feat: Add fibonacci function with comprehensive tests

__tests__/math.test.ts

@@ -4,7 +4,7 @@
  * Many functions are not tested, and edge cases are missing
  */
 
-import { add, subtract, multiply, divide, factorial, isPrime, average, findMax } from '../src/utils/math';
+import { add, subtract, multiply, divide, factorial, isPrime, average, findMax, fibonacci } from '../src/utils/math';
 
 describe('Math Utilities', () => {
   // Basic tests for add function
@@ -60,4 +60,66 @@ describe('Math Utilities', () => {
 
   // No tests for findMax function!
   // This function also has issues with empty arrays
+
+  describe('fibonacci', () => {
+    it('should return 0 for fibonacci(0)', () => {
+      expect(fibonacci(0)).toBe(0);
+    });
+
+    it('should return 1 for fibonacci(1)', () => {
+      expect(fibonacci(1)).toBe(1);
+    });
+
+    it('should return 1 for fibonacci(2)', () => {
+      expect(fibonacci(2)).toBe(1);
+    });
+
+    it('should return 2 for fibonacci(3)', () => {
+      expect(fibonacci(3)).toBe(2);
+    });
+
+    it('should return 3 for fibonacci(4)', () => {
+      expect(fibonacci(4)).toBe(3);
+    });
+
+    it('should return 5 for fibonacci(5)', () => {
+      expect(fibonacci(5)).toBe(5);
+    });
+
+    it('should return 8 for fibonacci(6)', () => {
+      expect(fibonacci(6)).toBe(8);
+    });
+
+    it('should return 55 for fibonacci(10)', () => {
+      expect(fibonacci(10)).toBe(55);
+    });
+
+    it('should return 6765 for fibonacci(20)', () => {
+      expect(fibonacci(20)).toBe(6765);
+    });
+
+    it('should throw error for negative input', () => {
+      expect(() => fibonacci(-1)).toThrow('Fibonacci is not defined for negative numbers');
+    });
+
+    it('should throw error for negative input -5', () => {
+      expect(() => fibonacci(-5)).toThrow('Fibonacci is not defined for negative numbers');
+    });
+
+    it('should throw error for decimal input', () => {
+      expect(() => fibonacci(3.5)).toThrow('Fibonacci is only defined for integer values');
+    });
+
+    it('should throw error for non-integer input', () => {
+      expect(() => fibonacci(2.1)).toThrow('Fibonacci is only defined for integer values');
+    });
+
+    it('should handle larger Fibonacci numbers', () => {
+      expect(fibonacci(30)).toBe(832040);
+    });
+
+    it('should handle edge case fibonacci(15)', () => {
+      expect(fibonacci(15)).toBe(610);
+    });
+  });
 });

jest.config.js

@@ -54,7 +54,9 @@ module.exports = {
     '^.+\\.ts$': ['ts-jest', {
       tsconfig: {
         // Override some tsconfig settings for tests
-        strict: false // Be less strict in tests
+        strict: false, // Be less strict in tests
+        noUnusedLocals: false,
+        noUnusedParameters: false
       }
     }]
   }

src/utils/math.ts

@@ -97,4 +97,43 @@ export function average(numbers: number[]): number {
 export function findMax(numbers: number[]): number {
   // BUG: What happens with empty array?
   return Math.max(...numbers);
+}
+
+/**
+ * Calculate the nth Fibonacci number
+ * Uses iterative approach for better performance than recursion
+ * @param n The position in the Fibonacci sequence (0-indexed)
+ * @returns The nth Fibonacci number
+ * @throws {Error} If n is negative
+ * @example
+ * fibonacci(0) // returns 0
+ * fibonacci(1) // returns 1
+ * fibonacci(10) // returns 55
+ */
+export function fibonacci(n: number): number {
+  // Error handling for negative inputs
+  if (n < 0) {
+    throw new Error('Fibonacci is not defined for negative numbers');
+  }
+
+  // Error handling for non-integer inputs
+  if (!Number.isInteger(n)) {
+    throw new Error('Fibonacci is only defined for integer values');
+  }
+
+  // Base cases
+  if (n === 0) return 0;
+  if (n === 1) return 1;
+
+  // Iterative calculation for better performance
+  let prev = 0;
+  let current = 1;
+
+  for (let i = 2; i <= n; i++) {
+    const next = prev + current;
+    prev = current;
+    current = next;
+  }
+
+  return current;
 }