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#!/usr/bin/env python3

"""

Double hashing is a collision resolving technique in Open Addressed Hash tables.

Double hashing uses the idea of applying a second hash function to key when a collision

occurs. The advantage of Double hashing is that it is one of the best form of probing,

producing a uniform distribution of records throughout a hash table. This technique

does not yield any clusters. It is one of effective method for resolving collisions.

Double hashing can be done using: (hash1(key) + i * hash2(key)) % TABLE_SIZE

Where hash1() and hash2() are hash functions and TABLE_SIZE is size of hash table.

Reference: https://en.wikipedia.org/wiki/Double_hashing

"""

from .hash_table import HashTable

from .number_theory.prime_numbers import is_prime, next_prime

class DoubleHash(HashTable):

"""

Hash Table example with open addressing and Double Hash

"""

def __init__(self, *args, **kwargs):

super().__init__(*args, **kwargs)

def __hash_function_2(self, value, data):

next_prime_gt = (

next_prime(value % self.size_table)

if not is_prime(value % self.size_table)

else value % self.size_table

) # gt = bigger than

return next_prime_gt - (data % next_prime_gt)

def __hash_double_function(self, key, data, increment):

return (increment * self.__hash_function_2(key, data)) % self.size_table

def _collision_resolution(self, key, data=None):

"""

Examples:

1. Try to add three data elements when the size is three

>>> dh = DoubleHash(3)

>>> dh.insert_data(10)

>>> dh.insert_data(20)

>>> dh.insert_data(30)

>>> dh.keys()

{1: 10, 2: 20, 0: 30}

2. Try to add three data elements when the size is two

>>> dh = DoubleHash(2)

>>> dh.insert_data(10)

>>> dh.insert_data(20)

>>> dh.insert_data(30)

>>> dh.keys()

{10: 10, 9: 20, 8: 30}

3. Try to add three data elements when the size is four

>>> dh = DoubleHash(4)

>>> dh.insert_data(10)

>>> dh.insert_data(20)

>>> dh.insert_data(30)

>>> dh.keys()

{9: 20, 10: 10, 8: 30}

"""

i = 1

new_key = self.hash_function(data)

while self.values[new_key] is not None and self.values[new_key] != key:

new_key = (

self.__hash_double_function(key, data, i)

if self.balanced_factor() >= self.lim_charge

else None

)

if new_key is None:

break

else:

i += 1

return new_key

if __name__ == "__main__":

import doctest

doctest.testmod()

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