1 files changed, 110 insertions, 0 deletions
diff --git a/src/day04.exs b/src/day04.exs
new file mode 100644
index 0000000..990e04f
--- /dev/null
+++ b/src/day04.exs
@@ -0,0 +1,110 @@
+defmodule Day04 do
+  def part1(data, word \\ ~c"XMAS") do
+    data = data
+    |> String.split("\n", trim: true)
+    |> Enum.map(&(:array.from_list(String.to_charlist(&1))))
+
+    count_word(0, data, word) +
+      count_word(0, data, Enum.reverse(word))
+  end
+
+  def count_word(acc, [], _) do
+    acc
+  end
+
+  def count_word(acc, data, word) do
+    count_word(acc + count_word_hd(data, word), tl(data), word)
+  end
+
+  def count_word_hd(data, word) do
+    0..:array.size(hd(data))-1
+    |> Enum.map(&(count_word_hd_col(data, word, &1)))
+    |> Enum.sum()
+  end
+
+  def count_word_hd_col(data, word, col) do
+    [check_word_hori(data, word, col),
+     check_word_vert(data, word, col),
+     check_word_diag_right(data, word, col),
+     check_word_diag_left(data, word, col)]
+    |> Enum.count(&(&1))
+  end
+
+  def check_word_hori(data, word, col) do
+    check_word(data, word, col, &(&1), &(&1 + 1))
+  end
+
+  def check_word_vert(data, word, col) do
+    check_word(data, word, col, &tl/1, &(&1))
+  end
+
+  def check_word_diag_right(data, word, col) do
+    check_word(data, word, col, &tl/1, &(&1 + 1))
+  end
+
+  def check_word_diag_left(data, word, col) do
+    check_word(data, word, col, &tl/1, &(&1 - 1))
+  end
+
+  def check_word(data, word, col, data_step_fn, col_step_fn) do
+    cond do
+      word == [] ->
+        true
+      data == [] or col < 0 or col >= :array.size(hd(data)) ->
+        false
+      hd(word) == :array.get(col, hd(data)) ->
+        check_word(
+          data_step_fn.(data),
+          tl(word),
+          col_step_fn.(col),
+          data_step_fn,
+          col_step_fn
+        )
+      true ->
+        false
+    end
+  end
+
+  def part2(data, word \\ ~c"MAS") do
+    data = data
+    |> String.split("\n", trim: true)
+    |> Enum.map(&(:array.from_list(String.to_charlist(&1))))
+
+    count_cross(0, data, word)
+  end
+
+  def count_cross(acc, [], _) do
+    acc
+  end
+
+  def count_cross(acc, data, word) do
+    count_cross(acc + count_cross_hd(data, word), tl(data), word)
+  end
+
+  def count_cross_hd(data, word) do
+    0..:array.size(hd(data))-1
+    |> Enum.count(&(check_cross_hd_col(data, word, &1)))
+  end
+
+  def check_cross_hd_col(data, word, col) do
+    (check_word_diag_right(data, word, col) and
+       check_word_diag_left(data, word, col+2)) or
+      (check_word_diag_right(data, Enum.reverse(word), col) and
+         check_word_diag_left(data, word, col+2)) or
+      (check_word_diag_right(data, word, col) and
+         check_word_diag_left(data, Enum.reverse(word), col+2)) or
+      (check_word_diag_right(data, Enum.reverse(word), col) and
+         check_word_diag_left(data, Enum.reverse(word), col+2))
+  end
+end
+
+data = IO.read(:stdio, :eof)
+
+{time1 , ans1} = :timer.tc(fn -> Day04.part1(data) end)
+IO.puts("Time  : #{time1 / 1000000}")
+IO.puts("Answer: #{ans1}")
+
+{time2 , ans2} = :timer.tc(fn -> Day04.part2(data) end)
+IO.puts("Time  : #{time2 / 1000000}")
+IO.puts("Answer: #{ans2}")
+