Project Euler 021

DIRECTORY.md

@@ -239,6 +239,7 @@
   * [Problem017](Project-Euler/Problem017.js)
   * [Problem018](Project-Euler/Problem018.js)
   * [Problem020](Project-Euler/Problem020.js)
+  * [Problem021](Project-Euler/Problem021.js)
   * [Problem023](Project-Euler/Problem023.js)
   * [Problem025](Project-Euler/Problem025.js)
   * [Problem028](Project-Euler/Problem028.js)

Project-Euler/Problem021.js

@@ -0,0 +1,35 @@
+/**
+ * Amicable numbers
+ *
+ * P.S.(Project Euler - 021):
+ * Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
+ * If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers.
+ *
+ * For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110;
+ * therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.
+ *
+ * Evaluate the sum of all the amicable numbers under 10000.
+ *
+ * @param {Number} maxNum
+ * @returns {Number} Sum of all amicable numbers under maxNum
+ */
+export const sumAmicableNums = (maxNum) => {
+  const amicNums = []
+
+  for (let i = 1; i < maxNum; i++) {
+    const left = d(i)
+    const right = d(left)
+    if (i === right && i !== left && amicNums.indexOf(i) === -1) {
+      amicNums.push(i, left)
+    }
+  }
+  return amicNums.reduce((a, b) => a + b, 0)
+}
+
+const d = (num) => {
+  const output = []
+  for (let i = 1; i < num; i++) {
+    if (num % i === 0) { output.push(i) }
+  }
+  return output.reduce((a, b) => a + b, 0)
+}

Project-Euler/test/Problem021.test.js

@@ -0,0 +1,8 @@
+import { sumAmicableNums } from '../Problem021.js'
+
+describe('Amicable numbers', () => {
+  // Project Euler Condition Check
+  test('if the limit is 10000', () => {
+    expect(sumAmicableNums(10000)).toBe(31626)
+  })
+})