Add LRU Cache solution
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Donne Martin
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7595e2a499
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/system-design-primer-primer)."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Design an LRU cache"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Constraints and assumptions\n",
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"\n",
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"* What are we caching?\n",
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" * We are cahing the results of web queries\n",
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"* Can we assume inputs are valid or do we have to validate them?\n",
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" * Assume they're valid\n",
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"* Can we assume this fits memory?\n",
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" * Yes"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Solution"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 1,
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"metadata": {
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"collapsed": false
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},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"Overwriting lru_cache.py\n"
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]
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}
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],
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"source": [
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"%%writefile lru_cache.py\n",
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"class Node(object):\n",
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"\n",
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" def __init__(self, results):\n",
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" self.results = results\n",
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" self.next = next\n",
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"\n",
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"\n",
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"class LinkedList(object):\n",
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"\n",
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" def __init__(self):\n",
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" self.head = None\n",
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" self.tail = None\n",
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"\n",
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" def move_to_front(self, node): # ...\n",
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" def append_to_front(self, node): # ...\n",
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" def remove_from_tail(self): # ...\n",
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"\n",
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"\n",
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"class Cache(object):\n",
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"\n",
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" def __init__(self, MAX_SIZE):\n",
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" self.MAX_SIZE = MAX_SIZE\n",
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" self.size = 0\n",
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" self.lookup = {} # key: query, value: node\n",
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" self.linked_list = LinkedList()\n",
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"\n",
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" def get(self, query)\n",
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" \"\"\"Get the stored query result from the cache.\n",
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" \n",
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" Accessing a node updates its position to the front of the LRU list.\n",
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" \"\"\"\n",
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" node = self.lookup[query]\n",
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" if node is None:\n",
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" return None\n",
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" self.linked_list.move_to_front(node)\n",
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" return node.results\n",
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"\n",
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" def set(self, results, query):\n",
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" \"\"\"Set the result for the given query key in the cache.\n",
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" \n",
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" When updating an entry, updates its position to the front of the LRU list.\n",
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" If the entry is new and the cache is at capacity, removes the oldest entry\n",
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" before the new entry is added.\n",
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" \"\"\"\n",
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" node = self.lookup[query]\n",
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" if node is not None:\n",
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" # Key exists in cache, update the value\n",
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" node.results = results\n",
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" self.linked_list.move_to_front(node)\n",
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" else:\n",
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" # Key does not exist in cache\n",
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" if self.size == self.MAX_SIZE:\n",
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" # Remove the oldest entry from the linked list and lookup\n",
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" self.lookup.pop(self.linked_list.tail.query, None)\n",
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" self.linked_list.remove_from_tail()\n",
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" else:\n",
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" self.size += 1\n",
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" # Add the new key and value\n",
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" new_node = Node(results)\n",
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" self.linked_list.append_to_front(new_node)\n",
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" self.lookup[query] = new_node"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 3",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.4.3"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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@ -0,0 +1,61 @@
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class Node(object):
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def __init__(self, results):
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self.results = results
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self.next = next
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class LinkedList(object):
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def __init__(self):
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self.head = None
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self.tail = None
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def move_to_front(self, node): # ...
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def append_to_front(self, node): # ...
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def remove_from_tail(self): # ...
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class Cache(object):
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def __init__(self, MAX_SIZE):
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self.MAX_SIZE = MAX_SIZE
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self.size = 0
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self.lookup = {} # key: query, value: node
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self.linked_list = LinkedList()
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def get(self, query)
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"""Get the stored query result from the cache.
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Accessing a node updates its position to the front of the LRU list.
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"""
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node = self.lookup[query]
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if node is None:
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return None
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self.linked_list.move_to_front(node)
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return node.results
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def set(self, results, query):
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"""Set the result for the given query key in the cache.
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When updating an entry, updates its position to the front of the LRU list.
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If the entry is new and the cache is at capacity, removes the oldest entry
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before the new entry is added.
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"""
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node = self.lookup[query]
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if node is not None:
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# Key exists in cache, update the value
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node.results = results
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self.linked_list.move_to_front(node)
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else:
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# Key does not exist in cache
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if self.size == self.MAX_SIZE:
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# Remove the oldest entry from the linked list and lookup
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self.lookup.pop(self.linked_list.tail.query, None)
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self.linked_list.remove_from_tail()
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else:
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self.size += 1
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# Add the new key and value
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new_node = Node(results)
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self.linked_list.append_to_front(new_node)
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self.lookup[query] = new_node
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