Euler 191

Mix.install([
  # Join the string so a copy of dayN to dayM doesn't destroy it.
  {:kino, "~> 0.1" <> "4.2"}
])

# Join the string so a copy of dayN to dayM doesn't destroy it.
IEx.Helpers.c("/Users/johnb/dev/2" <> "0" <> "2" <> "4adventOfCode/advent_of_code.ex")
alias AdventOfCode, as: AOC
alias Kino.Input

Solution

# A particular school offers cash rewards to children with good attendance and punctuality. 
# If they are absent for three consecutive days or late on more than one occasion then they
# forfeit their prize.
#
# During an n-day period a trinary string is formed for each child consisting of L's (late), 
#  O's (on time), and A's (absent).
# 
# Although there are eighty-one trinary strings for a 4-day period that can be formed, 
# exactly forty-three strings would lead to a prize:
#
# How many "prize" strings exist over a 30-day period?

defmodule Euler191 do
  def iterate(_days = 0, state), do: prizes(state)
  def iterate(days, %{absences: absences, lates: lates, string: string} = state) do
    o_result = prizes(days, "O", state)
    a_result = prizes(days, "A", %{o_result | absences: absences, lates: lates, string: string})
    l_result = prizes(days, "L", %{a_result | absences: absences, lates: lates, string: string})

    l_result
  end
  
  def prizes(n_days, "O", %{string: string} = state), do: 
    iterate(n_days - 1, %{state | absences: 0, string: string <> "O"})
  def prizes(n_days, "L", %{lates: 0, string: string} = state), do: 
    iterate(n_days - 1, %{state | absences: 0, lates: 1, string: string <> "L"})
  def prizes(_n_days, "L", state), 
    do: state
  def prizes(_n_days, "A", %{absences: 2} = state), 
    do: state
  def prizes(n_days, "A", %{absences: absences, string: string} = state), 
    do: iterate(n_days - 1, %{state | absences: absences + 1, string: string <> "A"})
  def prizes(%{prize_strings: prize_strings} = state), 
    do: %{state | prize_strings: prize_strings + 1} # adding [string] only works for small numbers
  def prizes(n_days) do
    empty_state = %{absences: 0, lates: 0, prize_strings: 0, string: ""}
    result = iterate(n_days, empty_state)

    # IO.inspect(result.prize_strings)
    IO.puts result.prize_strings
  end
end

# Example:

result = Euler191.prizes(30)
|> IO.inspect()

# 43 results expected for 4:
# OOOO OOOA OOOL OOAO OOAA OOAL OOLO OOLA OAOO OAOA
# OAOL OAAO OAAL OALO OALA OLOO OLOA OLAO OLAA AOOO
# AOOA AOOL AOAO AOAA AOAL AOLO AOLA AAOO AAOA AAOL
# AALO AALA ALOO ALOA ALAO ALAA LOOO LOOA LOAO LOAA
# LAOO LAOA LAAO