Auto merge of #79895 - Kerollmops:slice-group-by, r=m-ou-se

The return of the GroupBy and GroupByMut iterators on slice

According to https://github.com/rust-lang/rfcs/pull/2477#issuecomment-742034372, I am opening this PR again, this time I implemented it in safe Rust only, it is therefore much easier to read and is completely safe.

This PR proposes to add two new methods to the slice, the `group_by` and `group_by_mut`. These two methods provide a way to iterate over non-overlapping sub-slices of a base slice that are separated by the predicate given by the user (e.g. `Partial::eq`, `|a, b| a.abs() < b.abs()`).

```rust
let slice = &[1, 1, 1, 3, 3, 2, 2, 2];

let mut iter = slice.group_by(|a, b| a == b);
assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
assert_eq!(iter.next(), Some(&[3, 3][..]));
assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
assert_eq!(iter.next(), None);
```

[An RFC](https://github.com/rust-lang/rfcs/pull/2477) was open 2 years ago but wasn't necessary.

7 files changed, 330 insertions, 0 deletions

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 54b402faaeb..e6db66ac571 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -139,6 +139,7 @@
 #![feature(try_trait)]
 #![feature(type_alias_impl_trait)]
 #![feature(associated_type_bounds)]
+#![feature(slice_group_by)]
 // Allow testing this library
 
 #[cfg(test)]
diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index 064700fc72c..cb015b94930 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -110,6 +110,8 @@ pub use core::slice::{Chunks, Windows};
 pub use core::slice::{ChunksExact, ChunksExactMut};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::slice::{ChunksMut, Split, SplitMut};
+#[unstable(feature = "slice_group_by", issue = "80552")]
+pub use core::slice::{GroupBy, GroupByMut};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::slice::{Iter, IterMut};
 #[stable(feature = "rchunks", since = "1.31.0")]
diff --git a/library/alloc/tests/lib.rs b/library/alloc/tests/lib.rs
index b56437bd188..cd4174ed400 100644
--- a/library/alloc/tests/lib.rs
+++ b/library/alloc/tests/lib.rs
@@ -20,6 +20,7 @@
 #![feature(iter_map_while)]
 #![feature(int_bits_const)]
 #![feature(vecdeque_binary_search)]
+#![feature(slice_group_by)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};
diff --git a/library/alloc/tests/slice.rs b/library/alloc/tests/slice.rs
index a4f0fb415fb..777c10b1bf7 100644
--- a/library/alloc/tests/slice.rs
+++ b/library/alloc/tests/slice.rs
@@ -1898,3 +1898,61 @@ fn subslice_patterns() {
     m!(&mut v, [..] => ());
     m!(&mut v, [x, .., y] => c!((x, y), (&mut N, &mut N), (&mut N(0), &mut N(4))));
 }
+
+#[test]
+fn test_group_by() {
+    let slice = &[1, 1, 1, 3, 3, 2, 2, 2, 1, 0];
+
+    let mut iter = slice.group_by(|a, b| a == b);
+    assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
+    assert_eq!(iter.next(), Some(&[3, 3][..]));
+    assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
+    assert_eq!(iter.next(), Some(&[1][..]));
+    assert_eq!(iter.next(), Some(&[0][..]));
+    assert_eq!(iter.next(), None);
+
+    let mut iter = slice.group_by(|a, b| a == b);
+    assert_eq!(iter.next_back(), Some(&[0][..]));
+    assert_eq!(iter.next_back(), Some(&[1][..]));
+    assert_eq!(iter.next_back(), Some(&[2, 2, 2][..]));
+    assert_eq!(iter.next_back(), Some(&[3, 3][..]));
+    assert_eq!(iter.next_back(), Some(&[1, 1, 1][..]));
+    assert_eq!(iter.next_back(), None);
+
+    let mut iter = slice.group_by(|a, b| a == b);
+    assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
+    assert_eq!(iter.next_back(), Some(&[0][..]));
+    assert_eq!(iter.next(), Some(&[3, 3][..]));
+    assert_eq!(iter.next_back(), Some(&[1][..]));
+    assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
+    assert_eq!(iter.next_back(), None);
+}
+
+#[test]
+fn test_group_by_mut() {
+    let slice = &mut [1, 1, 1, 3, 3, 2, 2, 2, 1, 0];
+
+    let mut iter = slice.group_by_mut(|a, b| a == b);
+    assert_eq!(iter.next(), Some(&mut [1, 1, 1][..]));
+    assert_eq!(iter.next(), Some(&mut [3, 3][..]));
+    assert_eq!(iter.next(), Some(&mut [2, 2, 2][..]));
+    assert_eq!(iter.next(), Some(&mut [1][..]));
+    assert_eq!(iter.next(), Some(&mut [0][..]));
+    assert_eq!(iter.next(), None);
+
+    let mut iter = slice.group_by_mut(|a, b| a == b);
+    assert_eq!(iter.next_back(), Some(&mut [0][..]));
+    assert_eq!(iter.next_back(), Some(&mut [1][..]));
+    assert_eq!(iter.next_back(), Some(&mut [2, 2, 2][..]));
+    assert_eq!(iter.next_back(), Some(&mut [3, 3][..]));
+    assert_eq!(iter.next_back(), Some(&mut [1, 1, 1][..]));
+    assert_eq!(iter.next_back(), None);
+
+    let mut iter = slice.group_by_mut(|a, b| a == b);
+    assert_eq!(iter.next(), Some(&mut [1, 1, 1][..]));
+    assert_eq!(iter.next_back(), Some(&mut [0][..]));
+    assert_eq!(iter.next(), Some(&mut [3, 3][..]));
+    assert_eq!(iter.next_back(), Some(&mut [1][..]));
+    assert_eq!(iter.next(), Some(&mut [2, 2, 2][..]));
+    assert_eq!(iter.next_back(), None);
+}
diff --git a/library/core/src/slice/iter.rs b/library/core/src/slice/iter.rs
index e373936a6c7..a367b4737db 100644
--- a/library/core/src/slice/iter.rs
+++ b/library/core/src/slice/iter.rs
@@ -1,3 +1,4 @@
+// ignore-tidy-filelength
 //! Definitions of a bunch of iterators for `[T]`.
 
