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Python Program for KMP Algorithm for Pattern Searching[duplicate]

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  • Last Updated : 17 Dec, 2021

Python




# Python program for KMP Algorithm
def KMPSearch(pat, txt):
    M = len(pat)
    N = len(txt)
  
    # 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 i < N:
        if pat[j] == txt[i]:
            i += 1
            j += 1
  
        if j == M:
            print "Found pattern at index " + str(i-j)
            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
  
def computeLPSArray(pat, M, lps):
    len = 0 # length of the previous longest prefix suffix
  
    lps[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
  
txt = "ABABDABACDABABCABAB"
pat = "ABABCABAB"
KMPSearch(pat, txt)
  
# This code is contributed by Bhavya Jain

Please refer complete article on KMP Algorithm for Pattern Searching for more details!


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