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Add LRU Cache solution

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Donne Martin 2017-03-03 18:53:54 -08:00
parent 7595e2a499
commit ae9832c6b7
3 changed files with 201 additions and 0 deletions
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solutions/object_oriented_design/lru_cache/__init__.py Normal file
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solutions/object_oriented_design/lru_cache/lru_cache.ipynb Normal file
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{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"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)."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Design an LRU cache"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Constraints and assumptions\n",
"\n",
"* What are we caching?\n",
" * We are cahing the results of web queries\n",
"* Can we assume inputs are valid or do we have to validate them?\n",
" * Assume they're valid\n",
"* Can we assume this fits memory?\n",
" * Yes"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Solution"
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Overwriting lru_cache.py\n"
]
}
],
"source": [
"%%writefile lru_cache.py\n",
"class Node(object):\n",
"\n",
" def __init__(self, results):\n",
" self.results = results\n",
" self.next = next\n",
"\n",
"\n",
"class LinkedList(object):\n",
"\n",
" def __init__(self):\n",
" self.head = None\n",
" self.tail = None\n",
"\n",
" def move_to_front(self, node): # ...\n",
" def append_to_front(self, node): # ...\n",
" def remove_from_tail(self): # ...\n",
"\n",
"\n",
"class Cache(object):\n",
"\n",
" def __init__(self, MAX_SIZE):\n",
" self.MAX_SIZE = MAX_SIZE\n",
" self.size = 0\n",
" self.lookup = {} # key: query, value: node\n",
" self.linked_list = LinkedList()\n",
"\n",
" def get(self, query)\n",
" \"\"\"Get the stored query result from the cache.\n",
" \n",
" Accessing a node updates its position to the front of the LRU list.\n",
" \"\"\"\n",
" node = self.lookup[query]\n",
" if node is None:\n",
" return None\n",
" self.linked_list.move_to_front(node)\n",
" return node.results\n",
"\n",
" def set(self, results, query):\n",
" \"\"\"Set the result for the given query key in the cache.\n",
" \n",
" When updating an entry, updates its position to the front of the LRU list.\n",
" If the entry is new and the cache is at capacity, removes the oldest entry\n",
" before the new entry is added.\n",
" \"\"\"\n",
" node = self.lookup[query]\n",
" if node is not None:\n",
" # Key exists in cache, update the value\n",
" node.results = results\n",
" self.linked_list.move_to_front(node)\n",
" else:\n",
" # Key does not exist in cache\n",
" if self.size == self.MAX_SIZE:\n",
" # Remove the oldest entry from the linked list and lookup\n",
" self.lookup.pop(self.linked_list.tail.query, None)\n",
" self.linked_list.remove_from_tail()\n",
" else:\n",
" self.size += 1\n",
" # Add the new key and value\n",
" new_node = Node(results)\n",
" self.linked_list.append_to_front(new_node)\n",
" self.lookup[query] = new_node"
]
}
],
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"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
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"codemirror_mode": {
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"version": 3
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"file_extension": ".py",
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solutions/object_oriented_design/lru_cache/lru_cache.py Normal file
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class Node(object):
def __init__(self, results):
self.results = results
self.next = next
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 = {} # key: query, value: node
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.lookup[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(results)
self.linked_list.append_to_front(new_node)
self.lookup[query] = new_node