Added some static methods for simple captcha processing

app/model/Captcha.java

+package model;
+
+public class Captcha {
+
+    /**
+     * URLs of photos the user should select
+     */
+    public static String[] yesPhotos = {
+
+    };
+
+
+    /**
+     * URLs of photos the user should not select
+     */
+    public static String[] noPhotos = {
+
+    };
+
+    /**
+     * How many photos there are in total
+     */
+    public static int numPhotos() {
+        return yesPhotos.length + noPhotos.length;
+    }
+
+    /**
+     * The index of a random photo
+     */
+    public static int randomPhotoIdx() {
+        return (int)(Math.random() * numPhotos());
+    }
+
+    /**
+     * Get a photo with the specified index
+     */
+    public static String getPhoto(int idx) {
+
+        if (idx < 0 || idx >= numPhotos()) {
+            throw new IllegalArgumentException("index out of range");
+        }
+
+        if (idx < yesPhotos.length) {
+            return yesPhotos[idx];
+        } else {
+            return noPhotos[idx - yesPhotos.length];
+        }
+    }
+
+    /**
+     * Whether a photo selected by the user was in the "yes" set
+     */
+    public boolean isCorrect(int idx) {
+        return idx >= 0 && idx < yesPhotos.length;
+    }
+
+}