types/views: add SliceEqualAnyOrderFunc

Extracted from some code written in the other repo.

Updates tailscale/corp#25479

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I6df062fdffa1705524caa44ac3b6f2788cf64595

types/views/views.go

@@ -360,6 +360,41 @@ func SliceEqualAnyOrder[T comparable](a, b Slice[T]) bool {
 	return true
 }
 
+// SliceEqualAnyOrderFunc reports whether a and b contain the same elements,
+// regardless of order. The underlying slices for a and b can be nil.
+//
+// The provided function should return a comparable value for each element.
+func SliceEqualAnyOrderFunc[T any, V comparable](a, b Slice[T], cmp func(T) V) 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++ {
+		av := cmp(a.At(diffStart))
+		bv := cmp(b.At(diffStart))
+		if av != bv {
+			break
+		}
+	}
+	if diffStart == a.Len() {
+		return true
+	}
+
+	// count the occurrences of remaining values and compare
+	valueCount := make(map[V]int)
+	for i, n := diffStart, a.Len(); i < n; i++ {
+		valueCount[cmp(a.At(i))]++
+		valueCount[cmp(b.At(i))]--
+	}
+	for _, count := range valueCount {
+		if count != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // MapSlice is a view over a map whose values are slices.
 type MapSlice[K comparable, V any] struct {
 	// ж is the underlying mutable value, named with a hard-to-type

types/views/views_test.go

@@ -153,6 +153,43 @@ func TestViewUtils(t *testing.T) {
 		qt.Equals, true)
 }
 
+func TestSliceEqualAnyOrderFunc(t *testing.T) {
+	type nc struct {
+		_ structs.Incomparable
+		v string
+	}
+
+	// ncFrom returns a Slice[nc] from a slice of []string
+	ncFrom := func(s ...string) Slice[nc] {
+		var out []nc
+		for _, v := range s {
+			out = append(out, nc{v: v})
+		}
+		return SliceOf(out)
+	}
+
+	// cmp returns a comparable value for a nc
+	cmp := func(a nc) string { return a.v }
+
+	v := ncFrom("foo", "bar")
+	c := qt.New(t)
+
+	// Simple case of slice equal to itself.
+	c.Check(SliceEqualAnyOrderFunc(v, v, cmp), qt.Equals, true)
+
+	// Different order.
+	c.Check(SliceEqualAnyOrderFunc(v, ncFrom("bar", "foo"), cmp), qt.Equals, true)
+
+	// Different values, same length
+	c.Check(SliceEqualAnyOrderFunc(v, ncFrom("foo", "baz"), cmp), qt.Equals, false)
+
+	// Different values, different length
+	c.Check(SliceEqualAnyOrderFunc(v, ncFrom("foo"), cmp), qt.Equals, false)
+
+	// Nothing shared
+	c.Check(SliceEqualAnyOrderFunc(v, ncFrom("baz", "qux"), cmp), qt.Equals, false)
+}
+
 func TestSliceEqual(t *testing.T) {
 	a := SliceOf([]string{"foo", "bar"})
 	b := SliceOf([]string{"foo", "bar"})