#!/usr/bin/env python3
"""Fibonacci sequence calculator with colored output."""

import argparse
import sys

# ANSI color codes
GREEN = "\033[32m"
CYAN = "\033[36m"
RED = "\033[31m"
RESET = "\033[0m"


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


def main():
    parser = argparse.ArgumentParser(description="Fibonacci sequence calculator")
    parser.add_argument("n", type=int, help="The position n in the Fibonacci sequence")
    parser.add_argument("--output", "-o", help="Write output to a file instead of stdout")
    args = parser.parse_args()

    try:
        result = fibonacci(args.n)
    except ValueError as e:
        print(f"{RED}Error: {e}{RESET}")
        sys.exit(1)

    output_line = f"{CYAN}fibonacci({args.n}){RESET} {GREEN}={RESET} {GREEN}{result}{RESET}"

    if args.output:
        with open(args.output, "w") as f:
            f.write(output_line + "\n")
    else:
        print(output_line)


if __name__ == "__main__":
    main()