feat: add the Collatz conjecture

Author: yelinator

maths/collatz_sequence.ts

@@ -0,0 +1,36 @@
+/**
+ * Generates the Collatz sequence for a given positive integer.
+ *
+ * The Collatz conjecture sequence is defined as follows:
+ * - Start with any positive integer `n`.
+ * - If `n` is even, divide it by 2.
+ * - If `n` is odd, multiply it by 3 and add 1.
+ * - Repeat the process with the new value of `n` until `n` equals 1.
+ *
+ * @param n - A positive integer to generate the Collatz sequence.
+ * @throws {Error} - Throws an error if `n` is not a positive integer.
+ * @returns {number[]} - The Collatz sequence starting from `n`.
+ *
+ * @example
+ * collatzSequence(5); // Returns: [5, 16, 8, 4, 2, 1]
+ * collatzSequence(10); // Returns: [10, 5, 16, 8, 4, 2, 1]
+ */
+export const collatzSequence = (n: number): number[] => {
+  if (n <= 0 || !Number.isInteger(n)) {
+    throw new Error('Sequence only defined for positive integers.')
+  }
+
+  const result: number[] = []
+
+  while (n !== 1) {
+    result.push(n)
+    if (n % 2 === 0) {
+      n /= 2
+    } else {
+      n = n * 3 + 1
+    }
+  }
+
+  result.push(1)
+  return result
+}

maths/test/collatz_sequence.test.ts

@@ -0,0 +1,39 @@
+import { collatzSequence } from '../collatz_sequence'
+
+describe('collatzSequence', () => {
+  test('should generate the correct Collatz sequence for n = 5', () => {
+    const result = collatzSequence(5)
+    expect(result).toEqual([5, 16, 8, 4, 2, 1])
+  })
+
+  test('should generate the correct Collatz sequence for n = 1', () => {
+    const result = collatzSequence(1)
+    expect(result).toEqual([1])
+  })
+
+  test('should handle large inputs correctly (n = 27)', () => {
+    const result = collatzSequence(27)
+    // The exact sequence for n = 27 is very long; check key parts
+    expect(result[0]).toBe(27)
+    expect(result[result.length - 1]).toBe(1)
+    expect(result.length).toBeGreaterThan(100)
+  })
+
+  test('should throw an error for n = 0', () => {
+    expect(() => collatzSequence(0)).toThrow(
+      'Sequence only defined for positive integers.'
+    )
+  })
+
+  test('should throw an error for negative inputs (n = -5)', () => {
+    expect(() => collatzSequence(-5)).toThrow(
+      'Sequence only defined for positive integers.'
+    )
+  })
+
+  test('should throw an error for non-integer inputs (n = 5.5)', () => {
+    expect(() => collatzSequence(5.5)).toThrow(
+      'Sequence only defined for positive integers.'
+    )
+  })
+})