Day 6

Setup

input =
  File.read!("input.txt")
  |> String.replace(" ", "")
  |> String.trim("\n")
  |> String.split(",")
  |> Enum.map(fn num ->
    {x, _} = Integer.parse(num)
    x
  end)
  |> Enum.frequencies()

# input = Map.put_new(input, "dead", 0)
IO.inspect(input)

Part 1

defmodule FishLife do
  def next_day(fishlist) do
    newfish = fishlist[0]

    # remove dead fish
    newfishlist =
      Enum.filter(fishlist, fn {k, _} ->
        k != 0
      end)
      |> Enum.into(%{})
      |> Enum.map(fn {k, val} ->
        case k > 0 do
          true -> {k - 1, val}
          # filtered out above
          _ -> "should not happen"
        end
      end)
      |> Enum.into(%{})

    # increment 6 with new fish and set number of new fish at 8
    case newfish != nil and newfish > 0 do
      true ->
        Map.put(newfishlist, 6, Map.get(newfishlist, 6, 0) + newfish)
        |> Map.put(8, newfish)

      _ ->
        newfishlist
    end
  end

  def fish_days(fishlist, max_days) when max_days > 0 do
    newfishlist = next_day(fishlist)
    fish_days(newfishlist, max_days - 1)
  end

  def fish_days(fishlist, 0) do
    fishlist
  end
end

tot_fish = FishLife.fish_days(input, 80)

Enum.map(tot_fish, fn {_, v} ->
  v
end)
|> Enum.sum()

Part 2

tot_fish = FishLife.fish_days(input, 256)

Enum.map(tot_fish, fn {_, v} ->
  v
end)
|> Enum.sum()