def index_each_line
        positions = []
        size = 0
        each_line do |line|
          positions << [size, line.size]
          size += line.size
        end
        rewind
        positions
      end
def index_each_line
        positions = []
        size = 0
        each_line do |line|
          positions << [size, line.size]
          size += line.size
        end
        rewind
        positions
      end
Builds an Array of position and length of the current file.
 @return [Array[Array[Fixnum,Fixnum]]] array of position, line length pairs