shift (FasterCSV)

    # File lib/faster_csv.rb, line 1545
      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 = ""

        #
        # 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
          line  += @io.gets(@row_sep) rescue return nil
          # 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

          #
          # shave leading empty fields if needed, because the main parser chokes
          # on these
          #
          csv = if parse.sub!(@parsers[:leading_fields], "")
            [nil] * ($&.length / @col_sep.length)
          else
            Array.new
          end
          #
          # then parse the main fields with a hyper-tuned Regexp from
          # Mastering Regular Expressions, Second Edition
          #
          parse.gsub!(@parsers[:csv_row]) do
            csv << if $1.nil?     # we found an unquoted field
              if $2.empty?        # switch empty unquoted fields to +nil+...
                nil               # for CSV compatibility
              else
                # I decided to take a strict approach to CSV parsing...
                if $2.count("\r\n").zero?  # verify correctness of field...
                  $2
                else
                  # or throw an Exception
                  raise MalformedCSVError, "Unquoted fields do not allow " +
                                           "\\r or \\n (line #{lineno + 1})."
                end
              end
            else                  # we found a quoted field...
              $1.gsub(@quote_char * 2, @quote_char)  # unescape contents
            end
            ""  # gsub!'s replacement, clear the field
          end

          # if parse is empty?(), we found all the fields on the line...
          if parse.empty?
            @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 parse =~ @parsers[:bad_field]
            raise MalformedCSVError, "Illegal quoting on line #{lineno + 1}."
          elsif @field_size_limit and parse.length >= @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