1 files changed, 88 insertions, 0 deletions
diff --git a/tool/lrama/lib/lrama/grammar/code/rule_action.rb b/tool/lrama/lib/lrama/grammar/code/rule_action.rb
new file mode 100644
index 0000000000..d3c0eab64a
--- /dev/null
+++ b/tool/lrama/lib/lrama/grammar/code/rule_action.rb
@@ -0,0 +1,88 @@
+module Lrama
+  class Grammar
+    class Code
+      class RuleAction < Code
+        def initialize(type:, token_code:, rule:)
+          super(type: type, token_code: token_code)
+          @rule = rule
+        end
+
+        private
+
+        # * ($$) yyval
+        # * (@$) yyloc
+        # * ($:$) error
+        # * ($1) yyvsp[i]
+        # * (@1) yylsp[i]
+        # * ($:1) i - 1
+        #
+        #
+        # Consider a rule like
+        #
+        #   class: keyword_class { $1 } tSTRING { $2 + $3 } keyword_end { $class = $1 + $keyword_end }
+        #
+        # For the semantic action of original rule:
+        #
+        # "Rule"                class: keyword_class { $1 } tSTRING { $2 + $3 } keyword_end { $class = $1 + $keyword_end }
+        # "Position in grammar"                   $1     $2      $3          $4          $5
+        # "Index for yyvsp"                       -4     -3      -2          -1           0
+        # "$:n"                                  $:1    $:2     $:3         $:4         $:5
+        # "index of $:n"                          -5     -4      -3          -2          -1
+        #
+        #
+        # For the first midrule action:
+        #
+        # "Rule"                class: keyword_class { $1 } tSTRING { $2 + $3 } keyword_end { $class = $1 + $keyword_end }
+        # "Position in grammar"                   $1
+        # "Index for yyvsp"                        0
+        # "$:n"                                  $:1
+        def reference_to_c(ref)
+          case
+          when ref.type == :dollar && ref.name == "$" # $$
+            tag = ref.ex_tag || lhs.tag
+            raise_tag_not_found_error(ref) unless tag
+            "(yyval.#{tag.member})"
+          when ref.type == :at && ref.name == "$" # @$
+            "(yyloc)"
+          when ref.type == :index && ref.name == "$" # $:$
+            raise "$:$ is not supported"
+          when ref.type == :dollar # $n
+            i = -position_in_rhs + ref.index
+            tag = ref.ex_tag || rhs[ref.index - 1].tag
+            raise_tag_not_found_error(ref) unless tag
+            "(yyvsp[#{i}].#{tag.member})"
+          when ref.type == :at # @n
+            i = -position_in_rhs + ref.index
+            "(yylsp[#{i}])"
+          when ref.type == :index # $:n
+            i = -position_in_rhs + ref.index
+            "(#{i} - 1)"
+          else
+            raise "Unexpected. #{self}, #{ref}"
+          end
+        end
+
+        def position_in_rhs
+          # If rule is not derived rule, User Code is only action at
+          # the end of rule RHS. In such case, the action is located on
+          # `@rule.rhs.count`.
+          @rule.position_in_original_rule_rhs || @rule.rhs.count
+        end
+
+        # If this is midrule action, RHS is a RHS of the original rule.
+        def rhs
+          (@rule.original_rule || @rule).rhs
+        end
+
+        # Unlike `rhs`, LHS is always a LHS of the rule.
+        def lhs
+          @rule.lhs
+        end
+
+        def raise_tag_not_found_error(ref)
+          raise "Tag is not specified for '$#{ref.value}' in '#{@rule}'"
+        end
+      end
+    end
+  end
+end