feat(utilities): add percentage helper

This provides the user wiht a curried helper function. The first parameter is the percentage of 100
which we want to calculate the second is the total value so for example if we would like to calulate
30 percent of 1024 we would write `percentage(30, 1024)` or `percentage(30)(1024)`

Close DCOS-38640

Author: Poltergeist

packages/utilities/components/percentage.ts

@@ -0,0 +1,17 @@
+function calculate(a, b) {
+  return (b / 100) * a;
+}
+
+function percentage(a?: number, b?: number) {
+  if (arguments.length === 0) {
+    return percentage;
+  }
+  if (arguments.length === 1) {
+    return b => {
+      return calculate(a, b);
+    };
+  }
+  return calculate(a, b);
+}
+
+export default percentage;

packages/utilities/index.ts

@@ -1,2 +1,3 @@
 export { default as min } from "./components/min";
 export { default as max } from "./components/max";
+export { default as percentage } from "./components/percentage";

packages/utilities/tests/percentage.test.ts

@@ -0,0 +1,15 @@
+import { default as percentage } from "../components/percentage";
+
+describe("percentage", () => {
+  it("returns percentage of total", () => {
+    expect(percentage(30, 100)).toEqual(30);
+  });
+
+  it("returns function if only percentage is provided", () => {
+    expect(typeof percentage(30)).toBe("function");
+  });
+
+  it("returns calculates the percentage from curried function", () => {
+    expect(percentage(30)(100)).toEqual(30);
+  });
+});