src/main/java/ch/fritteli/maze/generator/algorithm/AbstractMazeGeneratorAlgorithm.java

@@ -0,0 +1,19 @@
+package ch.fritteli.maze.generator.algorithm;
+
+import ch.fritteli.maze.generator.model.Maze;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Random;
+
+public abstract class AbstractMazeGeneratorAlgorithm implements MazeGeneratorAlgorithm {
+    @NotNull
+    protected final Maze maze;
+    @NotNull
+    protected final Random random;
+
+
+    public AbstractMazeGeneratorAlgorithm(@NotNull final Maze maze) {
+        this.maze = maze;
+        this.random = new Random(maze.getRandomSeed());
+    }
+}

src/main/java/ch/fritteli/maze/generator/algorithm/MazeGeneratorAlgorithm.java

@@ -0,0 +1,5 @@
+package ch.fritteli.maze.generator.algorithm;
+
+public interface MazeGeneratorAlgorithm {
+    void run();
+}

src/main/java/ch/fritteli/maze/generator/algorithm/RandomDepthFirst.java

@@ -10,20 +10,13 @@ import org.jetbrains.annotations.Nullable;
 
 import java.util.Deque;
 import java.util.LinkedList;
-import java.util.Random;
 
-public class RandomDepthFirst {
-
-    @NotNull
-    private final Maze maze;
-    @NotNull
-    private final Random random;
+public class RandomDepthFirst extends AbstractMazeGeneratorAlgorithm {
     @NotNull
     private final Deque<Position> positions = new LinkedList<>();
 
     public RandomDepthFirst(@NotNull final Maze maze) {
-        this.maze = maze;
-        this.random = new Random(maze.getRandomSeed());
+        super(maze);
     }
 
