-
Notifications
You must be signed in to change notification settings - Fork 0
/
hash_table.py
313 lines (261 loc) · 11.4 KB
/
hash_table.py
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
""" Hash Table ADT
Defines a Hash Table using Linear Probing for conflict resolution.
That rehashes the primary cluster to handle deletion.
comments starting with # ----- has been added by student to keep track of collision, probes...
and are used in statistics method
"""
__author__ = "Derek Chukwudi Anyanwu"
__author__ = 'Brendon Taylor'
__docformat__ = 'reStructuredText'
__modified__ = '21/05/2020'
__since__ = '14/05/2020'
from referential_array import ArrayR
from typing import TypeVar, Generic
import unittest
T = TypeVar('T')
class LinearProbeHashTable(Generic[T]):
"""
Linear Probe Hash Table
constants:
MIN_CAPACITY: smallest valid table size
DEFAULT_TABLE_SIZE: default table size used in the __init__
DEFAULT_HASH_TABLE: default hash base used for the hash function
PRIMES: list of prime numbers to use for resizing
attributes:
count: number of elements in the hash table
table: used to represent our internal array
hash_base: base prime used in hash function
table_size: current size of the hash table
next_prime: next prime number to use when resizing
"""
MIN_CAPACITY = 1
DEFAULT_TABLE_SIZE = 17
DEFAULT_HASH_BASE = 31
PRIMES = [3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591, 17519, 21023,
25229, 30313, 36353, 43627, 52361, 62851, 75521, 90523, 108631, 130363, 156437, 187751, 225307, 270371,
324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263, 1674319, 2009191, 2411033,
2893249, 3471899, 4166287, 4999559, 5999471, 7199369]
def __init__(self, hash_base: int = DEFAULT_HASH_BASE, table_size: int = DEFAULT_TABLE_SIZE) -> None:
"""
:complexity: O(N) where N is the table_size
"""
self.count = 0
self.table = ArrayR(max(self.MIN_CAPACITY, table_size))
self.hash_base = hash_base
self.next_prime = 0
self.probeChains = []
self.rehash_count = 0
while LinearProbeHashTable.PRIMES[self.next_prime] <= table_size:
self.next_prime += 1
def __len__(self) -> int:
"""
Returns number of elements in the hash table
:complexity: O(1)
"""
return self.count
def __delitem__(self, key: str) -> None:
"""
Deletes an item from our hash table by rehashing the
remaining items in the current primary cluster
:raises KeyError: when the key doesn't exist
:complexity best: O(K) finds the position straight away and doesn't have to rehash
where K is the size of the key
:complexity worst: O(K + N) when it has to rehash all items in the hash table
where N is the table size
"""
position = self.__linear_probe(key, False)
self.table[position] = None
self.count -= 1
position = (position + 1) % len(self.table)
while self.table[position] is not None:
item = self.table[position]
self.table[position] = None
self.count -= 1
self[str(item[0])] = item[1]
position = (position + 1) % len(self.table)
def __rehash(self) -> None:
"""
Need to resize table and reinsert all values
:complexity:
"""
new_hash = LinearProbeHashTable(self.hash_base, LinearProbeHashTable.PRIMES[self.next_prime])
self.next_prime += 1
for i in range(len(self.table)):
if self.table[i] is not None:
new_hash[str(self.table[i][0])] = self.table[i][1]
self.count = new_hash.count
self.table = new_hash.table
self.rehash_count += 1 # ----update rehash_count
def __linear_probe(self, key: str, is_insert: bool) -> int:
"""
Find the correct position for this key in the hash table using linear probing
:complexity best: O(K) first position is empty
where K is the size of the key
:complexity worst: O(K + N) when we've searched the entire table
where N is the table_size
:raises KeyError: When a position can't be found
"""
position = self.hash(key) # get the position using hash
if is_insert and self.is_full():
raise KeyError(key)
for _ in range(len(self.table)): # start traversing
if self.table[position] is None: # found empty slot
if is_insert:
return position
else:
raise KeyError(key) # so the key is not in
elif self.table[position][0] == key: # found key
return position
else: # there is something but not the key, try next
position = (position + 1) % len(self.table)
self.probeChainLength += 1 # ----update probeChainLength
raise KeyError(key)
def __contains__(self, key: str) -> bool:
"""
Checks to see if the given key is in the Hash Table
:see: #self.__getitem__(self, key: str)
"""
try:
_ = self[key]
except KeyError:
return False
else:
return True
def __getitem__(self, key: str) -> T:
"""
Get the item at a certain key
:see: #self.__linear_probe(key: str, is_insert: bool)
:raises KeyError: when the item doesn't exist
"""
