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Day 15
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‎src/test/java/com/macasaet/Day15.java

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package com.macasaet;
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import org.junit.jupiter.api.Assertions;
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import org.junit.jupiter.api.Test;
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import java.util.Arrays;
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import java.util.Collections;
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import java.util.HashMap;
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import java.util.List;
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import java.util.Map;
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import java.util.function.IntPredicate;
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import java.util.stream.StreamSupport;
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/**
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* --- Day 15: Beacon Exclusion Zone ---
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* <a href="https://adventofcode.com/2022/day/15">https://adventofcode.com/2022/day/15</a>
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*/
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public class Day15 {
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record Coordinate(int x, int y) {
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static Coordinate parse(final String string) {
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final var components = string.split(", ");
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return new Coordinate(Integer.parseInt(components[1].replaceAll("^y=", "")),
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Integer.parseInt(components[0].replaceAll("^x=", "")));
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}
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Map<Integer, Item> getRow(final Map<Integer, Map<Integer, Item>> grid) {
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return grid.computeIfAbsent(x(), ignored -> new HashMap<>());
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}
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int distanceTo(final Coordinate other) {
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return Math.abs(x() - other.x()) + Math.abs(y() - other.y());
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}
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}
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public record Sensor(Coordinate location, Coordinate beaconLocation) {
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public static Sensor parse(final String line) {
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final var location = Coordinate.parse(line.replaceAll("^Sensor at ", "").replaceAll(": closest beacon is at .*$", ""));
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final var beaconLocation = Coordinate.parse(line.replaceAll("^.*: closest beacon is at ", ""));
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return new Sensor(location, beaconLocation);
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}
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void setSensor(final Map<Integer, Map<Integer, Item>> grid, IntPredicate includeRow, IntPredicate includeColumn) {
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if(includeRow.test(location().x()) && includeColumn.test(location().y())) {
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location().getRow(grid).put(location().y(), Item.SENSOR);
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}
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}
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void setBeacon(final Map<Integer, Map<Integer, Item>> grid, IntPredicate includeRow, IntPredicate includeColumn) {
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if(includeRow.test(beaconLocation().x()) && includeColumn.test(beaconLocation().y())) {
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beaconLocation().getRow(grid).put(beaconLocation().y(), Item.BEACON);
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}
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}
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void setCoverageArea(final Map<Integer, Map<Integer, Item>> grid, IntPredicate includeRow, IntPredicate includeColumn) {
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final var distance = distanceToBeacon();
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final var x = location().x();
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final var y = location().y();
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for(int i = 0; i <= distance; i++ ) {
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if(!includeRow.test(x + i) && !includeRow.test(x - i)) {
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continue;
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}
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final var lowerRow = includeRow.test(x + i)
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? grid.computeIfAbsent(x + i, ignored -> new HashMap<>())
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: new HashMap<Integer, Item>();
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final var upperRow = includeRow.test(x - i)
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? grid.computeIfAbsent(x - i, ignored -> new HashMap<>())
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: new HashMap<Integer, Item>();
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for(int j = 0; j <= distance - i; j++ ) {
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if(includeColumn.test(y + j)) {
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// SE
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lowerRow.putIfAbsent(y + j, Item.COVERED);
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// NE
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upperRow.putIfAbsent(y + j, Item.COVERED);
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}
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if(includeColumn.test(y - j)) {
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// SW
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lowerRow.putIfAbsent(y - j, Item.COVERED);
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// NW
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upperRow.putIfAbsent(y - j, Item.COVERED);
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}
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}
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}
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}
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int distanceToBeacon() {
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return location().distanceTo(beaconLocation());
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}
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}
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enum Item {
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SENSOR,
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BEACON,
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COVERED
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}
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public record CaveMap(Map<Integer, Map<Integer, Item>> grid, int minX, int maxX, int minY, int maxY) {
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public int countCoveredCellsInRow(final int x) {
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final var row = grid().getOrDefault(x, Collections.emptyMap());
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int result = 0;
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for(int j = minY(); j <= maxY(); j++) {
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final var cell = row.get(j);
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if(cell != null && cell != Item.BEACON) {
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result++;
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}
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}
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return result;
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}
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public static CaveMap fromSensors(final Iterable<? extends Sensor> sensors, IntPredicate includeRow, IntPredicate includeColumn) {
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int minX = Integer.MAX_VALUE;
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int maxX = Integer.MIN_VALUE;
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int minY = Integer.MAX_VALUE;
