2024/08 writeup

2024/Day08/README.md

@@ -1,8 +1,16 @@
 ## --- Day 8: Resonant Collinearity ---
-You find yourselves on the [roof](/2016/day/25) of a top-secret Easter Bunny installation.
+You find yourselves on the _roof_ of a top-secret Easter Bunny installation.
 
 While The Historians do their thing, you take a look at the familiar <em>huge antenna</em>. Much to your surprise, it seems to have been reconfigured to emit a signal that makes people 0.1% more likely to buy Easter Bunny brand Imitation Mediocre Chocolate as a Christmas gift! Unthinkable!
 
 Scanning across the city, you find that there are actually many such antennas. Each antenna is tuned to a specific <em>frequency</em> indicated by a single lowercase letter, uppercase letter, or digit. You create a map (your puzzle input) of these antennas. For example:
 
 Read the [full puzzle](https://adventofcode.com/2024/day/8).
+
+Continuing the steps I started yesterday, I extracted a common function (`GetUniquePositions`) that takes a parameter to generate antinode positions, representing the difference between part one and part two.
+
+`getAntinodes` returns the antinode positions of srcAntenna on the dstAntenna side. It doesn’t need to be symmetric — i.e., it doesn’t have to return the antinodes on the srcAntenna side — because GetUniquePositions will call it with the parameters swapped as well. This allows us to handle only one direction at a time.
+
+The generators are fairly straightforward: I simply take steps in the direction determined by the antennas, starting from the destination. Since I represented coordinates using complex numbers again, there’s no need for any special handling on the algebra side, regular + and - operations work.
+
+The `GetAntinodes` delegate is introduced only for documentation purposes. It looks better to my eyes than an ugly `Func<>` with four type parameters would in `GetUniquePositions`-s signature. 

2024/Day08/Solution.cs

@@ -13,7 +13,7 @@ class Solution : Solver {
     public object PartOne(string input) => GetUniquePositions(input, GetAntinodes1).Count();
     public object PartTwo(string input) => GetUniquePositions(input, GetAntinodes2).Count();
 
-    HashSet<Complex> GetUniquePositions(string input, GetAntinodes antinodePostions) {
+    HashSet<Complex> GetUniquePositions(string input, GetAntinodes getAntinodes) {
         var map = GetMap(input);
 
         var antennaLocations = (
@@ -26,18 +26,18 @@ select pos
              from srcAntenna in antennaLocations
              from dstAntenna in antennaLocations
              where srcAntenna != dstAntenna && map[srcAntenna] == map[dstAntenna]
-             from antinode in antinodePostions(srcAntenna, dstAntenna, map)
+             from antinode in getAntinodes(srcAntenna, dstAntenna, map)
              select antinode
          ).ToHashSet();
     }
 
-    // returns the antinode positions on the srcAntenna side induced by dstAntenna
+    // returns the antinode positions of srcAntenna on the dstAntenna side
     delegate IEnumerable<Complex> GetAntinodes(Complex srcAntenna, Complex dstAntenna, Map map);
 
     // in part 1 we just look at the immediate neighbour
     IEnumerable<Complex> GetAntinodes1(Complex srcAntenna, Complex dstAntenna, Map map) {
         var dir = dstAntenna - srcAntenna;
-        var antinote = srcAntenna - dir;
+        var antinote = dstAntenna + dir;
         if (map.Keys.Contains(antinote)) {
             yield return antinote;
         }
@@ -46,10 +46,10 @@ IEnumerable<Complex> GetAntinodes1(Complex srcAntenna, Complex dstAntenna, Map m
     // in part 2 this becomes a cycle, plus srcAntenna is also a valid position now
     IEnumerable<Complex> GetAntinodes2(Complex srcAntenna, Complex dstAntenna, Map map) {
         var dir = dstAntenna - srcAntenna;
-        var antinote = srcAntenna;
+        var antinote = dstAntenna;
         while (map.Keys.Contains(antinote)) {
             yield return antinote;
-            antinote -= dir;
+            antinote += dir;
         }
     }