int GA_POPSIZE= 300; // GA population size
  int GA_MAXITER= 200; // maximum iterations
  float GA_ELITRATE= 0.10; // elitism rate
  float GA_MUTATION= 0.25; // mutation rate
  String [] population = new String[GA_POPSIZE]; // array to hold possible solutions
  int [] population_fitness = new int[GA_POPSIZE]; // array to hold the fitness value
  String [] buffer = new String[GA_POPSIZE]; // a copy of population
  int [] buffer_fitness = new int[GA_POPSIZE]; // a copy of fitness values
  int esize = (int)( GA_POPSIZE * GA_ELITRATE); //elitism size
  String character = ""; // a dummy String used to create words
  int w = 30; //side of the bitmap (30x30)
  int tsize = w*w; // the size (length) of the target String
  int iter = 0;
  void setup(){
    size(200,200);
    // initialize the population: creates a number of randomom Strings
    for(int i=0; i< GA_POPSIZE; i++){
      character = "";
      population_fitness[i] = 0;
      for(int j=0; j< tsize; j++) // for tsize characters
        character += str(int(random(2))); //create a random String
      population[i] = character;
    }
  }
  void draw(){
    background(255);
    int i=0;
    for(int x=0; x<w*5; x+=5)
      for(int y=0; y<w*5; y+=5){
        if(population[0].charAt(i)=='1'){
          fill(0);
          rect(25+y,25+x,5,5);
        }
        i++;
      }
  }
  void keyPressed(){
    // calculate fitness
    for(int i=0; i<GA_POPSIZE; i++){
      int k=0;
      int pop_length = population[i].length();
      for(int idx=0; idx<pop_length/2; idx++)
        if(population[i].charAt((idx/15)*30+(idx%15))==
          population[i].charAt((idx/15)*30+(30-idx%15-1)))
          k++;
      population_fitness[i] = (w*w/2) - k;
    }
    // sort them (simple bubble sort)
    for(int i=1; i< GA_POPSIZE; i++)
      for(int j=0; j< GA_POPSIZE-1; j++)
        if( population_fitness[i] < population_fitness[j]){
          //swap values
          int temp = population_fitness[ i];
          population_fitness[i] = population_fitness[j];
          population_fitness[j] = temp;
          String stemp = population[i];
          population[i] = population[j];
          population[j] = stemp;
        }
    // print the best one
    println( (iter++) + " Best fitness: " + population_fitness[0] );
    // mate take half of the population and cross it with the other half
    int spos, i1, i2;
    for (int i=0; i< GA_POPSIZE; i++) {
      i1 = (int)(random(1.) * esize); //random position within the elite population
        i2 = (int)(random(1.) * esize); //random position within the elite population
        spos = (int)(random(1.) * tsize); //random position of character within a String
        buffer[i] = population[i1].substring(0, spos ) +
        population[i2].substring(spos, tsize); //crossover
      // mutate: take a tring and alter one of its characters
      if(random(1.) < GA_MUTATION) { //if chance is 1:mutation rate
        int ipos = (int)(random(1.) * tsize); //select a random character
        character = "";
        for(int j=0; j< tsize; j++)
          if( j== ipos)
            character += str(int(random(2))); //replace a random character in the String
            else
            character += buffer[i].charAt(j);
      }
      buffer[i] = character;
    }//for
    // swap buffers
    for (int i=0; i< GA_POPSIZE; i++) {
      population[i] = buffer[i];
      population_fitness[i] = buffer_fitness[i];
    }
  } // key pressed