AoC 2025 Day 4 - java

Author: pareronia

README.md

@@ -8,9 +8,9 @@
 <!-- @BEGIN:ImplementationsTable:2025@ -->
 | | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
 | ---| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
-| python3 | [✓](src/main/python/AoC2025_01.py) | [✓](src/main/python/AoC2025_02.py) | [✓](src/main/python/AoC2025_03.py) |  |  |  |  |  |  |  |  |  |
-| java | [✓](src/main/java/AoC2025_01.java) | [✓](src/main/java/AoC2025_02.java) | [✓](src/main/java/AoC2025_03.java) |  |  |  |  |  |  |  |  |  |
-| bash | [✓](src/main/bash/AoC2025_01.sh) | [✓](src/main/bash/AoC2025_02.sh) | [✓](src/main/bash/AoC2025_03.sh) |  |  |  |  |  |  |  |  |  |
+| python3 | [✓](src/main/python/AoC2025_01.py) | [✓](src/main/python/AoC2025_02.py) | [✓](src/main/python/AoC2025_03.py) | [✓](src/main/python/AoC2025_04.py) |  |  |  |  |  |  |  |  |
+| java | [✓](src/main/java/AoC2025_01.java) | [✓](src/main/java/AoC2025_02.java) | [✓](src/main/java/AoC2025_03.java) | [✓](src/main/java/AoC2025_04.java) |  |  |  |  |  |  |  |  |
+| bash | [✓](src/main/bash/AoC2025_01.sh) | [✓](src/main/bash/AoC2025_02.sh) | [✓](src/main/bash/AoC2025_03.sh) | [✓](src/main/bash/AoC2025_04.sh) |  |  |  |  |  |  |  |  |
 <!-- @END:ImplementationsTable:2025@ -->
 
 ## 2024

src/main/java/AoC2025_04.java

@@ -0,0 +1,84 @@
+import com.github.pareronia.aoc.CharGrid;
+import com.github.pareronia.aoc.Grid.Cell;
+import com.github.pareronia.aoc.solution.Sample;
+import com.github.pareronia.aoc.solution.Samples;
+import com.github.pareronia.aoc.solution.SolutionBase;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+@SuppressWarnings({"PMD.ClassNamingConventions", "PMD.NoPackage"})
+public final class AoC2025_04 extends SolutionBase<CharGrid, Long, Long> {
+
+    public static final char ROLL = '@';
+    public static final char EMPTY = '.';
+
+    private AoC2025_04(final boolean debug) {
+        super(debug);
+    }
+
+    public static AoC2025_04 create() {
+        return new AoC2025_04(false);
+    }
+
+    public static AoC2025_04 createDebug() {
+        return new AoC2025_04(true);
+    }
+
+    @Override
+    protected CharGrid parseInput(final List<String> inputs) {
+        return CharGrid.from(inputs);
+    }
+
+    private boolean isRemovable(final CharGrid grid, final Cell cell) {
+        return grid.getValue(cell) == ROLL
+                && grid.getAllNeighbours(cell).filter(n -> grid.getValue(n) == ROLL).count() < 4;
+    }
+
+    @Override
+    public Long solvePart1(final CharGrid grid) {
+        return grid.getCells().filter(cell -> isRemovable(grid, cell)).count();
+    }
+
+    @Override
+    public Long solvePart2(final CharGrid grid) {
+        final Deque<Cell> queue = new ArrayDeque<>();
+        grid.getCells().filter(cell -> isRemovable(grid, cell)).forEach(queue::add);
+        final Set<Cell> seen = new HashSet<>();
+        while (!queue.isEmpty()) {
+            final Cell cell = queue.poll();
+            if (seen.contains(cell)) {
+                continue;
+            }
+            seen.add(cell);
+            grid.setValue(cell, EMPTY);
+            grid.getAllNeighbours(cell).filter(n -> isRemovable(grid, n)).forEach(queue::add);
+        }
+        return (long) seen.size();
+    }
+
+    @Samples({
+        @Sample(method = "part1", input = TEST, expected = "13"),
+        @Sample(method = "part2", input = TEST, expected = "43"),
+    })
+    public static void main(final String[] args) throws Exception {
+        create().run();
+    }
+
+    private static final String TEST =
+            """
+            ..@@.@@@@.
+            @@@.@.@.@@
+            @@@@@.@.@@
+            @.@@@@..@.
+            @@.@@@@.@@
+            .@@@@@@@.@
+            .@.@.@.@@@
+            @.@@@.@@@@
+            .@@@@@@@@.
+            @.@.@@@.@.
+            """;
+}