Harsh lessons in binaryDecode

[Alex Tweedly]

I was trying to use binaryDecode to get some data out of a string; the 
format is simple
 - 4 digit number
 - a space
 - 4 digit number
 - a space
 - 12 digit number

However, the numbers may be completely blank (otherwise I'd just have 
used word 1, word 2 and word 3 :-)
So get some sample data ready ...

  put "1234 5678 123456789012 and some more" into lV

Rather than
   put char 1 to 4 of V into tVal1
   put char 6 to 9 of V into tVal2
   put char 11 to 22 of V into tVal3
I decided to have a go at using binaryDecode (thanks to Jim Ault for 
suggesting this), because it would be a single statement and potentially 
faster (the real case has more variables and decoding to do)
 
  get binaryDecode("a4xa4xa12", lV, tVal1, tVal2, tVal3)

i.e. put 4 chars into the first variable, skip one char, 4 chars into 
next var, skip, 12 chars into next var.

I heeded the warning about the fact that the variables must exist, so 
added local declarations.

Unfortunately, executing this code makes Rev crash immediately - no 
error, no warning, just "Boom" - gone.

It turns out (I think) that although the binaryDecode format of "x" is 
described as

> The formatsList consists of one or more dataTypes, each followed 
> optionally by an amount. A dataType is one of the following single 
> letters:
>
> x: skip next amount bytes of data
> a: convert next amount bytes of data to characters

etc.
"skip" doesn't mean skip - it means skip and use up one variable from 
the variable list !!

Therefore, there were too few variables in the variable list. A crash 
out of Rev seems a bit harsh even for too few variables, but I can live 
with that.

A quick scan in Bugzilla didn't show anything like this. So
 - should  I BZ it ?  (or this the way it's always been and should be 
forever :-)
 - BZ the docs ? (i.e. skip doesn't mean skip), since a change would 
risk backwards compatibility
 - BZ the crash separately ??


Here's the recipe - switch the commented out lines to see the problem ...

[ in the script of a button ]
local lV
on mouseUp
  put "1234 5678 123456789012 and some more" into lV
  doit
end mouseUp

on doit
  local tVal1, tVal2, tVal3
  local x1, x2
  --  get binaryDecode("a4xa4xa12", lV, tVal1, tVal2, tVal3)
  get binaryDecode("a4xa4xa12", lV, tVal1, x1, tVal2, x2, tVal3)
  put it && tVal1, tVal2, tVal3 & cr after msg
end doit


Moral of the day, for those who've made it this far ...
  I've spent more hours with this problem than I will ever save seconds 
in run time from this efficiency :-)

-- 
Alex Tweedly       http://www.tweedly.net



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