Day 14 Part 1

2020/day14/day14.ex

@@ -0,0 +1,102 @@
+defmodule Day14 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp test_input do
+    """
+    mask = XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
+    mem[8] = 11
+    mem[7] = 101
+    mem[8] = 0
+    """
+  end
+
+  @set_mask_pattern ~r/^mask = (?<mask>[01X]{36})$/
+  @assignment_pattern ~r/^mem\[(?<addr>\d+)\] = (?<value>\d+)$/
+
+  defp parse_program(text) do
+    String.split(text, "\n", trim: true)
+    |> Enum.map(fn line ->
+      cond do
+        Regex.match?(@set_mask_pattern, line) ->
+          %{"mask" => mask} = Regex.named_captures(@set_mask_pattern, line)
+          {:mask, parse_mask(mask)}
+
+        Regex.match?(@assignment_pattern, line) ->
+          %{"addr" => addr, "value" => value} = Regex.named_captures(@assignment_pattern, line)
+          {:mem, String.to_integer(addr), String.to_integer(value)}
+      end
+    end)
+  end
+
+  defp pow(_base, 0) do
+    1
+  end
+
+  defp pow(_base, exp) when exp < 0 do
+    raise ArithmeticError, "negative exponents not supported"
+  end
+
+  defp pow(base, exp) when exp > 0 do
+    List.duplicate(base, exp)
+    |> Enum.reduce(1, &(&1 * &2))
+  end
+
+  defp parse_mask(mask) do
+    String.codepoints(mask)
+    |> Enum.reverse()
+    |> Enum.with_index()
+    |> Enum.reduce([], fn {char, idx}, ops ->
+      case char do
+        "X" -> ops
+        "0" -> [{:and, Bitwise.bnot(pow(2, idx))} | ops]
+        "1" -> [{:or, pow(2, idx)} | ops]
+      end
+    end)
+  end
+
+  defp apply_mask([], num) do
+    num
+  end
+
+  defp apply_mask([{:and, val} | rest], num) do
+    apply_mask(rest, Bitwise.band(val, num))
+  end
+
+  defp apply_mask([{:or, val} | rest], num) do
+    apply_mask(rest, Bitwise.bor(val, num))
+  end
+
+  defp run([], {_, memory}) do
+    memory
+  end
+
+  defp run([{:mask, mask} | rest], {_, memory}) do
+    run(rest, {mask, memory})
+  end
+
+  defp run([{:mem, addr, value} | rest], {mask, memory}) do
+    run(rest, {mask, assign(memory, addr, value, mask)})
+  end
+
+  defp assign(mem, addr, val, mask) do
+    Map.put(mem, addr, apply_mask(mask, val))
+  end
+
+  def part1 do
+    read_input()
+    |> parse_program()
+    |> run({nil, %{}})
+    |> Enum.map(fn {_, val} -> val end)
+    |> Enum.sum()
+  end
+
+  def debug do
+    nil
+  end
+end
+
+Day14.part1() |> IO.inspect()
+Day14.debug() |> IO.inspect()

2020/template.ex

@@ -4,6 +4,11 @@ defmodule DayX do
     |> File.read!()
   end
 
+  defp test_input do
+    """
+    """
+  end
+
   def part1 do
     read_input()
   end