Can you use a number for the subject code and the athor code? If you used the following packing:

Year         2 bytes     (0..65535)
subject code 2 bytes(?)  (0..65535)
author code  2 bytes(?)  (0..65535)
chemicals:   2 bytes per (0..65535)
[download]

That allows you to have up to 11 chemicals. It's possible to do better (by using bits which aren't an even number of bytes), but you lose efficiency.

If the above is ok, let me know and I'll code it tonight.

If the above is not sufficient, let me know more precise ranges for the year, the subject codes, the author codes and chemical codes.

Update: Promised code:

sub encode_base32 {
   ... based on encode_base64 ...
}

sub decode_base32 {
   ... based on decode_base64 ...
}

sub compress_data {
   my ($year, $subject, $author, @chemicals) = @_;

   carp(...) if $year    < 0 || $year    > 65535;
   carp(...) if $subject < 0 || $subject > 65535;
   carp(...) if $author  < 0 || $author  > 65535;
   carp(...) if @chemicals > 11;
   carp(...) if grep { $_ == 65535 } @chemicals;

   push(@chemicals, (65535)x(11-@chemicals));

   return pack('n*', $year, $subject, $author, @chemicals);
}

sub decompress_data {
   my ($data) = @_;
   my ($year, $subject, $author, @chemicals) = unpack('n*', $data);
   @chemicals = grep { $_ != 65535 } @chemicals;
   return ($year, $subject, $author, @chemicals);
}


$file_name = encode_base32(compress_data(...));

(...) = decompress_data(decode_base32($file_name));
[download]

Untested.

Be aware that packing 0 .. 65536 into 2 bytes is going to result in filenames containing all manner of control characters and 8-bit characters that even if the filesystem accepts them will be nigh impossible for any user to type.

You'll probably need to hexify the numbers to make them acceptable for use, with the possible resultant greater length.

---

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Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

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You should reread the thread. In short, first you compress the data to 28 bytes (at which point "all manner of control characters and 8-bit characters" are perfectly acceptable), then you encode it into 45 safe characters using a 32 bit variant of Base64.

Also, if any of the fields are completely numeric, I think you would get a better result by compressing them separately.