how to round a number to the nearest multiple of 64

Question

I am trying to make a program that creates a square on top of a tile in a tile map. The tiles are 64 by 64. I was thinking that I need to get the coordinates of the mouse, round it to the nearest multiple of 64, and divide that by 64 to get the tile coordinate. Is there a better way? Should I make all of the tiles buttons? Thanks!

Answer 1

You can do this as follows:

int number = 445226;
int roundoff = (number+0x20)&(~0x3f);

It works as follows:

1. You first add 32 (0x20) to the number so that it is rounded up/down. This means everything below 32 will result in a value less than 64 and everything larger than 32 to a value larger than 64.
2. You mask out the last six bits.

In general bitwise operations are faster than division and multiplication. And since you ask for a power of two, it's a nice performance hack you can use.

But if you are interested in tile coordinate, you don't have to multiply it again by 64 I guess. In that case you can use:

int number = 445226;
int div64 = (number+0x20)>>0x06; //this is divided by 64, not rounded

About the user interface, as always mutliple answers are correct, but if you're going to paint some kind of map on it, I guess coordinates are better than buttons. Especially if later in the process, people will be able to click on items on the map that lie between two buttons.