euler_50.exs

# https://projecteuler.net/index.php?section=problems&id=50
defmodule Primes do
  def primes(ceiling), do: Enum.filter(1..ceiling, fn x -> is_prime(x) end)

  def is_prime(n), do: factors(n, div(n, 2)) == [1]

  def factors(1, _), do: [1]
  def factors(_, 1), do: [1]
  def factors(n, i) do
    if rem(n, i) == 0 do
      [i|factors(n, i-1)]
    else
      factors(n, i-1)
    end
  end


  def recursive_addition([], depth, total, ceiling), do: [ depth, total ]
  def recursive_addition([prime|remainder], depth, total, ceiling) do
    if (( total + prime) > ceiling ) do
      [ depth, total ]
    else
      recursive_addition(remainder, depth + 1, total + prime, ceiling)
    end
  end

  def primes_and_counts([], ceiling, high_score), do: high_score
  def primes_and_counts([prime|remainder], ceiling, high_score) do
    [count, total] = recursive_addition([prime|remainder], 0, 0, ceiling)
    new_high_score = if count > high_score.count && Primes.is_prime(total) do
      %{ prime: prime, total: total, count: count }
    else
      high_score
    end
    primes_and_counts(remainder, ceiling, new_high_score)
  end

end

ceiling = 1_000_000
primes = Primes.primes(ceiling)
answer = Primes.primes_and_counts(primes, ceiling, %{count: 0} )