"""
A fork of Python 3.6's stdlib lru_cache (found in Python's 'cpython/Lib/functools.py')
adapted into a data structure for single threaded uses.

https://github.com/python/cpython/blob/v3.6.12/Lib/functools.py


Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Python Software Foundation;

All Rights Reserved


PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2
--------------------------------------------

1. This LICENSE AGREEMENT is between the Python Software Foundation
("PSF"), and the Individual or Organization ("Licensee") accessing and
otherwise using this software ("Python") in source or binary form and
its associated documentation.

2. Subject to the terms and conditions of this License Agreement, PSF hereby
grants Licensee a nonexclusive, royalty-free, world-wide license to reproduce,
analyze, test, perform and/or display publicly, prepare derivative works,
distribute, and otherwise use Python alone or in any derivative version,
provided, however, that PSF's License Agreement and PSF's notice of copyright,
i.e., "Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Python Software Foundation;
All Rights Reserved" are retained in Python alone or in any derivative version
prepared by Licensee.

3. In the event Licensee prepares a derivative work that is based on
or incorporates Python or any part thereof, and wants to make
the derivative work available to others as provided herein, then
Licensee hereby agrees to include in any such work a brief summary of
the changes made to Python.

4. PSF is making Python available to Licensee on an "AS IS"
basis.  PSF MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR
IMPLIED.  BY WAY OF EXAMPLE, BUT NOT LIMITATION, PSF MAKES NO AND
DISCLAIMS ANY REPRESENTATION OR WARRANTY OF MERCHANTABILITY OR FITNESS
FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF PYTHON WILL NOT
INFRINGE ANY THIRD PARTY RIGHTS.

5. PSF SHALL NOT BE LIABLE TO LICENSEE OR ANY OTHER USERS OF PYTHON
FOR ANY INCIDENTAL, SPECIAL, OR CONSEQUENTIAL DAMAGES OR LOSS AS
A RESULT OF MODIFYING, DISTRIBUTING, OR OTHERWISE USING PYTHON,
OR ANY DERIVATIVE THEREOF, EVEN IF ADVISED OF THE POSSIBILITY THEREOF.

6. This License Agreement will automatically terminate upon a material
breach of its terms and conditions.

7. Nothing in this License Agreement shall be deemed to create any
relationship of agency, partnership, or joint venture between PSF and
Licensee.  This License Agreement does not grant permission to use PSF
trademarks or trade name in a trademark sense to endorse or promote
products or services of Licensee, or any third party.

8. By copying, installing or otherwise using Python, Licensee
agrees to be bound by the terms and conditions of this License
Agreement.

"""

SENTINEL = object()


# aliases to the entries in a node
PREV = 0
NEXT = 1
KEY = 2
VALUE = 3


class LRUCache:
    def __init__(self, max_size):
        assert max_size > 0

        self.max_size = max_size
        self.full = False

        self.cache = {}

        # root of the circularly linked list to keep track of
        # the least recently used key
        self.root = []  # type: ignore
        # the node looks like [PREV, NEXT, KEY, VALUE]
        self.root[:] = [self.root, self.root, None, None]

        self.hits = self.misses = 0

    def set(self, key, value):
        link = self.cache.get(key, SENTINEL)

        if link is not SENTINEL:
            # have to move the node to the front of the linked list
            link_prev, link_next, _key, _value = link

            # first remove the node from the lsnked list
            link_prev[NEXT] = link_next
            link_next[PREV] = link_prev

            # insert the node between the root and the last
            last = self.root[PREV]
            last[NEXT] = self.root[PREV] = link
            link[PREV] = last
            link[NEXT] = self.root

            # update the value
            link[VALUE] = value

        elif self.full:
            # reuse the root node, so update its key/value
            old_root = self.root
            old_root[KEY] = key
            old_root[VALUE] = value

            self.root = old_root[NEXT]
            old_key = self.root[KEY]

            self.root[KEY] = self.root[VALUE] = None

            del self.cache[old_key]

            self.cache[key] = old_root

        else:
            # insert new node after last
            last = self.root[PREV]
            link = [last, self.root, key, value]
            last[NEXT] = self.root[PREV] = self.cache[key] = link
            self.full = len(self.cache) >= self.max_size

    def get(self, key, default=None):
        link = self.cache.get(key, SENTINEL)

        if link is SENTINEL:
            self.misses += 1
            return default

        # have to move the node to the front of the linked list
        link_prev, link_next, _key, _value = link

        # first remove the node from the lsnked list
        link_prev[NEXT] = link_next
        link_next[PREV] = link_prev

        # insert the node between the root and the last
        last = self.root[PREV]
        last[NEXT] = self.root[PREV] = link
        link[PREV] = last
        link[NEXT] = self.root

        self.hits += 1

        return link[VALUE]