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quickest way to convert hex to a 16 bit signed integer

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looking for the quickest way to convert 16 bit hex into a singed 16 bit int
performance is key here.
I was doing it with a type cast before but appears very slow
Anyone have any ideas?
  1 Comment
James Tursa
James Tursa on 29 Jul 2021
Please provide details of exact input and desired output. Is the hex in a char array? What size? What is the exact code you have tried so far?

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Accepted Answer

Walter Roberson
Walter Roberson on 29 Jul 2021
Edited: Walter Roberson on 31 Jul 2021
format long g
N = 100000;
HC = ['0':'9', 'A':'F'];
data = HC(randi(16, N, 4));
timeit(@() typecast(uint16(hex2dec(data)),'int16'), 0)
ans =
0.078666781
timeit(@() typecast(uint16(sscanf(data.', '%4x')),'int16'), 0)
ans =
0.010088781
timeit(@() typecast(cell2mat(textscan(strjoin(cellstr(data),'\n'),'%xu16')),'int16'), 0)
ans =
0.114500781
timeit(@() cell2mat(textscan(strjoin(cellstr(data),'\n'),'%xs16')), 0)
ans =
0.114293781
timeit(@() via_ismember_typecast(data, HC), 0)
ans =
0.003229781
timeit(@() via_ismember_no_typecast(data, HC), 0)
ans =
0.004451781
timeit(@() via_math_typecast(data, HC), 0)
ans =
0.005474781
timeit(@() via_discretize_typecast(data, HC), 0)
ans =
0.005370781
timeit(@() via_lookup_typecast(data,HC), 0)
ans =
0.001278781
function num = via_ismember_no_typecast(data, HC)
[~, dec] = ismember(data, HC);
num = dec(:,1)*4096 + dec(:,2)*256 + dec(:,3) * 16 + dec(:,4);
mask = num > 32767;
num(mask) = num(mask) - 65536;
num = int16(num);
end
function num = via_ismember_typecast(data, HC)
[~, dec] = ismember(data, HC);
dec = dec-1; %bin numbers start with 1
num = typecast(uint16(dec(:,1)*4096 + dec(:,2)*256 + dec(:,3) * 16 + dec(:,4)),'int16');
end
function num = via_math_typecast(data, HC)
dec = data - '0';
mask = dec>9;
dec(mask) = dec(mask) - 7;
num = typecast(uint16(dec(:,1)*4096 + dec(:,2)*256 + dec(:,3) * 16 + dec(:,4)),'int16');
end
function num = via_discretize_typecast(data, HC)
dec = discretize(double(data), double(HC)) - 1; %bin numbers start with 1
num = typecast(uint16(dec(:,1)*4096 + dec(:,2)*256 + dec(:,3) * 16 + dec(:,4)),'int16');
end
function num = via_lookup_typecast(data, HC)
lookup(HC) = 0:15;
dec = lookup(data);
num = typecast(uint16(dec(:,1)*4096 + dec(:,2)*256 + dec(:,3) * 16 + dec(:,4)),'int16');
end
So via_lookup_typecast is the fastest of these, and via_ismember_typecast is second fastest out of all of these possibilities.
If you are doing a lot of these conversions, then the lookup table can be precomputed -- and it is easy to extend the lookup table to handle lowercase as well as upper case.
  6 Comments
Walter Roberson
Walter Roberson on 6 Aug 2021
To clarify the chars represent hex, so even though two chars takes 4 bytes of storage, two char is encoding one byte.

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