 #[macro_use] // import iterator! and forward_iterator!
@@ -2967,3 +2968,176 @@ unsafe impl<'a, T> TrustedRandomAccess for IterMut<'a, T> {
         false
     }
 }
+
+/// An iterator over slice in (non-overlapping) chunks separated by a predicate.
+///
+/// This struct is created by the [`group_by`] method on [slices].
+///
+/// [`group_by`]: ../../std/primitive.slice.html#method.group_by
+/// [slices]: ../../std/primitive.slice.html
+#[unstable(feature = "slice_group_by", issue = "80552")]
+pub struct GroupBy<'a, T: 'a, P> {
+    slice: &'a [T],
+    predicate: P,
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> GroupBy<'a, T, P> {
+    pub(super) fn new(slice: &'a [T], predicate: P) -> Self {
+        GroupBy { slice, predicate }
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> Iterator for GroupBy<'a, T, P>
+where
+    P: FnMut(&T, &T) -> bool,
+{
+    type Item = &'a [T];
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.slice.is_empty() {
+            None
+        } else {
+            let mut len = 1;
+            let mut iter = self.slice.windows(2);
+            while let Some([l, r]) = iter.next() {
+                if (self.predicate)(l, r) { len += 1 } else { break }
+            }
+            let (head, tail) = self.slice.split_at(len);
+            self.slice = tail;
+            Some(head)
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.slice.is_empty() { (0, Some(0)) } else { (1, Some(self.slice.len())) }
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<Self::Item> {
+        self.next_back()
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> DoubleEndedIterator for GroupBy<'a, T, P>
+where
+    P: FnMut(&T, &T) -> bool,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.slice.is_empty() {
+            None
+        } else {
+            let mut len = 1;
+            let mut iter = self.slice.windows(2);
+            while let Some([l, r]) = iter.next_back() {
+                if (self.predicate)(l, r) { len += 1 } else { break }
+            }
+            let (head, tail) = self.slice.split_at(self.slice.len() - len);
+            self.slice = head;
+            Some(tail)
+        }
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> FusedIterator for GroupBy<'a, T, P> where P: FnMut(&T, &T) -> bool {}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a + fmt::Debug, P> fmt::Debug for GroupBy<'a, T, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("GroupBy").field("slice", &self.slice).finish()
+    }
+}
+
+/// An iterator over slice in (non-overlapping) mutable chunks separated
+/// by a predicate.
+///
+/// This struct is created by the [`group_by_mut`] method on [slices].
+///
+/// [`group_by_mut`]: ../../std/primitive.slice.html#method.group_by_mut
+/// [slices]: ../../std/primitive.slice.html
+#[unstable(feature = "slice_group_by", issue = "80552")]
+pub struct GroupByMut<'a, T: 'a, P> {
+    slice: &'a mut [T],
+    predicate: P,
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> GroupByMut<'a, T, P> {
+    pub(super) fn new(slice: &'a mut [T], predicate: P) -> Self {
+        GroupByMut { slice, predicate }
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> Iterator for GroupByMut<'a, T, P>
+where
+    P: FnMut(&T, &T) -> bool,
+{
+    type Item = &'a mut [T];
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.slice.is_empty() {
+            None
+        } else {
+            let mut len = 1;
+            let mut iter = self.slice.windows(2);
+            while let Some([l, r]) = iter.next() {
+                if (self.predicate)(l, r) { len += 1 } else { break }
+            }
+            let slice = mem::take(&mut self.slice);
+            let (head, tail) = slice.split_at_mut(len);
+            self.slice = tail;
+            Some(head)
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.slice.is_empty() { (0, Some(0)) } else { (1, Some(self.slice.len())) }
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<Self::Item> {
+        self.next_back()
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> DoubleEndedIterator for GroupByMut<'a, T, P>
+where
+    P: FnMut(&T, &T) -> bool,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.slice.is_empty() {
+            None
+        } else {
+            let mut len = 1;
+            let mut iter = self.slice.windows(2);
+            while let Some([l, r]) = iter.next_back() {
+                if (self.predicate)(l, r) { len += 1 } else { break }
+            }
+            let slice = mem::take(&mut self.slice);
+            let (head, tail) = slice.split_at_mut(slice.len() - len);
+            self.slice = head;
+            Some(tail)
+        }
+    }
+}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a, P> FusedIterator for GroupByMut<'a, T, P> where P: FnMut(&T, &T) -> bool {}
+
+#[unstable(feature = "slice_group_by", issue = "80552")]
+impl<'a, T: 'a + fmt::Debug, P> fmt::Debug for GroupByMut<'a, T, P> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("GroupByMut").field("slice", &self.slice).finish()
+    }
+}
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index bb1014332a1..58bf74c8cf4 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -57,6 +57,9 @@ pub use iter::{ArrayChunks, ArrayChunksMut};
 #[unstable(feature = "array_windows", issue = "75027")]
 pub use iter::ArrayWindows;
 
