Add utility fn

app/lib/int_extension.dart

@@ -0,0 +1,15 @@
+extension IntExtension on int {
+  Iterable<int> until(int to, [int step = 1]) sync* {
+    if (step == 0) {
+      throw ArgumentError("step must not be zero");
+    }
+    final sign = (to - this).sign;
+    if (sign != step.sign) {
+      // e.g., 0.until(10, -1) or 0.until(-10, 1)
+      return;
+    }
+    for (var i = this; sign > 0 ? i < to : i > to; i += step) {
+      yield i;
+    }
+  }
+}

app/test/int_extension_test.dart

@@ -0,0 +1,48 @@
+import 'package:nc_photos/int_extension.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group("IntExtension", () {
+    group("until", () {
+      /// Expected: exception
+      test("step == 0", () {
+        expect(() => 1.until(10, 0), throwsArgumentError);
+      });
+
+      /// Expected: [1, ..., 9]
+      test("+to > +beg, default step", () {
+        expect(1.until(10).toList(), List.generate(9, (i) => i + 1));
+      });
+
+      /// Expected: [1, 3, ..., 9]
+      test("+to > +beg, step = 2", () {
+        expect(1.until(10, 2).toList(), List.generate(5, (i) => i + 1 + i));
+      });
+
+      /// Expected: []
+      test("+to > +beg, -step", () {
+        expect(1.until(10, -1).toList(), []);
+      });
+
+      /// Expected: []
+      test("+to < +beg, +step", () {
+        expect(10.until(1).toList(), []);
+      });
+
+      /// Expected: [10, ..., 2]
+      test("+to < +beg, -step", () {
+        expect(10.until(1, -1).toList(), List.generate(9, (i) => 10 - i));
+      });
+
+      /// Expected: [-10, ..., -2]
+      test("-to > -beg, +step", () {
+        expect((-10).until(-1, 1).toList(), List.generate(9, (i) => i + -10));
+      });
+
+      /// Expected: [-1, ..., -9]
+      test("-to < -beg, -step", () {
+        expect((-1).until(-10, -1).toList(), List.generate(9, (i) => -1 - i));
+      });
+    });
+  });
+}