Initial commit: fibonacci calculator

fibonacci.py

@@ -0,0 +1,41 @@
+#!/usr/bin/env python3
+"""Fibonacci sequence calculator."""
+
+
+def fibonacci(n: int) -> int:
+    """Return the nth Fibonacci number (0-indexed).
+    
+    fibonacci(0) = 0
+    fibonacci(1) = 1
+    fibonacci(2) = 1
+    fibonacci(3) = 2
+    ...
+    """
+    if n < 0:
+        raise ValueError(f"n must be non-negative, got {n}")
+    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
+
+
+if __name__ == "__main__":
+    import sys
+    
+    if len(sys.argv) != 2:
+        print(f"Usage: {sys.argv[0]} <n>")
+        sys.exit(1)
+    
+    try:
+        n = int(sys.argv[1])
+    except ValueError:
+        print(f"Error: '{sys.argv[1]}' is not a valid integer")
+        sys.exit(1)
+    
+    result = fibonacci(n)
+    print(f"fibonacci({n}) = {result}")