2021 day 6 part 2

2021/day6.exs

@@ -0,0 +1,84 @@
+defmodule Day6 do
+  def part1(input) do
+    input
+    |> run_days(80)
+    |> length()
+  end
+
+  defp run_days(school, 0), do: school
+  defp run_days(school, days), do: run_days(day(school), days - 1)
+
+  defp day(school), do: Enum.flat_map(school, &fish/1)
+
+  defp fish(0), do: [6, 8]
+  defp fish(age), do: [age - 1]
+
+  # Implementing part 2 completely differently to avoid exponential growth
+
+  def part2(input) do
+    counts = Map.new(0..8, fn age -> {age, 0} end)
+
+    input
+    |> Enum.frequencies()
+    |> Enum.into(counts)
+    |> run_days2(256)
+    |> Map.values()
+    |> Enum.sum()
+  end
+
+  defp run_days2(counts, 0), do: counts
+  defp run_days2(counts, days), do: run_days2(day2(counts), days - 1)
+
+  defp day2(counts) do
+    {breeders, counts} = Map.pop(counts, 0)
+
+    counts
+    |> Map.new(fn {age, count} -> {age - 1, count} end)
+    |> Map.update(6, 0, &(&1 + breeders))
+    |> Map.put(8, breeders)
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      input
+      |> String.trim()
+      |> String.split(",")
+      |> 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("#{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 day6.exs input_filename")
+  end
+end
+
+Day6.run()