4
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This code uses the Rabin-Karp algorithm to compare two text files. It takes a few minutes when both files contain around 1000 words. How can I improve?

class RollingHash:
    def __init__(self, text, sizeWord):
        self.text = text
        self.hash = 0 
        self.sizeWord = sizeWord

        for i in range(0, sizeWord):
            #subtract out the ASCII value of "a" to start the indexing at zero
            self.hash += (ord(self.text[i]) - ord("a")+1)*(26**(sizeWord - i -1))
        #start index of current window
        self.window_start = 0
        #end of index window
        self.window_end = sizeWord

    def move_window(self):
        if self.window_end <= len(self.text) - 1:

            self.hash -= (ord(self.text[self.window_start]) - ord("a")+1)*26**(self.sizeWord-1)
            self.hash *= 26
            self.hash += ord(self.text[self.window_end])- ord("a")+1
            self.window_start += 1
            self.window_end += 1

    def window_text(self):
        return self.text[self.window_start:self.window_end]

def rabin_karp(word, text):
    if word == "" or text == "":
        return None
    if len(word) > len(text):
        return None

    rolling_hash = RollingHash(text, len(word))
    word_hash = RollingHash(word, len(word))
    word_hash.move_window()

    for i in range(len(text) - len(word) + 1):
        if rolling_hash.hash == word_hash.hash:
            if rolling_hash.window_text() == word:
                return i
        rolling_hash.move_window()
    return None
\$\endgroup\$

migrated from stackoverflow.com May 29 at 19:11

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  • \$\begingroup\$ It is not Rabin-Karp but longest common substring what you need, look it in geeksforgeeks.org/longest-common-substring-dp-29 And your implementation is realy messy. Look in geeksforgeeks.org/rabin-karp-algorithm-for-pattern-searching And afaik (sorry it was long time ago) Rabin-Karp is not the best for substring search, google for suffix trees. \$\endgroup\$ – user8426627 May 29 at 23:30
  • \$\begingroup\$ @user8426627 My assignment required me to solve using Rabin-Karp. After using the code from geeksforgeeks.org/rabin-karp-algorithm-for-pattern-searching that you had mentioned above, the compile time reduce from 1m51sec to 44sec. Is it possible to modify the code to reduce more the compile time? \$\endgroup\$ – Chua May 30 at 4:35
  • 1
    \$\begingroup\$ "This code uses the Rabin-Karp algorithm to compare two text files." No, it does not -- I suppose you use this code as part of a solution that compares files. Posting the whole program and some data for testing might be a good idea. \$\endgroup\$ – Janne Karila May 30 at 5:56

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