Problem with bits in byte array....

[HenrikOlsson]

Hi Don,
Thanks for that. I've tried to figure your code out without much luck, that visual representation helps me understand how you intend it to work. First I got thrown off by the arrays not being indexed from 0 and I thoght that was by intention (some trick) to make it work but now I'm not sure and I have to ask if you tried it?

If I load arr1 with 77,97,110 I expect to get 19,22,5,46 (taken from Wikipedias example) but instead I get 19,0,4,0.

The reason, I think, is that you're trying to load two bytes from the array into one word in one go with w1=arr1[1] etc.

/Henrik.

EDIT: It's not near as small as yours but this works for me:

    Out[0] = ((In[0] >> 2) & 63)
    Out[1] = ((In[0] & 3) << 4) + ((In[1] & 240) >> 4) & 63
    Out[2] = ((In[1] & 15) << 2) + ((In[2] & 192) >> 6) & 63
    Out[3] = (In[2] & 63)

[amgen]

Henrik,
Yes, I mistakingly was thinking that some 'W=' takes the two consecutive bytes. Also array[0] and [1] confuses me at times with for-next etc. so I just consider array start at [1].
What you coded is good. The top and bottom (out[0] and out[3]) are simple but the ones where the 6 bit straddle 2 bytes take a little more. I tried to follow your calc on out[2] but my brain started smoking. Just wondering if '+' changes the value as opposed to the '>>' and '<<' and &. But, if it works, never argue with success.
Anyway, thats all a relatively small simple code.
Don

[HenrikOlsson]

Let's take the Wikipedia example where they show Man being encoded:

  In[0]    In[1]   In[2]
01001101.01100001.01101110

The bytes we are interested in, in the case of Out[2] are marked in red. We can break the line down in steps, making it easier to see what's going on.

Out[2] = In[1] & 15          ' Isolate the 4 lower bits (Out[2] now equals %00000001)
Out[2] = In[1] = In[1] << 2  ' Shift "up" two steps (Out[2] now equals %00000100)
Temp = (In[2] & 192)         ' Isolate the 2 top bits in In[2] (Temp now equals %01000000)
Temp = In[2] >> 6            ' Shift it down 6 steps (Temp now equals %00000001)
Out[2] = Out[2] + Temp       ' Add the two together (Out[2] now equals %00000101)
Out[2] = Out[2] & 63         ' Isolate the 6 lower bits

In fact, I wonder if the last step is actually needed....depends on what gets "shifted in" at the top but I guess that "should" be all zeros - perhaps room for improvement there....

/Henrik.