Python Hashing (Hash tables and hashlib)

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Hash Tables

When we talk about hash tables, we're actually talking about dictionary. While an array can be used to construct hash tables, array indexes its elements using integers. However, if we want to store data and use keys other than integer, such as 'string', we may want to use dictionary.

Dictionaries in Python are implemented using hash tables. It is an array whose indexes are obtained using a hash function on the keys.

We declare an empty dictionary like this:

>>> D = {}

Then, we can add its elements:

>>> D['a'] = 1
>>> D['b'] = 2
>>> D['c'] = 3
>>> D
{'a': 1, 'c': 3, 'b': 2}

It's a structure with (key, value) pair:

D[key] = value

The string used to "index" the hash table D is called the "key". To access the data stored in the table, we need to know the key:

>>> D['b'] 
2

How we loop through the hash table?

>>> for k in D.keys():
...     print D[k]
... 
1
3
2

If we want to print the (key, value) pair:

>>> for k,v in D.items():
...     print k,':',v
... 
a : 1
c : 3
b : 2

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Hashing from two arrays

Using two Arrays of equal length, create a Hash object where the elements from one array (the keys) are associated with the elements of the other (the values):

>>> keys = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>> hash = {k:v for k, v in zip(keys, values)}
>>> hash
{'a': 1, 'c': 3, 'b': 2}

Hashing

Here are some hashing samples using built-in hash function:

>>> map(hash, [0, 1, 2, 3])
[0, 1, 2, 3]
>>> map(hash, ['0','1','2','3'])
[6144018481, 6272018864, 6400019251, 6528019634]
>>> hash('0')
6144018481

As we can see from the example, Python is using different hash() function depending on the type of data.

Python provides hashlib for secure hashes and message digests:

md5(), sha*():

>>> import hashlib

>>> hashlib.md5('a')

>>> hashlib.md5('a').digest()
'\x0c\xc1u\xb9\xc0\xf1\xb6\xa81\xc3\x99\xe2iw&a'
>>> hashlib.md5('a').hexdigest()
'0cc175b9c0f1b6a831c399e269772661'

>>> hashlib.sha512('a')

>>> hashlib.sha512('a').digest()
'\x1f@\xfc\x92\xda$\x16\x94u\ty\xeel\xf5\x82\xf2\xd5\xd7\xd2\x8e\x183]\xe0Z\xbcT\xd0V\x0e\x0fS\x02\x86\x0ce+\xf0\x8dV\x02R\xaa^t!\x05F\xf3i\xfb\xbb\xce\x8c\x12\xcf\xc7\x95{&R\xfe\x9au'
>>> hashlib.sha512('a').hexdigest()
'1f40fc92da241694750979ee6cf582f2d5d7d28e18335de05abc54d0560e0f5302860c652bf08d560252aa5e74210546f369fbbbce8c12cfc7957b2652fe9a75'
>>>

Hashing example code

The following code is a revision from Sets (union/intersection) and itertools - Jaccard coefficient & shingling to check plagiarism. In this section, we used 64 bit integer (hash value from hash()) for the comparison of shingles instead of directly working on the string.

from __future__ import division
import itertools
import re
import hashlib

# a shingle in this code is a string with K-words
K = 4

def jaccard_set(s1, s2):
    " takes two sets and returns Jaccard coefficient"
    u = s1.union(s2)
    i = s1.intersection(s2)
    return len(i)/len(u)

def make_a_set_of_tokens(doc):
    """makes a set of K-tokens"""

    # replace non-alphanumeric char with a space, and then split
    tokens = re.sub("[^\w]", " ",  doc).split()

    sh = set()
    for i in range(len(tokens)-K):
        t = tokens[i]
        for x in tokens[i+1:i+K]:
            t += ' ' + x 
        sh.add(t)
    return sh

if __name__ == '__main__':

    documents = ["The legal system is made up of civil courts, criminal courts and specialty courts such as family law courts and bankruptcy court. Each court has its own jurisdiction, which refers to the cases that the court is allowed to hear. In some instances, a case can only be heard in one type of court. For example, a bankruptcy case must be heard in a bankruptcy court. In other instances, there may be several potential courts with jurisdiction. For example, a federal criminal court and a state criminal court would each have jurisdiction over a crime that is a federal drug offense but that is also an offense on the state level.",
      "The legal system is comprised of criminal and civil courts and specialty courts like bankruptcy and family law courts. Every one of the courts is vested with its own jurisdiction. Jurisdiction means the types of cases each court is permitted to rule on. Sometimes, only one type of court can hear a particular case. For instance, bankruptcy cases an be ruled on only in bankruptcy court. In other situations, it is possible for more than one court to have jurisdiction. For instance, both a state and federal criminal court could have authority over a criminal case that is illegal under federal and state drug laws.",
      "In many jurisdictions the judicial branch has the power to change laws through the process of judicial review. Courts with judicial review power may annul the laws and rules of the state when it finds them incompatible with a higher norm, such as primary legislation, the provisions of the constitution or international law. Judges constitute a critical force for interpretation and implementation of a constitution, thus de facto in common law countries creating the body of constitutional law."]
  
    shingles = []
    # handle documents one by one
    # makes a list of sets which are compresized of a list of K words string
    for doc in documents:
        # makes a set of tokens
        # sh = set([' ', ..., ' '])
        sh = make_a_set_of_tokens(doc)
        print("sh = %s") %(sh)

        # hasing
        bucket = map(hash, sh)

        # print("bucket = %s") %(bucket)
        
        # shingles : list of sets (sh)
        shingles.append(set(bucket))

    #print("shingles=%s") %(shingles)
    
    combinations = list( itertools.combinations([x for x in range(len(shingles))], 2) )
    print("combinations=%s") %(combinations)

    # compare each pair in combinations tuple of shingles
    for c in combinations:
        i1 = c[0]
        i2 = c[1]
        jac = jaccard_set(shingles[i1], shingles[i2])
        print("%s : jaccard=%s") %(c,jac)

Output is exactly the same as the one we got using string comparison:

combinations=[(0, 1), (0, 2), (1, 2)]
(0, 1) : jaccard=0.0196078431373
(0, 2) : jaccard=0.0
(1, 2) : jaccard=0.0

phymath999

Tuesday, April 19, 2016

python hash

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