2

I'm skimming through a very popular game's source code at the moment and I came across these variables inside of this class.

public static final BlockPos ORIGIN = new BlockPos(0, 0, 0);
private static final int NUM_X_BITS = 1 + MathHelper.calculateLogBaseTwo(MathHelper.roundUpToPowerOfTwo(30000000));
private static final int NUM_Z_BITS = NUM_X_BITS;
private static final int NUM_Y_BITS = 64 - NUM_X_BITS - NUM_Z_BITS;
private static final int Y_SHIFT = 0 + NUM_Z_BITS;
private static final int X_SHIFT = Y_SHIFT + NUM_Y_BITS;
private static final long X_MASK = (1L << NUM_X_BITS) - 1L;
private static final long Y_MASK = (1L << NUM_Y_BITS) - 1L;
private static final long Z_MASK = (1L << NUM_Z_BITS) - 1L;

What is the point of initializing these this way instead of just calculating it once and initializing with the answer?

Improve this question
1
  • 2
    Because these are static fields, the expressions are indeed "calculated once", since all of the expressions are calculated once and only once, when the class is loaded. They aren't like C macros, where the part to the right of the = is just substituted in the source code.
    – Hank D
    Commented Apr 30, 2016 at 4:33

1 Answer 1

Reset to default
5

The point in initializing them this way is that it makes it clear to the reader where the values come from.

I can look at the code and see that it is creating bitmasks for 64 bit values with parts X,Y,Z where the X and Z values must hold at least 30,000,000 and that is using the remaining space for the Y values.

Imagine the code looked like this:

private static final long X_MASK = 335544321
private static final long Y_MASK = 16383
private static final long Z_MASK = 335544321

Would it be as clear what the values meant? Have you memorized what '2^25' is, and even if you have, do you know why '2^25' was chosen?

Improve this answer
4
  • Oh okay I didn't even think about it that way. I'd be more inclined to just add a detailed comment explaining the relationship if it were so important. But if you're concerned with space or something I could see someone just defining it this way and letting the end user figure it out.
    – GregoryNeal
    Commented Apr 30, 2016 at 6:35
  • 1
    The comments would tend to look like this code. It's always preferable to make your code explain itself.
    – Karl Bielefeldt
    Commented Apr 30, 2016 at 16:44
  • @KarlBielefeldt Oh, no, not at all. In fact, private static final long X_MASK = 0x14000001 // Bitmask for the X part is a whole lot better than either of those forms.
    – Ross Patterson
    Commented Apr 30, 2016 at 23:44
  • Not really. What's the max value the author expected X to be?
    – user53141
    Commented May 1, 2016 at 1:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.