feat(factorial): Add factorial module

Author: sargalias

src/recursion-practice/factorial/factorial.js

@@ -0,0 +1,27 @@
+import { trampoline, identity } from '../utils';
+
+const factorial = n => {
+  if (n <= 1) return 1;
+  return n * factorial(n - 1);
+};
+
+const factorialPTC = (n, totalSoFar = 1) => {
+  if (n <= 1) return totalSoFar;
+  const newTotalSoFar = n * totalSoFar;
+  return factorialPTC(n - 1, newTotalSoFar);
+};
+
+const factorialCPS = (n, cont = identity) => {
+  if (n <= 1) return cont(1);
+  return factorialCPS(n - 1, remainingResult => cont(remainingResult * n));
+};
+
+const factorialTrampoline = trampoline(function _factorialTrampoline(
+  n,
+  totalSoFar = 1,
+) {
+  if (n <= 1) return totalSoFar;
+  return () => _factorialTrampoline(n - 1, n * totalSoFar);
+});
+
+export { factorial, factorialPTC, factorialCPS, factorialTrampoline };

src/recursion-practice/factorial/factorial.test.js

@@ -0,0 +1,23 @@
+import each from 'jest-each';
+import {
+  factorial,
+  factorialPTC,
+  factorialCPS,
+  factorialTrampoline,
+} from './factorial';
+
+[factorial, factorialPTC, factorialCPS, factorialTrampoline].forEach(fn => {
+  describe(`${fn.name}`, () => {
+    each`
+      n     | expected
+      ${0}  | ${1}
+      ${1}  | ${1}
+      ${2}  | ${2}
+      ${3}  | ${6}
+      ${4}  | ${24}
+      ${5}  | ${120}
+    `.test('should return $expected when called with $n', ({ n, expected }) => {
+      expect(fn(n)).toBe(expected);
+    });
+  });
+});