alien.py

# Python3 program for KMP Algorithm


def KMPSearch(pat, txt):
    M = len(pat)
    N = len(txt)
    a = 0

    # create lps[] that will hold the longest prefix suffix
    # values for pattern
    lps = [0] * M
    j = 0  # index for pat[]

    # Preprocess the pattern (calculate lps[] array)
    computeLPSArray(pat, M, lps)

    i = 0  # index for txt[]
    while (N - i) >= (M - j):
        if pat[j] == txt[i]:
            i += 1
            j += 1

        if j == M:
            # print("Found pattern at index " + str(i - j))
            a += 1
            j = lps[j - 1]

        # mismatch after j matches
        elif i < N and pat[j] != txt[i]:
            # Do not match lps[0..lps[j-1]] characters,
            # they will match anyway
            if j != 0:
                j = lps[j - 1]
            else:
                i += 1
    return a


# Function to compute LPS array
def computeLPSArray(pat, M, lps):
    len = 0  # length of the previous longest prefix suffix

    lps[0] = 0  # lps[0] is always 0
    i = 1

    # the loop calculates lps[i] for i = 1 to M-1
    while i < M:
        if pat[i] == pat[len]:
            len += 1
            lps[i] = len
            i += 1
        else:
            # This is tricky. Consider the example.
            # AAACAAAA and i = 7. The idea is similar
            # to search step.
            if len != 0:
                len = lps[len - 1]

            # Also, note that we do not increment i here
            else:
                lps[i] = 0
                i += 1


# Driver code
if __name__ == '__main__':
    txt = "ABABDABACDABABCABAB"
    pat = "ABABCABAB"
    if KMPSearch(pat, txt) < 2:

    print(KMPSearch(pat, txt))

# This code is contributed by Bhavya Jain