1. 2 years ago 

   Bitwise math, speed increase on item features

   I have learned a few things while programming for an iPod/iPhone. I’ll be applying some of what I learned to legacy of a warlord. The following tweak will definitely help when I allow NPCs to equip and use items.

   While profiling code for Legacy of a Warlord. I noticed that I was querying what features exist on an item, a lot! Not that this is a bad thing yet the way I was doing it could use a refactor.

   Features in items reside in a MAP data structure, I was using a map count to see if the feature existed.

   // old feature check queried map

   bool Item::QueryFeature(FEATURE_TYPES t_FeatureCheck) const
   {
       if (Features.count(t_FeatureCheck)>0){
           return true;
       }else{
           return false;
       }
   }

   I actually didn’t find out how many nanoseconds it took to return a count of the available features, however I am assuming that performing a bit-wise computation on an integer using a mask is magnitudes quicker than counting how many indexes are in a map data structure.

   First I added an integer on the item to store a feature mask.

   int FeatureMask=0;

   // Then there is a enumeration that stores the mask values.
   typedef enum FEATURE_TYPES{
       FEATURE_LOADERROR=  1«0,
       FEATURE_HOLDABLE=   1«1,
       FEATURE_TRADABLE=   1«2,
   …
       FEATURE_USABLE=  1«14
   } FEATURE_TYPES;

   As an item is generated, I set the flag of the features on the item that were specified in the XML file:

   As an example to add display and tradable to the item feature mask.

   FeatureMask |= FEATURE_DISPLAY;
   FeatureMask |= FEATURE_TRADABLE;

   This is the helper function I wrote to return whether a feature exists.

   // new feature check to check feature mask
   bool Item::QueryFeature(FEATURE_TYPES t_FeatureCheck) const {
       if ( (FeatureMask & t_FeatureCheck) != 0 ){
           return true;
       }else{
           return false;
       }
   }

   Now when the render engine needs to determine if the 100 items on the ground are visible it can call QueryFeature(FEATURE_DISPLAY) and know if it has to display the item or hide it. Also, this is called any time an item is created, destroyed, or equipped by any NPC/player. 

   I will bitmasks to other core engine features that need the speed optimized.

   edit: slight correction, the bit masks can easily be set using 1«n instead of 1«(2*n) … oops.

Currently developing Legacy of a Warlord. A graphical tile rogue like game. This blog is about my foray into C++ programming.

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