Quantity formula assignment and validation

app/models/quantity.rb

@@ -1,4 +1,6 @@
 class Quantity < ActiveRecord::Base
+  require 'ripper'
+
   enum domain: {
     diet: 0,
     measurement: 1,
@@ -13,6 +15,41 @@ class Quantity < ActiveRecord::Base
   validate if: -> { parent.present? } do
     errors.add(:parent, :parent_domain_mismatch) unless domain == parent.domain
   end
+  validate if: -> { formula.present? } do
+    # 1st: check if formula is valid Ruby code
+    tokenized_length = Ripper.tokenize(formula).join.length
+    unless tokenized_length == formula.length
+      errors.add(:formula, :invalid_formula, {part: formula[0...tokenized_length]})
+    end
+
+    # 2nd: check if formula contains only allowed token types
+    identifiers = []
+    Ripper.lex(formula).each do |location, ttype, token|
+      case
+      when [:on_ident, :on_tstring_content, :on_const].include?(ttype)
+        identifiers << token
+      when [:on_sp, :on_int, :on_rational, :on_float, :on_tstring_beg, :on_tstring_end,
+        :on_lparen, :on_rparen].include?(ttype)
+      when :on_op == ttype && '+-*/'.include?(token)
+      else
+        errors.add(:formula, :disallowed_token,
+                   {token: token, ttype: ttype, location: location})
+      end
+    end
+
+    # 3rd: check for disallowed function calls (they are not detected by Ripper.lex)
+    # FIXME: this is unreliable (?) detection of function calls, should be replaced
+    # with parsing Ripper.sexp if necessary
+    function = Ripper.slice(formula, 'ident [sp]* lparen')
+    errors.add(:formula, :disallowed_function_call, {function: function}) if function
+
+    # 4th: check if identifiers used in formula correspond to existing quantities
+    identifiers.uniq!
+    quantities = self.project.quantities.where(name: identifiers).pluck(:name)
+    if quantities.length != identifiers.length
+      errors.add(:formula, :unknown_quantity, {quantities: identifiers - quantities})
+    end
+  end
 
   after_initialize do
     if new_record?