# --
# Copyright (C) 2008-2009 10gen Inc.
#
# This program is free software: you can redistribute it and/or modify it
# under the terms of the GNU Affero General Public License, version 3, as
# published by the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful, but WITHOUT
# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
# FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License
# for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
# ++

require 'mongo/message'
require 'mongo/util/byte_buffer'
require 'mongo/util/bson'

module XGen
  module Mongo
    module Driver

      # A cursor over query results. Returned objects are hashes.
      class Cursor

        include Enumerable

        RESPONSE_HEADER_SIZE = 20

        def initialize(db, collection, num_to_return=0)
          @db, @collection, @num_to_return = db, collection, num_to_return
          @objects_returned = 0
          @objects = []
          @closed = false
          read_all
        end

        # Return +true+ if there are more records to retrieve. We do not check
        # @num_to_return; #each is responsible for doing that.
        def more?
          num_remaining > 0
        end

        # Return the next object. Raises an error if necessary.
        def next_object
          refill_via_get_more if num_remaining == 0
          o = @objects.shift
          raise o['$err'] if o['$err']
          o
        end

        # Iterate over each object, yielding it to the given block. At most
        # @num_to_return records are returned (or all of them, if
        # @num_to_return is 0).
        def each
          num_returned = 0
          while more? && (@num_to_return <= 0 || num_returned < @num_to_return)
            yield next_object()
            num_returned += 1
          end
        end

        # Close the cursor.
        def close
          @db.send_to_db(KillCursorMessage(@cursor_id)) if @cursor_id
          @objects = []
          @cursor_id = 0
          @closed = true
        end

        protected

        def read_all
          read_message_header
          read_response_header
          read_objects_off_wire
        end

        def read_objects_off_wire
          while doc = next_object_on_wire
            @objects << doc
          end
        end

        def read_message_header
          MessageHeader.new.read_header(@db.socket)
        end

        def read_response_header
          header_buf = ByteBuffer.new
          header_buf.put_array(@db.socket.recv(RESPONSE_HEADER_SIZE).unpack("C*"))
          raise "Short read for DB response header; expected #{RESPONSE_HEADER_SIZE} bytes, saw #{header_buf.length}" unless header_buf.length == RESPONSE_HEADER_SIZE
          header_buf.rewind
          @result_flags = header_buf.get_int
          @cursor_id = header_buf.get_long
          @starting_from = header_buf.get_int
          @n_returned = header_buf.get_int
          @n_remaining = @n_returned
        end

        def num_remaining
          refill_via_get_more if @objects.length == 0
          @objects.length
        end

        private

        def next_object_on_wire
          # if @n_remaining is 0 but we have a non-zero cursor, there are more
          # to fetch, so do a GetMore operation, but don't do it here - do it
          # when someone pulls an object out of the cache and it's empty
          return nil if @n_remaining == 0
          object_from_stream
        end

        def refill_via_get_more
          return if @cursor_id == 0
          @db.send_to_db(GetMoreMessage.new(@db.name, @collection, @cursor_id))
          read_all
        end

        def object_from_stream
          buf = ByteBuffer.new
          buf.put_array(@db.socket.recv(4).unpack("C*"))
          buf.rewind
          size = buf.get_int
          buf.put_array(@db.socket.recv(size-4).unpack("C*"), 4)
          @n_remaining -= 1
          buf.rewind
          BSON.new.deserialize(buf)
        end

        def to_s
          "DBResponse(flags=#@result_flags, cursor_id=#@cursor_id, start=#@starting_from, n_returned=#@n_returned)"
        end
      end
    end
  end
end