aoc2024

Advent of Code 2024
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day8_pt1.java (2816B)

      1 import java.nio.file.Files;
      2 import java.nio.file.Path;
      3 import java.io.IOException;
      4 import java.io.BufferedReader;
      5 import java.io.StringReader;
      6 import java.util.Arrays;
      7 import java.util.List;
      8 import java.util.HashMap;
      9 import java.util.HashSet;
     10 import java.util.ArrayList;
     11 
     12 public class day8_pt1 {
     13   record Point(int x, int y) {}
     14   
     15   public static void main(String[] args) throws IOException {
     16     var input = Files.readString(Path.of(args[0]));
     17     var grid = new BufferedReader(new StringReader(input))
     18       .lines()
     19       .toList();
     20     // Just gonna do this the naïve way
     21     // First pass get the sets of antennas
     22     
     23     var height = grid.size();
     24     var width = grid.getFirst().length();
     25     var antennas = new HashMap<Character, ArrayList<Point>>();
     26     for (var x = 0; x < width; x++) {
     27       for (var y = 0; y < height; y++) {
     28         var ch = grid.get(y).charAt(x);
     29         if (ch != '.') {
     30           var al = antennas.computeIfAbsent(ch, ignored -> new ArrayList<>());
     31           al.add(new Point(x, y));
     32         }
     33       }
     34     }
     35     //System.out.printf("antennas = %s\n", antennas);
     36     // Second pass calculate the distance to each antenna and check if we're an antinode
     37     var ans = new HashSet<Point>();
     38     var antss = antennas.values();
     39     for (var x = 0; x < width; x++) {
     40       for (var y = 0; y < height; y++) {
     41         var pt = new Point(x, y);
     42         for (var ants: antss) {
     43           if (isAn(ants, pt)) {
     44             ans.add(pt);
     45             break; // no need to test other antenna sets
     46           }
     47         }
     48       }
     49     }
     50   
     51     // for (var y = 0; y < height; y++) {
     52     //   for (var x = 0; x < width; x++) {
     53     //     if (ans.contains(new Point(x, y))) {
     54     //       System.out.print('#');
     55     //     } else {
     56     //       System.out.print(grid.get(y).charAt(x));
     57     //     }
     58     //   }
     59     //   System.out.print('\n');
     60     // }
     61     System.out.printf("day8, pt1 = %d\n", ans.size());
     62   }
     63 
     64   static boolean isAn(List<Point> ants, Point pt) {
     65     for (var i=0; i<ants.size(); i++) {
     66       for (var j=0; j<ants.size(); j++) {
     67         if (i == j) continue;
     68         var ant1 = ants.get(i);
     69         var ant2 = ants.get(j);
     70         // are we perfectly in line, AND twice the distance?
     71         // y distance has to be exactly twice and so does x distance
     72         // and signs have to be right (either in same or opposing quadrants)
     73         var d1x = ant1.x - pt.x;
     74         var d1y = ant1.y - pt.y;
     75         var d2x = ant2.x - pt.x;
     76         var d2y = ant2.y - pt.y;
     77         var s1 = sig(d1x) * sig(d1y);
     78         var s2 = sig(d2x) * sig(d2y);
     79         if (Math.abs(d1y) == 2 * Math.abs(d2y) && 
     80           Math.abs(d1x) == 2 * Math.abs(d2x) &&
     81           s1 == s2) {
     82           return true;
     83         }
     84       }
     85     }
     86     return false;
     87   }
     88   static int sig(int x) {
     89     return x >= 0 ? 1 : -1;
     90   }
     91 }