Fix #1: Implement fibonacci with edge-case ValueError handling

src/math_utils.py

@@ -9,3 +9,30 @@ def add(a, b):
 def multiply(a, b):
     """Multiply two numbers"""
     return a * b
+
+
+def fibonacci(n: int) -> int:
+    """
+    Calculate the nth Fibonacci number (0-indexed).
+
+    Args:
+        n (int): The 0-indexed position in the Fibonacci sequence.
+
+    Returns:
+        int: The nth Fibonacci number.
+
+    Raises:
+        ValueError: If n is negative, with message 'n must be non-negative'.
+    """
+    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(2, n + 1):
+        a, b = b, a + b
+    return b
+

tests/test_math_utils.py

@@ -1,8 +1,10 @@
 import sys
 from pathlib import Path
+import pytest
+
 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 +15,22 @@ def test_multiply():
     assert multiply(2, 3) == 6
     assert multiply(-1, 5) == -5
     assert multiply(0, 10) == 0
+
+def test_fibonacci_base_and_normal_cases():
+    assert fibonacci(0) == 0
+    assert fibonacci(1) == 1
+    assert fibonacci(2) == 1
+    assert fibonacci(3) == 2
+    assert fibonacci(4) == 3
+    assert fibonacci(5) == 5
+    assert fibonacci(6) == 8
+    assert fibonacci(10) == 55
+    assert fibonacci(20) == 6765
+
+def test_fibonacci_negative_edge_cases():
+    with pytest.raises(ValueError, match=r"^n must be non-negative$"):
+        fibonacci(-1)
+
+    with pytest.raises(ValueError, match=r"^n must be non-negative$"):
+        fibonacci(-10)
+