Added KMP and Z Algorithm for string pattern matching

python/string algorithms/kmp_string_matching.py

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+'''
+For details on the working of the algorithm, refer this video by Abdul Bari:
+https://www.youtube.com/watch?v=V5-7GzOfADQ
+'''
+
+'''
+Defining the KMP Search function
+'''
+def KMPSearchfn(pattern, input_text): 
+    len_pattern = len(pattern) 
+    len_input = len(input_text) 
+
+    pi = [0]*len_pattern #pi is the piArray that is used in KMP computation
+
+    j = 0 #j is the iterator for pattern
+
+    # computing the pi array of prefix-suffix values 
+    piArray(pattern, len_pattern, pi) 
+
+    i = 0 #iterator for input_text
+
+    while i < len_input:
+        #if the current letters match, we move both in text & pattern 
+        if pattern[j] == input_text[i]: 
+            i += 1
+            j += 1
+
+        #if we reach the end of the pattern, that means that the whole pattern
+        #is matched. Hence, we can return the index
+        if j == len_pattern: 
+            print ("Found pattern at index " + str(i-j) + " of the string") 
+            j = pi[j-1] 
+
+        #if neither of the above case AND the chars don't match,
+        #we move j backwards acc. to the pi array values
+        elif i < len_input and pattern[j] != input_text[i]: 
+
+            if j != 0: 
+                j = pi[j-1]
+
+            #in this subcase, j cannot be moves backwards, so we move i forwards      
+            else: 
+                i += 1
+
+    #if we complete our search and final value of j<len_pattern, then we
+    #can conclude that we didn't find the string
+    if(j<len_pattern):
+        print("The pattern is not present in the string")
+
+
+'''
+Defining the piArray auxillary function, which constructs the piArray
+used during KMP searching 
+'''
+
+
+def piArray(pattern, len_pattern, pi): 
+    l = 0 # length of the previous longest prefix suffix 
+
+    pi[0] # pi[0] is 0, since it is the starting 
+    i = 1
+
+    # the loop calculates pi[i] for i = 1 to (len_pattern-1)
+    while i < len_pattern: 
+        if pattern[i]== pattern[l]: 
+            l += 1
+            pi[i] = l
+            i += 1
+
+        else:  
+            if l != 0: 
+                l = pi[l-1] 
+
+            else: 
+                pi[i] = 0
+                i += 1
+
+
+if __name__ == "__main__":
+
+    print("_______This is the Knuth-Morris-Pratt pattern searching algo________\n\n")
+    print("Enter the text body to be searched IN:")
+    input_text = input()
+
+    print("Enter the string to be searched")
+    pattern = input()
+
+    KMPSearchfn(pattern, input_text)

python/string algorithms/z-algorithm.py

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+'''
+For details on the working of the algorithm, refer this video by Tushar Roy:
+https://www.youtube.com/watch?v=CpZh4eF8QBw
+'''
+
+
+def Z_algo_search(input_text, pattern): 
+
+    # First we concat the pattern, text and a unique character (i am using @) 
+    concat_string = pattern + "@" + input_text 
+    l = len(concat_string) 
+
+    # calling the function to build the Z array 
+    z = [0] * l 
+    getZarr(concat_string, z) 
+
+    # now looping through Z array for matching condition 
+    for i in range(l): 
+
+        # z[i] is the length of the matched region.
+        # if it equals the length of the pattern, we've found the pattern
+        if z[i] == len(pattern): 
+            print("Pattern found at index",  str(i - len(pattern) - 1) + " of the text")
+            return
+
+    print("Pattern not found in the input text")
+
+
+
+'''
+HIGHLY recommended to watch the video to understand how to build the Z array
+'''
+def getZarr(string, z): 
+    n = len(string) 
+
+    left, right, k = 0, 0, 0
+    for i in range(1, n): 
+
+        if i > right: 
+            left, right = i, i 
+
+            while right < n and string[right - left] == string[right]: 
+                right += 1
+            z[i] = right - left 
+            right -= 1
+
+        else: 
+
+            if z[k] < right - i + 1: 
+                z[i] = z[k] 
+
+            else: 
+                left = i 
+                while right < n and string[right - left] == string[right]: 
+                    right += 1
+                z[i] = right - left 
+                right -= 1
+
+
+if __name__ == "__main__":
+
+    print("_______This is the Knuth-Morris-Pratt pattern searching algo________\n\n")
+    print("Enter the text body to be searched IN:") 
+    input_text = input()
+
+    print("Enter the string to be searched")
+    pattern = input()
+    Z_algo_search(input_text, pattern)