Unit conversion

app/models/unit_conversion.rb

@@ -48,6 +48,20 @@ module UnitConversion
       unit.compatible_with? Unitwise(1, 'g/ml')
     end
 
+    def volume?(unit)
+      unit.compatible_with? Unitwise(1, 'ml')
+    end
+
+    def mass?(unit)
+      unit.compatible_with? Unitwise(1, 'g')
+    end
+
+    def get_density(str)
+      unit = parse(str)
+      raise UnknownUnitError, "#{str} expected to be a density" unless density?(unit)
+      unit
+    end
+
     def normalize_unit_names(unit_description)
       unit_description.downcase.gsub(/[a-z]+/) do |match|
         UNIT_ALIAS_MAP[match] || match
@@ -71,7 +85,7 @@ module UnitConversion
       end
     end
 
-    def convert(quantity, factor, input_unit, output_unit)
+    def convert(quantity, factor, input_unit, output_unit, density = nil)
 
       value = get_value(quantity)
       factor = get_value(factor)
@@ -80,23 +94,36 @@ module UnitConversion
         factor = factor.to_d(10)
       end
 
-      input_unit = normalize_unit_names(input_unit)
-      output_unit = normalize_unit_names(output_unit)
+      converted = value * factor
 
-      original = Unitwise(value, input_unit)
-      scaled = original * factor
+      input_unit = normalize_unit_names(input_unit) unless input_unit.nil?
+      output_unit = normalize_unit_names(output_unit) unless output_unit.nil?
 
-      converted = scaled.convert_to output_unit
+      if input_unit && output_unit && input_unit != output_unit
+        in_unit = Unitwise(1, input_unit)
+        out_unit = Unitwise(1, output_unit)
+        unit = Unitwise(converted, input_unit)
+
+        if volume?(in_unit) && mass?(out_unit)
+          unit_density = get_density(density)
+          unit = unit * unit_density
+        elsif mass?(in_unit) && volume?(out_unit)
+          unit_density = get_density(density)
+          unit = unit / unit_density
+        end
+
+        converted = unit.convert_to(output_unit).value
+      end
 
       if value.is_a? Rational
-        rational_val = converted.value.to_r.rationalize(0.001)
+        rational_val = converted.to_r.rationalize(0.001)
         if rational_val.denominator == 1
           rational_val.to_i.to_s
         else
           rational_val.to_s
         end
       else
-        "%g" % ("%.3f" % converted.value)
+        "%g" % ("%.3f" % converted)
       end
     end
 

spec/models/unit_conversion_spec.rb

@@ -40,11 +40,24 @@ RSpec.describe UnitConversion do
       expect(UnitConversion.convert('2.0', '1', 'tablespoon', 'cup')).to eq '0.125'
     end
 
+    it 'scales odd units without conversion' do
+      expect(UnitConversion.convert('1/2', '2', 'slices', 'slices')).to eq '1'
+      expect(UnitConversion.convert('4', '1/8', nil, nil)).to eq '1/2'
+    end
+
     it 'converts and scales' do
       expect(UnitConversion.convert('1/2', '2', 'cup', 'tbsp')).to eq '16'
       expect(UnitConversion.convert('2.0', '1 1/2', 'tablespoon', 'cup')).to eq '0.188'
       expect(UnitConversion.convert('2', '1 1/2', 'tablespoon', 'cup')).to eq '3/16'
     end
+
+    it 'converts from volume to mass' do
+      expect(UnitConversion.convert('5', '1', 'cup', 'ounce', '5 oz/c')).to eq '25'
+    end
+
+    it 'converts from mass to volume' do
+      expect(UnitConversion.convert('25', '1', 'ounce', 'cup', '5 oz/c')).to eq '5'
+    end
   end
 
   describe '.parse' do