142 lines
4.0 KiB
Python
142 lines
4.0 KiB
Python
{
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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)."
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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 caching 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.prev = None\n",
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" self.next = None\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.get(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.get(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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