> {
self.next_back()
Expand Down Expand Up @@ -371,6 +378,14 @@ macro_rules! iterator {
Some(next_back_unchecked!(self))
}
}

#[inline]
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) }
}
}

#[stable(feature = "fused", since = "1.26.0")]
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 @@ -134,6 +134,48 @@ 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