Added Jaccard similarity

Author: tdebatty

src/main/java/info/debatty/java/stringsimilarity/Jaccard.java

@@ -0,0 +1,91 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2015 tibo.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package info.debatty.java.stringsimilarity;
+
+/**
+ * 
+ * @author Thibault Debatty
+ */
+public class Jaccard implements StringSimilarityInterface {
+
+    /**
+     * @param args the command line arguments
+     */
+    public static void main(String[] args) {
+        Jaccard j2 = new Jaccard(2);
+        
+        // AB BC CD DE DF
+        // 1  1  1  1  0
+        // 1  1  1  0  1
+        // => 3 / 5 = 0.6
+        System.out.println(j2.similarity("ABCDE", "ABCDF"));
+    }
+    
+    private int k;
+    
+    /**
+     * The strings are first transformed into sets of k-shingles (sequences of k
+     * characters), then Jaccard index is computed as |A inter B| / |A union B|.
+     * The default value of k is 3.
+     * 
+     * @param k 
+     */
+    public Jaccard(int k) {
+        this.k = k;
+    }
+    
+    public Jaccard() {
+        this.k = 3;
+    }
+
+    public double similarity(String s1, String s2) {
+        KShingling ks = new KShingling(this.k);
+        ks.parse(s1);
+        ks.parse(s2);
+        
+        boolean[] v1 = ks.booleanVectorOf(s1);
+        boolean[] v2 = ks.booleanVectorOf(s2);
+        
+        int inter = 0;
+        int union = 0;
+        for (int i = 0; i < v1.length; i++) {
+            if (v1[i] || v2[i]) {
+                union++;
+                
+                if (v1[i] && v2[i]) {
+                    inter++;
+                }
+            }
+        }
+        
+        return (double) inter / union;
+        
+    }
+
+    public double distance(String s1, String s2) {
+        return 1.0 - similarity(s1, s2);
+    }
+    
+}

src/main/java/info/debatty/java/stringsimilarity/QGram.java

@@ -24,6 +24,8 @@ public static void main(String[] args) {
         // 2 / (3 + 3) = 0.33333
         System.out.println(dig.distance("ABCD", "ABCE"));
 
+        System.out.println(dig.similarity("", "QSDFGHJKLM"));
+        
         System.out.println(dig.similarity(
                 "High Qua1ityMedications   Discount On All Reorders = Best Deal Ever! Viagra50/100mg - $1.85 071",
                 "High Qua1ityMedications   Discount On All Reorders = Best Deal Ever! Viagra50/100mg - $1.85 7z3"));
@@ -55,7 +57,7 @@ public int absoluteDistance(String s1, String s2) {
 
     protected double dist(String s1, String s2, boolean abs) {
         if (s1.length() < n || s2.length() < n) {
-            return 0;
+            return 1;
         }
 
         KShingling sh = new KShingling(n);