Add min/max functionality

lib/src/duration.dart

@@ -107,3 +107,9 @@ extension PrettyDuration on Duration {
         maxUnits: maxUnits);
   }
 }
+
+/// Returns the lesser of two durations.
+Duration min(Duration a, Duration b) => a < b ? a : b;
+
+/// Returns the larger of two durations.
+Duration max(Duration a, Duration b) => a > b ? a : b;

test/min_max_test.dart

@@ -0,0 +1,45 @@
+import 'package:duration/duration.dart';
+import 'package:test/test.dart';
+
+void main() {
+  const a = Duration(minutes: 59, seconds: 59); // 3599 sec
+  const b = Duration(hours: 1, minutes: 1); // 3660 sec
+  const c = Duration(minutes: 66); // 3960
+
+  group('min/max', () {
+    test('min(positives)', () {
+      expect(min(a, b), a);
+      expect(min(a, c), a);
+      expect(min(b, c), b);
+      expect(min(a, min(b, c)), a);
+    });
+    test('max(positives)', () {
+      expect(max(a, b), b);
+      expect(max(a, c), c);
+      expect(max(b, c), c);
+      expect(max(a, max(b, c)), c);
+    });
+    test('min(negatives)', () {
+      expect(min(-a, -b), -b);
+      expect(min(-a, -c), -c);
+      expect(min(-b, -c), -c);
+      expect(min(-a, min(-b, -c)), -c);
+    });
+    test('max(negatives)', () {
+      expect(max(-a, -b), -a);
+      expect(max(-a, -c), -a);
+      expect(max(-b, -c), -b);
+      expect(max(-a, max(-b, -c)), -a);
+    });
+    test('equals', () {
+      expect(min(c, Duration(minutes: 66)), c);
+      expect(max(c, Duration(minutes: 66)), c);
+    });
+    test('+/0/-', () {
+      expect(min(a, min(Duration.zero, -a)), -a);
+      expect(min(-a, min(Duration.zero, a)), -a);
+      expect(max(a, max(Duration.zero, -a)), a);
+      expect(max(-a, max(Duration.zero, a)), a);
+    });
+  });
+}