Talk:Technical Reference:State Definition

From BCI2000 Wiki
Jump to: navigation, search

State Vector

The section on state vectors is totally wrong. After reverse-engineering some data files with the help of the BCI2000Viewer, it appears that the state vectors are actually encoded in reverse-big-endian. In other words, if smaller numbers represent lower order bits, then the state vector layout mentioned in this section would actually be encoded as follows:

State Vector Byte 1 State Vector Byte 2 State Vector Byte 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Running SourceTime unused
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15  

Which on the physical machine actually maps to:

State Vector Byte 1 State Vector Byte 2 State Vector Byte 3
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
SourceTime Running SourceTime unused SourceTime
6 5 4 3 2 1 0 0 14 13 12 11 10 9 8 7   15

Fixed section on state vector encoding

Thanks for pointing out the error. State vector encoding is actually little-endian (LSB first) rather than big-endian, and its layout on a little-endian machine is straightforward as a concatenation of little-endian encoded state values. I fixed the description accordingly.

Pseudocode reference

Do you think it would be helpful to readers to put (in pseudocode) a way to read the State Definition?

int ExtractStateValue(unsigned char[] vector, int byteoffset, int bitoffset, int numberofbits){

    int numberofbytes = numberofbits / 8 + 1;
    char bytes[numbytes] = vector[ byteoffset : byteoffset + numbytes ];

    int ret = bytes[0] >> bitoffset;
    for ( int i=1; i<numbytes; i++ ){
        ret += bytes[i] << ( 8 * i - bitoffset );
    }

    return ret % ( 1 << numbits );
}

This is not compilable C or C++ code, as far as I know. However, I think it might make things clearer.

Cstocks 21:15, 4 January 2010 (UTC)

Actual C++ code

The actual C++ code is here: http://www.bci2000.org/tracproj/browser/trunk/src/shared/types/StateVectorSample.cpp#L69 (log into trac if you get a message about browser view privileges)

Mellinger 15:00, 5 January 2010 (UTC)

Pseudocode

Thanks for pointing that out, but to someone (like me) who is not terribly familiar with C++ code, seeing the actual code is not very helpful. For example, I would have to start hunting through the code to find out what State::ValueType, mByteLength, mpData, et cetera mean.

Also, I am proud to note that my algorithm is at least 8 times more efficient than the C++ function you pointed me to.