defmodule Naiveical.Extractor do
  @moduledoc """
  This module allows the extraction of parts of a icalendar text.
  """

  alias Naiveical.Helpers

  @doc """
  Extract parts of an icalender text, such as all VALARMs.
    ## Examples:

  iex> Naiveical.Extractor.extract_sections_by_tag("BEGIN:XX\\nBEGIN:YY\\nA:aa\\nB:bb\\nEND:YY\\nBEGIN:YY\\nC:cc\\nD:dd\\nEND:YY\\nEND:XX", "XX")
  ["BEGIN:XX\\nBEGIN:YY\\nA:aa\\nB:bb\\nEND:YY\\nBEGIN:YY\\nC:cc\\nD:dd\\nEND:YY\\nEND:XX"]

  iex> Naiveical.Extractor.extract_sections_by_tag("BEGIN:XX\\nBEGIN:YY\\nA:aa\\nB:bb\\nEND:YY\\nBEGIN:YY\\nC:cc\\nD:dd\\nEND:YY\\nEND:XX", "YY")
  ["BEGIN:YY\\nA:aa\\nB:bb\\nEND:YY", "BEGIN:YY\\nC:cc\\nD:dd\\nEND:YY"]

  """
  def extract_sections_by_tag(ical_text, tag) do
    {:ok, regex} = Regex.compile("BEGIN:#{tag}")
    startings = Regex.scan(regex, ical_text, return: :index)
    {:ok, regex} = Regex.compile("END:#{tag}")
    endings = Regex.scan(regex, ical_text, return: :index)

    if length(startings) != length(endings),
      do: raise("No correct ical file, no matchin BEGIN/END for #{tag}")

    Enum.map(0..(length(startings) - 1), fn idx ->
      [{s, _len}] = Enum.at(startings, idx)
      [{e, len}] = Enum.at(endings, idx)
      String.slice(ical_text, s, e - s + len)
    end)
  end

  @doc """
  Extract a single content line from an icalendar text. It returns a tuple with `{tag-name, properties, value}`.

    ## Examples:

  iex> Naiveical.Extractor.extract_contentline_by_tag("BEGIN:XX\\nBEGIN:YY\\nA:aa\\nB:bb\\nEND:YY\\nBEGIN:YY\\nC:cc\\nD:dd\\nEND:YY\\nEND:XX", "A")
  {"A","","aa"}

  """
  def extract_contentline_by_tag(ical_text, tag) do
    {:ok, regex} = Regex.compile("^#{tag}[;]?(.*):(.*)$", [:multiline])
    ical_text = Helpers.unfold(ical_text)

    [_, properties, values] = Regex.run(regex, ical_text)
    {tag, String.trim(properties), String.trim(values)}
  end
end