Introduce the partition_dedup/by/by_key methods for slices

Author: Kerollmops

src/liballoc/vec.rs

@@ -947,10 +947,9 @@ impl<T> Vec<T> {
     /// Removes all but the first of consecutive elements in the vector satisfying a given equality
     /// relation.
     ///
-    /// The `same_bucket` function is passed references to two elements from the vector, and
-    /// returns `true` if the elements compare equal, or `false` if they do not. The elements are
-    /// passed in opposite order from their order in the vector, so if `same_bucket(a, b)` returns
-    /// `true`, `a` is removed.
+    /// The `same_bucket` function is passed references to two elements from the vector and
+    /// must determine if the elements compare equal. The elements are passed in opposite order
+    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
     ///
     /// If the vector is sorted, this removes all duplicates.
     ///
@@ -1533,7 +1532,8 @@ impl<'a> Drop for SetLenOnDrop<'a> {
 }
 
 impl<T: PartialEq> Vec<T> {
-    /// Removes consecutive repeated elements in the vector.
+    /// Removes consecutive repeated elements in the vector according to the
+    /// [`PartialEq`] trait implementation.
     ///
     /// If the vector is sorted, this removes all duplicates.
     ///

src/libcore/slice/mod.rs

@@ -1402,6 +1402,178 @@ impl<T> [T] {
         sort::quicksort(self, |a, b| f(a).lt(&f(b)));
     }
 
+    /// Moves all consecutive repeated elements to the end of the slice according to the
+    /// [`PartialEq`] trait implementation.
+    ///
+    /// Returns two slices. The first contains no consecutive repeated elements.
+    /// The second contains all the duplicates in no specified order.
+    ///
+    /// If the slice is sorted, the first returned slice contains no duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_partition_dedup)]
+    ///
+    /// let mut slice = [1, 2, 2, 3, 3, 2, 1, 1];
+    ///
+    /// let (dedup, duplicates) = slice.partition_dedup();
+    ///
+    /// assert_eq!(dedup, [1, 2, 3, 2, 1]);
+    /// assert_eq!(duplicates, [2, 3, 1]);
+    /// ```
+    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
+    #[inline]
+    pub fn partition_dedup(&mut self) -> (&mut [T], &mut [T])
+        where T: PartialEq
+    {
+        self.partition_dedup_by(|a, b| a == b)
+    }
+
+    /// Moves all but the first of consecutive elements to the end of the slice satisfying
+    /// a given equality relation.
+    ///
+    /// Returns two slices. The first contains no consecutive repeated elements.
+    /// The second contains all the duplicates in no specified order.
+    ///
+    /// The `same_bucket` function is passed references to two elements from the slice and
+    /// must determine if the elements compare equal. The elements are passed in opposite order
+    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is moved
+    /// at the end of the slice.
+    ///
+    /// If the slice is sorted, the first returned slice contains no duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_partition_dedup)]
+    ///
+    /// let mut slice = ["foo", "Foo", "BAZ", "Bar", "bar", "baz", "BAZ"];
+    ///
+    /// let (dedup, duplicates) = slice.partition_dedup_by(|a, b| a.eq_ignore_ascii_case(b));
+    ///
+    /// assert_eq!(dedup, ["foo", "BAZ", "Bar", "baz"]);
+    /// assert_eq!(duplicates, ["bar", "Foo", "BAZ"]);
+    /// ```
+    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
+    #[inline]
+    pub fn partition_dedup_by<F>(&mut self, mut same_bucket: F) -> (&mut [T], &mut [T])
+        where F: FnMut(&mut T, &mut T) -> bool
+    {
+        // Although we have a mutable reference to `self`, we cannot make
+        // *arbitrary* changes. The `same_bucket` calls could panic, so we
+        // must ensure that the slice is in a valid state at all times.
+        //
+        // The way that we handle this is by using swaps; we iterate
+        // over all the elements, swapping as we go so that at the end
+        // the elements we wish to keep are in the front, and those we
+        // wish to reject are at the back. We can then split the slice.
+        // This operation is still O(n).
+        //
+        // Example: We start in this state, where `r` represents "next
+        // read" and `w` represents "next_write`.
+        //
+        //           r
+        //     +---+---+---+---+---+---+
+        //     | 0 | 1 | 1 | 2 | 3 | 3 |
+        //     +---+---+---+---+---+---+
+        //           w
+        //
+        // Comparing self[r] against self[w-1], this is not a duplicate, so
+        // we swap self[r] and self[w] (no effect as r==w) and then increment both
+        // r and w, leaving us with:
+        //
+        //               r
+        //     +---+---+---+---+---+---+
+        //     | 0 | 1 | 1 | 2 | 3 | 3 |
+        //     +---+---+---+---+---+---+
+        //               w
+        //
+        // Comparing self[r] against self[w-1], this value is a duplicate,
+        // so we increment `r` but leave everything else unchanged:
+        //
+        //                   r
+        //     +---+---+---+---+---+---+
+        //     | 0 | 1 | 1 | 2 | 3 | 3 |
+        //     +---+---+---+---+---+---+
+        //               w
+        //
+        // Comparing self[r] against self[w-1], this is not a duplicate,
+        // so swap self[r] and self[w] and advance r and w:
+        //
+        //                       r
+        //     +---+---+---+---+---+---+
+        //     | 0 | 1 | 2 | 1 | 3 | 3 |
+        //     +---+---+---+---+---+---+
+        //                   w
+        //
+        // Not a duplicate, repeat:
+        //
+        //                           r
+        //     +---+---+---+---+---+---+
+        //     | 0 | 1 | 2 | 3 | 1 | 3 |
+        //     +---+---+---+---+---+---+
+        //                       w
+        //
+        // Duplicate, advance r. End of slice. Split at w.
+
+        let len = self.len();
+        if len <= 1 {
+            return (self, &mut [])
+        }
+
+        let ptr = self.as_mut_ptr();
+        let mut next_read: usize = 1;
+        let mut next_write: usize = 1;
+
+        unsafe {
+            // Avoid bounds checks by using raw pointers.
+            while next_read < len {
+                let ptr_read = ptr.add(next_read);
+                let prev_ptr_write = ptr.add(next_write - 1);
+                if !same_bucket(&mut *ptr_read, &mut *prev_ptr_write) {
+                    if next_read != next_write {
+                        let ptr_write = prev_ptr_write.offset(1);
+                        mem::swap(&mut *ptr_read, &mut *ptr_write);
+                    }
+                    next_write += 1;
+                }
+                next_read += 1;
+            }
+        }
+
+        self.split_at_mut(next_write)
+    }
+
+    /// Moves all but the first of consecutive elements to the end of the slice that resolve
+    /// to the same key.
+    ///
+    /// Returns two slices. The first contains no consecutive repeated elements.
+    /// The second contains all the duplicates in no specified order.
+    ///
+    /// If the slice is sorted, the first returned slice contains no duplicates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_partition_dedup)]
+    ///
+    /// let mut slice = [10, 20, 21, 30, 30, 20, 11, 13];
+    ///
+    /// let (dedup, duplicates) = slice.partition_dedup_by_key(|i| *i / 10);
+    ///
+    /// assert_eq!(dedup, [10, 20, 30, 20, 11]);
+    /// assert_eq!(duplicates, [21, 30, 13]);
+    /// ```
+    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
+    #[inline]
+    pub fn partition_dedup_by_key<K, F>(&mut self, mut key: F) -> (&mut [T], &mut [T])
+        where F: FnMut(&mut T) -> K,
+              K: PartialEq,
+    {
+        self.partition_dedup_by(|a, b| key(a) == key(b))
+    }
+
     /// Rotates the slice in-place such that the first `mid` elements of the
     /// slice move to the end while the last `self.len() - mid` elements move to
     /// the front. After calling `rotate_left`, the element previously at index

