From 0bc03b715713ae2c86831a317121214561f062cf Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Tue, 17 Dec 2024 16:59:45 +0900
Subject: [PATCH] 2024 Day 17 Part 1 + not working Part 2

---
 2024/day17.exs | 145 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 145 insertions(+)
 create mode 100755 2024/day17.exs

diff --git a/2024/day17.exs b/2024/day17.exs
new file mode 100755
index 0000000..2b44939
--- /dev/null
+++ b/2024/day17.exs
@@ -0,0 +1,145 @@
+#!/usr/bin/env elixir
+defmodule Day17 do
+  defmodule Computer do
+    defstruct a: 0, b: 0, c: 0, output: [], program: []
+  end
+
+  def part1(computer) do
+    process(computer.program, computer)
+    |> Map.fetch!(:output)
+    |> Enum.reverse()
+    |> Enum.join(",")
+  end
+
+  def process([], computer), do: computer
+
+  # adv div(A, 2^ combo operand) -> A
+  def process([0, operand | program], computer) do
+    value = div(computer.a, 2 ** combo(operand, computer))
+    computer = %Computer{computer | a: value}
+    process(program, computer)
+  end
+
+  # bxl B XOR literal operand -> B
+  def process([1, operand | program], computer) do
+    value = Bitwise.bxor(computer.b, operand)
+    computer = %Computer{computer | b: value}
+    process(program, computer)
+  end
+
+  # bst combo operand mod 8 -> B (only 3 lowest bits)
+  def process([2, operand | program], computer) do
+    value = rem(combo(operand, computer), 8)
+    computer = %Computer{computer | b: value}
+    process(program, computer)
+  end
+
+  # jnz noting if A is 0, otherwise set instruction to literal operand - don't inc by 2 if jump
+  def process([3, operand | program], computer) do
+    if computer.a == 0 do
+      process(program, computer)
+    else
+      program = Enum.drop(computer.program, operand)
+      process(program, computer)
+    end
+  end
+
+  # bxc B XOR C -> B (ignore operand)
+  def process([4, _operand | program], computer) do
+    value = Bitwise.bxor(computer.b, computer.c)
+    computer = %Computer{computer | b: value}
+    process(program, computer)
+  end
+
+  # out combo operand mod 8 + output (CSV)
+  def process([5, operand | program], computer) do
+    value = rem(combo(operand, computer), 8)
+    computer = %Computer{computer | output: [value | computer.output]}
+    process(program, computer)
+  end
+
+  # bdv same as adv but store in B
+  def process([6, operand | program], computer) do
+    value = div(computer.a, 2 ** combo(operand, computer))
+    computer = %Computer{computer | b: value}
+    process(program, computer)
+  end
+
+  # cdv same as adv but store in C
+  def process([7, operand | program], computer) do
+    value = div(computer.a, 2 ** combo(operand, computer))
+    computer = %Computer{computer | c: value}
+    process(program, computer)
+  end
+
+  def combo(literal, _computer) when literal in [0, 1, 2, 3], do: literal
+  def combo(4, computer), do: computer.a
+  def combo(5, computer), do: computer.b
+  def combo(6, computer), do: computer.c
+
+  def part2(computer) do
+    # TODO runs forever. Probably need to detect loops?
+    search_for_self(computer)
+  end
+
+  def search_for_self(computer, a \\ 0) do
+    computer = %Computer{computer | a: a}
+    output = process(computer.program, computer).output |> Enum.reverse()
+
+    if output == computer.program do
+      a
+    else
+      search_for_self(computer, a + 1)
+    end
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      "Register A: " <> input = input
+      {a, input} = Integer.parse(input)
+      "\nRegister B: " <> input = input
+      {b, input} = Integer.parse(input)
+      "\nRegister C: " <> input = input
+      {c, input} = Integer.parse(input)
+      "\n\nProgram: " <> input = input
+      program = input |> String.split([",", "\n"], trim: true) |> Enum.map(&String.to_integer/1)
+
+      %Computer{a: a, b: b, c: c, program: program}
+    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("#{inspect(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 day17.exs input_filename")
+  end
+end
+
+Day17.run()