the absolute bleed edge fastest way to strip out bits from 4 bytes of hex

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I am looking for the absolute top dog in speed to do the following
If i have an 8 byte char for example
test = {a a a a a a a a }
I need to be able to break it down into two pieces
x bits to y bits
m bits to n bits
So far i have done a speed test on an old process like this
answer = dec2bin(hex2dec(strcat(word_1,word_2)));
answer = answer(1:22);
answer = bin2dec(answer);
Then i ran a speed test on a newer function
Then using the bi2de function that i beleive is newer
Im seeing a 10X slow down in the new function
Can anyone confirm?
  1 Comment
DGM on 1 Aug 2021
Assuming these are character vectors, you can get a chunk of speed by just using
instead of strcat().
Given that dec2bin() and hex2dec() seem to be doing the math in m-code, I imagine you can roll a pedestrian hex2bin() function that's faster than the two together, but maybe not by much.

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Accepted Answer

DGM on 1 Aug 2021
Edited: DGM on 2 Aug 2021
There's this.
word_1 = 'DEAD';
word_2 = 'BEEF';
answer = hex2bin([word_1,word_2]);
answer = bin2dec(answer(1:22));
Using the simplified concatenation and the combined hex2bin conversion reduces execution time by about 70% for me. There are probably faster ways yet, but eh. I'll leave that to others ... probably.
Attached is a modified version of this FEX submission:
I thought it'd be quick enough to just grab it, but I ended up changing it to output character vectors and be case-insensitive. It's not really any faster.
If you use the attached version of bin2dec(), you can squeeze a bit more out. About the fastest I've managed is an 80% reduction in time. EDIT: updated to fix a dumb error! I guess I oversimplified a bit too much and didn't catch it.
DGM on 28 Feb 2023
I've been finding lately how many of the things I've learned have since left my mind. I spent much time using LUTs in C to avoid arithmetic on feeble microcontrollers, but I've since just forgotten that something with a small domain like this would be a prime use-case.

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More Answers (1)

Walter Roberson
Walter Roberson on 1 Aug 2021
The absolute top dog in speed is to have (y-x+n-m+2) different 8 dimensional lookup tables, each of which is (102 x 102 x 102 x 102 x 102 x 102 x 102 x 102) entries and each of which returns one bit. If you can promise upper case instead of lower case then the dimensions can be 70 instead of 102.
The memory requirements can be drastically reduced by very simple calculations, but that will not give you absolute top dog performance.
This approach requires more memory than any publicly known x64 implementation can use (the public x64 design only defines 48 address lines), but so what? It is faster than the alternatives.
When you ask for absolute top dog, you are asking for all of the safety measures to be taken out, including safety precautions against it requiring more hardware than you can possibly afford.
  1 Comment
Robert Scott
Robert Scott on 1 Aug 2021
really intertesting results Walter
I just got rid of all my hextobinaryvector calls and just went back to the code that is actually 5 lines longer and got a 10X speedup
Apparently, hextobinaryvector has to much overhead
answer = dec2bin(hex2dec(strcat(word_1,word_2)));
answer = answer(1:22);
answer = bin2dec(answer);
This worked much much faster

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