Idiom for guarded byte streams

[Jose E. Marchesi]

Hi people!

So today I was writing a pickle for the BER encoding of ASN-1 and came
with an interesting problem: how to best denote a sequence of bytes
ended by two consecutive 0UB bytes.

Sounds simple enough... but I really had to think for a while before
coming to this:

type GuardedBytes =
 struct
 {
   type Datum =
     union
     {
       byte[2] pair : pair[1] != 0UB;
       byte single : single != 0UB;
     };

   Datum[] data;
   byte[2] end : end == [0UB,0UB];
 };

It is easy to get an array with the bytes (at the cost of copying them)
with a function or a method like this:

 method get_bytes = byte[]:
 {
   var bytes = byte[]();
   for (d in data)
     try bytes += d.pair;
     catch if E_elem { bytes += [d.single]; }
   return bytes;
 }

And given a mapped GuardedBytes, it is easy to map other stuff in its
contents using data'offset and data'size.

If the end-of-data marker was three bytes instead of two, I guess I
would need to add a `triplet' alternative to the Datum union.  And so
on...

Can you think about a better solution for this kind of structures?  In
that case please share, I'm very interested :)