unpacking variable length bit fields

Skeeve has asked for the wisdom of the Perl Monks concerning the following question:

Hi fellow monks!

I'm searching for a more elegant way to solve this problem. I have a string of 3 byte which contain fields of information packed in bits. These fields have variable length. The bit pattern is: AAABBCCD EEEEFFGH IIJJKLMM.

What I do now is similar to:

( $a, $b, $c, $d, $e, $f, $g, $h, $i, $j, $k, $l, $m )=
  map { unpack 'C', pack 'b*',$_ }
    unpack 'A3A2A2A1A4A2A1A1A2A2A1A1A2',
      unpack 'B*',$x;
[download]

So you see, I do:

unpack my value to a string of '0' and '1's
"split" it according to the lengths of the fields
pack each individual substring to a string
unpack it as an integer

Can you advise me of a better / more elegant way to achieve this?

Update: I just noticed: My code seemed to work, but didn't work as I expected... I'll update again when I found a working solution :-(

Update #2: Thanks to BrowserUK for finding a working solution!

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

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Re: unpacking variable length bit fields by BrowserUk (Patriarch) on Oct 24, 2006 at 12:00 UTC
This is a little simpler. `map oct( "0b$_" ), unpack 'A3A2A2A1A4A2A1A1A2A2A1A1A2', unpack 'B*', +$x;;` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re^2: unpacking variable length bit fields by Skeeve (Parson) on Oct 24, 2006 at 12:18 UTC
Thanks! Not as elegant as I hoped but it has the big advantage: It works correct! `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re^3: unpacking variable length bit fields by BrowserUk (Patriarch) on Oct 24, 2006 at 12:53 UTC
I know what you mean. You want to be able to replace all the As with Bs and skip the first step, but of course each B starts a new byte. Maybe there could be something like `unpack 'B{3,2,2,1}B{4,2,1,1}B{2,2,1,1,2}', $x`? I've also had occassion to want to extract bit-ranges that cross byte boundaries and that gets real messy. Maybe an enhanced vec that allows non-powers-of-two sizes. That shouldn't be so hard to write. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]