expr

# File lib/rexml/xpath_parser.rb, line 164
    def expr( path_stack, nodeset, context=nil )
      # enter(:expr, path_stack, nodeset)
      return nodeset if path_stack.length == 0 || nodeset.length == 0
      while path_stack.length > 0
        # trace(:while, path_stack, nodeset)
        if nodeset.length == 0
          path_stack.clear
          return []
        end
        op = path_stack.shift
        case op
        when :document
          first_raw_node = nodeset.first.raw_node
          nodeset = [XPathNode.new(first_raw_node.root_node, position: 1)]
        when :self
          nodeset = step(path_stack) do
            [nodeset]
          end
        when :child
          nodeset = step(path_stack) do
            child(nodeset)
          end
        when :literal
          # trace(:literal, path_stack, nodeset)
          return path_stack.shift
        when :attribute
          nodeset = step(path_stack, any_type: :attribute) do
            nodesets = []
            nodeset.each do |node|
              raw_node = node.raw_node
              next unless raw_node.node_type == :element
              attributes = raw_node.attributes
              next if attributes.empty?
              nodesets << attributes.each_attribute.collect.with_index do |attribute, i|
                XPathNode.new(attribute, position: i + 1)
              end
            end
            nodesets
          end
        when :namespace
          pre_defined_namespaces = {
            "xml" => "http://www.w3.org/XML/1998/namespace",
          }
          nodeset = step(path_stack, any_type: :namespace) do
            nodesets = []
            nodeset.each do |node|
              raw_node = node.raw_node
              case raw_node.node_type
              when :element
                if @namespaces
                  nodesets << pre_defined_namespaces.merge(@namespaces)
                else
                  nodesets << pre_defined_namespaces.merge(raw_node.namespaces)
                end
              when :attribute
                if @namespaces
                  nodesets << pre_defined_namespaces.merge(@namespaces)
                else
                  nodesets << pre_defined_namespaces.merge(raw_node.element.namespaces)
                end
              end
            end
            nodesets
          end
        when :parent
          nodeset = step(path_stack) do
            nodesets = []
            nodeset.each do |node|
              raw_node = node.raw_node
              if raw_node.node_type == :attribute
                parent = raw_node.element
              else
                parent = raw_node.parent
              end
              nodesets << [XPathNode.new(parent, position: 1)] if parent
            end
            nodesets
          end
        when :ancestor
          nodeset = step(path_stack) do
            nodesets = []
            # new_nodes = {}
            nodeset.each do |node|
              raw_node = node.raw_node
              new_nodeset = []
              while raw_node.parent
                raw_node = raw_node.parent
                # next if new_nodes.key?(node)
                new_nodeset << XPathNode.new(raw_node,
                                             position: new_nodeset.size + 1)
                # new_nodes[node] = true
              end
              nodesets << new_nodeset unless new_nodeset.empty?
            end
            nodesets
          end
        when :ancestor_or_self
          nodeset = step(path_stack) do
            nodesets = []
            # new_nodes = {}
            nodeset.each do |node|
              raw_node = node.raw_node
              next unless raw_node.node_type == :element
              new_nodeset = [XPathNode.new(raw_node, position: 1)]
              # new_nodes[node] = true
              while raw_node.parent
                raw_node = raw_node.parent
                # next if new_nodes.key?(node)
                new_nodeset << XPathNode.new(raw_node,
                                             position: new_nodeset.size + 1)
                # new_nodes[node] = true
              end
              nodesets << new_nodeset unless new_nodeset.empty?
            end
            nodesets
          end
        when :descendant_or_self
          nodeset = step(path_stack) do
            descendant(nodeset, true)
          end
        when :descendant
          nodeset = step(path_stack) do
            descendant(nodeset, false)
          end
        when :following_sibling
          nodeset = step(path_stack) do
            nodesets = []
            nodeset.each do |node|
              raw_node = node.raw_node
              next unless raw_node.respond_to?(:parent)
              next if raw_node.parent.nil?
              all_siblings = raw_node.parent.children
              current_index = all_siblings.index(raw_node)
              following_siblings = all_siblings[(current_index + 1)..-1]
              next if following_siblings.empty?
              nodesets << following_siblings.collect.with_index do |sibling, i|
                XPathNode.new(sibling, position: i + 1)
              end
            end
            nodesets
          end
        when :preceding_sibling
          nodeset = step(path_stack, order: :reverse) do
            nodesets = []
            nodeset.each do |node|
              raw_node = node.raw_node
              next unless raw_node.respond_to?(:parent)
              next if raw_node.parent.nil?
              all_siblings = raw_node.parent.children
              current_index = all_siblings.index(raw_node)
              preceding_siblings = all_siblings[0, current_index].reverse
              next if preceding_siblings.empty?
              nodesets << preceding_siblings.collect.with_index do |sibling, i|
                XPathNode.new(sibling, position: i + 1)
              end
            end
            nodesets
          end
        when :preceding
          nodeset = step(path_stack, order: :reverse) do
            unnode(nodeset) do |node|
              preceding(node)
            end
          end
        when :following
          nodeset = step(path_stack) do
            unnode(nodeset) do |node|
              following(node)
            end
          end
        when :variable
          var_name = path_stack.shift
          return [@variables[var_name]]

