Added LCP Array

Author: KubaSz4

src/com/jwetherell/algorithms/data_structures/LCPArray.java

@@ -0,0 +1,84 @@
+package com.jwetherell.algorithms.data_structures;
+
+import java.util.ArrayList;
+
+/**
+ * In computer science, the longest common prefix array (LCP array) is an auxiliary
+ * data structure to the suffix array. It stores the lengths of the longest common
+ * prefixes (LCPs) between all pairs of consecutive suffixes in a sorted suffix array.
+ *
+ * https://en.wikipedia.org/wiki/LCP_array
+ *
+ * @author Jakub Szarawarski <kubaszarawarski@gmail.com>
+ */
+public class LCPArray {
+    private static final char DEFAULT_END_SEQ_CHAR = '$';
+    private char END_SEQ_CHAR;
+
+    private SuffixArray suffixArrayBuilder;
+
+    private ArrayList<Integer> LCP;
+
+    public LCPArray(CharSequence sequence){
+        this(sequence, DEFAULT_END_SEQ_CHAR);
+    }
+
+    public LCPArray(CharSequence sequence, char endChar) {
+        END_SEQ_CHAR = endChar;
+        suffixArrayBuilder = new SuffixArray(sequence, endChar);
+    }
+
+    public ArrayList<Integer> getLCPArray() {
+        if(LCP == null){
+            LCPAlgorithm();
+        }
+        return LCP;
+    }
+
+    private void LCPAlgorithm() {
+        ArrayList<Integer> LCPR = getLCPR();
+        getLCPfromLCPR(LCPR);
+    }
+
+    private ArrayList<Integer> getLCPR() {
+        ArrayList<Integer> KMRArray = suffixArrayBuilder.getKMRarray();
+        ArrayList<Integer> suffixArray = suffixArrayBuilder.getSuffixArray();
+        String string = suffixArrayBuilder.getString();
+        int length = KMRArray.size();
+
+        ArrayList<Integer> LCPR = new ArrayList<Integer>();             // helper array, LCP[i] = LCPR[suffixArray[i]]
+
+        int startingValue = 0;
+
+        for(int i=0; i<length; i++){
+            if(KMRArray.get(i).equals(0)){
+                LCPR.add(0);
+                startingValue = 0;
+            }
+            else{
+                int LCPRValue = startingValue;
+                int predecessor = suffixArray.get(KMRArray.get(i)-1);
+
+                while(string.charAt(i+LCPRValue) == string.charAt(predecessor+LCPRValue))
+                    LCPRValue++;
+
+                LCPR.add(LCPRValue);
+                startingValue = LCPRValue-1 > 0 ? LCPRValue-1 : 0;
+            }
+        }
+
+        return LCPR;
+    }
+
+    private void getLCPfromLCPR(ArrayList<Integer> LCPR) {
+        ArrayList<Integer> suffixArray = suffixArrayBuilder.getSuffixArray();
+        int length = suffixArray.size();
+
+        LCP = new ArrayList<Integer>();
+        LCP.add(null);                                                  //no value for LCP[0]
+
+        for(int i=1; i<length; i++){
+            LCP.add(LCPR.get(suffixArray.get(i)));
+        }
+    }
+}

src/com/jwetherell/algorithms/data_structures/SuffixArray.java

@@ -48,6 +48,10 @@ public ArrayList<Integer> getKMRarray() {
         return KMRarray;
     }
 
+    public String getString(){
+        return string;
+    }
+
     /**
      * Creates suffix array using KMR algorithm with O(n log^2 n) complexity.
      *