cmpopts: add EquateApproxDuration

cmp/cmpopts/equate.go

@@ -124,6 +124,30 @@ func (a timeApproximator) compare(x, y time.Time) bool {
 	return !x.Add(a.margin).Before(y)
 }
 
+// EquateApproxDuration returns a [cmp.Comparer] option that determines two
+// [time.Duration] values to be equal if they are within some margin of one
+// another. The margin must be non-negative.
+func EquateApproxDuration(margin time.Duration) cmp.Option {
+	if margin < 0 {
+		panic("margin must be a non-negative number")
+	}
+	a := durationApproximator{margin}
+	return cmp.Comparer(a.compare)
+}
+
+type durationApproximator struct {
+	margin time.Duration
+}
+
+func (a durationApproximator) compare(x, y time.Duration) bool {
+	if x > y {
+		x, y = y, x
+	}
+	// Avoid subtracting durations to prevent overflow when the
+	// difference is larger than the largest representable duration.
+	return !(x+a.margin < y)
+}
+
 // AnyError is an error that matches any non-nil error.
 var AnyError anyError
 

cmp/cmpopts/util_test.go

@@ -547,6 +547,48 @@ func TestOptions(t *testing.T) {
 		opts:      []cmp.Option{EquateApproxTime(3 * time.Second)},
 		wantEqual: false,
 		reason:    "time difference overflows time.Duration",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         time.Second,
+		y:         time.Second,
+		opts:      []cmp.Option{EquateApproxDuration(0)},
+		wantEqual: true,
+		reason:    "equal because durations are identical",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         time.Duration(0),
+		y:         3 * time.Second,
+		opts:      []cmp.Option{EquateApproxDuration(3 * time.Second)},
+		wantEqual: true,
+		reason:    "equal because duration is exactly at the allowed margin",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         3 * time.Second,
+		y:         time.Duration(0),
+		opts:      []cmp.Option{EquateApproxDuration(3 * time.Second)},
+		wantEqual: true,
+		reason:    "equal because duration is exactly at the allowed margin (negative)",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         3 * time.Second,
+		y:         time.Duration(0),
+		opts:      []cmp.Option{EquateApproxDuration(3*time.Second - 1)},
+		wantEqual: false,
+		reason:    "not equal because duration is outside allowed margin",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         time.Duration(0),
+		y:         3 * time.Second,
+		opts:      []cmp.Option{EquateApproxDuration(3*time.Second - 1)},
+		wantEqual: false,
+		reason:    "not equal because duration is outside allowed margin (negative)",
+	}, {
+		label:     "EquateApproxDuration",
+		x:         time.Duration(math.MaxInt64),
+		y:         time.Duration(math.MinInt64),
+		opts:      []cmp.Option{EquateApproxDuration(3 * time.Second)},
+		wantEqual: false,
+		reason:    "duration difference overflows time.Duration",
 	}, {
 		label:     "EquateErrors",
 		x:         nil,