util/uniq: add ModifySliceFunc (#5504)

Follow-up to #5491. This was used in control... oops!

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

util/uniq/slice.go

@@ -33,3 +33,31 @@ func ModifySlice[E comparable](slice *[]E) {
 		*slice = (*slice)[:end]
 	}
 }
+
+// ModifySliceFunc is the same as ModifySlice except that it allows using a
+// custom comparison function.
+//
+// eq should report whether the two provided elements are equal.
+func ModifySliceFunc[E any](slice *[]E, eq func(i, j E) bool) {
+	// Remove duplicates
+	dst := 0
+	for i := 1; i < len(*slice); i++ {
+		if eq((*slice)[dst], (*slice)[i]) {
+			continue
+		}
+		dst++
+		(*slice)[dst] = (*slice)[i]
+	}
+
+	// Zero out the elements we removed at the end of the slice
+	end := dst + 1
+	var zero E
+	for i := end; i < len(*slice); i++ {
+		(*slice)[i] = zero
+	}
+
+	// Truncate the slice
+	if end < len(*slice) {
+		*slice = (*slice)[:end]
+	}
+}

util/uniq/slice_test.go

@@ -12,22 +12,23 @@ import (
 	"tailscale.com/util/uniq"
 )
 
-func runTests(t *testing.T, cb func(*[]int)) {
+func runTests(t *testing.T, cb func(*[]uint32)) {
 	tests := []struct {
-		in   []int
+		// Use uint32 to be different from an int-typed slice index
-		want []int
+		in   []uint32
+		want []uint32
 	}{
-		{in: []int{0, 1, 2}, want: []int{0, 1, 2}},
+		{in: []uint32{0, 1, 2}, want: []uint32{0, 1, 2}},
-		{in: []int{0, 1, 2, 2}, want: []int{0, 1, 2}},
+		{in: []uint32{0, 1, 2, 2}, want: []uint32{0, 1, 2}},
-		{in: []int{0, 0, 1, 2}, want: []int{0, 1, 2}},
+		{in: []uint32{0, 0, 1, 2}, want: []uint32{0, 1, 2}},
-		{in: []int{0, 1, 0, 2}, want: []int{0, 1, 0, 2}},
+		{in: []uint32{0, 1, 0, 2}, want: []uint32{0, 1, 0, 2}},
-		{in: []int{0}, want: []int{0}},
+		{in: []uint32{0}, want: []uint32{0}},
-		{in: []int{0, 0}, want: []int{0}},
+		{in: []uint32{0, 0}, want: []uint32{0}},
-		{in: []int{}, want: []int{}},
+		{in: []uint32{}, want: []uint32{}},
 	}
 
 	for _, test := range tests {
-		in := make([]int, len(test.in))
+		in := make([]uint32, len(test.in))
 		copy(in, test.in)
 		cb(&test.in)
 		if !reflect.DeepEqual(test.in, test.want) {
@@ -44,11 +45,19 @@ func runTests(t *testing.T, cb func(*[]int)) {
 }
 
 func TestModifySlice(t *testing.T) {
-	runTests(t, func(slice *[]int) {
+	runTests(t, func(slice *[]uint32) {
 		uniq.ModifySlice(slice)
 	})
 }
 
+func TestModifySliceFunc(t *testing.T) {
+	runTests(t, func(slice *[]uint32) {
+		uniq.ModifySliceFunc(slice, func(i, j uint32) bool {
+			return i == j
+		})
+	})
+}
+
 func Benchmark(b *testing.B) {
 	benches := []struct {
 		name  string