Zero-prefix numbers

2020/day03/day3.ex

@@ -0,0 +1,92 @@
+defmodule TreeMap do
+  def start_link(map_text) do
+    Task.start_link(fn -> loop(parse(map_text)) end)
+  end
+
+  defp parse(text) do
+    String.split(text, "\n", trim: true)
+    |> Enum.with_index()
+    |> Enum.reduce(%{grid: %{}, width: 0, height: 0}, fn {line, r}, map ->
+      String.codepoints(line)
+      |> Enum.with_index()
+      |> Enum.reduce(map, fn {char, c}, map ->
+        %{
+          grid: Map.put(map.grid, {r, c}, char_to_content(char)),
+          width: max(map.width, c + 1),
+          height: max(map.height, r + 1)
+        }
+      end)
+    end)
+  end
+
+  defp loop(map) do
+    receive do
+      {:get, {r, c}, caller} ->
+        send(caller, Map.fetch(map.grid, wrap({r, c}, {map.width, map.height})))
+        loop(map)
+    end
+  end
+
+  defp wrap({r, c}, {w, _h}) do
+    {r, rem(c, w)}
+  end
+
+  defp char_to_content(char) do
+    case char do
+      "." -> :open
+      "#" -> :tree
+    end
+  end
+end
+
+defmodule Day3 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp content(tree_map, {r, c}) do
+    send(tree_map, {:get, {r, c}, self()})
+
+    receive do
+      {:ok, content} -> content
+      :error -> :error
+    end
+  end
+
+  defp trees(tree_map, slope) do
+    {dr, dc} = slope
+
+    Stream.iterate({0, 0}, fn {r, c} -> {r + dr, c + dc} end)
+    |> Enum.reduce_while(0, fn cell, tree_count ->
+      case content(tree_map, cell) do
+        :error -> {:halt, tree_count}
+        :open -> {:cont, tree_count}
+        :tree -> {:cont, tree_count + 1}
+      end
+    end)
+  end
+
+  def part1 do
+    {:ok, tree_map} = TreeMap.start_link(read_input())
+    trees(tree_map, {1, 3})
+  end
+
+  def part2 do
+    {:ok, tree_map} = TreeMap.start_link(read_input())
+
+    slopes = [
+      {1, 1},
+      {1, 3},
+      {1, 5},
+      {1, 7},
+      {2, 1}
+    ]
+
+    Enum.map(slopes, fn slope -> trees(tree_map, slope) end)
+    |> Enum.reduce(1, fn x, prod -> x * prod end)
+  end
+end
+
+Day3.part1() |> IO.inspect()
+Day3.part2() |> IO.inspect()