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_with_backoff(
    operation: Callable[P, T],
    *args: P.args,
    max_attempts: int = 3,
    initial_delay: float = 0.5,
    backoff_factor: float = 2.0,
    retry_on: tuple[type[Exception], ...] = (Exception,),
    sleep: Callable[[float], None] = time.sleep,
    **kwargs: P.kwargs,
) -> T:
    """Run an operation, retrying failures with exponential backoff."""
    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 backoff_factor < 1:
        raise ValueError("backoff_factor must be at least 1")

    delay = initial_delay

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

            # Wait progressively longer before each subsequent attempt.
            sleep(delay)
            delay *= backoff_factor

    raise RuntimeError("unreachable")


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

        def flaky_operation() -> str:
            nonlocal attempts
            attempts += 1
            if attempts < 3:
                raise OSError("temporary failure")
            return "success"

        result = retry_with_backoff(
            flaky_operation,
            max_attempts=4,
            initial_delay=0.25,
            backoff_factor=2,
            retry_on=(OSError,),
            sleep=delays.append,
        )

        self.assertEqual(result, "success")
        self.assertEqual(attempts, 3)
        self.assertEqual(delays, [0.25, 0.5])

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

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

        with self.assertRaisesRegex(ValueError, "permanent failure"):
            retry_with_backoff(
                always_fails,
                max_attempts=3,
                initial_delay=1,
                retry_on=(ValueError,),
                sleep=delays.append,
            )

        self.assertEqual(delays, [1, 2])


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