system-design-primer/solutions/system_design/query_cache/query_cache_snippets.py

# -*- coding: utf-8 -*-

class QueryApi(object):

    def __init__(self, memory_cache, reverse_index_cluster):
        self.memory_cache = memory_cache
        self.reverse_index_cluster = reverse_index_cluster

    def parse_query(self, query):
        """Remove markup, break text into terms, deal with typos,
        normalize capitalization, convert to use boolean operations.
        """
        ...

    def process_query(self, query):
        query = self.parse_query(query)
        results = self.memory_cache.get(query)
        if results is None:
            results = self.reverse_index_cluster.process_search(query)
            self.memory_cache.set(query, results)
        return results


class Node(object):

    def __init__(self, query, results):
        self.query = query
        self.results = results


class LinkedList(object):

    def __init__(self):
        self.head = None
        self.tail = None

    def move_to_front(self, node):
        ...

    def append_to_front(self, node):
        ...

    def remove_from_tail(self):
        ...


class Cache(object):

    def __init__(self, MAX_SIZE):
        self.MAX_SIZE = MAX_SIZE
        self.size = 0
        self.lookup = {}
        self.linked_list = LinkedList()

    def get(self, query):
        """Get the stored query result from the cache.

        Accessing a node updates its position to the front of the LRU list.
        """
        node = self.lookup[query]
        if node is None:
            return None
        self.linked_list.move_to_front(node)
        return node.results

    def set(self, results, query):
        """Set the result for the given query key in the cache.

        When updating an entry, updates its position to the front of the LRU list.
        If the entry is new and the cache is at capacity, removes the oldest entry
        before the new entry is added.
        """
        node = self.map[query]
        if node is not None:
            # Key exists in cache, update the value
            node.results = results
            self.linked_list.move_to_front(node)
        else:
            # Key does not exist in cache
            if self.size == self.MAX_SIZE:
                # Remove the oldest entry from the linked list and lookup
                self.lookup.pop(self.linked_list.tail.query, None)
                self.linked_list.remove_from_tail()
            else:
                self.size += 1
            # Add the new key and value
            new_node = Node(query, results)
            self.linked_list.append_to_front(new_node)
            self.lookup[query] = new_node