Day16

aoc2023/src/main/java/io/github/zebalu/aoc2023/days/Day16.java

@@ -0,0 +1,122 @@
+package io.github.zebalu.aoc2023.days;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.function.Predicate;
+
+import static java.util.function.Predicate.not;
+
+public class Day16 {
+    public static void main(String[] args) {
+        String input = readInput();
+        List<String> tiles = input.lines().toList();
+
+        System.out.println(countEnergized(tiles, new Beam(new Coord(0,0), Coord.RIGHT)));
+        List<Beam> starts = collectStartBeams(tiles.size(), tiles.getFirst().length());
+        System.out.println(starts.stream().parallel().mapToLong(start -> countEnergized(tiles, start)).max().orElseThrow());
+    }
+
+    private static List<Beam> collectStartBeams(int height, int width) {
+        List<Beam> starts = new ArrayList<>();
+        for(int y=0; y<height; ++y) {
+            starts.add(new Beam(new Coord(0,y), Coord.RIGHT));
+            starts.add(new Beam(new Coord(width-1,y), Coord.RIGHT));
+        }
+        for(int x=0; x<width; ++x) {
+            starts.add(new Beam(new Coord(x,0), Coord.DOWN));
+            starts.add(new Beam(new Coord(0, height-1), Coord.UP));
+        }
+        return starts;
+    }
+
+    private static long countEnergized(List<String> tiles, Beam start) {
+        int height = tiles.size();
+        int width = tiles.getFirst().length();
+        Predicate<Beam> isValid = beam -> 0<= beam.position().x && beam.position().x < width && 0<= beam.position().y() && beam.position().y() < height; 
+        List<Beam> beams = new ArrayList<>();
+        beams.add(start);
+        Set<Beam> visited = new HashSet<>();
+        while(beams.stream().anyMatch(not(visited::contains))) {
+            beams = beams.stream().filter(not(visited::contains)).peek(visited::add).map(beam->{
+                char tile = tiles.get(beam.position().y()).charAt(beam.position.x());
+                return switch (tile) {
+                case '.' -> List.of(beam.step());
+                case '|' -> {
+                    if(beam.direction.x() != 0) {
+                        yield List.of(new Beam(beam.position(), Coord.UP).step(), new Beam(beam.position(), Coord.DOWN).step());
+                    } else {
+                        yield List.of(beam.step());
+                    }
+                }
+                case '-' -> {
+                    if(beam.direction.y() != 0) {
+                        yield List.of(new Beam(beam.position(), Coord.LEFT).step(), new Beam(beam.position(), Coord.RIGHT).step());
+                    } else {
+                        yield List.of(beam.step());
+                    }
+                }
+                case '/' -> {
+                    if (beam.direction().x() != 0) {
+                        if (beam.direction().x() < 0) {
+                            yield List.of(new Beam(beam.position(), Coord.DOWN).step());
+                        } else {
+                            yield List.of(new Beam(beam.position(), Coord.UP).step());
+                        }
+                    } else {
+                        if (beam.direction().y() < 0) {
+                            yield List.of(new Beam(beam.position(), Coord.RIGHT).step());
+                        } else {
+                            yield List.of(new Beam(beam.position(), Coord.LEFT).step());
+                        }
+                    }
+                }
+                case '\\' -> {
+                    if (beam.direction().x() != 0) {
+                        if (beam.direction().x() < 0) {
+                            yield List.of(new Beam(beam.position(), Coord.UP).step());
+                        } else {
+                            yield List.of(new Beam(beam.position(), Coord.DOWN).step());
+                        }
+                    } else {
+                        if (beam.direction().y() < 0) {
+                            yield List.of(new Beam(beam.position(), Coord.LEFT).step());
+                        } else {
+                            yield List.of(new Beam(beam.position(), Coord.RIGHT).step());
+                        }
+                    }
+                }
+                default -> throw new IllegalArgumentException("Unexpected value: " + tile);
+                };
+            }).flatMap(List::stream).filter(isValid).toList();
+        }
+        return (visited.stream().map(Beam::position).distinct().count());
+    }
+
+    private static String readInput() {
+        try {
+            return Files.readString(Path.of("day16.txt").toAbsolutePath());
+        } catch (IOException e) {
+            throw new IllegalStateException(e);
+        }
+    }
+    private record Coord(int x, int y) {
+        private static Coord LEFT = new Coord(-1,0);
+        private static Coord RIGHT = new Coord(1,0);
+        private static Coord UP = new Coord(0,-1);
+        private static Coord DOWN = new Coord(0,1);
+        Coord add(Coord other) {
+            return new Coord(x+other.x, y+other.y);
+        }
+    }
+    private record Beam(Coord position, Coord direction) {
+        Beam step() {
+            return new Beam(position.add(direction), direction);
+        }
+    }
+
+}