[REBOL] char! maths and Request For Improvement for for
From: alex::pini::mclink::it at: 2-Sep-2000 0:42
>- Open Your Mind -<
Just wanted to warn...
Character-based arithmetics can be deceiving. Consider the following
>> for c #"^(00)" #"^(ff)" 1 [prin [c]]
which goes on forever (on REBOL/Core 2.3.0.1.1). Investigating for's source, I found
out the advancing instruction
start: start + bump
where 'start represents a char!, while 'bump represents the integer! 1; and the while-test
while [op end start]
where 'op represents :greater-or-equal?, 'end represents #"^(ff)" and start steps up
one by one starting from #"^(00)". Now, when 'start gets to the 255th character, we have
a wraparound:
>> #"^(ff)" + 1
== #"^@"
and when the while-test comes, it is equivalent to
>> greater-or-equal? #"^(ff)" #"^(00)"
== true
and here we go again! :-)
Be my guest (-:, have a few other examples:
>> #"^(ff)" + 67
== #"B"
>> #"h" - 255
== #"i"
>> #"h" - 256
== #"h"
>> #"h" - 257
== #"g"
So, stay alert whenever you do maths on char! data, *especially* when nearing boundaries.
Moreover, since the for function explicitly accepts char! values, and since it internally
treats series in a special way separated from all other datatypes, I'd like to see it
modified so that it treats the char! case in its own special way as well.
I've thought of at least three different methods to take care of this, involving bitsets,
strings or integers, but, though I definitely prefer the integer! way for elegance, I
can't say which one is faster: if there's one thing I've learned about speed tests on
the Amiga version of REBOL is that they don't correspond on the other versions... :-)
Of course, I'm sending this message to feedback, too... but I'm almost sure it's going
to bounce back. :-(
Alessandro Pini ([alex--pini--mclink--it])
I'm going in!
BOING! "Then again, maybe not!" (Paris)
