from __future__ import annotations

import threading
import time
import unittest
from collections.abc import Callable


class TokenBucket:
    """Thread-safe token-bucket rate limiter."""

    def __init__(
        self,
        capacity: float,
        refill_rate: float,
        clock: Callable[[], float] = time.monotonic,
    ) -> None:
        if capacity <= 0:
            raise ValueError("capacity must be positive")
        if refill_rate <= 0:
            raise ValueError("refill_rate must be positive")

        self.capacity = float(capacity)
        self.refill_rate = float(refill_rate)
        self._tokens = float(capacity)
        self._clock = clock
        self._last_refill = clock()
        self._lock = threading.Lock()

    def _refill(self, now: float) -> None:
        elapsed = max(0.0, now - self._last_refill)
        self._tokens = min(
            self.capacity,
            self._tokens + elapsed * self.refill_rate,
        )
        self._last_refill = now

    def try_acquire(self, tokens: float = 1.0) -> bool:
        """Consume tokens immediately, returning False when insufficient."""
        if tokens <= 0:
            raise ValueError("tokens must be positive")
        if tokens > self.capacity:
            return False

        with self._lock:
            self._refill(self._clock())
            if self._tokens < tokens:
                return False
            self._tokens -= tokens
            return True

    def retry_after(self, tokens: float = 1.0) -> float:
        """Return the seconds until the requested tokens are available."""
        if tokens <= 0:
            raise ValueError("tokens must be positive")
        if tokens > self.capacity:
            raise ValueError("requested tokens exceed bucket capacity")

        with self._lock:
            self._refill(self._clock())
            missing = max(0.0, tokens - self._tokens)
            return missing / self.refill_rate


class FakeClock:
    def __init__(self) -> None:
        self.now = 0.0

    def __call__(self) -> float:
        return self.now

    def advance(self, seconds: float) -> None:
        self.now += seconds


class TokenBucketTests(unittest.TestCase):
    def test_rejects_when_empty(self) -> None:
        clock = FakeClock()
        bucket = TokenBucket(capacity=2, refill_rate=1, clock=clock)

        self.assertTrue(bucket.try_acquire(2))
        self.assertFalse(bucket.try_acquire())

    def test_refills_over_time(self) -> None:
        clock = FakeClock()
        bucket = TokenBucket(capacity=2, refill_rate=2, clock=clock)

        self.assertTrue(bucket.try_acquire(2))
        self.assertAlmostEqual(bucket.retry_after(), 0.5)

        clock.advance(0.5)
        self.assertTrue(bucket.try_acquire())
        self.assertFalse(bucket.try_acquire())


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