1.11: Guttman-Rosler Transform (GRT) Sort

• Caches extracted keys in a single string for each input record
• String records can be stored in that string
• Reference records have their index stored in the cache string
  • The sorted indices are used to slice into the input array to create sorted records
• Key extraction is O( N )
• The comparison code is internal and uses C with no Perl callback (big win)
• More complex to use properly and efficiently
• GRT has special key description attributes for optimization
  • Numbers can be integer/float, signed/unsigned
  • Strings can be fixed/varying
  • Fastest sort style for larger sets of sort records
• Pass 'GRT' option to make_sorter

  sub {
          my $rec_ind = 0 ;
  
          return  @_[
              map unpack( 'N', substr( $_, -4 ) ), 
              sort
              map pack( "Z*Z*N",
                  do{ my( $val ) = $_->[0] ; ( $val ) },
                  do{ my( $val ) = $_->[1] ; ( $val ) },
                  $rec_ind++
              ), @_
          ] ;
  }