module JSONAPI
  class ResourceSerializer

    # Serializes a single resource, or an array of resources
    # include:
    #     Purpose: determines which objects will be side loaded with the source objects in a linked section
    #     Example: ['comments','author','comments.tags','author.posts']
    # fields:
    #     Purpose: determines which fields are serialized for a resource type. This encompasses both attributes and
    #              association ids in the links section for a resource. Fields are global for a resource type.
    #     Example: { people: [:id, :email, :comments], posts: [:id, :title, :author], comments: [:id, :body, :post]}
    def serialize(source, include = [], fields = {}, context = nil)
      @fields = fields
      @context = context
      @linked_objects = {}

      requested_associations = parse_includes(include)

      if source.respond_to?(:to_ary)
        return {} if source.size == 0
        @primary_class_name = source[0].class._serialize_as
      else
        @primary_class_name = source.class._serialize_as
      end

      process_primary(source, requested_associations)

      primary_class_name = @primary_class_name.to_sym
      primary_hash = {primary_class_name => []}

      linked_hash = {}
      @linked_objects.each do |class_name, objects|
        class_name = class_name.to_sym

        linked = []
        objects.each_value do |object|
          if object[:primary]
            primary_hash[primary_class_name].push(object[:object_hash])
          else
            linked.push(object[:object_hash])
          end
        end
        linked_hash[class_name] = linked unless linked.empty?
      end

      if linked_hash.size > 0
        primary_hash.merge!({linked: linked_hash})
      end

      return primary_hash
    end

    private
    # Convert an array of associated objects to include along with the primary document in the form of
    # ['comments','author','comments.tags','author.posts'] into a structure that tells what we need to include
    # from each association.
    def parse_includes(includes)
      requested_associations = {}
      includes.each do |include|
        include = include.to_s if include.is_a? Symbol

        pos = include.index('.')
        if pos
          association_name = include[0, pos].to_sym
          requested_associations[association_name] ||= {}
          requested_associations[association_name].store(:include_children, true)
          requested_associations[association_name].store(:include_related, parse_includes([include[pos+1, include.length]]))
        else
          association_name = include.to_sym
          requested_associations[association_name] ||= {}
          requested_associations[association_name].store(:include, true)
        end
      end if includes.is_a?(Array)
      return requested_associations
    end

    # Process the primary source object(s). This will then serialize associated object recursively based on the
    # requested includes. Fields are controlled fields option for each resource type, such
    # as fields: { people: [:id, :email, :comments], posts: [:id, :title, :author], comments: [:id, :body, :post]}
    # The fields options controls both fields and included links references.
    def process_primary(source, requested_associations)
      if source.respond_to?(:to_ary)
        source.each do |object|
          id = object.send(object.class._key)
          if already_serialized?(@primary_class_name, id)
            set_primary(@primary_class_name, id)
          end

          add_linked_object(@primary_class_name, id, object_hash(object,  requested_associations), true)
        end
      else
        id = source.send(source.class._key)
        # ToDo: See if this is actually needed
        # if already_serialized?(@primary_class_name, id)
        #   set_primary(@primary_class_name, id)
        # end

        add_linked_object(@primary_class_name, id, object_hash(source,  requested_associations), true)
      end
    end

    # Returns a serialized hash for the source object, with
    def object_hash(source, requested_associations)
      obj_hash = attribute_hash(source)
      links = links_hash(source, requested_associations)
      obj_hash.merge!({links: links}) unless links.empty?
      return obj_hash
    end

    def requested_fields(model)
      @fields[model] if @fields
    end

    def attribute_hash(source)
      requested = requested_fields(source.class._serialize_as)
      fields = source.class._attributes.to_a
      unless requested.nil?
        fields = requested & fields
      end

      source.fetchable(fields, @context).each_with_object({}) do |name, hash|
        hash[name] = source.send(name)
      end
    end

    # Returns a hash of links for the requested associations for a resource, filtered by the resource
    # class's fetchable method
    def links_hash(source, requested_associations)
      associations = source.class._associations
      requested = requested_fields(source.class._serialize_as)
      fields = associations.keys
      unless requested.nil?
        fields = requested & fields
      end

      field_set = Set.new(fields)

      included_associations = source.fetchable(associations.keys, @context)
      associations.each_with_object({}) do |(name, association), hash|
        if included_associations.include? name
          key = association.key

          if field_set.include?(name)
            hash[name] = source.send(key)
          end

          ia = requested_associations.is_a?(Hash) ? requested_associations[name] : nil

          include_linked_object = ia && ia[:include]
          include_linked_children = ia && ia[:include_children]

          type = association.serialize_type_name

          # If the object has been serialized once it will be in the related objects list,
          # but it's possible all children won't have been captured. So we must still go
          # through the associations.
          if include_linked_object || include_linked_children
            if association.is_a?(JSONAPI::Association::HasOne)
              object = source.send("_#{name}_object")

              id = object.send(association.primary_key)
              associations_only = already_serialized?(type, id)
              if include_linked_object && !associations_only
                add_linked_object(type, id, object_hash(object, ia[:include_related]))
              elsif include_linked_children || associations_only
                links_hash(object, ia[:include_related])
              end
            elsif association.is_a?(JSONAPI::Association::HasMany)
              objects = source.send("_#{name}_objects")
              objects.each do |object|
                id = object.send(association.primary_key)
                associations_only = already_serialized?(type, id)
                if include_linked_object && !associations_only
                  add_linked_object(type, id, object_hash(object, ia[:include_related]))
                elsif include_linked_children || associations_only
                  links_hash(object, ia[:include_related])
                end
              end
            end
          end
        end
      end
    end

    def already_serialized?(type, id)
      return @linked_objects.key?(type) && @linked_objects[type].key?(id)
    end

    # Sets that an object should be included in the primary document of the response.
    def set_primary(type, id)
      @linked_objects[type][id][:primary] = true
    end

    # Collects the hashes for all objects processed by the serializer
    def add_linked_object(type, id, object_hash, primary = false)
      unless @linked_objects.key?(type)
        @linked_objects[type] = {}
      end

      if already_serialized?(type, id)
        if primary
          set_primary(type, id)
        end
      else
        @linked_objects[type].store(id, {primary: primary, object_hash: object_hash})
      end
    end
  end
end