aoc2024

commit b1aab78ac3d97cc2c4ccf5cfdc7de126278e1f2f
parent 7968d9cf758aa41788c775127ff3bfd74129de41
Author: Martin Ashby <martin@ashbysoft.com>
Date:   Tue, 31 Dec 2024 00:11:19 +0000

Day 8 pt2

Diffstat:
A
src/main/java/day8_pt2.java
 | 
107
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

1 file changed, 107 insertions(+), 0 deletions(-)
diff --git a/src/main/java/day8_pt2.java b/src/main/java/day8_pt2.java
@@ -0,0 +1,107 @@
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.io.IOException;
+import java.io.BufferedReader;
+import java.io.StringReader;
+import java.util.Arrays;
+import java.util.List;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.ArrayList;
+
+public class day8_pt2 {
+  record Point(int x, int y) {
+    Vec diff(Point other) {
+      return new Vec(this.x - other.x, this.y - other.y);
+    }
+  }
+  record Vec(int x, int y) {
+    Vec mul(int scalar) {
+      return new Vec(this.x * scalar, this.y * scalar);
+    }
+    record DivRes(int scalar, boolean valid){}
+    // Divide one vector by another
+    // to return an exact integer result. Otherwise valid=false
+    DivRes div(Vec other) {
+      if (other.x == 0 && this.x != 0) {
+        return new DivRes(0, false);
+      } else if (other.y == 0 && this.y != 0) {
+        return new DivRes(0, false);
+      } else if (other.x == 0) {
+        return new DivRes(this.y / other.y, this.y % other.y == 0);
+      } else if (other.y == 0) {
+        return new DivRes(this.x / other.x, this.x % other.x == 0);
+      } else {
+        var my = this.y / other.y;
+        var mx = this.x / other.x;
+        return new DivRes(my, my == mx && this.y % other.y == 0 && this.x % other.x == 0);
+      }
+    }
+  }
+  
+  
+  public static void main(String[] args) throws IOException {
+    var input = Files.readString(Path.of(args[0]));
+    var grid = new BufferedReader(new StringReader(input))
+      .lines()
+      .toList();
+    // Just gonna do this the naïve way
+    // First pass get the sets of antennas    
+    var height = grid.size();
+    var width = grid.getFirst().length();
+    var antennas = new HashMap<Character, ArrayList<Point>>();
+    for (var x = 0; x < width; x++) {
+      for (var y = 0; y < height; y++) {
+        var ch = grid.get(y).charAt(x);
+        if (ch != '.') {
+          var al = antennas.computeIfAbsent(ch, ignored -> new ArrayList<>());
+          al.add(new Point(x, y));
+        }
+      }
+    }
+    //System.out.printf("antennas = %s\n", antennas);
+    // Second pass calculate the distance to each antenna and check if we're an antinode
+    var ans = new HashSet<Point>();
+    var antss = antennas.values();
+    for (var x = 0; x < width; x++) {
+      for (var y = 0; y < height; y++) {
+        var pt = new Point(x, y);
+        for (var ants: antss) {
+          if (isAn(ants, pt)) {
+            ans.add(pt);
+            break; // no need to test other antenna sets
+          }
+        }
+      }
+    }
+  
+    for (var y = 0; y < height; y++) {
+      for (var x = 0; x < width; x++) {
+        if (ans.contains(new Point(x, y))) {
+          System.out.print('#');
+        } else {
+          System.out.print(grid.get(y).charAt(x));
+        }
+      }
+      System.out.print('\n');
+    }
+    System.out.printf("day8, pt2 = %d\n", ans.size());
+  }
+
+  static boolean isAn(List<Point> ants, Point pt) {
+    for (var i=0; i<ants.size(); i++) {
+      for (var j=0; j<ants.size(); j++) {
+        if (i == j) continue;
+        var ant1 = ants.get(i);
+        var ant2 = ants.get(j);
+        var va = ant1.diff(ant2);
+        var v1 = pt.diff(ant1);
+        var dr = v1.div(va);
+        if (dr.valid) {
+          return true;
+        }
+      }
+    }
+    return false;
+  }
+}