     public void run() {

src/main/java/ch/fritteli/maze/generator/algorithm/Wilson.java

@@ -0,0 +1,297 @@
+package ch.fritteli.maze.generator.algorithm;
+
+import ch.fritteli.maze.generator.model.Direction;
+import ch.fritteli.maze.generator.model.Maze;
+import ch.fritteli.maze.generator.model.Position;
+import ch.fritteli.maze.generator.model.Tile;
+import io.vavr.Tuple2;
+import io.vavr.collection.Iterator;
+import io.vavr.collection.Seq;
+import io.vavr.collection.Stream;
+import io.vavr.collection.Vector;
+import io.vavr.control.Option;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.ArrayList;
+import java.util.EnumSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Random;
+
+/**
+ * An implementation of <a href="https://en.wikipedia.org/wiki/Maze_generation_algorithm#Wilson's_algorithm">Wilson's Algorithm</a>.
+ */
+public class Wilson extends AbstractMazeGeneratorAlgorithm {
+    public Wilson(@NotNull final Maze maze) {
+        super(maze);
+    }
+
+    @Override
+    public void run() {
+        final MyMaze myMaze = new MyMaze(this.maze.getWidth(), this.maze.getHeight());
+        // 1. Initialization: pick random location, add to maze
+        final Position position = myMaze.getRandomAvailablePosition(this.random).get();
+        myMaze.setPartOfMaze(position);
+
+        // 2. while locations-not-in-maze exist, repeat:
+
+        final List<MyMaze.Path> pths = new ArrayList<>();
+        Position startPosition;
+        while ((startPosition = myMaze.getRandomAvailablePosition(this.random).getOrNull()) != null) {
+            final MyMaze.Path path = myMaze.createPath(startPosition);
+            while (true) {
+                final Position nextPosition = path.nextRandomPosition(this.random);
+                if (path.contains(nextPosition)) {
+                    path.removeLoopUpTo(nextPosition);
+                } else {
+                    path.append(nextPosition);
+                    if (myMaze.isPartOfMaze(nextPosition)) {
+                        myMaze.setPartOfMaze(path);
+                        pths.add(path);
+                        break;
+                    }
+                }
+            }
+
+        }
+
+        applyToMaze(pths, maze);
+        solve(maze);
+        //    1. pick random location not in maze
+        //    2. perform random walk until you reach the maze (*)
+        //    3. add path to maze
+        // (*): random walk:
+        //    1. advance in random direction
+        //    2. if loop with current path is formed, remove loop, continue from last location before loop
+    }
+
+    private void applyToMaze(@NotNull final List<MyMaze.Path> pths, @NotNull final Maze maze) {
+        pths.forEach(path -> {
+            final Iterator<Direction> moves = Stream.ofAll(path.path)
+                    .sliding(2)
+                    .flatMap(poss -> poss.head().getDirectionTo(poss.get(1)));
+            Position position = path.path.getFirst();
+            while (moves.hasNext()) {
+                Tile tile = maze.getTileAt(position).get();
+                final Direction move = moves.next();
+                tile.digTo(move);
+                position = position.move(move);
+                maze.getTileAt(position).forEach(t -> t.digTo(move.invert()));
+            }
+        });
+        Direction direction = determineDirectionForDigging(maze.getStart());
+        Tile t = maze.getStartTile();
+        this.digTo(t, direction);
+        direction = determineDirectionForDigging(maze.getEnd());
+        t = maze.getEndTile();
+        this.digTo(t, direction);
+    }
+
+    @Nullable
+    private Direction determineDirectionForDigging(@NotNull final Position position) {
+        if (position.y() == 0) {
+            return Direction.TOP;
+        }
+        if (position.x() == 0) {
+            return Direction.LEFT;
+        }
+        if (position.y() == this.maze.getHeight() - 1) {
+            return Direction.BOTTOM;
+        }
+        if (position.x() == this.maze.getWidth() - 1) {
+            return Direction.RIGHT;
+        }
+        return null;
+    }
+
+    private void digTo(@NotNull final Tile tile, @NotNull final Direction direction) {
+        tile.enableDiggingToOrFrom(direction);
+        tile.digTo(direction);
+    }
+
+    private void solve(@NotNull final Maze maze) {
+        EnumSet<Direction>[][] remainingDirs = new EnumSet[maze.getWidth()][maze.getHeight()];
+        for (int x = 0; x < remainingDirs.length; x++) {
+            for (int y = 0; y < remainingDirs[x].length; y++) {
+                remainingDirs[x][y] = EnumSet.allOf(Direction.class);
+            }
+        }
+        Position p = maze.getStart();
+        final Direction direction = this.determineDirectionForDigging(p);
+        remainingDirs[p.x()][p.y()].remove(direction);
+        LinkedList<Position> solution = new LinkedList<>();
+        solution.add(p);
+        while (!p.equals(maze.getEnd())) {
+            final Tile tile = maze.getTileAt(p).get();
+            EnumSet<Direction> dirs = remainingDirs[p.x()][p.y()];
+            dirs.removeIf(tile::hasWallAt);
+            if (dirs.isEmpty()) {
+                solution.pop();
+                p = solution.peek();
+            } else {
+                final Direction nextDir = dirs.iterator().next();
+                final Position nextPos = p.move(nextDir);
+                solution.push(nextPos);
+                remainingDirs[p.x()][p.y()].remove(nextDir);
+                remainingDirs[nextPos.x()][nextPos.y()].remove(nextDir.invert());
+                p = nextPos;
+            }
+        }
+        solution.forEach(s -> maze.getTileAt(s).forEach(Tile::setSolution));
+    }
+
+    private static class MyMaze {
+        private final int width;
+        private final int height;
+        private final boolean[][] partOfMaze;
+        private final boolean[] completeColumns;
+
+        MyMaze(final int width, final int height) {
+            this.width = width;
+            this.height = height;
+            this.partOfMaze = new boolean[this.width][this.height];
+            for (int x = 0; x < this.width; x++) {
+                this.partOfMaze[x] = new boolean[this.height];
+            }
+            this.completeColumns = new boolean[this.width];
+        }
+
+        boolean isPartOfMaze(final int x, final int y) {
+            return this.partOfMaze[x][y];
+        }
+
+        boolean isPartOfMaze(@NotNull final Position position) {
+            return this.isPartOfMaze(position.x(), position.y());
+        }
+
+        void setPartOfMaze(final int x, final int y) {
+            this.partOfMaze[x][y] = true;
+            this.checkCompleteColumn(x);
+        }
+
+        void setPartOfMaze(@NotNull final Position position) {
+            this.setPartOfMaze(position.x(), position.y());
+        }
+
+        void setPartOfMaze(@NotNull final Path path) {
+            path.path.forEach(this::setPartOfMaze);
+        }
+
+        void checkCompleteColumn(final int x) {
+            if (this.completeColumns[x]) {
+                return;
+            }
+            for (int y = 0; y < this.height; y++) {
+                if (!this.isPartOfMaze(x, y)) {
+                    return;
+                }
+            }
+            this.completeColumns[x] = true;
+        }
+
+        Option<Position> getRandomAvailablePosition(@NotNull final Random random) {
+            final Seq<Integer> allowedColumns = Vector.ofAll(this.completeColumns)
+                    .zipWithIndex()
+                    .reject(Tuple2::_1)
+                    .map(Tuple2::_2);
+            if (allowedColumns.isEmpty()) {
+                return Option.none();
+            }
+            final int x = allowedColumns.get(random.nextInt(allowedColumns.size()));
+            final boolean[] column = partOfMaze[x];
+
+            final Seq<Integer> allowedRows = Vector.ofAll(column)
+                    .zipWithIndex()
+                    .reject(Tuple2::_1)
+                    .map(Tuple2::_2);
+            if (allowedRows.isEmpty()) {
+                return Option.none();
+            }
+            final int y = allowedRows.get(random.nextInt(allowedRows.size()));
+            return Option.some(new Position(x, y));
+        }
+
+        public Path createPath(@NotNull final Position position) {
+            return new Path(position);
+        }
+
+        private class Path {
+            @NotNull
+            private final List<Position> path = new LinkedList<>();
+
+            Path(@NotNull final Position position) {
+                this.path.add(position);
+            }
+
+            @NotNull
+            Position nextRandomPosition(@NotNull final Random random) {
+                final Direction direction = this.getRandomDirectionFromLastPosition(random);
+                return this.path.getLast().move(direction);
+            }
+
+            boolean contains(@NotNull final Position position) {
+                return this.path.contains(position);
+            }
+
+            void removeLoopUpTo(@NotNull final Position position) {
+                while (!this.path.removeLast().equals(position)) {
+                }
+                this.path.add(position);
+            }
+
+            public void append(@NotNull final Position nextPosition) {
+                this.path.add(nextPosition);
+            }
+
+            @NotNull
+            private Direction getRandomDirectionFromLastPosition(@NotNull final Random random) {
+                final EnumSet<Direction> validDirections = this.getValidDirectionsFromLastPosition();
+                if (validDirections.isEmpty()) {
+                    throw new IllegalStateException("WE MUST NOT GET HERE! analyze why it happened!!!");
+                }
+                if (validDirections.size() == 1) {
+                    return validDirections.iterator().next();
+                }
+                final Direction[] directionArray = validDirections.toArray(Direction[]::new);
+                final int index = random.nextInt(directionArray.length);
+                return directionArray[index];
+            }
+
+            @NotNull
+            private EnumSet<Direction> getValidDirectionsFromLastPosition() {
+                final Position fromPosition = this.path.getLast();
+                final EnumSet<Direction> validDirections = EnumSet.allOf(Direction.class);
+                if (this.path.size() > 1) {
+                    final Position prevP = this.path.get(this.path.size() - 2);
+                    fromPosition.getDirectionTo(prevP)
+                            .forEach(validDirections::remove);
+                }
+                boolean canLeft = fromPosition.x() > 0;
+                boolean canRight = fromPosition.x() < width - 1;
+                boolean canUp = fromPosition.y() > 0;
+                boolean canDown = fromPosition.y() < height - 1;
+                if (!canLeft) {
+                    validDirections.remove(Direction.LEFT);
+                }
+                if (!canRight) {
+                    validDirections.remove(Direction.RIGHT);
+                }
+                if (!canUp) {
+                    validDirections.remove(Direction.TOP);
+                }
+                if (!canDown) {
+                    validDirections.remove(Direction.BOTTOM);
+                }
+                return validDirections;
+            }
+
+            @Override
+            public String toString() {
+                return Stream.ofAll(this.path)
+                        .map(position -> "(%s,%s)".formatted(position.x(), position.y()))
+                        .mkString("Path[", "->", "]");
+            }
+        }
+    }
+}

