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

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


class LRUCache(Generic[K, V]):
    """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
        self._items.move_to_end(key)  # Mark as recently used.
        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

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

    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)
    assert cache.get("a") == 1

    cache.put("c", 3)
    assert cache.get("b") is None  # "b" was least recently used.
    assert cache.get("c") == 3
    assert len(cache) == 2

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


if __name__ == "__main__":
    main()