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Day 13
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‎src/test/java/com/macasaet/Day13.java

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package com.macasaet;
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import java.util.*;
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import java.util.stream.Collectors;
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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 13: Transparent Origami ---
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*/
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public class Day13 {
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protected Stream<String> getInput() {
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return StreamSupport
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.stream(new LineSpliterator("day-13.txt"),
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false);
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}
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/**
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* A point on the translucent sheet of paper. Note that <em>x</em> and <em>y</em> correspond to a particular
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* {@link Axis}.
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*/
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public record Point(int x, int y) {
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}
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/**
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* An axis of the translucent sheet of paper
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*/
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public enum Axis {
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/**
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* The axis that increases to the right
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*/
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X,
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/**
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* The axis that increases downward
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*/
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Y,
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}
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/**
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* An equation for a line
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*/
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public record Line(Axis axis, int value) {
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public String toString() {
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return switch (axis()) {
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case X -> "x=" + value;
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case Y -> "y=" + value;
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};
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}
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}
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public record Input(Collection<Point> points, List<Line> folds, int maxX, int maxY) {
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public Sheet getSheet() {
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final boolean[][] grid = new boolean[maxY + 1][];
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for (int i = grid.length; --i >= 0; ) {
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grid[i] = new boolean[maxX + 1];
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}
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for (final var point : points) {
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/* The first value, x, increases to the right. The second value, y, increases downward. */
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grid[point.y()][point.x()] = true;
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}
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return new Sheet(grid);
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}
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}
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/**
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* A sheet of translucent paper
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*/
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public record Sheet(boolean[][] grid) {
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public int countDots() {
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int result = 0;
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final var grid = grid();
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for (int i = grid.length; --i >= 0; ) {
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for (int j = grid[i].length; --j >= 0; ) {
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if (grid[i][j]) {
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result++;
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}
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}
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}
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return result;
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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 (final var row : grid) {
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for (final var cell : row) {
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builder.append(cell ? '#' : '.');
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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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public Sheet fold(final Line line) {
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// note, value is always positive
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return switch (line.axis()) {
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case X -> {
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// fold along the x-axis (vertical line)
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final var newGrid = new boolean[grid.length][];
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for (int i = newGrid.length; --i >= 0; ) {
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final var newRow = new boolean[line.value() + 1];
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for (int j = newRow.length; --j >= 0; newRow[j] = grid[i][j]) ;
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for(int j = grid[i].length - line.value(); --j > 0; ) {
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if(grid[i][line.value() + j]) {
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newRow[line.value() - j] = true;
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}
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}
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newGrid[i] = newRow;
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}
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yield new Sheet(newGrid);
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}
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case Y -> {
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// fold along the y-axis (horizontal line)
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final var newGrid = new boolean[line.value()][];
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for (int i = newGrid.length; --i >= 0; ) {
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final var newRow = new boolean[grid[i].length];
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for (int j = grid[i].length; --j >= 0; newRow[j] = grid[i][j]) ;
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newGrid[i] = newRow;
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}
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for (int i = grid.length - line.value(); --i > 0; ) {
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final var oldRow = grid[line.value() + i];
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for (int j = oldRow.length;
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--j >= 0;
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newGrid[line.value() - i][j] |= oldRow[j])
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;
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}
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yield new Sheet(newGrid);
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}
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};
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}
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}
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public Input parseInput() {
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int section = 0;
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final var points = new HashSet<Point>();
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final var folds = new ArrayList<Line>();
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int maxX = Integer.MIN_VALUE;
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int maxY = Integer.MIN_VALUE;
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for (final var line : getInput().collect(Collectors.toList())) {
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if (line.isBlank()) {
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section++;
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continue;
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}
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if (section == 0) { // points
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final var components = line.split(",");
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final var x = Integer.parseInt(components[0]);
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maxX = Math.max(x, maxX);
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final var y = Integer.parseInt(components[1]);
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maxY = Math.max(y, maxY);
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final var point = new Point(x, y);
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points.add(point);
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} else { // commands
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final var equation = line.replaceFirst("fold along ", "");
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final var components = equation.split("=");
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final var axis = Axis.valueOf(components[0].toUpperCase(Locale.ROOT));
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final var value = Integer.parseInt(components[1]);
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final var fold = new Line(axis, value);
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folds.add(fold);
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}
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}
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return new Input(points, folds, maxX, maxY);
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}
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@Test
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public final void part1() {
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final var input = parseInput();
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final var sheet = input.getSheet();
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final var firstFold = input.folds().get(0);
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final var result = sheet.fold(firstFold);
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System.out.println("Part 1: " + result.countDots());
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}
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@Test
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public final void part2() {
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final var input = parseInput();
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var sheet = input.getSheet();
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for (final var fold : input.folds()) {
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sheet = sheet.fold(fold);
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}
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System.out.println("Part 2:\n" + sheet);
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}
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}

‎src/test/resources/sample/day-13.txt

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6,10
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0,14
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9,10
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0,3
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10,4
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4,11
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6,0
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6,12
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4,1
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0,13
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10,12
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3,4
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3,0
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8,4
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1,10
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2,14
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8,10
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9,0
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fold along y=7
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fold along x=5

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