Added kmp_string_pattern_search.py

Language/Python/kmp_string_pattern_search.py

@@ -0,0 +1,44 @@
+# KMP String Pattern Searching Algorithm
+
+def compute_lps_array(pattern, pat_len, longest_prefix_suffix):
+    pat_index = 1
+    len_prev = 0 # length of the previous longest prefix suffix
+    while pat_index < pat_len:
+        if pattern[pat_index] == pattern[len_prev]:
+            len_prev += 1
+            longest_prefix_suffix[pat_index] = len_prev
+            pat_index += 1
+        else:
+            if len_prev != 0:
+                len_prev = longest_prefix_suffix[len_prev-1]
+            else:
+                longest_prefix_suffix[pat_index] = 0
+                pat_index += 1
+
+def kmp_search(txt, pattern):
+    text_len = len(txt)
+    pat_len = len(pattern)
+
+    # create list that will hold the longest prefix suffix values for pattern
+    longest_prefix_suffix = [0]*pat_len
+    # Preprocess the pattern
+    compute_lps_array(pattern, pat_len, longest_prefix_suffix)
+
+    txt_index = 0
+    pat_index = 0
+    while txt_index < text_len:
+        if pattern[pat_index] == txt[txt_index]:
+            pat_index += 1
+            txt_index += 1
+        if pat_index == pat_len:
+            print("Found pattern at index " + str(txt_index - pat_index))
+            pat_index = longest_prefix_suffix[pat_index-1]
+        elif txt_index < text_len and pattern[pat_index] != txt[txt_index]:
+            if pat_index != 0:
+                pat_index = longest_prefix_suffix[pat_index-1]
+            else:
+                txt_index += 1
+
+txt = input("Enter the string to be searched: ") #"AAAAAACAAAAAAFAAAAAB"
+pattern = input("Enter the substring to look for: ") #"AAAB"
+kmp_search(txt, pattern)