module UnitConversion

  UNIT_PARSING_REGEX = /^(?<value>(?:-?[0-9]+)?(?:\.?[0-9]*)?)\s*(?<unit>[a-z\/.\-()0-9]+)$/

  class UnparseableUnitError < StandardError
  end

  class UnknownUnitError < UnparseableUnitError
  end

  class << self

    def parse(unit_string)
      match = UNIT_PARSING_REGEX.match(unit_string.to_s.strip)

      if match && match[:value].present? && match[:unit].present?
        begin
          Unitwise(match[:value].to_f, match[:unit])
        rescue Unitwise::ExpressionError => err
          raise UnknownUnitError, err.message
        end
      else
        raise UnparseableUnitError, "'#{unit_string}' does not appear to be a valid measurement of the form <value> <units> (ie '5 cup' or '223 gram/cup')"
      end
    end

    def density?(unit)
      unit.compatible_with? Unitwise(1, 'g/ml')
    end

  end
end