Day 9

l0s · l0s · commit 84d347b68d79 · 2021-12-08T22:42:50.000-08:00
diff --git a/src/test/java/com/macasaet/Day09.java b/src/test/java/com/macasaet/Day09.java
@@ -0,0 +1,177 @@
+package com.macasaet;
+
+import java.util.*;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * --- Day 9: Smoke Basin ---
+ */
+public class Day09 {
+
+    protected Stream<String> getInput() {
+        return StreamSupport
+                .stream(new LineSpliterator("day-09.txt"),
+                        false);
+    }
+
+    public HeightMap getHeightMap() {
+        final var list = getInput().map(line -> {
+            final var chars = line.toCharArray();
+            final var ints = new int[chars.length];
+            for (int i = chars.length; --i >= 0; ints[i] = chars[i] - '0') ;
+            return ints;
+        }).collect(Collectors.toList());
+        final int[][] grid = new int[list.size()][];
+        for (int i = list.size(); --i >= 0; grid[i] = list.get(i)) ;
+        return new HeightMap(grid);
+    }
+
+    /**
+     * A height map of the floor of the nearby caves generated by the submarine
+     */
+    public record HeightMap(int[][] grid) { // FIXME use bytes
+
+        public Stream<Point> points() {
+            return IntStream.range(0, grid().length)
+                    .boxed()
+                    .flatMap(i -> IntStream.range(0, grid()[i].length)
+                            .mapToObj(j -> new Point(i, j)));
+        }
+
+        /**
+         * A location on the floor of a nearby cave
+         */
+        public class Point {
+            final int x;
+            final int y;
+
+            public Point(final int x, final int y) {
+                this.x = x;
+                this.y = y;
+            }
+
+            public int x() {
+                return this.x;
+            }
+
+            public int y() {
+                return this.y;
+            }
+
+            public int getBasinSize() {
+                return getBasinPoints().size();
+            }
+
+            /**
+             * Identify all the higher points that are also part of the same basin, assuming this location is part of a
+             * basin.
+             *
+             * @return all the higher points, if any, that are part of the same basin.
+             */
+            public Set<Point> getBasinPoints() {
+                if (getHeight() >= 9) {
+                    return Collections.emptySet();
+                }
+                final var result = new HashSet<Point>();
+                result.add(this);
+                final Function<Point, Stream<Point>> basinPointRetriever = neighbour -> {
+                    if (neighbour.getHeight() >= 9 || neighbour.getHeight() <= getHeight() || result.contains(neighbour)) {
+                        return Stream.empty();
+                    }
+                    return neighbour.getBasinPoints().stream();
+                };
+                above().stream().flatMap(basinPointRetriever).forEach(result::add);
+                below().stream().flatMap(basinPointRetriever).forEach(result::add);
+                left().stream().flatMap(basinPointRetriever).forEach(result::add);
+                right().stream().flatMap(basinPointRetriever).forEach(result::add);
+                return Collections.unmodifiableSet(result);
+            }
+
+            /**
+             * @return true if and only if this location is lower than all of its adjacent locations (up to four,
+             * diagonals do not count)
+             */
+            public boolean isLowPoint() {
+                final var compareTo = new ArrayList<Point>(4);
+                above().ifPresent(compareTo::add);
+                below().ifPresent(compareTo::add);
+                left().ifPresent(compareTo::add);
+                right().ifPresent(compareTo::add);
+                return compareTo.stream().allMatch(neighbour -> neighbour.getHeight() > getHeight());
+            }
+
+            /**
+             * @return an assessment of the risk from smoke flowing through the cave
+             */
+            public int getRiskLevel() {
+                return getHeight() + 1;
+            }
+
+            /**
+             * @return the height of this particular location, from 0-9
+             */
+            public int getHeight() {
+                return grid()[x()][y()];
+            }
+
+            public Optional<Point> above() {
+                return x() > 0 ? Optional.of(new Point(x() - 1, y())) : Optional.empty();
+            }
+
+            public Optional<Point> below() {
+                return x() < grid().length - 1 ? Optional.of(new Point(x() + 1, y())) : Optional.empty();
+            }
+
+            public Optional<Point> left() {
+                return y() > 0 ? Optional.of(new Point(x(), y() - 1)) : Optional.empty();
+            }
+
+            public Optional<Point> right() {
+                return y() < grid()[x()].length - 1 ? Optional.of(new Point(x(), y() + 1)) : Optional.empty();
+            }
+
+            public int hashCode() {
+                return Objects.hash(x(), y());
+            }
+
+            public boolean equals(final Object o) {
+                try {
+                    final Point other = (Point) o;
+                    return this.x() == other.x() && this.y() == other.y();
+                } catch (final ClassCastException cce) {
+                    return false;
+                }
+            }
+        }
+
+    }
+
+    @Test
+    public final void part1() {
+        final var map = getHeightMap();
+        final int sum = map.points()
+                .filter(HeightMap.Point::isLowPoint)
+                .mapToInt(HeightMap.Point::getRiskLevel)
+                .sum();
+        System.out.println("Part 1: " + sum);
+    }
+
+    @Test
+    public final void part2() {
+        final var map = getHeightMap();
+        final var basinSizes = map.points()
+                .filter(HeightMap.Point::isLowPoint)
+                .mapToInt(HeightMap.Point::getBasinSize)
+                .collect(() -> new TreeSet<Integer>(Comparator.reverseOrder()), SortedSet::add, SortedSet::addAll);
+        final var iterator = basinSizes.iterator();
+        final var result = iterator.next() * iterator.next() * iterator.next();
+        System.out.println("Part 2: " + result);
+    }
+
+}
diff --git a/src/test/resources/sample/day-09.txt b/src/test/resources/sample/day-09.txt
@@ -0,0 +1,5 @@
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
