expr

# File lib/rexml/xpath_parser.rb, line 153
    def expr( path_stack, nodeset, context=nil )
      node_types = ELEMENTS
      return nodeset if path_stack.length == 0 || nodeset.length == 0
      while path_stack.length > 0
        if nodeset.length == 0
          path_stack.clear
          return []
        end
        case (op = path_stack.shift)
        when :document
          nodeset = [ nodeset[0].root_node ]

        when :qname
          prefix = path_stack.shift
          name = path_stack.shift
          nodeset.delete_if do |node|
            # FIXME: This DOUBLES the time XPath searches take
            ns = get_namespace( node, prefix )
            if node.node_type == :element
              if node.name == name
              end
            end
            !(node.node_type == :element and
              node.name == name and
              node.namespace == ns )
          end
          node_types = ELEMENTS

        when :any
          nodeset.delete_if { |node| !node_types.include?(node.node_type) }

        when :self
          # This space left intentionally blank

        when :processing_instruction
          target = path_stack.shift
          nodeset.delete_if do |node|
            (node.node_type != :processing_instruction) or
            ( target!='' and ( node.target != target ) )
          end

        when :text
          nodeset.delete_if { |node| node.node_type != :text }

        when :comment
          nodeset.delete_if { |node| node.node_type != :comment }

        when :node
          # This space left intentionally blank
          node_types = ALL

        when :child
          new_nodeset = []
          nt = nil
          nodeset.each do |node|
            nt = node.node_type
            new_nodeset += node.children if nt == :element or nt == :document
          end
          nodeset = new_nodeset
          node_types = ELEMENTS

        when :literal
          return path_stack.shift

        when :attribute
          new_nodeset = []
          case path_stack.shift
          when :qname
            prefix = path_stack.shift
            name = path_stack.shift
            for element in nodeset
              if element.node_type == :element
                attrib = element.attribute( name, get_namespace(element, prefix) )
                new_nodeset << attrib if attrib
              end
            end
          when :any
            for element in nodeset
              if element.node_type == :element
                new_nodeset += element.attributes.to_a
              end
            end
          end
          nodeset = new_nodeset

        when :parent
          nodeset = nodeset.collect{|n| n.parent}.compact
          #nodeset = expr(path_stack.dclone, nodeset.collect{|n| n.parent}.compact)
          node_types = ELEMENTS

        when :ancestor
          new_nodeset = []
          nodeset.each do |node|
            while node.parent
              node = node.parent
              new_nodeset << node unless new_nodeset.include? node
            end
          end
          nodeset = new_nodeset
          node_types = ELEMENTS

        when :ancestor_or_self
          new_nodeset = []
          nodeset.each do |node|
            if node.node_type == :element
              new_nodeset << node
              while ( node.parent )
                node = node.parent
                new_nodeset << node unless new_nodeset.include? node
              end
            end
          end
          nodeset = new_nodeset
          node_types = ELEMENTS

        when :predicate
          new_nodeset = []
          subcontext = { :size => nodeset.size }
          pred = path_stack.shift
          nodeset.each_with_index { |node, index|
            subcontext[ :node ] = node
            subcontext[ :index ] = index+1
            pc = pred.dclone
            result = expr( pc, [node], subcontext )
            result = result[0] if result.kind_of? Array and result.length == 1
            if result.kind_of? Numeric
              new_nodeset << node if result == (index+1)
            elsif result.instance_of? Array
              if result.size > 0 and result.inject(false) {|k,s| s or k}
                new_nodeset << node if result.size > 0
              end
            else
              new_nodeset << node if result
            end
          }
          nodeset = new_nodeset
          predicate = path_stack.shift
          ns = nodeset.clone
          result = expr( predicate, ns )
          if result.kind_of? Array
            nodeset = result.zip(ns).collect{|m,n| n if m}.compact
          else
            nodeset = result ? nodeset : []
          end

        when :descendant_or_self
          rv = descendant_or_self( path_stack, nodeset )
          path_stack.clear
          nodeset = rv
          node_types = ELEMENTS

        when :descendant
          results = []
          nt = nil
          nodeset.each do |node|
            nt = node.node_type
            results += expr( path_stack.dclone.unshift( :descendant_or_self ),
              node.children ) if nt == :element or nt == :document
          end
          nodeset = results
          node_types = ELEMENTS

        when :following_sibling
          results = []
          nodeset.each do |node|
            next if node.parent.nil?
            all_siblings = node.parent.children
            current_index = all_siblings.index( node )
            following_siblings = all_siblings[ current_index+1 .. -1 ]
            results += expr( path_stack.dclone, following_siblings )
          end
          nodeset = results

        when :preceding_sibling
          results = []
          nodeset.each do |node|
            next if node.parent.nil?
            all_siblings = node.parent.children
            current_index = all_siblings.index( node )
            preceding_siblings = all_siblings[ 0, current_index ].reverse
            results += preceding_siblings
          end
          nodeset = results
          node_types = ELEMENTS

        when :preceding
          new_nodeset = []
          nodeset.each do |node|
            new_nodeset += preceding( node )
          end
          nodeset = new_nodeset
          node_types = ELEMENTS

        when :following
          new_nodeset = []
          nodeset.each do |node|
            new_nodeset += following( node )
          end
          nodeset = new_nodeset
          node_types = ELEMENTS

        when :namespace
          new_nodeset = []
          prefix = path_stack.shift
          nodeset.each do |node|
            if (node.node_type == :element or node.node_type == :attribute)
              if @namespaces
                namespaces = @namespaces
              elsif (node.node_type == :element)
                namespaces = node.namespaces
              else
                namespaces = node.element.namesapces
              end
              if (node.namespace == namespaces[prefix])
                new_nodeset << node
              end
            end
          end
          nodeset = new_nodeset

        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 )
          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
          left = Functions::number(expr(path_stack.shift, nodeset, context)).to_f
          right = Functions::number(expr(path_stack.shift, nodeset, context)).to_f
          return (left / right)

        when :mod
          left = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          right = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          return (left % right)

        when :mult
          left = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          right = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          return (left * right)

        when :plus
          left = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          right = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          return (left + right)

        when :minus
          left = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          right = Functions::number(expr(path_stack.shift, nodeset, context )).to_f
          return (left - right)

        when :union
          left = expr( path_stack.shift, nodeset, context )
          right = expr( path_stack.shift, nodeset, context )
          return (left | right)

        when :neg
          res = expr( path_stack, nodeset, context )
          return -(res.to_f)

        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 { |n, i|
            if subcontext
              subcontext[:node]  = n
              subcontext[:index] = i
              cont = subcontext
            end
            arg_clone = arguments.dclone
            args = arg_clone.collect { |arg|
              expr( arg, [n], cont )
            }
            Functions.context = cont
            res << Functions.send( func_name, *args )
          }
          return res

        end
      end # while
      return nodeset
    end