Merge pull request #59 from devNoiseConsulting/develop

Factorial

Author: jhowardjr

lib/factorial.js

@@ -0,0 +1,39 @@
+// https://stackoverflow.com/a/3959275
+
+const { isPositiveInteger } = require('./isPositiveInteger');
+
+const handleErrors = (params) => {
+  if (params.length === 0) throw new Error('Must provide one or more paramters');
+  if (params.some(param => !Number.isInteger(param))) {
+    throw new Error('One of your parameters does not evaluate to a integer');
+  }
+  // Has to be a non negative integer
+  if (params.some(param => !isPositiveInteger(param) && param !== 0)) {
+    throw new Error('One of your parameters does not evaluate to a positive integer');
+  }
+  // JS can only safely represent integers less than Number.MAX_SAFE_INTEGER
+  if (params.some(param => param > 18)) {
+    throw new Error('Cannot reliably return numbers larger than 9,007,199,254,740,991');
+  }
+};
+
+const factorialCache = [1, 1];
+
+const caclulateFactorial = (num) => {
+  if (typeof factorialCache[num] !== 'undefined') {
+    return factorialCache[num];
+  }
+
+  const start = factorialCache.length;
+  for (let i = start; i <= num; i += 1) {
+    factorialCache[i] = factorialCache[i - 1] * i;
+  }
+
+  return factorialCache[num];
+};
+
+exports.factorial = (...params) => {
+  handleErrors(params);
+
+  return params.map(caclulateFactorial);
+};

spec/factorialSpec.js

@@ -0,0 +1,24 @@
+const { factorial } = require('../lib/factorial');
+
+describe('factorial', () => {
+  it('should return the factorial of all parameters, given that all parameters evaluate to numbers', () => {
+    const result = factorial(0, 1, 2, 3, 4);
+    expect(result).toEqual([1, 1, 2, 6, 24]);
+  });
+
+  it('should throw an error when no parameters are provided', () => {
+    expect(factorial).toThrow();
+  });
+
+  it('should throw an error when at least one parameter does not evaluate to a number', () => {
+    expect(() => factorial(1, 2, 'yo')).toThrow();
+  });
+
+  it('should throw an error when at least one parameter evaluates to a negative number', () => {
+    expect(() => factorial(1, 2, -1)).toThrow();
+  });
+
+  it('should throw an error when at least one parameter evaluates to a number larger than 18', () => {
+    expect(() => factorial(1, 2, Number.MAX_SAFE_INTEGER)).toThrow();
+  });
+});