Boyer Moore Algorithm in Python

# Python3 Program for Bad Character Heuristic
# of Boyer Moore String Matching Algorithm

NO_OF_CHARS = 256


def badCharHeuristic(string, size):
    '''
    The preprocessing function for
    Boyer Moore's bad character heuristic
    '''

    # Initialize all occurrence as -1
    badChar = [-1]*NO_OF_CHARS

    # Fill the actual value of last occurrence
    for i in range(size):
        badChar[ord(string[i])] = i

    # return initialized list
    return badChar


def search(txt, pat):
    '''
    A pattern searching function that uses Bad Character
    Heuristic of Boyer Moore Algorithm
    '''
    m = len(pat)
    n = len(txt)

    # create the bad character list by calling
    # the preprocessing function badCharHeuristic()
    # for given pattern
    badChar = badCharHeuristic(pat, m)

    # s is shift of the pattern with respect to text
    s = 0
    while(s <= n-m):
        j = m-1

        # Keep reducing index j of pattern while
        # characters of pattern and text are matching
        # at this shift s
        while j >= 0 and pat[j] == txt[s+j]:
            j -= 1

        # If the pattern is present at current shift,
        # then index j will become -1 after the above loop
        if j < 0:
            print("Pattern occur at shift = {}".format(s))

            ''' 
                Shift the pattern so that the next character in text
                    aligns with the last occurrence of it in pattern.
                The condition s+m < n is necessary for the case when
                pattern occurs at the end of text
            '''
            s += (m-badChar[ord(txt[s+m])] if s+m < n else 1)
        else:
            '''
            Shift the pattern so that the bad character in text
            aligns with the last occurrence of it in pattern. The
            max function is used to make sure that we get a positive
            shift. We may get a negative shift if the last occurrence
            of bad character in pattern is on the right side of the
            current character.
            '''
            s += max(1, j-badChar[ord(txt[s+j])])


# Driver program to test above function
def main():
    txt = "ABAAABCD"
    pat = "ABC"
    search(txt, pat)


if __name__ == '__main__':
    main()