Mini Shell

"""
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.

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analyze, test, perform and/or display publicly, prepare derivative works,
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All Rights Reserved" are retained in Python alone or in any derivative version
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"""

SENTINEL = object()


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


class LRUCache(object):
    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]