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Day 16, part 1
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‎2024-java/src/main/java/pl/apostaremczak/aoc/Day16.java

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package pl.apostaremczak.aoc;
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import pl.apostaremczak.aoc.util.Coord2D;
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import pl.apostaremczak.aoc.util.DirectionSupport;
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import pl.apostaremczak.aoc.util.Map2D;
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import java.util.*;
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public class Day16 extends PuzzleSolution {
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public Day16(String inputFilename) {
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super(inputFilename);
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}
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@Override
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public Long solvePart1() {
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Maze maze = new Maze(inputLines);
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return maze.getBestPathScore();
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}
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@Override
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public Long solvePart2() {
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Maze maze = new Maze(inputLines);
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return maze.countTilesOnBestPaths();
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}
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public static void main(String[] args) {
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Day16 day16 = new Day16("src/main/resources/16.txt");
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Long part1Solution = day16.solvePart1();
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System.out.println("Part 1: " + part1Solution);
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Long part2Solution = day16.solvePart2();
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System.out.println("Part 2: " + part2Solution);
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}
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}
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class MazeTile implements Comparable<MazeTile>, DirectionSupport {
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private Coord2D position;
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private LinkedList<StackEntry> shortestPath = new LinkedList<>();
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public final Set<MazeTile> allPotentialPathEntries = new HashSet<>();
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private Integer distance = Integer.MAX_VALUE;
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public Integer getDistance() {
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return distance;
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}
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public void setDistance(Integer newDistance) {
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this.distance = newDistance;
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}
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public Coord2D getPosition() {
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return this.position;
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}
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public LinkedList<StackEntry> getShortestPath() {
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return this.shortestPath;
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}
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public void setShortestPath(LinkedList<StackEntry> shortestPath) {
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this.shortestPath = shortestPath;
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}
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public static MazeTile fromPosition(Coord2D tilePosition) {
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MazeTile tile = new MazeTile();
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tile.position = tilePosition;
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return tile;
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}
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public Long countPotentialPaths() {
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return (long) allPotentialPathEntries.size();
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}
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public String printShortestPath() {
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StringBuilder representation = new StringBuilder();
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for (StackEntry entry : this.getShortestPath()) {
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Coord2D position = entry.tile().getPosition();
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String positionRepresentation = "[" + position.row() + ", " + position.column() + "]";
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Coord2D direction = entry.direction();
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representation.append(DirectionSupport.directionToString(direction));
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representation.append(positionRepresentation);
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representation.append(" ");
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}
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return representation.toString();
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}
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@Override
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public int compareTo(MazeTile o) {
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return Integer.compare(this.getDistance(), o.getDistance());
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}
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}
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class Maze implements DirectionSupport {
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private final Coord2D StartTilePosition;
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private final Coord2D EndTilePosition;
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private final Coord2D StartDirection;
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private final Map<Coord2D, MazeTile> Tiles;
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private final Integer ShortestDistance;
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public Maze(String[] inputLines) {
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Map2D<Character> mazeAsArray = Map2D.fromStringInputLines(inputLines);
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Coord2D startTilePosition = null;
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Coord2D endTilePosition = null;
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Map<Coord2D, MazeTile> tiles = new HashMap<>();
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for (int rowIdx = 0; rowIdx <= mazeAsArray.MAX_ROW_INDEX; rowIdx++) {
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for (int colIdx = 0; colIdx <= mazeAsArray.MAX_COLUMN_INDEX; colIdx++) {
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Coord2D position = new Coord2D(rowIdx, colIdx);
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Character value = mazeAsArray.getAt(position);
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switch (value) {
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case 'S':
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startTilePosition = position;
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tiles.put(position, MazeTile.fromPosition(position));
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break;
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case 'E':
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endTilePosition = position;
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tiles.put(position, MazeTile.fromPosition(position));
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break;
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case '.':
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tiles.put(position, MazeTile.fromPosition(position));
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break;
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}
