feat: add Tribonacci number algorithm

Maths/TribonacciNumber.js

@@ -0,0 +1,28 @@
+/**
+ * @function tribonacciNumber
+ * @description The Tribonacci sequence is similar to Fibonacci but each
+ *   number is the sum of the three preceding ones.
+ *   Sequence: 0, 0, 1, 1, 2, 4, 7, 13, 24, 44...
+ * @param {number} n - The position in the Tribonacci sequence (0-indexed)
+ * @returns {number} - The nth Tribonacci number
+ * @see https://en.wikipedia.org/wiki/Generalizations_of_Fibonacci_numbers#Tribonacci_numbers
+ * @example tribonacciNumber(0) => 0
+ * @example tribonacciNumber(4) => 2
+ * @example tribonacciNumber(7) => 13
+ */
+const tribonacciNumber = (n) => {
+  if (!Number.isInteger(n) || n < 0) {
+    throw new TypeError('Input must be a non-negative integer')
+  }
+  if (n === 0 || n === 1) return 0
+  if (n === 2) return 1
+  let a = 0
+  let b = 0
+  let c = 1
+  for (let i = 3; i <= n; i++) {
+    ;[a, b, c] = [b, c, a + b + c]
+  }
+  return c
+}
+
+export { tribonacciNumber }

Maths/test/TribonacciNumber.test.js

@@ -0,0 +1,43 @@
+import { tribonacciNumber } from '../TribonacciNumber.js'
+
+describe('tribonacciNumber', () => {
+  test('tribonacciNumber(0) returns 0', () => {
+    expect(tribonacciNumber(0)).toBe(0)
+  })
+
+  test('tribonacciNumber(1) returns 0', () => {
+    expect(tribonacciNumber(1)).toBe(0)
+  })
+
+  test('tribonacciNumber(2) returns 1', () => {
+    expect(tribonacciNumber(2)).toBe(1)
+  })
+
+  test('tribonacciNumber(3) returns 1', () => {
+    expect(tribonacciNumber(3)).toBe(1)
+  })
+
+  test('tribonacciNumber(4) returns 2', () => {
+    expect(tribonacciNumber(4)).toBe(2)
+  })
+
+  test('tribonacciNumber(5) returns 4', () => {
+    expect(tribonacciNumber(5)).toBe(4)
+  })
+
+  test('tribonacciNumber(7) returns 13', () => {
+    expect(tribonacciNumber(7)).toBe(13)
+  })
+
+  test('tribonacciNumber(8) returns 24', () => {
+    expect(tribonacciNumber(8)).toBe(24)
+  })
+
+  test('throws TypeError for negative input', () => {
+    expect(() => tribonacciNumber(-1)).toThrow(TypeError)
+  })
+
+  test('throws TypeError for float input', () => {
+    expect(() => tribonacciNumber(1.5)).toThrow(TypeError)
+  })
+})