        # :and, :or, :eq, :neq, :lt, :lteq, :gt, :gteq
        # TODO: Special case for :or and :and -- not evaluate the right
        # operand if the left alone determines result (i.e. is true for
        # :or and false for :and).
        when :eq, :neq, :lt, :lteq, :gt, :gteq, :or
          left = expr( path_stack.shift, nodeset.dup, context )
          right = expr( path_stack.shift, nodeset.dup, context )
          res = equality_relational_compare( left, op, right )
          # trace(op, left, right, res)
          return res

        when :and
          left = expr( path_stack.shift, nodeset.dup, context )
          return [] unless left
          if left.respond_to?(:inject) and !left.inject(false) {|a,b| a | b}
            return []
          end
          right = expr( path_stack.shift, nodeset.dup, context )
          res = equality_relational_compare( left, op, right )
          return res

        when :div, :mod, :mult, :plus, :minus
          left = expr(path_stack.shift, nodeset, context)
          right = expr(path_stack.shift, nodeset, context)
          left = unnode(left) if left.is_a?(Array)
          right = unnode(right) if right.is_a?(Array)
          left = Functions::number(left)
          right = Functions::number(right)
          case op
          when :div
            return left / right
          when :mod
            return left % right
          when :mult
            return left * right
          when :plus
            return left + right
          when :minus
            return left - right
          else
            raise "[BUG] Unexpected operator: <#{op.inspect}>"
          end
        when :union
          left = expr( path_stack.shift, nodeset, context )
          right = expr( path_stack.shift, nodeset, context )
          left = unnode(left) if left.is_a?(Array)
          right = unnode(right) if right.is_a?(Array)
          return (left | right)
        when :neg
          res = expr( path_stack, nodeset, context )
          res = unnode(res) if res.is_a?(Array)
          return -Functions.number(res)
        when :not
        when :function
          func_name = path_stack.shift.tr('-','_')
          arguments = path_stack.shift
          subcontext = context ? nil : { :size => nodeset.size }

          res = []
          cont = context
          nodeset.each_with_index do |node, i|
            if subcontext
              if node.is_a?(XPathNode)
                subcontext[:node]  = node.raw_node
                subcontext[:index] = node.position
              else
                subcontext[:node]  = node
                subcontext[:index] = i
              end
              cont = subcontext
            end
            arg_clone = arguments.dclone
            args = arg_clone.collect do |arg|
              result = expr( arg, [node], cont )
              result = unnode(result) if result.is_a?(Array)
              result
            end
            Functions.context = cont
            res << Functions.send( func_name, *args )
          end
          return res

        else
          raise "[BUG] Unexpected path: <#{op.inspect}>: <#{path_stack.inspect}>"
        end
      end # while
      return nodeset
    # ensure
    #   leave(:expr, path_stack, nodeset)
    end