#!/usr/bin/env python3
"""Fibonacci number verification - concrete testable artifact."""

def fib(n: int) -> int:
    """Compute the nth Fibonacci number."""
    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

def verify_fibs():
    """Verify known Fibonacci numbers."""
    # Known Fibonacci values
    expected = {
        0: 0,
        1: 1,
        2: 1,
        3: 2,
        4: 3,
        5: 5,
        6: 8,
        7: 13,
        8: 21,
        9: 34,
        10: 55,
        15: 610,
        20: 6765,
    }
    
    errors = []
    for n, expected_val in expected.items():
        actual = fib(n)
        if actual != expected_val:
            errors.append(f"fib({n}) = {actual}, expected {expected_val}")
    
    if errors:
        for e in errors:
            print(f"ERROR: {e}")
        return False
    
    print("All Fibonacci verifications passed")
    return True

if __name__ == "__main__":
    import sys
    success = verify_fibs()
    sys.exit(0 if success else 1)