2 files changed, 29 insertions, 0 deletions
diff --git a/src/client/react/lib/withinRange.js b/src/client/react/lib/withinRange.js
new file mode 100644
index 0000000..d3d4f24
--- /dev/null
+++ b/src/client/react/lib/withinRange.js
@@ -0,0 +1,11 @@
+export default function withinRange(number, rangeMax) {
+  if (number < 0) {
+    return withinRange(number + (rangeMax + 1), rangeMax);
+  }
+
+  if (number > rangeMax) {
+    return withinRange(number - (rangeMax + 1), rangeMax);
+  }
+
+  return number;
+}
diff --git a/src/client/react/lib/withinRange.test.js b/src/client/react/lib/withinRange.test.js
new file mode 100644
index 0000000..baec122
--- /dev/null
+++ b/src/client/react/lib/withinRange.test.js
@@ -0,0 +1,18 @@
+import withinRange from './withinRange';
+
+describe('withinRange', () => {
+  describe('the number is within range', () => {
+    it('just returns the number', () => {
+      expect(withinRange(5, 7)).toEqual(5);
+    });
+  });
+
+  describe('the number is outside the range', () => {
+    it('wraps the number until within range', () => {
+      expect(withinRange(-1, 6)).toEqual(6);
+      expect(withinRange(7, 6)).toEqual(0);
+      expect(withinRange(-20, 6)).toEqual(1);
+      expect(withinRange(20, 6)).toEqual(6);
+    });
+  });
+});