Day 6

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

input = Kino.Input.textarea("Problem input")

parsed_input =
  input
  |> Kino.Input.read()
  |> String.split(",")
  |> Stream.map(&String.to_integer/1)

Part 1 and 2

initial_ages =
  0..8
  |> Enum.map(fn x -> {x, 0} end)
  |> Enum.into(%{})
  |> Map.merge(parsed_input |> Enum.frequencies())

defmodule Solver do
  def decrement(ages) do
    {
      ages[0],
      ages
      |> Enum.map(fn
        {0, v} -> {6, v}
        {k, v} -> {k - 1, v}
      end)
      |> Enum.reduce(%{}, fn {k, v}, m ->
        Map.put(m, k, (m[k] || 0) + v)
      end)
    }
  end

  def solve(initial_ages, time_limit) do
    Enum.reduce(1..time_limit, initial_ages, fn _day, ages ->
      {new_fishes, ages} = decrement(ages)
      Map.put(ages, 8, ages[8] || 0 + new_fishes)
    end)
    |> Map.values()
    |> Enum.sum()
  end
end

Solver.solve(initial_ages, 256)