from __future__ import annotations

import time
import unittest
from collections.abc import Callable
from typing import ParamSpec, TypeVar

P = ParamSpec("P")
T = TypeVar("T")


def retry(
    function: Callable[P, T],
    *args: P.args,
    attempts: int = 3,
    initial_delay: float = 0.1,
    backoff_factor: float = 2.0,
    max_delay: float | None = None,
    retry_on: tuple[type[Exception], ...] = (Exception,),
    sleep: Callable[[float], None] = time.sleep,
    **kwargs: P.kwargs,
) -> T:
    """Call a function, retrying failures with exponential backoff."""
    if attempts < 1:
        raise ValueError("attempts must be at least 1")
    if initial_delay < 0:
        raise ValueError("initial_delay cannot be negative")
    if backoff_factor < 1:
        raise ValueError("backoff_factor must be at least 1")
    if max_delay is not None and max_delay < 0:
        raise ValueError("max_delay cannot be negative")

    delay = initial_delay

    for attempt in range(1, attempts + 1):
        try:
            return function(*args, **kwargs)
        except retry_on:
            if attempt == attempts:
                raise

            sleep(delay)
            delay *= backoff_factor
            if max_delay is not None:
                delay = min(delay, max_delay)

    raise AssertionError("unreachable")


class RetryTests(unittest.TestCase):
    def test_retries_with_exponential_backoff(self) -> None:
        calls = 0
        delays: list[float] = []

        def unreliable() -> str:
            nonlocal calls
            calls += 1
            if calls < 3:
                raise RuntimeError("temporary failure")
            return "success"

        result = retry(
            unreliable,
            attempts=4,
            initial_delay=0.5,
            sleep=delays.append,
        )

        self.assertEqual(result, "success")
        self.assertEqual(calls, 3)
        self.assertEqual(delays, [0.5, 1.0])

    def test_raises_after_final_attempt(self) -> None:
        def always_fails() -> None:
            raise ValueError("permanent failure")

        with self.assertRaisesRegex(ValueError, "permanent failure"):
            retry(always_fails, attempts=2, initial_delay=0, sleep=lambda _: None)


if __name__ == "__main__":
    unittest.main()