defmodule TaxCalculator do
  def calculate_tax(tax_rates, orders) 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