Quantity computation from formula

app/models/ingredient.rb

@@ -71,7 +71,9 @@ class Ingredient < ActiveRecord::Base
             i,
             input_q.map do |i_q|
               nutrient_data = (completed_q[i_q.name] || nutrients[i_q.name])[i.id]
-              [i_q.name, (nutrient_data || [nil, nil])[0]]
+              # FIXME: result for computation with nil values (substituted with 0s)
+              # should be marked as not precise
+              [i_q.name, (nutrient_data || [0, nil])[0]]
             end.to_h
           ]
         end

app/models/quantity.rb

@@ -70,6 +70,15 @@ class Quantity < ActiveRecord::Base
   end
 
   def calculate(inputs)
-    inputs.map { |i, values| [i, 1.0] }
+    paramed_formula = Ripper.lex(formula).map do |*, ttype, token|
+      QUANTITY_TTYPES.include?(ttype) ? "params['#{token}']" : token
+    end.join
+    inputs.map { |i, values| [i, get_binding(values).eval(paramed_formula)] }
+  end
+
+  private
+
+  def get_binding(params)
+    binding
   end
 end