src/libcore/tests/lib.rs

@@ -39,6 +39,7 @@
 #![feature(inner_deref)]
 #![feature(slice_internals)]
 #![feature(option_replace)]
+#![feature(slice_partition_dedup)]
 #![feature(copy_within)]
 
 extern crate core;

src/libcore/tests/slice.rs

@@ -1001,6 +1001,65 @@ fn test_align_to_empty_mid() {
     }
 }
 
+#[test]
+fn test_slice_partition_dedup_by() {
+    let mut slice: [i32; 9] = [1, -1, 2, 3, 1, -5, 5, -2, 2];
+
+    let (dedup, duplicates) = slice.partition_dedup_by(|a, b| a.abs() == b.abs());
+
+    assert_eq!(dedup, [1, 2, 3, 1, -5, -2]);
+    assert_eq!(duplicates, [5, -1, 2]);
+}
+
+#[test]
+fn test_slice_partition_dedup_empty() {
+    let mut slice: [i32; 0] = [];
+
+    let (dedup, duplicates) = slice.partition_dedup();
+
+    assert_eq!(dedup, []);
+    assert_eq!(duplicates, []);
+}
+
+#[test]
+fn test_slice_partition_dedup_one() {
+    let mut slice = [12];
+
+    let (dedup, duplicates) = slice.partition_dedup();
+
+    assert_eq!(dedup, [12]);
+    assert_eq!(duplicates, []);
+}
+
+#[test]
+fn test_slice_partition_dedup_multiple_ident() {
+    let mut slice = [12, 12, 12, 12, 12, 11, 11, 11, 11, 11, 11];
+
+    let (dedup, duplicates) = slice.partition_dedup();
+
+    assert_eq!(dedup, [12, 11]);
+    assert_eq!(duplicates, [12, 12, 12, 12, 11, 11, 11, 11, 11]);
+}
+
+#[test]
+fn test_slice_partition_dedup_partialeq() {
+    #[derive(Debug)]
+    struct Foo(i32, i32);
+
+    impl PartialEq for Foo {
+        fn eq(&self, other: &Foo) -> bool {
+            self.0 == other.0
+        }
+    }
+
+    let mut slice = [Foo(0, 1), Foo(0, 5), Foo(1, 7), Foo(1, 9)];
+
+    let (dedup, duplicates) = slice.partition_dedup();
+
+    assert_eq!(dedup, [Foo(0, 1), Foo(1, 7)]);
+    assert_eq!(duplicates, [Foo(0, 5), Foo(1, 9)]);
+}
+
 #[test]
 fn test_copy_within() {
     // Start to end, with a RangeTo.