1 files changed, 134 insertions, 0 deletions
diff --git a/src/day08.exs b/src/day08.exs
new file mode 100644
index 0000000..0d312e1
--- /dev/null
+++ b/src/day08.exs
@@ -0,0 +1,134 @@
+defmodule Day08 do
+  defmodule Grid do
+    defstruct [:grid, :height, :width]
+
+    def from_data(data) do
+      grid = data
+      |> String.split("\n", trim: true)
+      |> Enum.map(& &1 |> String.codepoints() |> :array.from_list())
+      |> :array.from_list()
+
+      height = :array.size(grid)
+
+      width = :array.size(:array.get(0, grid))
+
+      %Grid{grid: grid, height: height, width: width}
+    end
+
+    def at(grid, i, j) do
+      :array.get(j, :array.get(i, grid.grid))
+    end
+
+    def in?(grid, i, j) do
+      i >= 0 and i < grid.height and j >= 0 and j < grid.width
+    end
+
+    def find_elements(grid, regex) do
+      for i <- 0..grid.height-1,
+          j <- 0..grid.width-1,
+          v = at(grid, i, j) do
+        if String.match?(v, regex) do
+          {v, i, j}
+        end
+      end
+      |> Enum.filter(& &1 != nil)
+    end
+  end
+
+  def part1(data) do
+    grid = data
+    |> Grid.from_data()
+
+    antennas = grid
+    |> Grid.find_elements(~r/[[:alnum:]]/)
+    |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
+
+    antennas
+    |> Enum.map(fn {_, a} -> antinodes([], a) end)
+    |> Enum.flat_map(& &1)
+    |> Enum.uniq()
+    |> Enum.filter(fn {i, j} -> Grid.in?(grid, i, j) end)
+    |> Enum.count()
+  end
+
+  defp antinodes(acc, [a | rest]) do
+    rest
+    |> Enum.reduce(acc, fn b, acc -> antinodes(acc, a, b) end)
+    |> antinodes(rest)
+  end
+
+  defp antinodes(acc, []), do: acc
+
+  defp antinodes(acc, {ai, aj}, {bi, bj}) do
+    di = abs(ai - bi)
+    dj = abs(aj - bj)
+    [{min(ai, bi) - di, if aj < bj do aj - dj else aj + dj end} |
+       [{max(ai, bi) + di, if aj < bj do bj + dj else bj - dj end} |
+          acc]]
+  end
+
+  def part2(data) do
+    grid = data
+    |> Grid.from_data()
+
+    antennas = grid
+    |> Grid.find_elements(~r/[[:alnum:]]/)
+    |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
+
+    antennas
+    |> Enum.map(fn {_, a} -> antinodes2([], a, grid) end)
+    |> Enum.flat_map(& &1)
+    |> Enum.uniq()
+    |> Enum.count()
+
+  end
+
+  defp antinodes2(acc, [a | rest], grid) do
+    rest
+    |> Enum.reduce(acc, fn b, acc -> antinodes2(acc, a, b, grid) end)
+    |> antinodes2(rest, grid)
+  end
+
+  defp antinodes2(acc, [], _), do: acc
+  
+  defp antinodes2(acc, {ai, aj}, {bi, bj}, grid) do
+    di = abs(ai - bi)
+    dj = abs(aj - bj)
+
+    cond do
+      di == 0 ->
+        acc
+        |> antinodes2(ai, aj, 0, -1, grid)
+        |> antinodes2(ai, aj, 0, 1, grid)
+      dj == 0 ->
+        acc
+        |> antinodes2(ai, aj, -1, 0, grid)
+        |> antinodes2(ai, aj, 1, 0, grid)
+      true ->
+        g = Integer.gcd(di, dj)
+        di = div(di, g)
+        dj = div(dj, g)
+        acc
+        |> antinodes2(ai, aj, -di, if aj < bj do -dj else dj end, grid)
+        |> antinodes2(ai, aj, di, if aj < bj do dj else -dj end, grid)
+    end
+  end
+
+  defp antinodes2(acc, i, j, di, dj, grid) do
+    if Grid.in?(grid, i, j) do
+      antinodes2([{i, j} | acc], i+di, j+dj, di, dj, grid)
+    else
+      acc
+    end
+  end
+end
+
+data = IO.read(:stdio, :eof)
+
+{time1 , ans1} = :timer.tc(fn -> Day08.part1(data) end)
+IO.puts("Time  : #{time1 / 1000000}")
+IO.puts("Answer: #{ans1}")
+
+{time2 , ans2} = :timer.tc(fn -> Day08.part2(data) end)
+IO.puts("Time  : #{time2 / 1000000}")
+IO.puts("Answer: #{ans2}")