from collections import OrderedDict
from typing import Generic, Optional, TypeVar

K = TypeVar("K")
V = TypeVar("V")


class LRUCache(Generic[K, V]):
    """A fixed-capacity least-recently-used cache."""

    def __init__(self, capacity: int) -> None:
        if capacity <= 0:
            raise ValueError("capacity must be positive")
        self.capacity = capacity
        self._items: OrderedDict[K, V] = OrderedDict()

    def get(self, key: K) -> Optional[V]:
        if key not in self._items:
            return None

        # Mark the key as most recently used.
        self._items.move_to_end(key)
        return self._items[key]

    def put(self, key: K, value: V) -> None:
        if key in self._items:
            self._items.move_to_end(key)

        self._items[key] = value

        # Evict the least recently used item.
        if len(self._items) > self.capacity:
            self._items.popitem(last=False)

    def __len__(self) -> int:
        return len(self._items)

    def __contains__(self, key: object) -> bool:
        return key in self._items


def _run_tests() -> None:
    cache: LRUCache[str, int] = LRUCache(2)
    cache.put("a", 1)
    cache.put("b", 2)
    assert cache.get("a") == 1

    cache.put("c", 3)
    assert cache.get("b") is None
    assert cache.get("a") == 1
    assert cache.get("c") == 3
    assert len(cache) == 2

    cache.put("a", 10)
    assert cache.get("a") == 10


if __name__ == "__main__":
    _run_tests()