module Mongoid::History
  module Trackable
    extend ActiveSupport::Concern

    module ClassMethods
      def track_history(options={})
        model_name = self.name.tableize.singularize.to_sym
        default_options = {
          :on             =>  :all,
          :except         =>  [:created_at, :updated_at],
          :modifier_field =>  :modifier,
          :version_field  =>  :version,
          :scope          =>  model_name,
          :track_create   =>  false,
          :track_update   =>  true,
          :track_destroy  =>  false,
        }

        options = default_options.merge(options)

        # normalize except fields
        # manually ensure _id, id, version will not be tracked in history
        options[:except] = [options[:except]] unless options[:except].is_a? Array
        options[:except] << options[:version_field]
        options[:except] << "#{options[:modifier_field]}_id".to_sym
        options[:except] += [:_id, :id]
        options[:except] = options[:except].map(&:to_s).flatten.compact.uniq
        options[:except].map(&:to_s)

        # normalize fields to track to either :all or an array of strings
        if options[:on] != :all
          options[:on] = [options[:on]] unless options[:on].is_a? Array
          options[:on] = options[:on].map(&:to_s).flatten.uniq
        end

        field options[:version_field].to_sym, :type => Integer
        referenced_in options[:modifier_field].to_sym, :class_name => Mongoid::History.modifier_class_name

        include MyInstanceMethods
        extend SingletonMethods

        delegate :history_trackable_options, :to => 'self.class'
        delegate :track_history?, :to => 'self.class'

        before_update :track_update if options[:track_update]
        before_create :track_create if options[:track_create]
        before_destroy :track_destroy if options[:track_destroy]

        Mongoid::History.trackable_class_options ||= {}
        Mongoid::History.trackable_class_options[model_name] = options
      end

      def track_history?
        enabled = Thread.current[track_history_flag]
        enabled.nil? ? true : enabled
      end

      def disable_tracking(&block)
        begin
          Thread.current[track_history_flag] = false
          yield
        ensure
          Thread.current[track_history_flag] = true
        end
      end

      def track_history_flag
        "mongoid_history_#{self.name.underscore}_trackable_enabled".to_sym
      end
    end

    module MyInstanceMethods
      def history_tracks
        @history_tracks ||= Mongoid::History.tracker_class.where(:scope => history_trackable_options[:scope], :association_chain => association_hash)
      end
      
      #  undo :from => 1, :to => 5
      #  undo 4
      #  undo :last => 10
      def undo!(modifier, options_or_version=nil)
        versions = get_versions_criteria(options_or_version).to_a
        versions.sort!{|v1, v2| v2.version <=> v1.version}

        versions.each do |v|
          undo_attr = v.undo_attr(modifier)
          self.attributes = v.undo_attr(modifier)
        end
        save!
      end

      def redo!(modifier, options_or_version=nil)
        versions = get_versions_criteria(options_or_version).to_a
        versions.sort!{|v1, v2| v1.version <=> v2.version}

        versions.each do |v|
          redo_attr = v.redo_attr(modifier)
          self.attributes = redo_attr
        end
        save!
      end

    private
      def get_versions_criteria(options_or_version)
        if options_or_version.is_a? Hash
          options = options_or_version
          if options[:from] && options[:to]
            lower = options[:from] >= options[:to] ? options[:to] : options[:from]
            upper = options[:from] <  options[:to] ? options[:to] : options[:from]
            versions = history_tracks.where( :version.in => (lower .. upper).to_a )
          elsif options[:last]
            versions = history_tracks.limit( options[:last] )
          else
            raise "Invalid options, please specify (:from / :to) keys or :last key."
          end
        else
          options_or_version = options_or_version.to_a if options_or_version.is_a?(Range)
          version_field_name = history_trackable_options[:version_field]
          version = options_or_version || self.attributes[version_field_name] || self.attributes[version_field_name.to_s]
          version = [ version ].flatten
          versions = history_tracks.where(:version.in => version)
        end
        versions.desc(:version)
      end

      def should_track_update?
        track_history? && !modified_attributes_for_update.blank?
      end

      def traverse_association_chain(node=self)
        list = node._parent ? traverse_association_chain(node._parent) : []
        list << association_hash(node)
        list
      end

      def association_hash(node=self)
        # get all reflections of embedded_in association metadata
        # and find the first association that matches _parent.
        if node._parent
          meta = node.reflect_on_all_associations(:embedded_in).find do |meta|
            node._parent == node.send(meta.key)
          end

          inverse = node._parent.reflect_on_association(meta.inverse)
        end

        # if root node has no meta, and should use class name instead
        name = meta ? meta.inverse.to_s : node.class.name

        { 'name' => name, 'id' => node.id}
      end

      def modified_attributes_for_update
        @modified_attributes_for_update ||= if history_trackable_options[:on] == :all
          changes.reject do |k, v|
            history_trackable_options[:except].include?(k)
          end
        else
          changes.reject do |k, v|
            !history_trackable_options[:on].include?(k)
          end

        end
      end

      def modified_attributes_for_create
        @modified_attributes_for_create ||= attributes.inject({}) do |h, pair|
          k,v =  pair
          h[k] = [nil, v]
          h
        end.reject do |k, v|
          history_trackable_options[:except].include?(k)
        end
      end

      def modified_attributes_for_destroy
        @modified_attributes_for_destroy ||= attributes.inject({}) do |h, pair|
          k,v =  pair
          h[k] = [nil, v]
          h
        end
      end

      def history_tracker_attributes(method)
        return @history_tracker_attributes if @history_tracker_attributes

        @history_tracker_attributes = {
          :association_chain  => traverse_association_chain,
          :scope              => history_trackable_options[:scope],
          :modifier        => send(history_trackable_options[:modifier_field])
        }

        original, modified = transform_changes(case method
          when :destroy then modified_attributes_for_destroy
          when :create then modified_attributes_for_create
          else modified_attributes_for_update
        end)

        @history_tracker_attributes[:original] = original
        @history_tracker_attributes[:modified] = modified
        @history_tracker_attributes
      end

      def track_update
        return unless should_track_update?
        current_version = (self.send(history_trackable_options[:version_field]) || 0 ) + 1
        self.send("#{history_trackable_options[:version_field]}=", current_version)
        Mongoid::History.tracker_class.create!(history_tracker_attributes(:update).merge(:version => current_version, :action => "update", :trackable => self))
        clear_memoization
      end

      def track_create
        return unless track_history?
        current_version = (self.send(history_trackable_options[:version_field]) || 0 ) + 1
        self.send("#{history_trackable_options[:version_field]}=", current_version)
        Mongoid::History.tracker_class.create!(history_tracker_attributes(:create).merge(:version => current_version, :action => "create", :trackable => self))
        clear_memoization
      end

      def track_destroy
        return unless track_history?
        current_version = (self.send(history_trackable_options[:version_field]) || 0 ) + 1
        Mongoid::History.tracker_class.create!(history_tracker_attributes(:destroy).merge(:version => current_version, :action => "destroy", :trackable => self))
        clear_memoization
      end

      def clear_memoization
        @history_tracker_attributes =  nil
        @modified_attributes_for_create = nil
        @modified_attributes_for_update = nil
        @history_tracks = nil
      end

      def transform_changes(changes)
        original = {}
        modified = {}
        changes.each_pair do |k, v|
          o, m = v
          original[k] = o unless o.nil?
          modified[k] = m unless m.nil?
        end

        return original.easy_diff modified
      end

    end

    module SingletonMethods
      def history_trackable_options
        @history_trackable_options ||= Mongoid::History.trackable_class_options[self.name.tableize.singularize.to_sym]
      end
    end

  end
end