module Scrivito
module Backend

module ObjDataCache

  # in the cache, missing objs (no obj exists with the given id) are
  # represented as an empty hash to distinguish them from cache misses.
  # nil => cache miss for given id
  # {} => cache hit for given id, no obj with this id exists
  NONEXISTENT_OBJ = {}

  def self.convert_from_backend(obj_data)
    if obj_data.blank? || obj_data["_deleted"]
      ObjDataCache::NONEXISTENT_OBJ
    else
      obj_data
    end
  end

  def self.view_for_revision(revision)
    if revision.base
      SimpleCacheView.new(revision.id)
    else
      view_for_workspace(revision.workspace.id, revision.content_state_node)
    end
  end

  def self.view_for_workspace(workspace_id, content_state_node)
      IncrementalCacheView.new(workspace_id, content_state_node)
  end

  # builds a "change index", i.e. a hash mapping from `_id` to
  # CmsDataCache-Tags that reference the data of the Obj
  def self.changes_index_from(objs)
    obj_ids = objs.map { |obj| obj["_id"] || obj["_deleted"] }

    tags = objs.
      map(&method(:convert_from_backend)).
      map(&CmsDataCache.method(:write_data_to_tag))

    Hash[obj_ids.zip(tags)]
  end


  class SimpleCacheView < Struct.new(:cache_id)

    def read_obj(id)
      if tag = CmsDataCache.read_obj_data_rest(cache_id, "id", id)
        CmsDataCache.read_data_from_tag(tag)
      end
    end

    def write_obj(id, data)
      write_cache("id", id, CmsDataCache.write_data_to_tag(data))
    end

    def read_index(index, key)
      index_access
    end

    def write_index(index, key, data)
      index_access
    end

    private

    def index_access
      # SimpleCacheView is currently only used for base revision that do not
      # support index queries.
      raise InternalError
    end

    def write_cache(index, key, data)
      raise InternalError unless data

      CmsDataCache.write_obj_data_rest(cache_id, index, key, data)
    end

  end

  class IncrementalCacheView < Struct.new(:cache_id, :viewed_state)

    def read_index(index, key, &update_function)
      if hit = CmsDataCache.read_obj_data_rest(cache_id, index, key)

        cached_at = hit.first
        cached_result = hit.second

        # if no update needed, just return
        return cached_result if cached_at == viewed_state.content_state_id

        updated_result = update_result(
            key, cached_result, cached_at, &update_function)

        return nil unless updated_result

        if cached_at != csid_to_write_to
          write_cache(index, key, updated_result)
        end

        updated_result
      end
    end

    def write_index(index, key, data)
      write_cache(index, key, data)
    end

    def read_obj(id)
      if hit = CmsDataCache.read_obj_data_rest(cache_id, "id", id)
        cached_at, cached_tag = hit

        tag_in_changes = viewed_state.obj_from_changes(id, cached_at)

        updated_tag =
          case tag_in_changes
          when ContentStateNode::CACHE_MISS
            # could not reconstruct all changes since `cached_at`
            # so we don't know if data is still valid.
            nil
          when ContentStateNode::NOT_FOUND
            # there were no relevant changes, so the data is still valid
            cached_tag
          else
            # the changes revealed a newer version, return it instead
            tag_in_changes
          end

        return nil unless updated_tag

        if cached_at != csid_to_write_to
          write_cache("id", id, updated_tag)
        end

        CmsDataCache.read_data_from_tag(updated_tag)
      end
    end

    def write_obj(id, data)
      write_obj_tag(id, CmsDataCache.write_data_to_tag(data))
    end

    def write_obj_tag(id, tag)
      write_cache("id", id, tag)
    end

    private

    def write_cache(index, key, data)
      raise InternalError unless data

      CmsDataCache.write_obj_data_rest(
          cache_id, index, key, [csid_to_write_to, data])
    end

    # tries to update an outdated cache result using the changes recorded in the
    # ContentStateNode history.
    # returns `nil` (= cache-miss) if update is not possible.
    def update_result(key, cached_result, cached_at, &update_function)
      # if index does not support update, treat as cache-miss
      return nil unless update_function

      change_index = viewed_state.change_index_for(cached_at)

      return nil if change_index == ContentStateNode::CACHE_MISS

      expanded_index = expand_index(change_index)

      # if the data of changed objs was evicted, treat as cache-miss
      return nil if !expanded_index

      update_function.call(key, cached_result, expanded_index)
    end

    # accepts a "change index" (id --> tag), looks up each
    # tag in the cache and return an "expanded index" (id --> data).
    # return `nil` if any tag cannot be found in the cache.
    def expand_index(change_index)
      # using "merge(self)" to map over a hash
      change_index.merge(change_index) do |id, tag|
        CmsDataCache.read_data_from_tag(tag) or return nil
      end
    end

    def csid_to_write_to
      viewed_state.next_stable_node.content_state_id
    end

  end

end

end
end