KMP algorithm

lvncnt · web-flow · commit 79a73aebb41d · 2017-10-01T17:45:16.000-06:00
diff --git a/Misc/KMP.java b/Misc/KMP.java
@@ -0,0 +1,55 @@
+
+/*
+Implementation of Knuth–Morris–Pratt algorithm
+Usage: 
+final String T = "AAAAABAAABA";
+final String P = "AAAA";
+KMPmatcher(T, P);
+*/
+public class KMP {
+
+    // find the starting index in string T[] that matches the search word P[]
+    public void KMPmatcher(final String T, final String P) {
+        final int m = T.length();
+        final int n = P.length();
+        final int[] pi = computePrefixFunction(P);
+        int q = 0;
+        for (int i = 0; i < m; i++) {
+            while (q > 0 && T.charAt(i) != P.charAt(q)) {
+                q = pi[q - 1]; 
+            }
+
+            if (T.charAt(i) == P.charAt(q)) {
+                q++;
+            }
+
+            if (q == n) {
+                System.out.println("Pattern starts: " + (i + 1 - n));
+                q = pi[q - 1];  
+            }
+        }
+
+    }
+
+    // return the prefix function 
+    private int[] computePrefixFunction(final String P) {
+        final int n = P.length();
+        final int[] pi = new int[n];
+        pi[0] = 0;
+        int q = 0;
+        for (int i = 1; i < n; i++) {
+            while (q > 0 && P.charAt(q) != P.charAt(i)) {
+                q = pi[q - 1];
+            }
+
+            if (P.charAt(q) == P.charAt(i)) {
+                q++;
+            }
+
+            pi[i] = q;
+
+        }
+
+        return pi;
+    }
+}