position = self.__linear_probe(key, False)
return self.table[position][1]
def __setitem__(self, key: str, data: T) -> None:
"""
Set an (key, data) pair in our hash table
:see: #self.__linear_probe(key: str, is_insert: bool)
:see: #self.__rehash()
"""
self.probeChainLength = 0 # -----initialize probe length to 0
if self.count > len(self.table)//2:
self.__rehash()
# if self.is_full():
# self.__rehash()
# here the probeChainLength is updated depending how many slot it has to go through
# in finding an empty slot of the same key
position = self.__linear_probe(key, True)
if self.table[position] is None:
self.count += 1
# here probeChain list append probeChainLength
self.probeChains.append(self.probeChainLength) # ----append probeChainLength
self.table[position] = (key, data)
def is_empty(self):
"""
Returns whether the hash table is empty
:complexity: O(1)
"""
return self.count == 0
def is_full(self):
"""
Returns whether the hash table is full
:complexity: O(1)
"""
# ____rehashing when hash table is above 50% of the table size
# ____return True if self.count > (0.5 * len(self.table)) else False
return self.count == len(self.table)
# return self.count == 0.5 * len(self.table)
def hash(self, key: str) -> int:
"""
Universal Hash function
:post: returns a valid position (0 <= value < table_size)
:complexity: O(K) where K is the size of the key
"""
value = 0
for c in key:
value = (value * self.hash_base + ord(c)) % len(self.table)
return value
def insert(self, key: str, data: T) -> None:
"""
Utility method to call our setitem method
:see: #__setitem__(self, key: str, data: T)
"""
self[key] = data
def statistics(self) -> tuple:
""" Method which re- turns a tuple (collision count, probe total, probe max, rehash count).
:complexity: O(K) where K is the size of the key
"""
# self.probeChains is list of different probechainLength
collision_count = sum([1 for chain in self.probeChains if chain != 0]) # the total number of collisions
probe_total = sum(self.probeChains) # the sum of all the probe chain lengths
probe_max = max(self.probeChains) # the length of the longest probe chain
rehashCount = self.rehash_count # the number of times rehash has been called
return collision_count, probe_total, probe_max, rehashCount
def __str__(self) -> str:
"""
Returns all they key/value pairs in our hash table (no particular order)
:complexity: O(N) where N is the table size
"""
result = ""
for item in self.table:
if item is not None:
(key, value) = item
result += "(" + str(key) + "," + str(value) + ")\n"
return result
def __iter__(self):
return iter(self.table)
class TestLinearProbeHashTable(unittest.TestCase):
def test_init(self):
""" Basic test to ensure the table is initialised """
dictionary = LinearProbeHashTable()
self.assertEqual(len(dictionary), 0, "Dictionary should be empty")
def test_is_empty(self):
""" Testing when the table is empty and non-empty """
dictionary = LinearProbeHashTable()
self.assertTrue(dictionary.is_empty())
dictionary["test"] = "test"
self.assertFalse(dictionary.is_empty())
def test_is_full(self):
""" Testing when the hash table is full and empty """
dictionary = LinearProbeHashTable(31, 10)
self.assertFalse(dictionary.is_full())
for i in range(10):
dictionary[str(i)] = i
self.assertTrue(len(dictionary) == 10)
def test_hash(self):
""" Testing the get item and contains """
dictionary = LinearProbeHashTable(31, 5)
for i in range(10):
dictionary[str(i)] = i
for i in range(10):
self.assertEqual(dictionary[str(i)], i, "Could not find item: " + str(i))
self.assertTrue(str(i) in dictionary, "Could not find item: " + str(i))
def test_len(self):
""" Testing an empty Hash Table and non-empty Hash Table """
dictionary = LinearProbeHashTable(5)
self.assertEqual(len(dictionary), 0, "Dictionary should be empty")
for i in range(3):
dictionary[str(i)] = i
self.assertEqual(len(dictionary), 3, "Dictionary should contain 3 items")
def test_del(self):
""" Adding 10 items, removing the first 5 and ensuring the state of the Hash Table is correct afterwards """
dictionary = LinearProbeHashTable(5)
for i in range(10):
dictionary[str(i)] = i
for i in range(5):
del dictionary[str(i)]
for i in range(10):
if i < 5:
with self.assertRaises(KeyError):
_ = dictionary[str(i)]
else:
self.assertEqual(dictionary[str(i)], i, "Could not find item: " + str(i))
def test_str(self):
""" Testing an empty table and one with 5 elements """
dictionary = LinearProbeHashTable(31, 5)
self.assertEqual(str(dictionary), "", "Dictionary should be empty")
for i in range(5):
dictionary[str(i)] = i
for i in range(5):
self.assertIn("(" + str(i) + "," + str(i) + ")", str(dictionary))
if __name__ == '__main__':
unittest.main()