from collections import OrderedDict
from typing import Generic, 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) -> V:
        """Return a value and mark its key as recently used."""
        value = self._items.pop(key)
        self._items[key] = value
        return value

    def put(self, key: K, value: V) -> None:
        """Insert or update a value, evicting the oldest if necessary."""
        if key in self._items:
            self._items.pop(key)
        self._items[key] = value

        if len(self._items) > self.capacity:
            self._items.popitem(last=False)

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

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


def main() -> None:
    cache: LRUCache[str, int] = LRUCache(2)
    cache.put("a", 1)
    cache.put("b", 2)
    cache.put("c", 3)
    assert "a" not in cache and cache.get("c") == 3

    cache.get("b")  # Refresh "b", making "c" the least recently used.
    cache.put("d", 4)
    assert "c" not in cache and cache.get("b") == 2


if __name__ == "__main__":
    main()