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

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


def retry_with_backoff(
    function: Callable[P, T],
    *args: P.args,
    max_attempts: int = 3,
    initial_delay: float = 0.5,
    multiplier: float = 2.0,
    retry_on: tuple[type[Exception], ...] = (Exception,),
    sleep: Callable[[float], None] = time.sleep,
    **kwargs: P.kwargs,
) -> T:
    """Call a function until it succeeds or the retry limit is reached."""
    if max_attempts < 1:
        raise ValueError("max_attempts must be at least 1")
    if initial_delay < 0:
        raise ValueError("initial_delay cannot be negative")
    if multiplier < 1:
        raise ValueError("multiplier must be at least 1")

    delay = initial_delay
    for attempt in range(1, max_attempts + 1):
        try:
            return function(*args, **kwargs)
        except retry_on:
            if attempt == max_attempts:
                raise
            sleep(delay)
            delay *= multiplier

    raise RuntimeError("unreachable")


class RetryWithBackoffTests(unittest.TestCase):
    def test_retries_with_exponential_delays(self) -> None:
        attempts = 0
        delays: list[float] = []

        def flaky() -> str:
            nonlocal attempts
            attempts += 1
            if attempts < 3:
                raise ValueError("temporary failure")
            return "ok"

        result = retry_with_backoff(
            flaky,
            max_attempts=3,
            initial_delay=0.25,
            multiplier=2,
            sleep=delays.append,
        )

        self.assertEqual(result, "ok")
        self.assertEqual(delays, [0.25, 0.5])

    def test_reraises_after_final_attempt(self) -> None:
        delays: list[float] = []

        def always_fails() -> None:
            raise ConnectionError("unavailable")

        with self.assertRaisesRegex(ConnectionError, "unavailable"):
            retry_with_backoff(
                always_fails,
                max_attempts=2,
                initial_delay=1,
                sleep=delays.append,
            )

        self.assertEqual(delays, [1])


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