frankfurter/lib/roundable.rb

# frozen_string_literal: true

module Roundable
  # To paraphrase Wikipedia, most currency pairs are quoted to four decimal
  # places. An exception to this is exchange rates with a value of less than
  # 1.000, which are quoted to five or six decimal places. Exchange rates
  # greater than around 20 are usually quoted to three decimal places and
  # exchange rates greater than 80 are quoted to two decimal places.
  # Currencies over 5000 are usually quoted with no decimal places.
  #
  # https://en.wikipedia.org/wiki/Exchange_rate#Quotations
  def round(value)
    if value > 5000
      value.round
    elsif value > 80
      Float(format('%<value>.2f', value:))
    elsif value > 20
      Float(format('%<value>.3f', value:))
    elsif value > 1
      Float(format('%<value>.4f', value:))
    # I had originally opted to round smaller numbers simply to five decimal
    # places but introduced this refinement to handle an edge case where a
    # lower-rate base currency like IDR produces less precise quotes.
    elsif value > 0.0001
      Float(format('%<value>.5f', value:))
    else
      Float(format('%<value>.6f', value:))
    end
  end
end