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}
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}
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assert startTilePosition != null : "Could not find the start tile";
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assert endTilePosition != null : "Could not find the end tile";
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this.StartTilePosition = startTilePosition;
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this.EndTilePosition = endTilePosition;
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// The Reindeer start on the Start Tile (marked S) facing East
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this.StartDirection = RIGHT;
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this.Tiles = tiles;
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this.dijkstra();
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this.ShortestDistance = this.Tiles.get(this.EndTilePosition).getDistance();
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}
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public Long getBestPathScore() {
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return (long) this.ShortestDistance;
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}
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public Long countTilesOnBestPaths() {
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return this.Tiles.get(this.EndTilePosition).countPotentialPaths();
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}
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private void dijkstra() {
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MazeTile startTile = this.Tiles.get(this.StartTilePosition);
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startTile.setDistance(0);
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startTile.allPotentialPathEntries.add(startTile);
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Set<StackEntry> settledTiles = new HashSet<>();
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PriorityQueue<StackEntry> unsettledTiles = new PriorityQueue<>();
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unsettledTiles.add(new StackEntry(startTile, StartDirection));
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while (!unsettledTiles.isEmpty()) {
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StackEntry currentTile = unsettledTiles.poll();
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unsettledTiles.remove(currentTile);
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// Get all the possible next steps
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// Try continuing in the same direction or rotating 90 degrees clockwise and counterclockwise
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Coord2D currentDirection = currentTile.direction();
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Coord2D[] potentialDirections = {currentDirection, currentDirection.rotateRightAngle(90), currentDirection.rotateRightAngle(-90)};
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Coord2D currentPosition = currentTile.tile().getPosition();
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for (int i = 0; i < 3; i++) {
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Coord2D direction = potentialDirections[i];
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Coord2D nextTilePosition = currentPosition.add(direction);
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if (this.Tiles.containsKey(nextTilePosition)) {
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MazeTile nextTile = this.Tiles.get(nextTilePosition);
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StackEntry nextTileEntry = new StackEntry(nextTile, direction);
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if (!settledTiles.contains(nextTileEntry)) {
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unsettledTiles.add(nextTileEntry);
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calculateMinimumDistance(nextTile, currentTile.tile(), direction);
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}
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}
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}
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settledTiles.add(currentTile);
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}
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}
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private void calculateMinimumDistance(MazeTile evaluationTile, MazeTile sourceTile, Coord2D direction) {
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int sourceDistance = sourceTile.getDistance();
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int newEdgeWeight = sourceDistance + 1;
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LinkedList<StackEntry> shortestPath = new LinkedList<>(sourceTile.getShortestPath());
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Coord2D lastDirection = !shortestPath.isEmpty() ? shortestPath.getLast().direction() : StartDirection;
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if (!direction.equals(lastDirection)) {
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newEdgeWeight = sourceDistance + 1001;
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}
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if (newEdgeWeight <= evaluationTile.getDistance()) {
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evaluationTile.setDistance(newEdgeWeight);
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shortestPath.add(new StackEntry(sourceTile, direction));
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evaluationTile.setShortestPath(shortestPath);
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}
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}
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}
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record StackEntry(MazeTile tile, Coord2D direction) implements Comparable<StackEntry> {
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@Override
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public int compareTo(StackEntry o) {
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return this.tile.compareTo(o.tile);
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}
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}

‎2024-java/src/test/java/pl/apostaremczak/aoc/Day16Tests.java

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package pl.apostaremczak.aoc;
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import org.junit.jupiter.api.Test;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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public class Day16Tests {
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private final Day16 day16 = new Day16("src/test/resources/16.txt");
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@Test
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public void testSolvePart1() {
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Long result = day16.solvePart1();
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assertEquals(7036L, result);
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}
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@Test
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public void testSolvePart1SecondExample() {
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Day16 day = new Day16("src/test/resources/16_2.txt");
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Long result = day.solvePart1();
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assertEquals(11048L, result);
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}
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@Test
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public void testSolvePart2() {
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Long result = day16.solvePart2();
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assertEquals(45L, result);
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}
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@Test
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public void testSolvePart2SecondExample() {
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Day16 day = new Day16("src/test/resources/16_2.txt");
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Long result = day.solvePart2();
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assertEquals(64L, result);
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}
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}

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