"""LRU cache with O(1) average get and put."""

from collections import OrderedDict


class LRUCache:
    """A least-recently-used cache holding at most `capacity` keys.

    Backed by an OrderedDict, which keeps keys in insertion/use order and
    provides O(1) average-time membership, lookup, insertion, deletion, and
    reordering (move_to_end / popitem). The most recently used key is kept at
    the end; the least recently used key is at the front.
    """

    def __init__(self, capacity):
        if capacity < 0:
            raise ValueError("capacity must be a non-negative integer")
        self.capacity = capacity
        self._store = OrderedDict()

    def get(self, key):
        """Return the value for `key`, or -1 if absent.

        A hit marks the key as most recently used.
        """
        if key not in self._store:
            return -1
        self._store.move_to_end(key)
        return self._store[key]

    def put(self, key, value):
        """Insert or update `key`, marking it most recently used.

        Updating an existing key does not grow the cache. If inserting a new
        key would exceed `capacity`, the least recently used key is evicted
        first. When `capacity` is 0, this is a no-op.
        """
        if self.capacity == 0:
            return
        if key in self._store:
            self._store[key] = value
            self._store.move_to_end(key)
            return
        self._store[key] = value
        if len(self._store) > self.capacity:
            self._store.popitem(last=False)