Question

I would like to swap a segment of bits between 2 unsigned char_type variables:a and b. How to address it in C language if the segment is not sequential? For example, a is 011*001*10, input segment length:5, start position pa=5, the segment is from bit7~bit5 and bit1~bit0, which is 011 10; b is 10*011*001, position pb=6, the segment is from bit6~bit7 and bit2~bit0, which is 10 001. The 2 segments have the same length. The expect result result should be like that: a1= 11000100, b1= 11011001

I tried to use 2 temp variables x and y to store swap bits in b and a, and then use ^ to finish swap. The method is too complex. Is there any tricky idea?

Answer 1

Spoiler: (I use bigger types than char)

void swapmask(unsigned *a, unsigned *b, unsigned posa, unsigned posb, unsigned len)
{
unsigned mask, fraga, fragb;

#include <limits.h>
if (len >= CHAR_BIT* sizeof *a) return ;
if (posa >= CHAR_BIT* sizeof *a) return ;
if (posb >= CHAR_BIT* sizeof *b) return ;

mask = (1u << len) -1;
fraga = (*a >> posa) & mask;
fragb = (*b >> posb) & mask;

fprintf(stderr, "Mask=%x Fa=%x, Fb=%x\n", mask, fraga, fragb);

*a = (*a & ~(mask << posa)) | (fragb << posa);
*b = (*b & ~(mask << posb)) | (fraga << posb);
}