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Day 11
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‎src/test/java/com/macasaet/Day11.java

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package com.macasaet;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.stream.Stream;
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import java.util.stream.StreamSupport;
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import org.junit.jupiter.api.Test;
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/**
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* --- Day 11: Dumbo Octopus ---
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*/
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public class Day11 {
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protected Stream<String> getInput() {
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return StreamSupport
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.stream(new LineSpliterator("day-11.txt"),
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false);
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}
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/**
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* @return a spatial grid of the cavern indicating the location of the octopuses
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*/
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protected Octopus[][] getOctopusGrid() {
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final var list = getInput().map(line -> {
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final var chars = line.toCharArray();
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final byte[] result = new byte[chars.length];
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for (int i = chars.length; --i >= 0; result[i] = (byte) (chars[i] - '0')) ;
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return result;
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}).collect(ArrayList<byte[]>::new, List::add, List::addAll);
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final var result = new Octopus[list.size()][];
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for (int i = list.size(); --i >= 0; ) {
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final var row = list.get(i);
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result[i] = new Octopus[row.length];
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for (int j = row.length; --j >= 0; result[i][j] = new Octopus(i, j, row[j])) ;
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}
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return result;
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}
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/**
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* A rare bioluminescent dumbo octopus
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*/
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public static class Octopus {
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private final int x, y;
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private byte energyLevel;
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public Octopus(final int x, final int y, final byte energyLevel) {
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this.x = x;
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this.y = y;
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this.energyLevel = energyLevel;
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}
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/**
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* Increase the octopus' energy level and, if appropriate, propagate side effects to its neighbours.
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*
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* @param population the full population of octopuses
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*/
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public void prepareStep(final Population population) {
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if (this.energyLevel > 9) {
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// "An octopus can only flash at most once per step."
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return;
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}
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// "First, the energy level of each octopus increases by 1."
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this.energyLevel++;
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if (this.energyLevel > 9) {
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// "Then, any octopus with an energy level greater than 9 flashes. This increases the energy level of
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// all adjacent octopuses by 1, including octopuses that are diagonally adjacent."
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final var grid = population.grid();
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final var hasRowAbove = x > 0;
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final var hasRowBelow = x < grid.length - 1;
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final var hasColumnToLeft = y > 0;
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final var hasColumnToRight = y < grid[x].length - 1;
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if (hasRowAbove) {
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grid[x - 1][y].prepareStep(population);
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if (hasColumnToLeft) {
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grid[x - 1][y - 1].prepareStep(population);
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}
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if (hasColumnToRight) {
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grid[x - 1][y + 1].prepareStep(population);
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}
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}
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if (hasColumnToLeft) {
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grid[x][y - 1].prepareStep(population);
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}
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if (hasColumnToRight) {
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grid[x][y + 1].prepareStep(population);
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}
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if (hasRowBelow) {
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grid[x + 1][y].prepareStep(population);
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if (hasColumnToLeft) {
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grid[x + 1][y - 1].prepareStep(population);
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}
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if (hasColumnToRight) {
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grid[x + 1][y + 1].prepareStep(population);
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}
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}
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}
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}
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/**
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* Complete the step and finalise any side effects.
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*
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* @return true if and only if the octopus flashed during this step.
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*/
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public boolean finishStep() {
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if (this.energyLevel > 9) {
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// "Finally, any octopus that flashed during this step has its energy level set to 0, as it used all of
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// its energy to flash."
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this.energyLevel = 0;
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return true;
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}
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return false;
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}
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}
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/**
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* The full population of dumbo octopuses. The population members will be modified with each step.
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*/
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public record Population(Octopus[][] grid) {
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public int step() {
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for (int i = grid.length; --i >= 0; ) {
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final var row = grid[i];
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for (int j = row.length; --j >= 0; ) {
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row[j].prepareStep(this);
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}
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}
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int flashes = 0;
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for (int i = grid.length; --i >= 0; ) {
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final var row = grid[i];
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for (int j = row.length; --j >= 0; ) {
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if (row[j].finishStep()) flashes++;
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}
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}
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return flashes;
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}
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}
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@Test
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public final void part1() {
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final var energyLevels = getOctopusGrid();
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final var population = new Population(energyLevels);
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int flashes = 0;
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for (int step = 0; step < 100; step++) {
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flashes += population.step();
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}
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System.out.println("Part 1: " + flashes);
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}
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@Test
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public final void part2() {
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final var energyLevels = getOctopusGrid();
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final var population = new Population(energyLevels);
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int step = 0;
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while(true) {
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final int flashes = population.step();
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step++;
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if(flashes == 100) {
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System.out.println("Part 2: " + step);
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break;
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}
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}
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}
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}

‎src/test/resources/sample/day-11.txt

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5483143223
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2745854711
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5264556173
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6141336146
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6357385478
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4167524645
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2176841721
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6882881134
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4846848554
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5283751526

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