require 'util/visitor'

# An EnterLeaveVisitor is a Visitor that visits objects both on the way down
# and on the way up when traversing an object structure/hierarchy.
# It will first call enter_X(x) if available, then visit_X(x) if avilable
# then traverse down into children objects and then call leave_X(x) if
# available.
# Objects use visit_all to indicate structure. The first argument to visit_all
# is a parent object and the rest are children objects.
# NOTE! If they simply call visit the leave method will never be called but
# the enter and visit methods will.
module EnterLeaveVisitor
  include Visitor

  def visit(obj)
    visit_me_then_children(obj)
  end

  def enter(obj)
    method = _enter_method(obj)
    self.send(method, obj) if method
  end

  def leave(obj)
    method = _leave_method(obj)
    self.send(method, obj) if method
  end

  # Each children should be an array with children
  def visit_me_then_children(parent, *children)
    enter_method, visit_method, leave_method = _evl_methods(parent)
    self.send(enter_method, parent) if enter_method
    self.send(visit_method, parent) if visit_method
    children.each {|cs| cs.each {|c| c.accept_visitor(self)}}
    self.send(leave_method, parent) if leave_method
  end

  def _evl_methods(obj)
    obj.class.ancestors.map {|c| _evl_methods_for_class(c)}.compact.first
  end

  def _evl_methods_for_class(klass)
    # Memoized so not dynamic, maybe we should clear cache when a method is
    # added?
    (@_memoized_methods ||= Hash.new)[klass] ||= _methodnames_from_class(klass)
  end

  def _methodname_from_classname(klassName, prefix)
    name = prefix + klassName
    self.class.instance_methods.include?(name) ? name : nil
  end

  def _methodnames_from_class(klass)
    class_name = klass.inspect.split("::").last
    l = ["enter_", "visit_", "leave_"].map do |prefix| 
      _methodname_from_classname(class_name, prefix)
    end
    # Return nil unless we found at least one method to visit
    (l.compact.length > 0) ? l : nil
  end
end

module EnterLeaveVisitable
  include Visitable
end

class Object
  include EnterLeaveVisitable
end