public String simulatePreviousGeneration(String currentGeneration) {

        //current individuals
        String cg = currentGeneration.replaceAll("\\+", "");
        //previous generation
        char[] prev = new char[size];
        Arrays.fill(prev, '+');

        //iterate over current individuals
        for (int i = 0; i < cg.length(); i++) {
            //50/50 if they're from the previous generation
            if (random.nextBoolean()) {
                //find a random place
                int r = random.nextInt(size);
                while (prev[r] != '+')
                    r = random.nextInt(size);
                prev[r] = cg.charAt(i);
            }
        }

        //catch the case that none make it by chance
        // /at least 1 parent must exist
        if (new String(prev).replaceAll("\\+", "").length() < 1)
            prev[random.nextInt(size)] = cg.charAt(random.nextInt(cg.length()));

        return new String(prev);
    }