from __future__ import annotations

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

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


def retry_with_exponential_backoff(
    func: Callable[P, T],
    *args: P.args,
    attempts: int = 3,
    initial_delay: float = 0.1,
    multiplier: float = 2.0,
    max_delay: float = 10.0,
    jitter: float = 0.0,
    retry_on: tuple[type[BaseException], ...] = (Exception,),
    sleep: Callable[[float], None] = time.sleep,
    **kwargs: P.kwargs,
) -> T:
    """Call func until it succeeds or retry attempts are exhausted."""
    if attempts < 1:
        raise ValueError("attempts must be at least 1")
    if initial_delay < 0:
        raise ValueError("initial_delay must be non-negative")
    if multiplier < 1:
        raise ValueError("multiplier must be at least 1")
    if max_delay < 0:
        raise ValueError("max_delay must be non-negative")
    if jitter < 0:
        raise ValueError("jitter must be non-negative")

    delay = initial_delay

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

            # Jitter helps avoid synchronized retries across many callers.
            actual_delay = delay + random.uniform(0, jitter) if jitter else delay
            sleep(actual_delay)
            delay = min(delay * multiplier, max_delay)

    raise RuntimeError("unreachable")


class RetryWithBackoffTests(unittest.TestCase):
    def test_succeeds_after_transient_failures(self) -> None:
        calls = 0
        sleeps: list[float] = []

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

        result = retry_with_exponential_backoff(
            flaky,
            attempts=4,
            initial_delay=0.5,
            multiplier=2.0,
            sleep=sleeps.append,
        )

        self.assertEqual(result, "ok")
        self.assertEqual(calls, 3)
        self.assertEqual(sleeps, [0.5, 1.0])

    def test_raises_after_attempts_are_exhausted(self) -> None:
        sleeps: list[float] = []

        def always_fails() -> None:
            raise ConnectionError("still down")

        with self.assertRaises(ConnectionError):
            retry_with_exponential_backoff(
                always_fails,
                attempts=3,
                initial_delay=1.0,
                multiplier=3.0,
                max_delay=2.0,
                sleep=sleeps.append,
            )

        self.assertEqual(sleeps, [1.0, 2.0])


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