2021 day10 part 2

2021/day10.exs

@@ -0,0 +1,95 @@
+defmodule Day10 do
+  def part1(input) do
+    input
+    |> Enum.map(&parse_line(&1, []))
+    |> Enum.filter(&match?({:corrupted, _}, &1))
+    |> Enum.map(fn {:corrupted, char} -> score(char) end)
+    |> Enum.sum()
+  end
+
+  defp parse_line(_, {:corrupted, char}), do: {:corrupted, char}
+  defp parse_line("", stack), do: {:incomplete, stack}
+  defp parse_line(<<char::utf8, rest::binary>>, []), do: parse_line(rest, [char])
+
+  defp parse_line(<<char::utf8, next::binary>>, [prev | popped] = stack) do
+    stack =
+      case char do
+        char when char in '([{<' -> [char | stack]
+        ?) when prev == ?( -> popped
+        ?] when prev == ?[ -> popped
+        ?} when prev == ?{ -> popped
+        ?> when prev == ?< -> popped
+        char -> {:corrupted, char}
+      end
+
+    parse_line(next, stack)
+  end
+
+  defp score(?)), do: 3
+  defp score(?]), do: 57
+  defp score(?}), do: 1197
+  defp score(?>), do: 25137
+
+  def part2(input) do
+    scores =
+      input
+      |> Enum.map(&parse_line(&1, []))
+      |> Enum.filter(&match?({:incomplete, _}, &1))
+      |> Enum.map(fn {_, chars} -> score2(chars) end)
+      |> Enum.sort()
+
+    Enum.at(scores, trunc(length(scores) / 2))
+  end
+
+  defp score2(chars) do
+    for char <- chars, reduce: 0 do
+      score -> score * 5 + points(char)
+    end
+  end
+
+  defp points(?(), do: 1
+  defp points(?[), do: 2
+  defp points(?{), do: 3
+  defp points(?<), do: 4
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      String.split(input, "\n", trim: true)
+    else
+      _ -> :error
+    end
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day10.exs input_filename")
+  end
+end
+
+Day10.run()