from __future__ import annotations

import time
import unittest
from collections.abc import Callable


class TokenBucket:
    """Token-bucket rate limiter using a monotonic clock."""

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

        self.rate = rate
        self.capacity = capacity
        self._tokens = capacity
        self._clock = clock
        self._updated_at = clock()

    def _refill(self) -> None:
        now = self._clock()
        elapsed = max(0.0, now - self._updated_at)
        self._tokens = min(self.capacity, self._tokens + elapsed * self.rate)
        self._updated_at = now

    def consume(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

        self._refill()
        if self._tokens < tokens:
            return False

        self._tokens -= tokens
        return True

    def retry_after(self, tokens: float = 1.0) -> float:
        """Return seconds until the requested tokens should be available."""
        if tokens <= 0:
            raise ValueError("tokens must be positive")
        if tokens > self.capacity:
            raise ValueError("tokens cannot exceed capacity")

        self._refill()
        return max(0.0, (tokens - self._tokens) / self.rate)


class TokenBucketTests(unittest.TestCase):
    def setUp(self) -> None:
        self.now = 0.0
        self.bucket = TokenBucket(2.0, 3.0, lambda: self.now)

    def test_capacity_is_enforced(self) -> None:
        self.assertTrue(self.bucket.consume(3.0))
        self.assertFalse(self.bucket.consume())

    def test_tokens_refill_over_time(self) -> None:
        self.assertTrue(self.bucket.consume(3.0))
        self.now += 0.5
        self.assertTrue(self.bucket.consume())
        self.assertFalse(self.bucket.consume())
        self.assertAlmostEqual(self.bucket.retry_after(), 0.5)


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