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Implement nth_back for RChunks(Exact)(Mut) #60555

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merged 1 commit into from
May 29, 2019
Merged

Implement nth_back for RChunks(Exact)(Mut) #60555

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72 changes: 72 additions & 0 deletions src/libcore/slice/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4643,6 +4643,23 @@ impl<'a, T> DoubleEndedIterator for RChunks<'a, T> {
Some(fst)
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
let len = self.len();
if n >= len {
self.v = &[];
None
} else {
// can't underflow because `n < len`
let offset_from_end = (len - 1 - n) * self.chunk_size;
let end = self.v.len() - offset_from_end;
let start = end.saturating_sub(self.chunk_size);
let nth_back = &self.v[start..end];
self.v = &self.v[end..];
Some(nth_back)
}
}
}

#[stable(feature = "rchunks", since = "1.31.0")]
Expand Down Expand Up @@ -4768,6 +4785,24 @@ impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
Some(head)
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
let len = self.len();
if n >= len {
self.v = &mut [];
None
} else {
// can't underflow because `n < len`
let offset_from_end = (len - 1 - n) * self.chunk_size;
let end = self.v.len() - offset_from_end;
let start = end.saturating_sub(self.chunk_size);
let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
let (_, nth_back) = tmp.split_at_mut(start);
self.v = tail;
Some(nth_back)
}
}
}

#[stable(feature = "rchunks", since = "1.31.0")]
Expand Down Expand Up @@ -4892,6 +4927,24 @@ impl<'a, T> DoubleEndedIterator for RChunksExact<'a, T> {
Some(fst)
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
let len = self.len();
if n >= len {
self.v = &[];
None
} else {
// now that we know that `n` corresponds to a chunk,
// none of these operations can underflow/overflow
let offset = (len - n) * self.chunk_size;
let start = self.v.len() - offset;
let end = start + self.chunk_size;
let nth_back = &self.v[start..end];
self.v = &self.v[end..];
Some(nth_back)
}
}
}

#[stable(feature = "rchunks", since = "1.31.0")]
Expand Down Expand Up @@ -5010,6 +5063,25 @@ impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
Some(head)
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
let len = self.len();
if n >= len {
self.v = &mut [];
None
} else {
// now that we know that `n` corresponds to a chunk,
// none of these operations can underflow/overflow
let offset = (len - n) * self.chunk_size;
let start = self.v.len() - offset;
let end = start + self.chunk_size;
let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
let (_, nth_back) = tmp.split_at_mut(start);
self.v = tail;
Some(nth_back)
}
}
}

#[stable(feature = "rchunks", since = "1.31.0")]
Expand Down
52 changes: 52 additions & 0 deletions src/libcore/tests/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -356,6 +356,19 @@ fn test_rchunks_nth() {
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_nth_back() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
let mut c = v.rchunks(2);
assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
assert_eq!(c.next_back().unwrap(), &[4, 5]);

let v2: &[i32] = &[0, 1, 2, 3, 4];
let mut c2 = v2.rchunks(3);
assert_eq!(c2.nth_back(1).unwrap(), &[2, 3, 4]);
assert_eq!(c2.next_back(), None);
}

#[test]
fn test_rchunks_last() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
Expand Down Expand Up @@ -407,6 +420,19 @@ fn test_rchunks_mut_nth() {
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_mut_nth_back() {
let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
let mut c = v.rchunks_mut(2);
assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
assert_eq!(c.next_back().unwrap(), &[4, 5]);

let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
let mut c2 = v2.rchunks_mut(3);
assert_eq!(c2.nth_back(1).unwrap(), &[2, 3, 4]);
assert_eq!(c2.next_back(), None);
}

#[test]
fn test_rchunks_mut_last() {
let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
Expand Down Expand Up @@ -460,6 +486,19 @@ fn test_rchunks_exact_nth() {
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_exact_nth_back() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
let mut c = v.rchunks_exact(2);
assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
assert_eq!(c.next_back().unwrap(), &[4, 5]);

let v2: &[i32] = &[0, 1, 2, 3, 4, 5, 6];
let mut c2 = v2.rchunks_exact(3);
assert_eq!(c2.nth_back(1).unwrap(), &[4, 5, 6]);
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_exact_last() {
let v: &[i32] = &[0, 1, 2, 3, 4, 5];
Expand Down Expand Up @@ -518,6 +557,19 @@ fn test_rchunks_exact_mut_nth() {
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_exact_mut_nth_back() {
let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
let mut c = v.rchunks_exact_mut(2);
assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
assert_eq!(c.next_back().unwrap(), &[4, 5]);

let v2: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
let mut c2 = v2.rchunks_exact_mut(3);
assert_eq!(c2.nth_back(1).unwrap(), &[4, 5, 6]);
assert_eq!(c2.next(), None);
}

#[test]
fn test_rchunks_exact_mut_last() {
let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
Expand Down