defmodule ExJira.QueryParams do

  @moduledoc """
  Helper module to convert parameters passed as a keyword list into a querystring.
  """

  @doc """
  Converts a keyword list and a list of atoms into a querystring containing only
  entries that exist in both the keyword list and the list of atoms.

  ## Examples

      iex> ExJira.QueryParams.convert([asdf: 123, hjkl: 456], [:asdf, :hjkl])
      "asdf=123&hjkl=456"

      iex> ExJira.QueryParams.convert([asdf: 123, hjkl: 456], [:asdf])
      "asdf=123"

      iex> ExJira.QueryParams.convert([asdf: 123, hjkl: 456], [:hjkl])
      "hjkl=456"

      iex> ExJira.QueryParams.convert([asdf: 123, hjkl: 456], [:qwerty])
      ""

      iex> ExJira.QueryParams.convert([asdf: 123, hjkl: 456], [])
      ""

  """
  @spec convert([{atom, String.t}], [atom]) :: String.t
  def convert(_params, []), do: ""
  def convert(params, [h | t]) do
    amp = case t do
      [] -> ""
      _  -> "&"
    end
    cond do
      Keyword.has_key?(params, h) -> "#{to_string(h)}=#{params[h]}#{amp}#{convert(params, t)}"
      true -> convert(params, t)
    end
  end

end