module UnitConversion

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

  UNIT_ALIASES = {
      cup: %w(cups c),
      tablespoon: %w(tbsp tbs tablespoons),
      teaspoon: %w(tsp teaspoons),
      ounce: %w(oz ounces),
      pound: %w(pounds lb lbs),
      pint: %w(pints),
      quart: %w(quarts),
      gallon: %w(gallons ga),

      gram: %w(grams g),
      kilogram: %w(kilograms kg)
  }

  UNIT_ALIAS_MAP = Hash[UNIT_ALIASES.map { |unit, values| values.map { |v| [v, unit] } }.flatten(1)]

  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

    def normalize_unit_names(unit_description)
      result = unit_description.dup

      result = result.downcase.gsub(/[a-z]+/) do |match|
        UNIT_ALIAS_MAP[match] || match
      end

      result
    end

  end
end