KMP Algorithm for pattern searching

Author: Smarty-hero

KMP Algorithm for pattern Searching

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+#include <bits/stdc++.h>
+ 
+void computeLPSArray(char* pat, int M, int* lps);
+ 
+// Prints occurrences of txt[] in pat[]
+void KMPSearch(char* pat, char* txt)
+{
+    int M = strlen(pat);
+    int N = strlen(txt);
+ 
+    // create lps[] that will hold the longest prefix suffix
+    // values for pattern
+    int lps[M];
+ 
+    // Preprocess the pattern (calculate lps[] array)
+    computeLPSArray(pat, M, lps);
+ 
+    int i = 0; // index for txt[]
+    int j = 0; // index for pat[]
+    while ((N - i) >= (M - j)) {
+        if (pat[j] == txt[i]) {
+            j++;
+            i++;
+        }
+ 
+        if (j == M) {
+            printf("Found pattern at index %d ", i - j);
+            j = lps[j - 1];
+        }
+ 
+        // mismatch after j matches
+        else if (i < N && 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 = i + 1;
+        }
+    }
+}
+ 
+// Fills lps[] for given pattern pat[0..M-1]
+void computeLPSArray(char* pat, int M, int* lps)
+{
+    // length of the previous longest prefix suffix
+    int len = 0;
+ 
+    lps[0] = 0; // lps[0] is always 0
+ 
+    // the loop calculates lps[i] for i = 1 to M-1
+    int i = 1;
+    while (i < M) {
+        if (pat[i] == pat[len]) {
+            len++;
+            lps[i] = len;
+            i++;
+        }
+        else // (pat[i] != pat[len])
+        {
+            // 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 // if (len == 0)
+            {
+                lps[i] = 0;
+                i++;
+            }
+        }
+    }
+}
+ 
+// Driver program to test above function
+int main()
+{
+    char txt[] = "ABABDABACDABABCABAB";
+    char pat[] = "ABABCABAB";
+    KMPSearch(pat, txt);
+    return 0;
+}