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Hwo to read bit and set same bit in different byte

Misery · 2012-10-18T15:21:28

Hello, I am using integers as bit containers representing bool values. How do I make a fast instruction that reads some bit from one integer and sets its value to another. Lets say: there are two 64 bit integers. One of them x has some bits set. I want to rewrite every second bit from x to y (from beginning). How do I make that avoiding code similar to that: y=0; //clear y t=0; for (int i=0;i<64;i+=2) if (x & (CONTAINER_INT(1) << (i%ContainerSize))) { y |= CONTAINER_INT(1) << (t%ContainerSize); t++; } I mean I want to avoid if instruction. Regards.

General and Gameplay Programming Programming

Started by Misery October 16, 2012 11:52 AM

21 comments, last by Misery 11 years, 6 months ago

patrrr

1,323

October 17, 2012 11:12 AM

#define COPY_BIT(x, i, y, j) (y) = ((y) & ~(1 << (j))) | ((((x) >> (i)) & 1) << (j))

Something like that...? Can probably be optimized better.

alvaro

21,604

October 17, 2012 12:20 PM

What patrrr wrote seems correct. I would do it with a function, though:

unsigned copy_bit(unsigned x, int i, unsigned y, int j) {

  y &= ~(1u << j); // Clear y's j-th bit

  y |= ((x >> i) & 1u) << j; // Copy x's i-th bit into position j

  return y;

}

The fact that you need to use some funky name for the type and that you are going for lots of generality is only confusing matters, so start without that.