from __future__ import annotations

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

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


class LRUCache(Generic[K, V]):
    """Small LRU cache backed by insertion order."""

    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, default: V | None = None) -> V | None:
        """Return a value and mark it as recently used."""
        if key not in self._items:
            return default

        self._items.move_to_end(key)
        return self._items[key]

    def put(self, key: K, value: V) -> None:
        """Insert or update an item, evicting the oldest when full."""
        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)

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

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

    def __iter__(self) -> Iterator[K]:
        return iter(self._items)

    def __repr__(self) -> str:
        return f"LRUCache(capacity={self.capacity}, items={dict(self._items)!r})"


def test_evicts_least_recently_used() -> None:
    cache: LRUCache[str, int] = LRUCache(capacity=2)

    cache.put("a", 1)
    cache.put("b", 2)
    cache.put("c", 3)

    assert cache.get("a") is None
    assert cache.get("b") == 2
    assert cache.get("c") == 3


def test_get_refreshes_usage() -> None:
    cache: LRUCache[str, int] = LRUCache(capacity=2)

    cache.put("a", 1)
    cache.put("b", 2)
    assert cache.get("a") == 1

    cache.put("c", 3)

    assert cache.get("a") == 1
    assert cache.get("b") is None
    assert cache.get("c") == 3


if __name__ == "__main__":
    test_evicts_least_recently_used()
    test_get_refreshes_usage()