Zero-prefix numbers

2020/day08/day8.ex

@@ -0,0 +1,92 @@
+defmodule Day8 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse_program(text) do
+    String.split(String.trim(text), "\n")
+    |> Enum.map(fn line ->
+      [op, val] = String.split(line)
+      {op, String.to_integer(val)}
+    end)
+  end
+
+  defp tick(program, pc, acc) do
+    case Enum.at(program, pc) do
+      {"nop", _} -> {pc + 1, acc}
+      {"acc", int} -> {pc + 1, acc + int}
+      {"jmp", int} -> {pc + int, acc}
+    end
+  end
+
+  defp find_loop(program, pc, acc, history) do
+    {next_pc, next_acc} = tick(program, pc, acc)
+    count = Map.get(history, next_pc, 0)
+
+    if count > 0 do
+      acc
+    else
+      find_loop(program, next_pc, next_acc, Map.put(history, next_pc, count + 1))
+    end
+  end
+
+  def part1 do
+    program = parse_program(read_input())
+    find_loop(program, 0, 0, %{})
+  end
+
+  def run(program, pc, acc, history, term_pc) do
+    {next_pc, next_acc} = tick(program, pc, acc)
+    count = Map.get(history, next_pc, 0)
+
+    cond do
+      count > 0 -> :loop
+      next_pc == term_pc -> {:term, next_acc}
+      next_pc > term_pc -> :wtf
+      true -> run(program, next_pc, next_acc, Map.put(history, next_pc, count + 1), term_pc)
+    end
+  end
+
+  def mod(program, ridx) do
+    Enum.with_index(program)
+    |> Enum.map(fn {{op, val}, idx} ->
+      if idx == ridx do
+        case op do
+          "nop" -> {"jmp", val}
+          "jmp" -> {"nop", val}
+          _ -> {op, val}
+        end
+      else
+        {op, val}
+      end
+    end)
+  end
+
+  def mods(program) do
+    Stream.with_index(program)
+    |> Stream.filter(fn {{op, _val}, _idx} ->
+      case op do
+        "acc" -> false
+        "nop" -> true
+        "jmp" -> true
+      end
+    end)
+    |> Stream.map(fn {_, idx} -> mod(program, idx) end)
+  end
+
+  def part2 do
+    program = parse_program(read_input())
+    term_pc = Enum.count(program)
+
+    Enum.reduce_while(mods(program), nil, fn modded_program, _ ->
+      case run(modded_program, 0, 0, %{}, term_pc) do
+        :loop -> {:cont, nil}
+        {:term, acc} -> {:halt, acc}
+      end
+    end)
+  end
+end
+
+Day8.part1() |> IO.inspect()
+Day8.part2() |> IO.inspect()