# File lib/faster_csv.rb, line 1553
  def shift
    #########################################################################
    ### This method is purposefully kept a bit long as simple conditional ###
    ### checks are faster than numerous (expensive) method calls.         ###
    #########################################################################
    
    # handle headers not based on document content
    if header_row? and @return_headers and
       [Array, String].include? @use_headers.class
      if @unconverted_fields
        return add_unconverted_fields(parse_headers, Array.new)
      else
        return parse_headers
      end
    end
    
    # begin with a blank line, so we can always add to it
    line = String.new

    # 
    # it can take multiple calls to <tt>@io.gets()</tt> to get a full line,
    # because of \r and/or \n characters embedded in quoted fields
    # 
    loop do
      # add another read to the line
      begin
        line  += @io.gets(@row_sep)
      rescue
        return nil
      end
      # copy the line so we can chop it up in parsing
      parse =  line.dup
      parse.sub!(@parsers[:line_end], "")
      
      # 
      # I believe a blank line should be an <tt>Array.new</tt>, not 
      # CSV's <tt>[nil]</tt>
      # 
      if parse.empty?
        @lineno += 1
        if @skip_blanks
          line = ""
          next
        elsif @unconverted_fields
          return add_unconverted_fields(Array.new, Array.new)
        elsif @use_headers
          return FasterCSV::Row.new(Array.new, Array.new)
        else
          return Array.new
        end
      end

      # parse the fields with a mix of String#split and regular expressions
      csv           = Array.new
      current_field = String.new
      field_quotes  = 0
      parse.split(@col_sep, -1).each do |match|
        if current_field.empty? && match.count(@quote_and_newlines).zero?
          csv           << (match.empty? ? nil : match)
        elsif(current_field.empty? ? match[0] : current_field[0]) == @quote_char[0]
          current_field << match
          field_quotes += match.count(@quote_char)
          if field_quotes % 2 == 0
            in_quotes = current_field[@parsers[:quoted_field], 1]
            raise MalformedCSVError unless in_quotes
            current_field = in_quotes
            current_field.gsub!(@quote_char * 2, @quote_char) # unescape contents
            csv           << current_field
            current_field =  String.new
            field_quotes  =  0
          else # we found a quoted field that spans multiple lines
            current_field << @col_sep
          end
        elsif match.count("\r\n").zero?
          raise MalformedCSVError, "Illegal quoting on line #{lineno + 1}."
        else
          raise MalformedCSVError, "Unquoted fields do not allow " +
                                   "\\r or \\n (line #{lineno + 1})."
        end
      end

      # if parse is empty?(), we found all the fields on the line...
      if field_quotes % 2 == 0
        @lineno += 1

        # save fields unconverted fields, if needed...
        unconverted = csv.dup if @unconverted_fields

        # convert fields, if needed...
        csv = convert_fields(csv) unless @use_headers or @converters.empty?
        # parse out header rows and handle FasterCSV::Row conversions...
        csv = parse_headers(csv)  if     @use_headers

        # inject unconverted fields and accessor, if requested...
        if @unconverted_fields and not csv.respond_to? :unconverted_fields
          add_unconverted_fields(csv, unconverted)
        end

        # return the results
        break csv
      end
      # if we're not empty?() but at eof?(), a quoted field wasn't closed...
      if @io.eof?
        raise MalformedCSVError, "Unclosed quoted field on line #{lineno + 1}."
      elsif @field_size_limit and current_field.size >= @field_size_limit
        raise MalformedCSVError, "Field size exceeded on line #{lineno + 1}."
      end
      # otherwise, we need to loop and pull some more data to complete the row
    end
  end