Day 8. A bit ugly, but got it done quickly

day8/data.txt

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+.....................U.........w..................
+l.................................................
+...........o.a................U...w...............
+............................................W.....
+..........T....................s.............7....
+.............................................W....
+.........T..............4....n.d.H.........5......
+......T.....oj...U.....n...w......H...........z...
+.G..x..........................E.....V..H.........
+.........a....................d....s.......7w.....
+...j....r.............o.............V.......d...W.
+.......r..J.Goa.U...............n................z
+.........Jj.........M..........Pv.................
+...J...........t..3..M..............sLV...........
+...................t................n.............
+....r...........X...........M........v............
+...x....t......I......a.PM...............W........
+...........1.Bj....I........vO.h.dL...............
+.........6....Rr......B...X........h..5v.L..z.....
+......1G...........x.....3B.......5...............
+.................B....0..........4..E.............
+.....................X.....5..h....P....f.....D...
+.......1........J.....eK..........................
+..................I....R....K...........k.........
+......G..................O........................
+...........H...9...............K8.P.4..k..E.......
+............1....3.............8.F.............f..
+.........................4........................
+.l...........X............9.......................
+....N.................R...t.e.....................
+...g............3..R.........e....h.........f.....
+...........................e......i...............
+................2...I.7..9..O.....s.........k.....
+....................6...9E.............F..O.......
+........................KN........................
+.......g......................Z.........F..f...Y..
+...........................A....i.................
+...........6g...b........8.......y.....S..........
+..l.....6.....m...............8...................
+....u..m...b...............p...A..................
+..............b.p........................k........
+....m......2...........Z..y....i..................
+........g2.....b.........i....D..ZF...............
+......2.0...........p............N..........A.....
+...m.............S...y........A...Z...N...........
+..S..l..........................................Y.
+........S....0u.................y......DY.........
+...........0.........................D............
+.................u...................p...Y........
+.......u..........................................

day8/main.exs

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+defmodule Day7 do
+  def run do
+    map =
+      File.read!("data.txt")
+      |> String.split("\n")
+      |> Enum.drop(-1)
+      |> Enum.map(&String.graphemes(&1))
+
+    height = length(map)
+    width = length(hd(map))
+
+    dbg(map)
+
+    # generate list of antennas
+    # flatten the map, add indices, if char isn't a dot, grab width and height, stick them in a map
+    antennas =
+      Enum.concat(map)
+      |> Enum.with_index()
+      |> Enum.reduce(%{}, fn {char, idx}, acc ->
+        if char != "." do
+          location = {div(idx, height), rem(idx, width)}
+          Map.update(acc, char, [location], fn list -> [location | list] end)
+        else
+          acc
+        end
+      end)
+
+    dbg(antennas, charlists: :as_lists)
+
+    # generate all possible pairs of locations for this antenna
+    pairs =
+      Enum.map(antennas, fn {char, locations} ->
+        pairs =
+          for x <- 0..(length(locations) - 2),
+              y <- (x + 1)..(length(locations) - 1),
+              x != y,
+              do: {Enum.at(locations, x), Enum.at(locations, y)}
+
+        {char, pairs}
+      end)
+
+    dbg(pairs, charlists: :as_lists)
+
+    # find antinodes
+    antinodes =
+      Enum.reduce(pairs, MapSet.new(), fn {char, nodes}, acc ->
+        Enum.reduce(nodes, acc, fn {{x1, y1}, {x2, y2}}, acc2 ->
+          {dx, dy} = {x2 - x1, y2 - y1}
+
+          new_points =
+            if (dx > 0 and dy > 0) or (dx < 0 and dy < 0) do
+              # slope \
+              [
+                {min(x1, x2) - abs(dx), min(y1, y2) - abs(dy)},
+                {max(x1, x2) + abs(dx), max(y1, y2) + abs(dy)}
+              ]
+            else
+              # slope /
+              [
+                {max(x1, x2) + abs(dx), min(y1, y2) - abs(dy)},
+                {min(x1, x2) - abs(dx), max(y1, y2) + abs(dy)}
+              ]
+            end
+            # ensure in bounds
+            |> Enum.filter(fn {x, y} -> x >= 0 and y >= 0 and x < height and y < width end)
+
+          # add each new point to the MapSet
+          Enum.reduce(new_points, acc2, fn point, acc3 ->
+            MapSet.put(acc3, point)
+          end)
+        end)
+      end)
+
+    dbg(MapSet.size(antinodes))
+
+    antinodes2 =
+      Enum.reduce(pairs, MapSet.new(), fn {char, nodes}, acc ->
+        Enum.reduce(nodes, acc, fn {{x1, y1}, {x2, y2}}, acc2 ->
+          {dx, dy} = {x2 - x1, y2 - y1}
+
+          # generate points along these lines until out of bounds
+          # roughly how many times do we need to loop? we'll stop early if both points are out of bounds
+          loops = trunc(min(abs(width / dy), abs(height / dx)))
+
+          new_points =
+            Enum.reduce_while(1..(loops + 1), [], fn idx, acc ->
+              points =
+                if (dx > 0 and dy > 0) or (dx < 0 and dy < 0) do
+                  # slope \
+                  [
+                    {min(x1, x2) - abs(dx) * idx, min(y1, y2) - abs(dy) * idx},
+                    {max(x1, x2) + abs(dx) * idx, max(y1, y2) + abs(dy) * idx}
+                  ]
+                else
+                  # slope /
+                  [
+                    {max(x1, x2) + abs(dx) * idx, min(y1, y2) - abs(dy) * idx},
+                    {min(x1, x2) - abs(dx) * idx, max(y1, y2) + abs(dy) * idx}
+                  ]
+                end
+                |> Enum.filter(fn {x, y} -> x >= 0 and y >= 0 and x < height and y < width end)
+
+              if length(points) == 0 do
+                {:halt, points ++ acc}
+              else
+                {:cont, points ++ acc}
+              end
+            end)
+
+          # add each new point to the MapSet
+          Enum.reduce(new_points, acc2, fn point, acc3 ->
+            MapSet.put(acc3, point)
+          end)
+        end)
+      end)
+
+    # add all the original antennae to the MapSet
+    part2 =
+      Enum.reduce(antennas, antinodes2, fn {char, points}, acc ->
+        Enum.reduce(points, acc, fn point, acc2 ->
+          MapSet.put(acc2, point)
+        end)
+      end)
+
+    dbg(MapSet.size(part2))
+  end
+end
+
+Day7.run()

day8/test.txt

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+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............

day8/test2.txt

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+T.........
+...T......
+.T........
+..........
+..........
+..........
+..........
+..........
+..........
+..........