Day10.java

public class Day10 {

	public final static int ITERATIONS_PART_1 = 40;
	public final static int ITERATIONS_PART_2 = 50;
	
	public static void main(String[] args) {

		String input = "1321131112";

		System.out.println("Part 1: " + runSequence(input, ITERATIONS_PART_1));
		System.out.println("Part 2: " + runSequence(input, ITERATIONS_PART_2));

	}

	public static String processString(String input) {

		StringBuilder sb = new StringBuilder();

		int pos = 1;
		int count = 1;

		char ch = input.charAt(0);

		while (pos < input.length()) {
			if (ch == input.charAt(pos)) { // same letter as stored
				count++;
				pos++;
			} else { // different letter
				sb.append(count);
				sb.append(ch);
				ch = input.charAt(pos);
				pos++;
				count = 1;
			}
		}
		sb.append(count);
		sb.append(ch);

		return sb.toString();

	}
	
	public static long runSequence(String input, int iterations) {
		
		String output = input;

		for (int i = 0; i < iterations; i++) {
			output = processString(output);
		}
		
		return output.length();
		
	}
	
	

}