from __future__ import annotations

import threading
import time
import unittest
from dataclasses import dataclass
from typing import Callable


Clock = Callable[[], float]


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

    capacity: float
    refill_rate: float
    clock: Clock = time.monotonic

    def __post_init__(self) -> None:
        if self.capacity <= 0:
            raise ValueError("capacity must be positive")
        if self.refill_rate <= 0:
            raise ValueError("refill_rate must be positive")

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

    @property
    def tokens(self) -> float:
        with self._lock:
            self._refill()
            return self._tokens

    def consume(self, amount: float = 1.0) -> bool:
        """Consume tokens immediately, returning False if the bucket is short."""
        if amount <= 0:
            raise ValueError("amount must be positive")
        if amount > self.capacity:
            raise ValueError("amount cannot exceed capacity")

        with self._lock:
            self._refill()
            if self._tokens < amount:
                return False

            self._tokens -= amount
            return True

    def time_until_available(self, amount: float = 1.0) -> float:
        """Return seconds until enough tokens exist for the requested amount."""
        if amount <= 0:
            raise ValueError("amount must be positive")
        if amount > self.capacity:
            raise ValueError("amount cannot exceed capacity")

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

    def wait_and_consume(self, amount: float = 1.0) -> None:
        """Block until tokens are available, then consume them."""
        while True:
            delay = self.time_until_available(amount)
            if delay == 0.0 and self.consume(amount):
                return
            time.sleep(delay)

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


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_consume_until_empty(self) -> None:
        clock = FakeClock()
        bucket = TokenBucket(capacity=2.0, refill_rate=1.0, clock=clock)

        self.assertTrue(bucket.consume())
        self.assertTrue(bucket.consume())
        self.assertFalse(bucket.consume())

    def test_refills_over_time_without_exceeding_capacity(self) -> None:
        clock = FakeClock()
        bucket = TokenBucket(capacity=3.0, refill_rate=2.0, clock=clock)

        self.assertTrue(bucket.consume(3.0))
        clock.advance(0.5)
        self.assertAlmostEqual(bucket.tokens, 1.0)

        clock.advance(10.0)
        self.assertAlmostEqual(bucket.tokens, 3.0)
        self.assertTrue(bucket.consume(3.0))


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