feat(math_utils): add fibonacci function with edge case handling (Closes #1)

src/math_utils.py

@@ -9,3 +9,16 @@ def add(a, b):
 def multiply(a, b):
     """Multiply two numbers"""
     return a * b
+
+def fibonacci(n):
+    """Calculate the nth Fibonacci number (0-indexed)."""
+    if n < 0:
+        raise ValueError("n must be non-negative")
+    if n == 0:
+        return 0
+    if n == 1:
+        return 1
+    a, b = 0, 1
+    for _ in range(n):
+        a, b = b, a + b
+    return a

tests/test_math_utils.py

@@ -1,8 +1,9 @@
 import sys
+import pytest
 from pathlib import Path
 sys.path.insert(0, str(Path(__file__).parent.parent / 'src'))
 
-from math_utils import add, multiply
+from math_utils import add, multiply, fibonacci
 
 def test_add():
     assert add(2, 3) == 5
@@ -13,3 +14,15 @@ def test_multiply():
     assert multiply(2, 3) == 6
     assert multiply(-1, 5) == -5
     assert multiply(0, 10) == 0
+
+def test_fibonacci_normal_cases():
+    assert fibonacci(0) == 0
+    assert fibonacci(1) == 1
+    assert fibonacci(5) == 5
+    assert fibonacci(10) == 55
+    assert fibonacci(20) == 6765
+
+def test_fibonacci_edge_case():
+    with pytest.raises(ValueError) as exc_info:
+        fibonacci(-1)
+    assert "n must be non-negative" in str(exc_info.value)