aoc2024

Advent of Code 2024
Log | Files | Refs | README

day7_pt2.java (1413B)

      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 
      9 public class day7_pt2 {
     10   public static void main(String[] args) throws IOException {
     11     var input = Files.readString(Path.of(args[0]));
     12     System.out.printf("day7, pt2 = %d\n", new BufferedReader(new StringReader(input))
     13       .lines()
     14       .mapToLong(line -> {
     15         var arr = line.split(":");
     16         var target = Long.parseLong(arr[0]);
     17         var nums = Arrays.stream(arr[1].trim().split(" ")).map(Long::parseLong).toList();
     18         if (fn(target, nums.getFirst(), nums.subList(1, nums.size()))) {
     19           return target;
     20         } else {
     21           return 0;
     22         }
     23       })
     24       .sum());
     25   }
     26 
     27   static boolean fn(long target, long res, List<Long> rem) {
     28     // short circuit dead ends
     29     // I checked the puzzle input for "* 0" and it doesn't occur, so
     30     // this seems to be valid... 
     31     if (res > target) {
     32       return false;
     33     }
     34     if (rem.size() == 0) {
     35       return res == target;
     36     } else {
     37       var arg = rem.getFirst();
     38       var rem2 = rem.subList(1, rem.size());
     39       return fn(target, res * arg, rem2) || fn(target, res + arg, rem2) || fn(target, cat(res, arg), rem2);
     40     }
     41   }
     42 
     43   static long cat(long l1, long l2) {
     44     return Long.parseLong(String.format("%d%d", l1, l2));
     45   }
     46 }