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MARS_999

Bit mask math

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How can I combine this into one statement? 

 

I tried this but no luck

if(id &= ~(NX::COLLISION_ID_IsPickable | NX::COLLISION_ID_IsHighlightable) == 200)

//do something 

 

 

irr::s32 id = selectedNode->getID();
if(id &= ~NX::COLLISION_ID_IsPickable)
if(id &= ~NX::COLLISION_ID_IsHighlightable)
if(id == 200)
{
//do something
}

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if((id &= ~(NX::COLLISION_ID_IsPickable | NX::COLLISION_ID_IsHighlightable)) == 200)

 

sigh fat fingered my ()

 

now it works

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Surely this is more readable?!:

id &= ~(NX::COLLISION_ID_IsPickable | NX::COLLISION_ID_IsHighlightable);
if (id == 200)
Edited by iMalc

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To elaborate on Sean's, I have to admit I have bought myself quite some peace of mind since I've restricted bitmask usage to serialization only. Just use structs.

auto props = selectedNode->getFlags();
if(props.canPick && props.canHilight) {
//do something
}

The profiler will tell you if code is hot or cold. Cold code has no point in not having maintainance and readability as priorities. Even if the code is hot, I would be careful in considering this an optimization.

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C++ wizardry like that drives me nuts, and is a clear sign of an inexperienced programmer. SeanMiddleditch is totally right.
Clarity always wins over a couple of cycles worth of optimization, especially in a team environment where you aren't the only one dealing with the code. Basically, if you're trying to reduce line counts for arbitrary reasons, then you're doing it wrong. Bit masking and bit shifting is almost always a sign of over-complication.
 

Edited by Scarabus2

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I've programmed C++ for nearly 10 years and I have no idea what "id &= ~NX::COLLISION_ID_IsPickable" even does. (I do, but I'd have to look it up to be sure.)

 

Well, you must not have done a whole lot of bit manipulations in your 10 years of C++. In any case, a helper function can add clarity to the code, so I would do something like this:

 

void clear_bits(irr::s32 &x, irr::s32 bits_to_erase) {
  x &= ~bits_to_erase;
}
 
//...
clear_bits(id, NX::COLLISION_ID_IsPickable);

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Bit fields might also help in this situation, since you can retain the packed nature of having bit-based data but instead accessing the data as C++ struct members for clarity.

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Bit fields might also help in this situation, since you can retain the packed nature of having bit-based data but instead accessing the data as C++ struct members for clarity.

 

One annoying detail to keep in mind: the order in which bits are assigned is not defined. Not an issue if everything is happening on one machine and within the same application.

 

Still, not knowing if setting a particular field will effectively result in 0x01 or in 0x80 is always making me nervous.

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Álvaro nice function. Like that...

 

The 200 isn't a permanent thing, it was more of a hurry up and post it!

 

I am using irrlicht and the id is a unsigned int and they are bit masking with the pipe | to do collision detection. So I am looking for ways to set and clear and change them without much headache...

 

Thanks!

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