Swap bits in a number in C [duplicate]

Answer 1

C++-like pseudocode (can be easily rewritten to not use temporary variables):

int firstPart = source & 0xF;
int offsetToHigherPart = sizeof( source ) * CHAR_BIT - 4;
int secondPart = ( source >> offsetToHigherPart ) & 0xF;
int maskToSeparateMiddle = -1 & ( ~0xF ) & ( ~( 0xF << offsetToHigherPart );
int result = ( firstPart << offsetToHigherPart ) | secondPart | (source & maskToSeparateMiddle);

This will require CHAR_BIT to be defined. It is usually in limits.h and is defined as 8 bits but is strictly speaking platform-dependent and can be not defined at all in the headers.