Day12.java

import java.nio.file.Files;
import java.nio.file.Path;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.function.Function;

class Day12 {

    public static void main(String ... args) throws Exception {
        var map = readMap(Files.readString(Path.of("../input/day12.txt")));
        XY start = map.keySet().stream().filter(k -> map.get(k) == 'S').findFirst().get();
        XY end = map.keySet().stream().filter(k -> map.get(k) == 'E').findFirst().get();
        map.put(start, (byte) 'a');
        map.put(end, (byte) 'z');

        System.out.printf("part1: %d\n",
            shortestPath(map, new HashMap<>(), start, dh -> dh <= 1, p -> p.equals(end)));
        System.out.printf("part2: %d\n",
            shortestPath(map, new HashMap<>(), end, dh -> dh >= -1, p -> map.get(p) == 'a'));
    }
 
    static int shortestPath(Map<XY,Byte> map, Map<XY,Integer> visits, XY start,
         Function<Byte,Boolean> ok, Function<XY,Boolean> target)  {        
        var visited = new HashMap<XY,Integer>();
        var toVisit = new LinkedList<Visit>();
        toVisit.add(new Visit(start, 0));
        while(!toVisit.isEmpty()) {
            var c = toVisit.removeFirst();
            if (!visited.containsKey(c.xy) || visited.get(c.xy) > c.distance) { // not visited or longer distance
                visited.put(c.xy, c.distance); // visit
                moves.stream().map(m -> c.move(m)) // next location
                    .filter(n -> map.containsKey(n.xy)) // is inside map
                    .filter(n -> ok.apply((byte) (map.get(n.xy) - map.get(c.xy)))) // ok to climb
                    .forEach(n -> toVisit.add(n));  // add to visit list
            }
        }

        return visited.keySet().stream()
            .filter(p -> target.apply(p)) // match or target
            .map(p -> visited.get(p)) // map to distance
            .sorted().findFirst().get(); // get minimum
    }

    static Map<XY,Byte> readMap(String input) {
        Map<XY,Byte> map = new HashMap<>();
        var lines = input.lines().toList();
        for (int y = 0; y < lines.size(); y++) {
            for (int x = 0; x < lines.get(y).length(); x++) {
                map.put(new XY(x,y), (byte) lines.get(y).charAt(x));
            }
        }
        return map;
    }

    record Visit(XY xy, int distance) {
        Visit move(XY v) { return new Visit(new XY(xy.x+v.x, xy.y+v.y), distance + 1); }
    }

    record XY(int x, int y) {}

    static List<XY> moves = List.of(new XY(1,0), new XY(-1,0), new XY(0,1), new XY(0,-1));
}