Tiff: algorithm to decode PackBits - image-processing

Question

While working on a routine to open TIFF files generated by a microscope software in Matlab I got stuck on reading compressed images. At this point it's a matter of honour for me to understand how to decode tiff data in the PackBits format.
With little experience in real computer science, I have troubles understanding the guidelines in the TIFF documentation, more specifically:
In the inverse routine, it is best to encode a 2-byte repeat run as a
replicate run except when preceded and followed by a literal run. In
that case, it is best to merge the three runs into one literal run.
Always encode 3-byte repeats as replicate runs. That is the essence of
the algorithm. Here are some additional rules:
• Pack each row
separately. Do not compress across row boundaries.
• The number of uncompressed bytes per row is defined to be (ImageWidth + 7) / 8. If the uncompressed bitmap is required to have an even number of bytes per row, decompress into word-aligned buffers.
• If a run is larger than 128 bytes, encode the remainder of the run as one or more additional replicate runs.
source: https://www.fileformat.info/format/tiff/corion-packbits.htm
I understand how to implement the pseudocode, and decode a sample string compressed with PackBits in Matlab. However, I'm lost during parsing a chunk of 16 bit, greyscale Tiff file. My question is how do I go about it? I don't really understand what it means in replicate run, neither what is a word-aligned buffer.
When I start decoding the data form form the first byte, I just get nonsense.
Help with understanding the logic of decompression will be appreciated, also a link to code decompressing the Tiff PackBits will be helpful.
~Jakub
Edit: I got the decompression algorithm to work, my error was to interpret the bytes wrongly, here is a code, if anyone will be interested in a similar problem in the future.
Tiff_file = 'compressed.tiff';
%open and read tiff file file
imInfo = imfinfo(Tiff_file);
fId = fopen(Tiff_file);
im = fread(fId);
fclose(fId);
%parse the file
output = zeros(1,imInfo.Width * imInfo.Height * 2);%preallocate
thisLoc = 1;
for strip = 1:length(imInfo.StripOffsets)
thisLength = imInfo.StripByteCounts(strip);
thisOffset = imInfo.StripOffsets(strip);
thisStrip = im(thisOffset + 1 : thisOffset + thisLength);
pntr = 1; %start at the first byte
%loop throught the coded data
while pntr < thisLength
key = thisStrip(pntr);
if key >= 129
key = 257 - key;
datTmp = repmat(thisStrip(pntr+1), [1 key]);
output(thisLoc:thisLoc+key-1) = datTmp;
thisLoc = thisLoc+key;
pntr = pntr + 2;
elseif key == 128 %nope
pntr = pntr + 1;
else
datTmp = thisStrip(pntr + 1 : pntr + 1 + key);
output(thisLoc:thisLoc+key) = datTmp;
thisLoc = thisLoc + key+1;
pntr = pntr + key + 2;
end
end
end
im = typecast(uint8(output),'uint16');
%reshape decoded data.
im = reshape(im,[imInfo.Width imInfo.Height])';