Moved formula code/validation to separate module

lib/body_tracking/formula.rb

@@ -0,0 +1,64 @@
+module BodyTracking
+  module Formula
+    require 'ripper'
+    QUANTITY_TTYPES = [:on_ident, :on_tstring_content, :on_const]
+
+    class Formula
+      def initialize(project, formula)
+        @project = project
+        @formula = formula
+      end
+
+      def validate(options = {})
+        errors = []
+
+        # 1st: check if formula is valid Ruby code
+        tokenized_length = Ripper.tokenize(@formula).join.length
+        unless tokenized_length == @formula.length
+          errors << [: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 QUANTITY_TTYPES.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 << [: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 << [:disallowed_function_call, {function: function}] if function
+
+        # 4th: check if identifiers used in formula correspond to existing quantities
+        identifiers.uniq!
+        quantities = @project.quantities.where(name: identifiers).pluck(:name)
+        if quantities.length != identifiers.length
+          errors << [:unknown_quantity, {quantities: identifiers - quantities}]
+        end
+
+        errors
+      end
+    end
+
+    class FormulaValidator < ActiveModel::EachValidator
+      def initialize(options)
+        super(options)
+      end
+
+      def validate_each(record, attribute, value)
+        Formula.new(record.project, value).validate(options).each do |message, params|
+          record.errors.add(attribute, message, params)
+        end
+      end
+    end
+  end
+end