types/views: add MapViewsEqual and MapViewsEqualFunc

Extracted from some code written in the other repo.

Updates tailscale/corp#25479

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I92c97a63a8f35cace6e89a730938ea587dcefd9b

types/views/views.go

@@ -513,6 +513,47 @@ func (m Map[K, V]) AsMap() map[K]V {
 	return maps.Clone(m.ж)
 }
 
+// NOTE: the type constraints for MapViewsEqual and MapViewsEqualFunc are based
+// on those for maps.Equal and maps.EqualFunc.
+
+// MapViewsEqual returns whether the two given [Map]s are equal. Both K and V
+// must be comparable; if V is non-comparable, use [MapViewsEqualFunc] instead.
+func MapViewsEqual[K, V comparable](a, b Map[K, V]) bool {
+	if a.Len() != b.Len() || a.IsNil() != b.IsNil() {
+		return false
+	}
+	if a.IsNil() {
+		return true // both nil; can exit early
+	}
+
+	for k, v := range a.All() {
+		bv, ok := b.GetOk(k)
+		if !ok || v != bv {
+			return false
+		}
+	}
+	return true
+}
+
+// MapViewsEqualFunc returns whether the two given [Map]s are equal, using the
+// given function to compare two values.
+func MapViewsEqualFunc[K comparable, V1, V2 any](a Map[K, V1], b Map[K, V2], eq func(V1, V2) bool) bool {
+	if a.Len() != b.Len() || a.IsNil() != b.IsNil() {
+		return false
+	}
+	if a.IsNil() {
+		return true // both nil; can exit early
+	}
+
+	for k, v := range a.All() {
+		bv, ok := b.GetOk(k)
+		if !ok || !eq(v, bv) {
+			return false
+		}
+	}
+	return true
+}
+
 // MapRangeFn is the func called from a Map.Range call.
 // Implementations should return false to stop range.
 type MapRangeFn[K comparable, V any] func(k K, v V) (cont bool)

types/views/views_test.go

@@ -15,6 +15,7 @@ import (
 	"unsafe"
 
 	qt "github.com/frankban/quicktest"
+	"tailscale.com/types/structs"
 )
 
 type viewStruct struct {
@@ -501,3 +502,87 @@ func TestMapFnIter(t *testing.T) {
 		t.Errorf("got %q; want %q", got, want)
 	}
 }
+
+func TestMapViewsEqual(t *testing.T) {
+	testCases := []struct {
+		name string
+		a, b map[string]string
+		want bool
+	}{
+		{
+			name: "both_nil",
+			a:    nil,
+			b:    nil,
+			want: true,
+		},
+		{
+			name: "both_empty",
+			a:    map[string]string{},
+			b:    map[string]string{},
+			want: true,
+		},
+		{
+			name: "one_nil",
+			a:    nil,
+			b:    map[string]string{"a": "1"},
+			want: false,
+		},
+		{
+			name: "different_length",
+			a:    map[string]string{"a": "1"},
+			b:    map[string]string{"a": "1", "b": "2"},
+			want: false,
+		},
+		{
+			name: "different_values",
+			a:    map[string]string{"a": "1"},
+			b:    map[string]string{"a": "2"},
+			want: false,
+		},
+		{
+			name: "different_keys",
+			a:    map[string]string{"a": "1"},
+			b:    map[string]string{"b": "1"},
+			want: false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := MapViewsEqual(MapOf(tc.a), MapOf(tc.b))
+			if got != tc.want {
+				t.Errorf("MapViewsEqual: got=%v, want %v", got, tc.want)
+			}
+
+			got = MapViewsEqualFunc(MapOf(tc.a), MapOf(tc.b), func(a, b string) bool {
+				return a == b
+			})
+			if got != tc.want {
+				t.Errorf("MapViewsEqualFunc: got=%v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestMapViewsEqualFunc(t *testing.T) {
+	// Test that we can compare maps with two different non-comparable
+	// values using a custom comparison function.
+	type customStruct1 struct {
+		_      structs.Incomparable
+		Field1 string
+	}
+	type customStruct2 struct {
+		_      structs.Incomparable
+		Field2 string
+	}
+
+	a := map[string]customStruct1{"a": {Field1: "1"}}
+	b := map[string]customStruct2{"a": {Field2: "1"}}
+
+	got := MapViewsEqualFunc(MapOf(a), MapOf(b), func(a customStruct1, b customStruct2) bool {
+		return a.Field1 == b.Field2
+	})
+	if !got {
+		t.Errorf("MapViewsEqualFunc: got=%v, want true", got)
+	}
+}