src/main/java/ch/fritteli/maze/generator/model/Direction.java

@@ -1,11 +1,14 @@
 package ch.fritteli.maze.generator.model;
 
+import org.jetbrains.annotations.NotNull;
+
 public enum Direction {
     TOP,
     RIGHT,
     BOTTOM,
     LEFT;
 
+    @NotNull
     public Direction invert() {
         return switch (this) {
             case TOP -> BOTTOM;

src/main/java/ch/fritteli/maze/generator/model/Position.java

@@ -1,10 +1,12 @@
 package ch.fritteli.maze.generator.model;
 
+import io.vavr.control.Option;
 import lombok.With;
 import org.jetbrains.annotations.NotNull;
 
 @With
 public record Position(int x, int y) {
+    @NotNull
     public Position move(@NotNull final Direction direction) {
         return switch (direction) {
             case BOTTOM -> this.withY(this.y + 1);
@@ -13,4 +15,20 @@ public record Position(int x, int y) {
             case TOP -> this.withY(this.y - 1);
         };
     }
+
+    @NotNull
+    public Option<Direction> getDirectionTo(@NotNull final Position position) {
+        final int xDiff = position.x - this.x;
+        final int yDiff = position.y - this.y;
+        return switch (xDiff) {
+            case -1 -> Option.when(yDiff == 0, Direction.LEFT);
+            case 0 -> switch (yDiff) {
+                case -1 -> Option.some(Direction.TOP);
+                case 1 -> Option.some(Direction.BOTTOM);
+                default -> Option.none();
+            };
+            case 1 -> Option.when(yDiff == 0, Direction.RIGHT);
+            default -> Option.none();
+        };
+    }
 }

src/main/java/ch/fritteli/maze/generator/renderer/pdf/Generator.java

@@ -233,7 +233,7 @@ class Generator {
                 if (position.equals(previousPosition)) {
                     continue;
                 }
-                if (tileAtPosition.map(Tile::isSolution).getOrElse(false)) {
+                if (tileAtPosition.exists(Tile::isSolution)) {
                     return position;
                 }
             }

src/test/java/ch/fritteli/maze/generator/algorithm/WilsonTest.java

@@ -0,0 +1,15 @@
+package ch.fritteli.maze.generator.algorithm;
+
+import ch.fritteli.maze.generator.model.Maze;
+import ch.fritteli.maze.generator.renderer.text.TextRenderer;
+import org.junit.jupiter.api.Test;
+
+class WilsonTest {
+    @Test
+    void foo() {
+        final Maze maze = new Maze(50, 10, 0);
+        final Wilson wilson = new Wilson(maze);
+        wilson.run();
+        System.out.println(TextRenderer.newInstance().render(maze));
+    }
+}