"""
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


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2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Python Software Foundation;

All Rights Reserved


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

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"""

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]
