From 53b1f667eeadbc1af929a9bf7fddb651aec2b49d Mon Sep 17 00:00:00 2001
From: Praneeth Jain <praneethjain005@gmail.com>
Date: Sat, 12 Aug 2023 19:48:14 +0530
Subject: [PATCH] Project Euler 021 (#1347)

* feat: Project Euler Problem 21

* test: Project Euler 21

* fix: test description in Project Euler 21
---
 Project-Euler/Problem021.js           | 33 +++++++++++++++++++++++++++
 Project-Euler/test/Problem021.test.js | 14 ++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 Project-Euler/Problem021.js
 create mode 100644 Project-Euler/test/Problem021.test.js

diff --git a/Project-Euler/Problem021.js b/Project-Euler/Problem021.js
new file mode 100644
index 000000000..34a5517bd
--- /dev/null
+++ b/Project-Euler/Problem021.js
@@ -0,0 +1,33 @@
+import { aliquotSum } from '../Maths/AliquotSum.js'
+
+/**
+ * Problem 21 - Amicable numbers
+ *
+ * @see {@link https://projecteuler.net/problem=21}
+ *
+ * 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 amicable numbers under 10000
+ *
+ * @author PraneethJain
+ */
+
+function problem21 (n) {
+  if (n < 2) {
+    throw new Error('Invalid Input')
+  }
+
+  let result = 0
+  for (let a = 2; a < n; ++a) {
+    const b = aliquotSum(a) // Sum of all proper divisors of a
+    // Check if b > a to ensure each pair isn't counted twice, and check if sum of proper divisors of b is equal to a
+    if (b > a && aliquotSum(b) === a) {
+      result += a + b
+    }
+  }
+  return result
+}
+
+export { problem21 }
diff --git a/Project-Euler/test/Problem021.test.js b/Project-Euler/test/Problem021.test.js
new file mode 100644
index 000000000..411f6da0d
--- /dev/null
+++ b/Project-Euler/test/Problem021.test.js
@@ -0,0 +1,14 @@
+import { problem21 } from '../Problem021.js'
+
+describe('check sum of amicable numbers under n', () => {
+  test('should be invalid input if number is negative', () => {
+    expect(() => problem21(-1)).toThrowError('Invalid Input')
+  })
+  test('should be invalid input if number is 0', () => {
+    expect(() => problem21(0)).toThrowError('Invalid Input')
+  })
+  // Project Euler Condition Check
+  test('if the number is greater or equal to 1', () => {
+    expect(problem21(10000)).toBe(31626)
+  })
+})