TheRiddle

import java.awt.image.BufferedImage;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

/**
 * Tuenti Programming Contest
 * Challenge 18: The Riddle
 * @author alefhidalgo [at] gmail [dot] com
 */
public class TheRiddle {

    /* Secret text constants*/
    private static final String HIDDEN_BEGIN_STRING =  "you can also answer this simple question to solve the challenge: how much is";
    private static final String HIDDEN_END_STRING =  "? Crack!";
    private BufferedImage image;

    /**
     * Constructor
     * @param image
     */
    public TheRiddle(BufferedImage image){
        this.image = image;
    }

    /**
     * Get Hidden solution from image
     *  - Read binary value for all stripes pixels until red begin (skipping green background)
     *  - Convert binary string to legible ascii grouping in bytes
     *  - Search the problem statement ( A operation B)
     *
     * @return The solution for the given expression.
     */
    public String getHiddenSolution(){
        int x = image.getWidth();
        int y = image.getHeight();
        StringBuilder sbDecoded = new StringBuilder();
    f1: for (int i = 0; i < x; i++) {
            for (int j = 0; j < y; j++) {
                int pixel = image.getRGB(i, j); //Get pixel value
                int r = (pixel >> 16) & 0xff;
                int g  =(pixel >> 8) & 0xff;
                int b = (pixel) & 0xff;
                if(r==255) break f1; /* until red */
                if(g!=255){ /* skip green */
                    //append 8byte -> ascii chars
                    sbDecoded.append((char)r);
                    sbDecoded.append((char)g);
                    sbDecoded.append((char)b);
                }
            }

        }
        //Get secret expression to solve (A operation B)
        int indexBeginCode= sbDecoded.indexOf(TheRiddle.HIDDEN_BEGIN_STRING) + TheRiddle.HIDDEN_BEGIN_STRING.length();
        int indexEndCode = sbDecoded.indexOf(TheRiddle.HIDDEN_END_STRING);
        return evalExpression(sbDecoded.substring(indexBeginCode,indexEndCode).trim());
    }

    /**
     * evalExpression
     *  Eval expression using JS engine
     *      A + B
     *      A - C
     *      A / C
     *      ...
     * @param expression to solve
     * @return solved expression value
     */
    private String evalExpression(String expression){
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("js");
        Double ret = null;
        try {
            ret = (Double) engine.eval(expression);
        } catch (ScriptException e) {
            /* Never should happen */
            return "NOT A VALID EXPRESSION";
        }
        return String.valueOf(ret.intValue());
    }


    public static void main(String[] s) {
        BufferedImage image= null;
        BASE64DecoderStream decoder = new BASE64DecoderStream(System.in);
        try {
            image = ImageIO.read(decoder);
        } catch (IOException e) {
            /* Never should happen */
        }
        TheRiddle theRiddle = new TheRiddle(image);
        System.out.println(theRiddle.getHiddenSolution());
    }

}