Subject:
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Re: Rotating through a four-bit nibble
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 20 Sep 2005 17:45:52 GMT
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Original-From:
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T. Alexander Popiel <POPIEL@WOLFSKEEPsaynotospam.COM>
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Viewed:
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1233 times
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In message: <In4Hp7.qGD@lugnet.com>
"Brian Davis" <brdavis@iusb.edu> writes:
> I'm working in NQC, and I need to rotate the bit series in a 4-bit
> (or 8-bit) nibble embedded in a 16-bit word. Annoying. Any suggestions?
What are you optimizing for? Code clarity? Code speed? Code length?
There are (to my knowledge) no truly pretty solutions to this.
I would tend to go with:
int rotbits(int input, int numbits, int offset, int rotdist) {
int mask = (~((~0) << numbits)) << offset;
int left = rotdist % numbits;
int right = (numbits - left) % numbits;
int temp = input & mask;
return (input & ~mask) | (((temp << left) | (temp >> right)) & mask);
}
This generic solution is (of course) slower (and a bit harder to read)
than a solution specifically tuned for a particular size, offset, and
rotation distance... but it's 7 lines of code that can then be called
wherever you need to do these sub-word manipulations. Depending on
how good the NQC optimizer has gotten, it might even simplify down to
the faster versions when given constants for the size and offset...
- Alex
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Message is in Reply To:
 | | Rotating through a four-bit nibble
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| No, it's not pornographic (look over in rtlToronto - I hear it's a fun thread). But I'm working in NQC, and I need to rotate the bit series in a 4-bit (or 8-bit) nibble embedded in a 16-bit word. Annoying. Any suggestions? Masking the desired four (...) (19 years ago, 20-Sep-05, to lugnet.robotics)
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