Refactor XPath compiler support for predicates

Handling of predicates is delegated to 3 different methods:

* on_predicate_direct: for predicates such as foo[bar] and foo[x < y].
* on_predicate_temporary: for predicates that use the last() function
  somewhere.
* on_predicate_index: for predicates that only contain a literal index,
  foo[10] being an example.

This enables the compiler to use more optimized code depending on the
type of predicate while still being able to support last() and
position().

The code is currently still quite rough on the edges, this will be taken
care of in following commits.

lib/oga/xpath/compiler.rb

@@ -52,6 +52,9 @@ module Oga
       # Resets the internal state.
       def reset
         @literal_id = 0
+
+        @predicate_nodesets = []
+        @predicate_indexes  = []
       end
 
       ##
@@ -77,7 +80,6 @@ module Oga
 
           if return_nodeset?(ast)
             body = matched.assign(literal(XML::NodeSet).new)
-              .followed_by(input_assign)
               .followed_by(ruby_ast)
               .followed_by(matched)
           else
@@ -117,6 +119,9 @@ module Oga
         ast.children.reverse_each.with_index do |child, index|
           # The first block should operate on the variable set in "input", all
           # others should operate on the child variables ("node").
+          #
+          # FIXME: this currently basically only works by accident due to
+          # various bits of code also using "node_literal".
           input_var = index == last_index ? input : var_name
 
           # The last segment of the path should add the code that actually
@@ -415,52 +420,139 @@ module Oga
       def on_predicate(ast, input, &block)
         test, predicate = *ast
 
+        index_var = unique_literal(:index)
+
         if number?(predicate)
-          on_predicate_index(test, predicate, input, &block)
+          method = :on_predicate_index
+        elsif has_call_node?(predicate, 'last')
+          method = :on_predicate_temporary
         else
-          on_predicate_expression(test, predicate, input, &block)
+          method = :on_predicate_direct
         end
+
+        @predicate_indexes << index_var
+
+        ast = index_var.assign(literal(1)).followed_by do
+          send(method, input, test, predicate, &block)
+        end
+
+        @predicate_indexes.pop
+
+        ast
       end
 
       ##
-      # Processes an index predicate such as `foo[10]`.
+      # Processes a predicate that requires a temporary NodeSet.
       #
+      # @param [Oga::Ruby::Node] input
       # @param [AST::Node] test
       # @param [AST::Node] predicate
-      # @param [Oga::Ruby::Node] input
       # @return [Oga::Ruby::Node]
       #
-      def on_predicate_index(test, predicate, input)
-        int1      = literal(1)
-        index     = to_int(predicate)
-        index_var = literal(:index)
+      def on_predicate_temporary(input, test, predicate)
+        temp_set   = unique_literal(:temp_set)
+        pred_node  = unique_literal(:pred_node)
+        pred_var   = unique_literal(:pred_var)
+        conversion = literal(Conversion)
 
-        index_var.assign(int1)
+        index_var  = predicate_index
+        index_step = literal(1)
+
+        @predicate_nodesets << temp_set
+
+        ast = temp_set.assign(literal(XML::NodeSet).new)
           .followed_by do
-            process(test, input) do |matched_test_node|
-              index_var.eq(index)
-                .if_true { yield matched_test_node }
-                .followed_by(index_var.assign(index_var + int1))
+            process(test, input) { |node| temp_set << node }
+          end
+          .followed_by do
+            temp_set.each.add_block(pred_node) do
+              pred_ast = process(predicate, pred_node)
+
+              pred_var.assign(pred_ast)
+                .followed_by do
+                  pred_var.is_a?(Numeric).if_true do
+                    pred_var.assign(pred_var.to_i.eq(index_var))
+                  end
+                end
+                .followed_by do
+                  conversion.to_boolean(pred_var).if_true { yield pred_node }
+                end
+                .followed_by do
+                  index_var.assign(index_var + index_step)
                 end
             end
           end
 
+        @predicate_nodesets.pop
+
+        ast
+      end
+
       ##
-      # Processes a predicate using an expression (e.g. a path).
+      # Processes a predicate that doesn't require temporary NodeSet.
       #
+      # @param [Oga::Ruby::Node] input
       # @param [AST::Node] test
       # @param [AST::Node] predicate
-      # @param [Oga::Ruby::Node] input
       # @return [Oga::Ruby::Node]
       #
-      def on_predicate_expression(test, predicate, input)
+      def on_predicate_direct(input, test, predicate)
+        pred_var   = unique_literal(:pred_var)
+        index_var  = predicate_index
+        index_step = literal(1)
+        conversion = literal(Conversion)
+
         process(test, input) do |matched_test_node|
-          catch_message(:predicate_matched) do
+          if return_nodeset?(predicate)
+            pred_ast = catch_message(:predicate_matched) do
               process(predicate, matched_test_node) do
                 throw_message(:predicate_matched, self.true)
               end
             end
-          .if_true { yield matched_test_node }
+          else
+            pred_ast = process(predicate, matched_test_node)
+          end
+
+          pred_var.assign(pred_ast)
+            .followed_by do
+              pred_var.is_a?(Numeric).if_true do
+                pred_var.assign(pred_var.to_i.eq(index_var))
+              end
+            end
+            .followed_by do
+              conversion.to_boolean(pred_var).if_true do
+                yield matched_test_node
+              end
+            end
+            .followed_by do
+              index_var.assign(index_var + index_step)
+            end
+        end
+      end
+
+      ##
+      # Processes a predicate that uses a literal index.
+      #
+      # @param [Oga::Ruby::Node] input
+      # @param [AST::Node] test
+      # @param [AST::Node] predicate
+      # @return [Oga::Ruby::Node]
+      #
+      def on_predicate_index(input, test, predicate)
+        pred_var   = unique_literal(:pred_var)
+        index_var  = predicate_index
+        index_step = literal(1)
+
+        index = process(predicate, input).to_i
+
+        process(test, input) do |matched_test_node|
+          index_var.eq(index)
+            .if_true do
+              yield matched_test_node
+            end
+            .followed_by do
+              index_var.assign(index_var + index_step)
+            end
         end
       end
 
@@ -1169,6 +1261,28 @@ module Oga
           end
       end
 
+      # @return [Oga::Ruby::Node]
+      def on_call_last(*)
+        set = predicate_nodeset
+
+        unless set
+          raise 'last() can only be used in a predicate'
+        end
+
+        set.length.to_f
+      end
+
+      # @return [Oga::Ruby::Node]
+      def on_call_position(*)
+        index = predicate_index
+
+        unless index
+          raise 'position() can only be used in a predicate'
+        end
+
+        index.to_f
+      end
+
       ##
       # Delegates type tests to specific handlers.
       #
@@ -1448,6 +1562,34 @@ module Oga
       def return_nodeset?(ast)
         RETURN_NODESET.include?(ast.type)
       end
+
+      # @param [AST::Node] ast
+      # @return [TrueClass|FalseClass]
+      def has_call_node?(ast, name)
+        visit = [ast]
+
+        until visit.empty?
+          current = visit.pop
+
+          return true if current.type == :call && current.children[0] == name
+
+          current.children.each do |child|
+            visit << child if child.is_a?(AST::Node)
+          end
+        end
+
+        false
+      end
+
+      # @return [Oga::Ruby::Node]
+      def predicate_index
+        @predicate_indexes.last
+      end
+
+      # @return [Oga::Ruby::Node]
+      def predicate_nodeset
+        @predicate_nodesets.last
+      end
     end # Compiler
   end # XPath
 end # Oga