+#[unstable(feature = "slice_group_by", issue = "80552")]
+pub use iter::{GroupBy, GroupByMut};
+
 #[unstable(feature = "split_inclusive", issue = "72360")]
 pub use iter::{SplitInclusive, SplitInclusiveMut};
 
@@ -1208,6 +1211,96 @@ impl<T> [T] {
         RChunksExactMut::new(self, chunk_size)
     }
 
+    /// Returns an iterator over the slice producing non-overlapping runs
+    /// of elements using the predicate to separate them.
+    ///
+    /// The predicate is called on two elements following themselves,
+    /// it means the predicate is called on `slice[0]` and `slice[1]`
+    /// then on `slice[1]` and `slice[2]` and so on.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_group_by)]
+    ///
+    /// let slice = &[1, 1, 1, 3, 3, 2, 2, 2];
+    ///
+    /// let mut iter = slice.group_by(|a, b| a == b);
+    ///
+    /// assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
+    /// assert_eq!(iter.next(), Some(&[3, 3][..]));
+    /// assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    ///
+    /// This method can be used to extract the sorted subslices:
+    ///
+    /// ```
+    /// #![feature(slice_group_by)]
+    ///
+    /// let slice = &[1, 1, 2, 3, 2, 3, 2, 3, 4];
+    ///
+    /// let mut iter = slice.group_by(|a, b| a <= b);
+    ///
+    /// assert_eq!(iter.next(), Some(&[1, 1, 2, 3][..]));
+    /// assert_eq!(iter.next(), Some(&[2, 3][..]));
+    /// assert_eq!(iter.next(), Some(&[2, 3, 4][..]));
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    #[unstable(feature = "slice_group_by", issue = "80552")]
+    #[inline]
+    pub fn group_by<F>(&self, pred: F) -> GroupBy<'_, T, F>
+    where
+        F: FnMut(&T, &T) -> bool,
+    {
+        GroupBy::new(self, pred)
+    }
+
+    /// Returns an iterator over the slice producing non-overlapping mutable
+    /// runs of elements using the predicate to separate them.
+    ///
+    /// The predicate is called on two elements following themselves,
+    /// it means the predicate is called on `slice[0]` and `slice[1]`
+    /// then on `slice[1]` and `slice[2]` and so on.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_group_by)]
+    ///
+    /// let slice = &mut [1, 1, 1, 3, 3, 2, 2, 2];
+    ///
+    /// let mut iter = slice.group_by_mut(|a, b| a == b);
+    ///
+    /// assert_eq!(iter.next(), Some(&mut [1, 1, 1][..]));
+    /// assert_eq!(iter.next(), Some(&mut [3, 3][..]));
+    /// assert_eq!(iter.next(), Some(&mut [2, 2, 2][..]));
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    ///
+    /// This method can be used to extract the sorted subslices:
+    ///
+    /// ```
+    /// #![feature(slice_group_by)]
+    ///
+    /// let slice = &mut [1, 1, 2, 3, 2, 3, 2, 3, 4];
+    ///
+    /// let mut iter = slice.group_by_mut(|a, b| a <= b);
+    ///
+    /// assert_eq!(iter.next(), Some(&mut [1, 1, 2, 3][..]));
+    /// assert_eq!(iter.next(), Some(&mut [2, 3][..]));
+    /// assert_eq!(iter.next(), Some(&mut [2, 3, 4][..]));
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    #[unstable(feature = "slice_group_by", issue = "80552")]
+    #[inline]
+    pub fn group_by_mut<F>(&mut self, pred: F) -> GroupByMut<'_, T, F>
+    where
+        F: FnMut(&T, &T) -> bool,
+    {
+        GroupByMut::new(self, pred)
+    }
+
     /// Divides one slice into two at an index.
     ///
     /// The first will contain all indices from `[0, mid)` (excluding
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index fba3294e0bb..e01aaa4cbf1 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -72,6 +72,7 @@
 #![feature(nonzero_leading_trailing_zeros)]
 #![feature(const_option)]
 #![feature(integer_atomics)]
+#![feature(slice_group_by)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;