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So, I wanted to play around with quickly accessing data in an array, and figured
I could use a hash function to calculate semi-unique array indices.
I made a stab at converting c++ code from here:
http://www.azillionmonkeys.com/qed/hash.html
and just made some naive assumptions about some of the memory-location specific
operations, but I think I got the rest of it correct.
Then something interesting happened.
I tried to run a loop to process a series of input values, but my loop only
executed one iteration.
I then sent the value of my iterator to the #debug stream after the loop exited,
and it's greater than my maximum/exit value!
Perhaps someone could verify this behaviour on their machine/OS and look over my
conversion of c++ to SDL and see if something crazy is happening.
Am I writing to a memory location by using bitwise operators in SDL?
If so: WHOA.
Scene file is attached.
- Bald "POV-Ray breaker" Eagle
Post a reply to this message
Attachments:
Download 'superfasthash.pov.txt' (5 KB)
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From: Cousin Ricky
Subject: Re: Bitwise operators corrupting SDL parse-time execution of POV-Ray?
Date: 3 Feb 2023 18:43:34
Message: <63dd9c26$1@news.povray.org>
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On 2023-02-03 18:33 (-4), Bald Eagle wrote:
>
> I tried to run a loop to process a series of input values, but my loop only
> executed one iteration.
>
> I then sent the value of my iterator to the #debug stream after the loop exited,
> and it's greater than my maximum/exit value!
If you pass an identifier as a macro argument, POV-Ray treats it as
pass-by-reference. Macro SuperFastHash() makes assignments to its
arguments, data and len. This hoses your loop counter i during the
first call to SuperFastHash().
You can fix this by assigning data and len to new local variables within
SuperFastHash(), and using only those new variables.
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Cousin Ricky <ric### [at] yahoo com> wrote:
> You can fix this by assigning data and len to new local variables within
> SuperFastHash(), and using only those new variables.
UGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGH!
I got snookered by this *AGAIN* !!!
Thank you, Richard.
jr and I grappled with this maddening behaviour before, but I forgot :(
Any idea if I can modify the code to only give positive results?
Bitshift to the right by one position to clean the sign bit?
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hi,
"Bald Eagle" <cre### [at] netscapenet> wrote:
> Cousin Ricky <ric### [at] yahoocom> wrote:
>
> > You can fix this by assigning data and len to new local variables within
> > SuperFastHash(), and using only those new variables.
alternatively, you could invoke the macro as 'SuperFastHash (i, 0 + 10)'. the
expression forcing POV-Ray to pass in a value.
regards, jr.
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"jr" <cre### [at] gmailcom> wrote:
early morning crap..
> ...
> alternatively, you could invoke the macro as 'SuperFastHash (i, 0 + 10)'. the
> expression forcing POV-Ray to pass in a value.
tja, did not look at the code, close enough and/or in time :-(
so the '#for (loop, len, 0, -1)' needs to be re arranged, as per docs: '#for
(Identifier, Start, End [, Step])'.
regards, jr.
Post a reply to this message
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From: Cousin Ricky
Subject: Re: Bitwise operators corrupting SDL parse-time execution of POV-Ray?
Date: 4 Feb 2023 13:29:41
Message: <63dea415$1@news.povray.org>
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On 2023-02-04 04:01 (-4), jr wrote:
>> Cousin Ricky <ric### [at] yahoocom> wrote:
>>
>>> You can fix this by assigning data and len to new local variables within
>>> SuperFastHash(), and using only those new variables.
>
> alternatively, you could invoke the macro as 'SuperFastHash (i, 0 + 10)'. the
> expression forcing POV-Ray to pass in a value.
It would have to be 'SuperFastHash (i + 0, 10)', if you choose to do it
that way. But I prefer not requiring the caller to babysit the macro.
Post a reply to this message
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