defmodule TaxCalculator do
  
  def calculate_tax_from_file(order_filename) do
    calculate_tax_from_file(order_filename, [ NC: 0.075, TX: 0.08 ])
  end
  
  def calculate_tax_from_file(order_filename, tax_rates) do
    read(order_filename)
    |> Stream.map(&TaxCalculator.parse/1)
    |> calculate_tax(tax_rates)
    |> Enum.to_list
  end
  
  def read(filename) do
    {:ok, file} = File.open(filename)
    data = IO.read(file, :line)
    IO.stream(file, :line)
  end
  
  def parse(line) do
    regex = ~r/(?<id>\d*?),:(?<ship_to>\w{2}),(?<net_amount>.*?)$/
    named_captures = Regex.named_captures(regex, line)
    
    [
      id: String.to_integer(named_captures["id"]),
      ship_to: String.to_atom(named_captures["ship_to"]),
      net_amount: String.to_float(named_captures["net_amount"])
    ]
  end
    
	def calculate_tax(orders, tax_rates) do
    for order <- orders,
        do:
          Keyword.merge(
            order,
            TaxCalculator.calculate_total_amount(tax_rates[order[:ship_to]], order[:net_amount])
          )
  end

  def calculate_total_amount(nil, net_amount), do: [total_amount: Float.round(net_amount, 2)]

  def calculate_total_amount(tax_rate, net_amount) do
    total_amount = (1 + tax_rate) * net_amount
    [total_amount: Float.round(total_amount, 2)]
  end
end