day15.ex

defmodule Day15 do
  def play(%{ :history => history, :last_value => value, :last_index => index }) do

    next_history = seen(history, value, index)
    next_value = get_value(next_history,  value)

    %{
      :history       => next_history,
      :last_index    => index + 1,
      :last_value    => next_value,
    }
  end

  def seen(history, value, index) do
    Map.update(history, value, [index, nil], fn
      [idx1, nil]  -> [index, idx1]
      [idx1, idx2] -> [index, idx1]
    end)
  end

  def get_value(history, value) do
    case Map.get(history, value) do
      [idx1, nil] -> 0
      [idx1, idx2] -> idx1 - idx2
    end
  end

  def make_history(values) do
    values
    |> Enum.with_index
    |> Enum.map(fn { val, idx } -> %{ val => [idx + 1, nil] } end)
    |> Enum.reduce(&Map.merge/2)
  end

  def read_input(values) do
    last_value = values |> Enum.take(-1) |> Enum.at(0)
    last_index = values |> Enum.count
    %{
      :history => make_history(values),
      :last_value => last_value,
      :last_index => last_index,
    }
  end

  def part1 do
    read_input([5,1,9,18,13,8,0])
    |> Stream.iterate(&play/1)
    |> Enum.find(fn %{:last_index => last_index} -> last_index == 30000000 end)
    |> IO.inspect
  end
  
end