day06 for AoC 2023 (part 2)

Took 30 minutes

Author: albertominetti

src/main/java/aminetti/adventofcode2024/day06/Day06.java

@@ -2,7 +2,9 @@
 
 import aminetti.adventofcode2023.day17.Day17;
 import org.apache.commons.lang3.tuple.ImmutablePair;
+import org.apache.commons.lang3.tuple.ImmutableTriple;
 import org.apache.commons.lang3.tuple.Pair;
+import org.apache.commons.lang3.tuple.Triple;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -17,6 +19,7 @@ public class Day06 {
     private List<String> input;
     private int ROWS;
     private int COLS;
+    private Pair<Integer, Integer> start;
 
     public Day06() {
     }
@@ -25,10 +28,15 @@ public void parseInput(List<String> input) {
         this.input = input;
         ROWS = input.size();
         COLS = input.getFirst().length();
+        start = findStart();
     }
 
     public long solvePart1() {
-        Pair<Integer, Integer> start = findStart();
+        Set<Pair<Integer, Integer>> steps = findSteps();
+        return steps.size();
+    }
+
+    private Set<Pair<Integer, Integer>> findSteps() {
         Set<Pair<Integer, Integer>> steps = new HashSet<>();
 
         Pair<Integer, Integer> current = start;
@@ -39,17 +47,16 @@ public long solvePart1() {
 
             Pair<Integer, Integer> next = new ImmutablePair<>(current.getLeft() + d.x, current.getRight() + d.y);
 
-            if (inMap(next) && input.get(next.getLeft()).charAt(next.getRight()) == '#') {
-                LOGGER.info("Can't go {}, so rotating 90°", d);
+            while (inMap(next) && input.get(next.getLeft()).charAt(next.getRight()) == '#') {
+                LOGGER.debug("Can't go {}, so rotating 90°", d);
                 d = Direction.values()[(d.ordinal() + 1) % 4];
                 next = new ImmutablePair<>(current.getLeft() + d.x, current.getRight() + d.y);
             }
 
             current = next;
-            LOGGER.info("Now position is {}", current);
+            LOGGER.debug("Now position is {}", current);
         }
-
-        return steps.size();
+        return steps;
     }
 
     public enum Direction {
@@ -80,7 +87,51 @@ private Pair<Integer, Integer> findStart() {
     }
 
     public long solvePart2() {
+        Set<Pair<Integer, Integer>> obstructionsForLoops = new HashSet<>();
+        for (Pair<Integer, Integer> step : findSteps()) {
+            Pair<Integer, Integer> o = step;
+
+            if (o.equals(start)) {
+                continue;
+            }
+            LOGGER.info("Obstruction at {}", o);
+
+            if (isLoop(start, o)) {
+                obstructionsForLoops.add(o);
+            }
+
+        }
+
+        return obstructionsForLoops.size();
+    }
+
+    private boolean isLoop(Pair<Integer, Integer> start, Pair<Integer, Integer> o) {
+        Set<Triple<Integer, Integer, Direction>> steps = new HashSet<>();
+
+        Pair<Integer, Integer> current = start;
+        Direction d = UP;
+
+        while (inMap(current)) {
+            ImmutableTriple<Integer, Integer, Direction> posAndDir = new ImmutableTriple<>(current.getLeft(), current.getRight(), d);
+            if (steps.contains(posAndDir)) {
+                return true;
+            }
+            steps.add(posAndDir);
+
+            Pair<Integer, Integer> next = new ImmutablePair<>(current.getLeft() + d.x, current.getRight() + d.y);
+
+            while (inMap(next) && (
+                    input.get(next.getLeft()).charAt(next.getRight()) == '#' || next.equals(o)
+            )) {
+                LOGGER.debug("Can't go {}, so rotating 90°", d);
+                d = Direction.values()[(d.ordinal() + 1) % 4];
+                next = new ImmutablePair<>(current.getLeft() + d.x, current.getRight() + d.y);
+            }
+
+            current = next;
+            LOGGER.debug("Now position is {}", current);
+        }
 
-        return 0;
+        return false;
     }
 }

src/test/java/aminetti/adventofcode2024/day06/Day06Test.java

@@ -9,6 +9,7 @@
 import static org.apache.commons.io.IOUtils.readLines;
 import static org.apache.commons.io.IOUtils.resourceToString;
 import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.is;
 
 class Day06Test {
@@ -24,7 +25,7 @@ void actualInputPart1() throws IOException {
         long l = solver.solvePart1();
 
         // then
-        assertThat(l, is(0L));
+        assertThat(l, is(4977L));
     }
 
     @Test
@@ -52,7 +53,9 @@ void actualInputPart2() throws IOException {
         long l = solver.solvePart2();
 
         // then
-        assertThat(l, is(0L));
+        assertThat(l, greaterThan(1650L));
+        assertThat(l, greaterThan(1651L));
+        assertThat(l, is(1729L));
     }
 
     @Test
@@ -66,7 +69,7 @@ void testInputPart2() throws IOException {
         long l = solver.solvePart2();
 
         // then
-        assertThat(l, is(0L));
+        assertThat(l, is(6L));
     }