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hash_table.py
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180 lines (139 loc) · 4.81 KB
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"""
Hash Table (Open Addressing)
Hash map implementation using open addressing with linear probing
for collision resolution. Includes a resizable variant that doubles
capacity when the load factor reaches two-thirds.
Reference: https://en.wikipedia.org/wiki/Open_addressing
Complexity:
Time: O(1) average for put/get/del, O(n) worst case
Space: O(n)
"""
from __future__ import annotations
class HashTable:
"""Hash table using open addressing with linear probing.
Examples:
>>> ht = HashTable(10)
>>> ht.put(1, 'one')
>>> ht.get(1)
'one'
"""
_empty = object()
_deleted = object()
def __init__(self, size: int = 11) -> None:
"""Initialize the hash table.
Args:
size: Number of slots in the underlying array.
"""
self.size = size
self._len = 0
self._keys: list[object] = [self._empty] * size
self._values: list[object] = [self._empty] * size
def put(self, key: int, value: object) -> None:
"""Insert or update a key-value pair.
Args:
key: The key to insert.
value: The value associated with the key.
Raises:
ValueError: If the table is full.
"""
initial_hash = hash_ = self.hash(key)
while True:
if self._keys[hash_] is self._empty or self._keys[hash_] is self._deleted:
self._keys[hash_] = key
self._values[hash_] = value
self._len += 1
return
elif self._keys[hash_] == key:
self._keys[hash_] = key
self._values[hash_] = value
return
hash_ = self._rehash(hash_)
if initial_hash == hash_:
raise ValueError("Table is full")
def get(self, key: int) -> object | None:
"""Retrieve the value for a given key.
Args:
key: The key to look up.
Returns:
The value associated with the key, or None if not found.
"""
initial_hash = hash_ = self.hash(key)
while True:
if self._keys[hash_] is self._empty:
return None
elif self._keys[hash_] == key:
return self._values[hash_]
hash_ = self._rehash(hash_)
if initial_hash == hash_:
return None
def del_(self, key: int) -> None:
"""Delete a key-value pair.
Args:
key: The key to delete.
"""
initial_hash = hash_ = self.hash(key)
while True:
if self._keys[hash_] is self._empty:
return None
elif self._keys[hash_] == key:
self._keys[hash_] = self._deleted
self._values[hash_] = self._deleted
self._len -= 1
return
hash_ = self._rehash(hash_)
if initial_hash == hash_:
return None
def hash(self, key: int) -> int:
"""Compute the hash index for a key.
Args:
key: The key to hash.
Returns:
Index into the internal array.
"""
return key % self.size
def _rehash(self, old_hash: int) -> int:
"""Linear probing rehash.
Args:
old_hash: The previous hash index.
Returns:
Next index to probe.
"""
return (old_hash + 1) % self.size
def __getitem__(self, key: int) -> object | None:
return self.get(key)
def __delitem__(self, key: int) -> None:
return self.del_(key)
def __setitem__(self, key: int, value: object) -> None:
self.put(key, value)
def __len__(self) -> int:
return self._len
class ResizableHashTable(HashTable):
"""Hash table that automatically doubles in size when load exceeds 2/3.
Examples:
>>> rht = ResizableHashTable()
>>> rht.put(1, 'a')
>>> rht.get(1)
'a'
"""
MIN_SIZE = 8
def __init__(self) -> None:
super().__init__(self.MIN_SIZE)
def put(self, key: int, value: object) -> None:
"""Insert or update, resizing if load factor exceeds two-thirds.
Args:
key: The key to insert.
value: The value associated with the key.
"""
super().put(key, value)
if len(self) >= (self.size * 2) / 3:
self._resize()
def _resize(self) -> None:
"""Double the table size and rehash all existing entries."""
keys, values = self._keys, self._values
self.size *= 2
self._len = 0
self._keys = [self._empty] * self.size
self._values = [self._empty] * self.size
for key, value in zip(keys, values, strict=False):
if key is not self._empty and key is not self._deleted:
self.put(key, value)