#!/usr/bin/env python3
"""Fibonacci calculator with memoization.

Verifiable: f(10) = 55, f(20) = 6765
"""
from functools import lru_cache


@lru_cache(maxsize=None)
def fib(n: int) -> int:
    """Return the nth Fibonacci number (0-indexed)."""
    if n < 0:
        raise ValueError("n must be non-negative")
    if n <= 1:
        return n
    return fib(n - 1) + fib(n - 2)


if __name__ == "__main__":
    # Test cases
    assert fib(0) == 0, "fib(0) should be 0"
    assert fib(1) == 1, "fib(1) should be 1"
    assert fib(10) == 55, "fib(10) should be 55"
    assert fib(20) == 6765, "fib(20) should be 6765"
    
    print("All tests passed!")
    print(f"fib(10) = {fib(10)}")
    print(f"fib(20) = {fib(20)}")