task1.ex

defmodule Task1 do
  @doc """
  A function that receives two sequences: A and B of integers and returns one sequence C.
  Sequence C should contain all elements from sequence A (maintaining the order) except those,
  that are present in sequence B p times, where p is a prime number.

  Example:
  A=[2,3,9,2,5,1,3,7,10]
  B=[2,1,3,4,3,10,6,6,1,7,10,10,10]
  C=[2,9,2,5,7,10]
  """

  @spec call(seq1 :: [integer()], seq2 :: [integer()], p :: integer()) :: [integer()]
  def call(seq1, seq2, prime) do
    prime
    |> prime?()
    |> maybe_filter(seq1, seq2, prime)
  end

  defp prime?(number)
       when number <= 2,
       do: true

  defp prime?(number), do: Enum.all?(range(number), &dividable?(number, &1))

  defp range(number), do: 2..(number |> :math.sqrt() |> round())

  defp dividable?(dividend, divisor), do: rem(dividend, divisor) != 0

  defp maybe_filter(false, _seq1, _seq2, prime), do: {:error, "#{prime} must be a prime number"}

  defp maybe_filter(true, seq1, seq2, prime) do
    seq2
    |> histogram()
    |> drop_occurences(prime)
    |> take_values()
    |> reject(seq1)
  end

  defp histogram(list) when is_list(list),
    do: Enum.reduce(list, %{}, &count/2)

  defp count(element, occurences),
    do: Map.update(occurences, element, 1, &(&1 + 1))

  defp drop_occurences(histogram, prime), do: Enum.filter(histogram, fn {_k, v} -> v == prime end)

  defp take_values(seq), do: Enum.map(seq, &elem(&1, 0))

  defp reject(to_remove, seq), do: Enum.reject(seq, &Enum.member?(to_remove, &1))
end

iex(1)> Task1.call([2,3,9,2,5,1,3,7,10], [2,1,3,4,3,10,6,6,1,7,10,10,10], 2)
[2, 9, 2, 5, 7, 10]