shift

# File lib/csv.rb, line 1772
  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

    #
    # 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
    #
    in_extended_col = false
    csv             = Array.new

    loop do
      # add another read to the line
      unless parse = @io.gets(@row_sep)
        return nil
      end

      parse.sub!(@parsers[:line_end], "")

      if csv.empty?
        #
        # I believe a blank line should be an <tt>Array.new</tt>, not Ruby 1.8
        # CSV's <tt>[nil]</tt>
        #
        if parse.empty?
          @lineno += 1
          if @skip_blanks
            next
          elsif @unconverted_fields
            return add_unconverted_fields(Array.new, Array.new)
          elsif @use_headers
            return self.class::Row.new(Array.new, Array.new)
          else
            return Array.new
          end
        end
      end

      next if @skip_lines and @skip_lines.match parse

      parts =  parse.split(@col_sep, -1)
      if parts.empty?
        if in_extended_col
          csv[-1] << @col_sep   # will be replaced with a @row_sep after the parts.each loop
        else
          csv << nil
        end
      end

      # This loop is the hot path of csv parsing. Some things may be non-dry
      # for a reason. Make sure to benchmark when refactoring.
      parts.each do |part|
        if in_extended_col
          # If we are continuing a previous column
          if part[-1] == @quote_char && part.count(@quote_char) % 2 != 0
            # extended column ends
            csv.last << part[0..-2]
            if csv.last =~ @parsers[:stray_quote]
              raise MalformedCSVError,
                    "Missing or stray quote in line #{lineno + 1}"
            end
            csv.last.gsub!(@quote_char * 2, @quote_char)
            in_extended_col = false
          else
            csv.last << part
            csv.last << @col_sep
          end
        elsif part[0] == @quote_char
          # If we are staring a new quoted column
          if part[-1] != @quote_char || part.count(@quote_char) % 2 != 0
            # start an extended column
            csv             << part[1..-1]
            csv.last        << @col_sep
            in_extended_col =  true
          else
            # regular quoted column
            csv << part[1..-2]
            if csv.last =~ @parsers[:stray_quote]
              raise MalformedCSVError,
                    "Missing or stray quote in line #{lineno + 1}"
            end
            csv.last.gsub!(@quote_char * 2, @quote_char)
          end
        elsif part =~ @parsers[:quote_or_nl]
          # Unquoted field with bad characters.
          if part =~ @parsers[:nl_or_lf]
            raise MalformedCSVError, "Unquoted fields do not allow " +
                                     "\\r or \\n (line #{lineno + 1})."
          else
            raise MalformedCSVError, "Illegal quoting in line #{lineno + 1}."
          end
        else
          # Regular ole unquoted field.
          csv << (part.empty? ? nil : part)
        end
      end

      # Replace tacked on @col_sep with @row_sep if we are still in an extended
      # column.
      csv[-1][-1] = @row_sep if in_extended_col

      if in_extended_col
        # if we're 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 csv.last.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
      else
        @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 CSV::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
    end
  end