types/views: add SliceEqualAnyOrder func

This is based on the tagsEqual func from corp/control/control.go, moved
here so that it can be reused in other places.

Signed-off-by: Will Norris <will@tailscale.com>

types/views/views.go

@@ -170,6 +170,37 @@ func SliceContains[T comparable](v Slice[T], e T) bool {
 	return false
 }
 
+// SliceEqualAnyOrder reports whether a and b contain the same elements, regardless of order.
+// The underlying slices for a and b can be nil.
+func SliceEqualAnyOrder[T comparable](a, b Slice[T]) bool {
+	if a.Len() != b.Len() {
+		return false
+	}
+
+	var diffStart int // beginning index where a and b differ
+	for n := a.Len(); diffStart < n; diffStart++ {
+		if a.At(diffStart) != b.At(diffStart) {
+			break
+		}
+	}
+	if diffStart == a.Len() {
+		return true
+	}
+
+	// count the occurrences of remaining values and compare
+	valueCount := make(map[T]int)
+	for i, n := diffStart, a.Len(); i < n; i++ {
+		valueCount[a.At(i)]++
+		valueCount[b.At(i)]--
+	}
+	for _, count := range valueCount {
+		if count != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // IPPrefixSlice is a read-only accessor for a slice of netip.Prefix.
 type IPPrefixSlice struct {
 	ж Slice[netip.Prefix]

types/views/views_test.go

@@ -125,4 +125,8 @@ func TestViewUtils(t *testing.T) {
 	c.Check(v.IndexFunc(func(s string) bool { return strings.HasPrefix(s, "z") }), qt.Equals, -1)
 	c.Check(SliceContains(v, "bar"), qt.Equals, true)
 	c.Check(SliceContains(v, "baz"), qt.Equals, false)
+	c.Check(SliceEqualAnyOrder(v, v), qt.Equals, true)
+	c.Check(SliceEqualAnyOrder(v, SliceOf([]string{"bar", "foo"})), qt.Equals, true)
+	c.Check(SliceEqualAnyOrder(v, SliceOf([]string{"foo"})), qt.Equals, false)
+	c.Check(SliceEqualAnyOrder(SliceOf([]string{"a", "a", "b"}), SliceOf([]string{"a", "b", "b"})), qt.Equals, false)
 }