Day13.java

public class Day13 {
    public static void main(String[] args) throws Exception {
//        Iterable<String> input = Files.readAllLines(Paths.get("H:/Data/day13-test.txt"));
        Iterable<String> input = Files.readAllLines(Paths.get("H:/Data/day13.txt"));

        int max = 0;

        Map<Integer, Integer> scanners = new HashMap<>();
        for(String line : input){
            String[] tokens = line.split(": ");

            int layer = Integer.parseInt(tokens[0]);
            int depth = Integer.parseInt(tokens[1]);
            scanners.put(layer, depth);

            if(layer > max)
                max = layer;
        }

        System.out.println(computeSeverity(scanners, max));
        System.out.println(findDelay(scanners));
    }

    private static int findDelay(Map<Integer, Integer> scanners) {
        int delay = 0;
        boolean done = false;

        while(!done){
            delay++;

            skip: while(!done){
                for(Map.Entry<Integer, Integer> entry : scanners.entrySet()){
                    if((delay + entry.getKey()) % ((entry.getValue() - 1)*2) == 0)
                        break skip;
                }

                done = true;
            }
        }
        return delay;
    }

    private static int computeSeverity(Map<Integer, Integer> scanners, int max) {
        List<Layer> layers = new ArrayList<>();

        for(int i=0; i<= max; i++){
            if(scanners.containsKey(i)){
                int depth = scanners.get(i);

                layers.add(new Layer(depth));
            }
            else
                layers.add(new Layer(0));
        }

        int severity = 0;

        for(int location = 0; location < layers.size(); location++){
            Layer layer = layers.get(location);
            if(layer.location == 0)
                severity += location * layer.depth;
            layers.forEach(Layer::move);
        }

        return severity;
    }

    static class Layer{
        public int location = 0;
        int depth;
        boolean movingForward = true;

        public Layer(int depth){
            this.depth = depth;
        }

        public void move(){
            if(depth == 0)
                return;

            if(movingForward)
                location++;
            else
                location--;

            if(location == 0)
                movingForward = true;
            if(location == depth-1)
                movingForward = false;
        }

        @Override
        public String toString(){
            return String.format("%d (%d)", location, depth);
        }
    }
}