iframe-proxy | Sunbelt Computer Software

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#!/usr/bin/env python3

"""

This program calculates the nth Fibonacci number in O(log(n)).

It's possible to calculate F(1_000_000) in less than a second.

"""

from __future__ import annotations

import sys

def fibonacci(n: int) -> int:

"""

return F(n)

>>> [fibonacci(i) for i in range(13)]

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]

"""

if n < 0:

raise ValueError("Negative arguments are not supported")

return _fib(n)[0]

# returns (F(n), F(n-1))

def _fib(n: int) -> tuple[int, int]:

if n == 0: # (F(0), F(1))

return (0, 1)

# F(2n) = F(n)[2F(n+1) - F(n)]

# F(2n+1) = F(n+1)^2+F(n)^2

a, b = _fib(n // 2)

c = a * (b * 2 - a)

d = a * a + b * b

return (d, c + d) if n % 2 else (c, d)

if __name__ == "__main__":

n = int(sys.argv[1])

print(f"fibonacci({n}) is {fibonacci(n)}")

Sunbelt Computer Software