slice: deprecate Dedup, shim to the equivalent stdlib slices.Compact

There is a slight semantic change: elements beyond the end of the compacted
slice are zeroed in slices.Compact. While this is a semantic change, it should
be a strict improvement since people don't generally intend to hide old values
(and pin pointers, from the GC's POV) in the unused part of a backing array.

slice/slice.go

@@ -1,6 +1,8 @@
 // Package slice implements some useful functions for slices.
 package slice
 
+import "slices"
+
 // Partition rearranges the elements of vs in-place so that all the elements v
 // for which keep(v) is true precede all those for which it is false.  It
 // returns the prefix of vs that contains the kept elements.  It takes time
@@ -69,57 +71,11 @@ func Partition[T any](vs []T, keep func(T) bool) []T {
 
 // Dedup rearranges the elements of vs in-place to deduplicate consecutive runs
 // of identical elements.  It returns a prefix of vs that contains the first
-// element of each run found, in their original relative order.  It takes time
-// proportional to len(vs) and does not allocate storage outside the slice.
+// element of each run found.
 //
-// The returned slice will contain (non-consecutive) duplicates only if
-// vs is not in sorted order at input. If vs is sorted at input (in either
-// direction), the elements of the prefix are exactly the unique first elements
-// of the input.
+// Deprecated: Use the equivalent [slices.Compact] instead.
 func Dedup[T comparable](vs []T) []T {
-	if len(vs) <= 1 {
-		return vs
-	}
-
-	// Setup:
-	//   i  : the location of the first element of the next run
-	//   j  : runs forward from i looking for the end of the run
-	//
-	i, j := 0, 1
-	for {
-		// Scan forward from i for an element different from vs[i].
-		for j < len(vs) && vs[i] == vs[j] {
-			j++
-		}
-
-		// If there are no further distinct elements, we're done.  The item at
-		// position i is the beginning of the last run in the slice.
-		if j == len(vs) {
-			return vs[:i+1]
-		}
-
-		// Reaching here, the slice looks like this:
-		//
-		//   [a b c d d d d d d d e ? ? ?]
-		//    0     i             j       n
-		//
-		// where a, b, c, d are distinct from their neighbors.
-
-		// Otherwise, we have found the first item of a new run at j.
-		// Move i forward to the next slot and (if necessary) swap vs[j] into it.
-		i++
-		if j > i {
-			// A swap is unnecessary (though harmless) if j is already the next slot.
-			vs[i], vs[j] = vs[j], vs[i]
-		}
-		j++
-
-		// Now:
-		//               swapped
-		//            v-----------v
-		//   [a b c d e d d d d d d ? ? ?]
-		//    0       i             j     n
-	}
+	return slices.Compact(vs)
 }
 
 // Reverse reverses the contents of vs in-place.

slice/slice_test.go

@@ -62,27 +62,6 @@ func TestPartition(t *testing.T) {
 	}
 }
 
-func TestDedup(t *testing.T) {
-	for _, test := range []testCase[int]{
-		{"Nil", nil, nil, nil},
-		{"Empty", []int{}, nil, nil},
-		{"One", []int{100}, []int{100}, nil},
-		{"NoRuns", []int{1, 3, 2, 4}, []int{1, 3, 2, 4}, nil},
-		{"Single", []int{5, 5, 5, 5, 5}, []int{5}, nil},
-		{"Two", []int{2, 2, 2, 3, 3, 3}, []int{2, 3}, nil},
-		{"Repeat", []int{1, 3, 3, 1}, []int{1, 3, 1}, nil},
-		{"NoRunsAsc", []int{0, 1, 2, 3, 4}, []int{0, 1, 2, 3, 4}, nil},
-		{"NoRunsDesc", []int{10, 9, 8, 7}, []int{10, 9, 8, 7}, nil},
-		{"RunsAsc", []int{0, 1, 1, 1, 2, 2, 3}, []int{0, 1, 2, 3}, nil},
-		{"RunsDesc", []int{5, 5, 5, 3, 3, 1, 1, 0}, []int{5, 3, 1, 0}, nil},
-
-		// Runs:           a  b---  c  b---  d---  e---------
-		{"Unsorted", []int{1, 0, 0, 9, 0, 0, 3, 3, 2, 2, 2, 2}, []int{1, 0, 9, 0, 3, 2}, nil},
-	} {
-		t.Run(test.desc, test.dedup)
-	}
-}
-
 func TestReverse(t *testing.T) {
 	tests := []struct {
 		name  string
@@ -542,20 +521,6 @@ func (tc *testCase[T]) partition(t *testing.T) {
 	}
 }
 
-func (tc *testCase[T]) dedup(t *testing.T) {
-	t.Helper()
-
-	cp := copyOf(tc.input)
-	t.Logf("Input: %+v", cp)
-
-	got := slice.Dedup(cp)
-	t.Logf("After dedup: %+v ~ %+v", got, cp[len(got):])
-	diff := cmp.Diff(tc.want, got, cmpopts.EquateEmpty())
-	if diff != "" {
-		t.Errorf("Dedup result (-want, +got)\n%s", diff)
-	}
-}
-
 func copyOf[T any](vs []T) []T {
 	out := make([]T, len(vs))
 	copy(out, vs)