Least recently used dictionary

lrucache.py

#!/usr/bin/env python3
import collections
import random
from typing import Hashable, Any, Optional, Dict, Tuple


class LruCache:
    """Dictionary like storage of most recently inserted values"""

    def __init__(self, size: int = 1000):
        """:param size number of cached entries"""
        assert isinstance(size, int)
        self.size = size
        self.insert_counter = 0
        self.oldest = 0
        self._data : Dict[Hashable,Tuple[Any,int]]= {} # store values and age index
        self._lru: Dict[int, Hashable] = {} # age counter dictionary

    def insert(self, key: Hashable, value: Any) -> None:
        """Insert into dictionary"""
        existing = self._data.get(key, None)
        self._data[key] = (value, self.insert_counter)
        self._lru[self.insert_counter] = key
        if existing is not None:
            self._lru.pop(existing[1], None)  # remove old counter value, if it exists
        self.insert_counter += 1
        if (sz := len(self._data)) > self.size:  # is cache full?
            assert sz == self.size + 1
            while (
            key := self._lru.get(self.oldest, None)) is None:  # index may not be present, if value was reinserted
                self.oldest += 1
            del self._data[key]  # remove oldest key / value from dictionary
            del self._lru[self.oldest]
            self.oldest += 1  # next oldest index
        assert len(self._lru) == len(self._data)

    def get(self, key: Hashable) -> Optional[Any]:
        """Get value or return None if not in cache"""
        if (tpl := self._data.get(key, None)) is not None:
            return tpl[0]
        return None


if __name__ == "__main__":
    CACHE_SIZE = 1000
    TEST_SIZE = 1_000_000
    cache = LruCache(size=CACHE_SIZE)

    all = []
    for i in range(TEST_SIZE):
        all.append(random.randint(-5000, 5000))

    summary = collections.defaultdict(int)
    for value in all:
        cache.insert(value, value * value)
        summary[value] += 1
    smallest = TEST_SIZE
    largest = -TEST_SIZE
    total = 0
    for value, count in summary.items():
        smallest = min(smallest, count)
        largest = max(largest, count)
        total += count
    avg = total / len(summary)
    print(f"{len(summary)} values occurrences range from {smallest} to {largest}, average {avg:.1f}")

    recent = set()  # recent most recent entries
    for i in range(len(all) - 1, -1, -1):  # loop backwards to get the most recent entries
        value = all[i]
        if len(recent) < CACHE_SIZE:
            recent.add(value)
        if value in recent:
            if (r := cache.get(value)) != value * value:
                raise ValueError(f"Cache missing recent {value} {r}")
        else:
            if cache.get(value) != None:
                raise ValueError(f"Cache includes old {value}")

Copyright 2022 Gerard Weatherby

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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