Day 12 Part 1

2020/day12/day12.ex

@@ -0,0 +1,83 @@
+defmodule Day12 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse_instructions(text) do
+    String.split(text, "\n", trim: true)
+    |> Enum.map(fn line ->
+      {
+        String.first(line) |> String.to_atom(),
+        String.slice(line, 1..-1) |> String.to_integer()
+      }
+    end)
+  end
+
+  defp steer([], _heading, position) do
+    position
+  end
+
+  defp steer([instruction | rest], heading, position = {x, y}) do
+    {new_heading, new_position} =
+      case instruction do
+        {:N, dy} -> {heading, {x, y + dy}}
+        {:S, dy} -> {heading, {x, y - dy}}
+        {:E, dx} -> {heading, {x + dx, y}}
+        {:W, dx} -> {heading, {x - dx, y}}
+        {:L, deg} -> {turn(heading, deg), position}
+        {:R, deg} -> {turn(heading, -deg), position}
+        {:F, dist} -> {heading, move(heading, position, dist)}
+      end
+
+    steer(rest, new_heading, new_position)
+  end
+
+  defp turn(heading, degrees) do
+    hd = %{E: 0, N: 90, W: 180, S: 270}
+    dh = Enum.into(hd, %{}, fn {h, d} -> {d, h} end)
+
+    new_deg = Map.fetch!(hd, heading) + degrees
+    Map.fetch!(dh, rem(new_deg + 360, 360))
+  end
+
+  defp move(heading, {x, y}, distance) do
+    case heading do
+      :N -> {x, y + distance}
+      :S -> {x, y - distance}
+      :E -> {x + distance, y}
+      :W -> {x - distance, y}
+    end
+  end
+
+  defp manhattan({x1, y1}, {x2, y2}) do
+    abs(x1 - x2) + abs(y1 - y2)
+  end
+
+  defp test_input do
+    """
+    F10
+    N3
+    F7
+    R90
+    F11
+    """
+  end
+
+  def part1 do
+    read_input()
+    |> parse_instructions()
+    |> steer(:E, {0, 0})
+    |> manhattan({0, 0})
+  end
+
+  def debug do
+    for h <- [:N, :S, :E, :W],
+        d <- [0, 90, 180, 270, 360, -90, -180, -270] do
+      turn(h, d)
+    end
+  end
+end
+
+Day12.part1() |> IO.inspect()
+# Day12.debug() |> IO.inspect()