slices: add DeleteFunc

DeleteFunc was added to the standard library for the 1.21 release.
Add it here in x/exp for people still using earlier releases.

For golang/go#54768
Fixes golang/go#61327

Change-Id: I3c37051c289f46b0068bc1ee5da610149c59cd22
Reviewed-on: https://go-review.googlesource.com/c/exp/+/509236
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>

slices/slices.go

@@ -168,6 +168,30 @@ func Delete[S ~[]E, E any](s S, i, j int) S {
 	return append(s[:i], s[j:]...)
 }
 
+// DeleteFunc removes any elements from s for which del returns true,
+// returning the modified slice.
+// When DeleteFunc removes m elements, it might not modify the elements
+// s[len(s)-m:len(s)]. If those elements contain pointers you might consider
+// zeroing those elements so that objects they reference can be garbage
+// collected.
+func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
+	// Don't start copying elements until we find one to delete.
+	for i, v := range s {
+		if del(v) {
+			j := i
+			for i++; i < len(s); i++ {
+				v = s[i]
+				if !del(v) {
+					s[j] = v
+					j++
+				}
+			}
+			return s[:j]
+		}
+	}
+	return s
+}
+
 // Replace replaces the elements s[i:j] by the given v, and returns the
 // modified slice. Replace panics if s[i:j] is not a valid slice of s.
 func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {

slices/slices_test.go

@@ -498,6 +498,52 @@ func TestDelete(t *testing.T) {
 	}
 }
 
+var deleteFuncTests = []struct {
+	s    []int
+	fn   func(int) bool
+	want []int
+}{
+	{
+		nil,
+		func(int) bool { return true },
+		nil,
+	},
+	{
+		[]int{1, 2, 3},
+		func(int) bool { return true },
+		nil,
+	},
+	{
+		[]int{1, 2, 3},
+		func(int) bool { return false },
+		[]int{1, 2, 3},
+	},
+	{
+		[]int{1, 2, 3},
+		func(i int) bool { return i > 2 },
+		[]int{1, 2},
+	},
+	{
+		[]int{1, 2, 3},
+		func(i int) bool { return i < 2 },
+		[]int{2, 3},
+	},
+	{
+		[]int{10, 2, 30},
+		func(i int) bool { return i >= 10 },
+		[]int{2},
+	},
+}
+
+func TestDeleteFunc(t *testing.T) {
+	for i, test := range deleteFuncTests {
+		copy := Clone(test.s)
+		if got := DeleteFunc(copy, test.fn); !Equal(got, test.want) {
+			t.Errorf("DeleteFunc case %d: got %v, want %v", i, got, test.want)
+		}
+	}
+}
+
 func panics(f func()) (b bool) {
 	defer func() {
 		if x := recover(); x != nil {