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int maxY = Integer.MIN_VALUE;
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final var grid = new HashMap<Integer, Map<Integer, Item>>();
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for(final var sensor : sensors) {
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minX = Math.min(minX, sensor.location().x() - sensor.distanceToBeacon());
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maxX = Math.max(maxX, sensor.location().x() + sensor.distanceToBeacon());
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minY = Math.min(minY, sensor.location().y() - sensor.distanceToBeacon());
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maxY = Math.max(maxY, sensor.location().y() + sensor.distanceToBeacon());
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sensor.setCoverageArea(grid, includeRow, includeColumn);
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sensor.setBeacon(grid, includeRow, includeColumn);
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sensor.setSensor(grid, includeRow, includeColumn);
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}
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return new CaveMap(grid, minX, maxX, minY, maxY);
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}
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public String toString() {
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final var builder = new StringBuilder();
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for(int i = minX(); i <= maxX(); i++) {
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builder.append(i).append('\t');
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final var row = grid().getOrDefault(i, Collections.emptyMap());
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for(int j = minY(); j <= maxY(); j++) {
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final var item = row.get(j);
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final var marker = item == null
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? '.'
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: item == Item.BEACON
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? 'B'
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: item == Item.SENSOR
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? 'S'
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: '#';
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builder.append(marker);
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}
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builder.append('\n');
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}
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return builder.toString();
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}
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}
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protected CaveMap getInput(final IntPredicate includeRow, IntPredicate includeColumn) {
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final var sensors = getSensors();
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return CaveMap.fromSensors(sensors, includeRow, includeColumn);
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}
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protected static List<Sensor> getSensors() {
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return StreamSupport.stream(new LineSpliterator("day-15.txt"), false)
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.map(Sensor::parse)
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.toList();
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}
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@Test
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public final void part1() {
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final int rowOfInterest = 2_000_000;
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final var map = getInput(row -> row == rowOfInterest, _column -> true);
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final var result = map.countCoveredCellsInRow(rowOfInterest);
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System.out.println("Part 1: " + result);
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}
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@Test
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public final void part2() {
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final int max = 4_000_000;
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final var sensors = getSensors();
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for(final var sensor : sensors) {
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final var x = sensor.location().x();
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final var y = sensor.location().y();
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final var reach = sensor.distanceToBeacon();
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// Find all the points just outside this sensor's reach
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for(int horizontalOffset = 0; horizontalOffset <= reach + 1; horizontalOffset++) {
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final var verticalOffset = reach + 1 - horizontalOffset;
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Assertions.assertEquals(horizontalOffset + verticalOffset, reach + 1);
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for(final var candidate : Arrays.asList(new Coordinate(x + verticalOffset, y + horizontalOffset), // SE
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new Coordinate(x + verticalOffset, y - horizontalOffset), // SW
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new Coordinate(x - verticalOffset, y + horizontalOffset), // NE
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new Coordinate(x - verticalOffset, y - horizontalOffset))) { // NW
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if(candidate.x() < 0 || candidate.y() < 0 || candidate.x() > max || candidate.y() > max) {
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continue;
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}
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Assertions.assertTrue(candidate.distanceTo(sensor.location()) > sensor.distanceToBeacon());
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// Check if the point is also outside the reach of every other sensor
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if(sensors.stream().allMatch(other -> candidate.distanceTo(other.location()) > other.distanceToBeacon())) {
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final long result = (long)candidate.y() * 4_000_000l + (long)candidate.x();
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System.out.println("Part 2: " + result);
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return;
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}
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}
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}
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}
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throw new IllegalStateException("No uncovered point found");
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}
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}

‎src/test/resources/sample/day-15.txt

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Sensor at x=2, y=18: closest beacon is at x=-2, y=15
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Sensor at x=9, y=16: closest beacon is at x=10, y=16
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Sensor at x=13, y=2: closest beacon is at x=15, y=3
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Sensor at x=12, y=14: closest beacon is at x=10, y=16
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Sensor at x=10, y=20: closest beacon is at x=10, y=16
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Sensor at x=14, y=17: closest beacon is at x=10, y=16
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Sensor at x=8, y=7: closest beacon is at x=2, y=10
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Sensor at x=2, y=0: closest beacon is at x=2, y=10
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Sensor at x=0, y=11: closest beacon is at x=2, y=10
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Sensor at x=20, y=14: closest beacon is at x=25, y=17
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Sensor at x=17, y=20: closest beacon is at x=21, y=22
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Sensor at x=16, y=7: closest beacon is at x=15, y=3
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Sensor at x=14, y=3: closest beacon is at x=15, y=3
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Sensor at x=20, y=1: closest beacon is at x=15, y=3

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