2024 Day 11 part 1 (and naive slow part 2, come back to it...)

2024/day11.exs

@@ -0,0 +1,76 @@
+#!/usr/bin/env elixir
+defmodule Day11 do
+  def part1(stones) do
+    1..25
+    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
+    |> Enum.count()
+  end
+
+  def blink([]), do: []
+  def blink([0 | rest]), do: [1 | blink(rest)]
+
+  def blink([stone | rest]) do
+    string = Integer.to_string(stone)
+    size = byte_size(string)
+
+    if rem(size, 2) == 0 do
+      string
+      |> String.split_at(div(size, 2))
+      |> Tuple.to_list()
+      |> Enum.map(&String.to_integer/1)
+      |> Kernel.++(blink(rest))
+    else
+      [stone |> Kernel.*(2024) | blink(rest)]
+    end
+  end
+
+  # probably have to track the changes rather than keep the whole list?
+  def part2(stones) do
+    1..75
+    |> Enum.reduce(stones, fn _, stones -> blink(stones) end)
+    |> Enum.count()
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.split([" ", "\n"], trim: true)
+      |> Enum.map(&String.to_integer/1)
+    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 day11.exs input_filename")
+  end
+end
+
+Day11.run()