reverse bit order

[Jaime Guevara]

How do I reverse the bit order for a 16 bit word? For example, if a number is represented as 0011 1011 1010 1110 then it has to be written to a file with the least significant bit first as 0111 0101 1101 1100. Also I'm devleloping in C.

Thanks,
Jaime Guevara

[NMTop40]

Here is one way

unsigned short BitReverse( unsigned short x )
{
unsigned short xReverse = 0;
for ( int cBits = 16; cBits--; )
{
xReverse <<= 1;
xReverse |= (x&1);
x >>= 1;
}
return xReverse;
}




Note in particular your function should use unsigned variables. It assumes that short is at least 16 bits, not exactly that length.


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[Jaime Guevara]

Thanks for the tip.

Since I need to transmit that 16-bit word serially in that order should I write the the data as:

b = (BYTE)(reversedWord >> 8); //Upper Byte
WriteFile(handle, &b, 1, &numBytesWritten, NULL);
b = (BYTE)reversedWord & 0x000000ff; // lower byte
WriteFile(handle, &b, 1, &numBytesWritten, NULL);






Thanks Again,
Jaime Guevara

[NMTop40]

sending it one byte at a time like that would ensure a standard byte order on all platforms, except that of course WriteFile is not a standard command. fwrite() is the ANSI-standard in C.

You could write a reverseByte function although when I had to do this in a project last year I created a table and looked up the reverse on the table.

I did that by writing a one-only reverseByte function which wrote the output to a file in hexadecimal and then used that output in the code. This my program wrote something like

0x00,
0x80,
0x40,
0x60
...

then on any platform you can convert a word as follows:

void ReverseWord[ unsigned char[2] target, unsigned short source )
{
target[0] = ReverseByte[(source >> 8) & 0xff];
target[1] = ReverseByte[ source & 0xff ];
}





This is assuming you are implementing ReverseByte as a const array, not as a function. (If a function, simply use ( ) instead of [ ] )


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