module UnitConversion

  class Conversion
  end

  class ScaleConversion < Conversion
    def initialize(parsed_factor)
      @factor = parsed_factor
    end

    def convert(value_unit)
      value = @factor.value * value_unit.value.value
      ValueUnit.for(value, value_unit.unit, value_unit.formatter)
    end
  end

  class ConvertConversion < Conversion
    def initialize(parsed_unit, density_unit_value = nil)
      @target_unit = parsed_unit

      raise UnknownUnitError, "#{density_unit_value} is not a density" if density_unit_value && !density_unit_value.density?
      @density = density_unit_value
    end

    def convert(value_unit)

      input = value_unit.unitwise

      if value_unit.volume? && @target_unit.mass?
        raise MissingDensityError, "Cannot convert #{value_unit.unit} to #{@target_unit} without density" unless @density
        input = input * @density.unitwise
      elsif value_unit.mass? && @target_unit.volume?
        raise MissingDensityError, "Cannot convert #{value_unit.unit} to #{@target_unit} without density" unless @density
        input = input / @density.unitwise
      end

      input = input.convert_to @target_unit.unit

      formatter = @target_unit.metric? ? DecimalFormatter.new : value_unit.formatter

      ValueUnit.for(input.value, @target_unit, formatter)

    end
  end

  class AutoUnitConversion < Conversion

    def convert(value_unit)

      unless known_auto_unit?(value_unit.unit)
        return value_unit
      end

      value = value_unit.raw_value
      unit = value_unit.unit.unit
      new_unit = unit

      unit_orders = UNIT_ORDERS.values.detect { |orders| orders.include?(unit.to_sym) }
      unit_range = UNIT_RANGES[unit.to_sym]

      if value < unit_range.first
        unit_orders = unit_orders.reverse
      end

      idx = unit_orders.index(new_unit.to_sym)

      while !unit_range.include?(value) && idx < (unit_orders.length - 1)
        idx += 1
        next_unit = unit_orders[idx]
        value = Unitwise(value, new_unit).convert_to(next_unit).value
        new_unit = next_unit.to_s
        unit_range = UNIT_RANGES[new_unit.to_sym]
      end

      ValueUnit.for(value, new_unit, value_unit.formatter)
    end

    def known_auto_unit?(unit)
      unit && UNIT_ORDERS.values.flatten.include?(unit.unit.to_sym)
    end
  end

end