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42 lines
1.9 KiB
42 lines
1.9 KiB
7 years ago
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## Rabin-Karp Rolling Hash
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## Implementation of: https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm#Hash_function_used
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##
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## This rolling hash function is useful when you need to compute the hash of successive substrings
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## of text. E.g. note that going from 'abcd' to 'bcde', we drop the 'a' from the back and add an 'e'
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## on the right. The rolling hash function thus allows us to update the hash in-place O(1) instead of
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## recomputing the full hash of the substring O(m), where m is the length of the substring.
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##
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## NOTE: The implementation below takes in a tuple of integers, to be as general as possible. For use
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## with strings, simply take the ASCII value of each character before passing into the functions.
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BASE = 101 # Arbitrary prime number
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def rk_hash_init(tpl):
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'''Initializes the hash with a tuple of integers.'''
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return sum(n * BASE ** i for i, n in enumerate(reversed(tpl)))
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def rk_hash_update(curr_hash, size, add_n, rem_n):
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'''Updates the hash by removing an integer from the left and appending
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an integer to the right.
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curr_hash: The previous hash
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size: The size of the rolling window
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add_n: The integer appended to the right
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rem_n: The integer removed from the left'''
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return (curr_hash - (rem_n * BASE ** (size - 1))) * BASE + add_n
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abc_hash = rk_hash_init(tuple(map(ord, 'abc'))) # Init the hash with 'abc'
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print('abc:', abc_hash)
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bcd_hash_1 = rk_hash_update(abc_hash, 3, ord('d'), ord('a')) # Add a 'd' to the right, remove an 'a' from the left
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print('bcd 1:', bcd_hash_1)
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zbc_hash = rk_hash_init(tuple(map(ord, 'zbc'))) # Init the hash with 'zbc'
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print('zbc:', zbc_hash)
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bcd_hash_2 = rk_hash_update(zbc_hash, 3, ord('d'), ord('z')) # Add a 'd' to the right, remove a 'z' from the left
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print('bcd 2:', bcd_hash_2)
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# Notice that both hash values are the same despite arriving via different paths
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print(bcd_hash_1 == bcd_hash_2)
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