Improve hash function for names.

PiperOrigin-RevId: 619173787
Change-Id: I7889ea71ebe7bdbe682042842c521d6631521a35

src/engine/engine_support.c

@@ -1050,11 +1050,11 @@ static int _getnumadr(const mjModel* m, mjtObj type, int** padr, int* mapadr) {
 }
 
 // get string hash, see http://www.cse.yorku.ca/~oz/hash.html
-uint64_t mj_hashdjb2(const char* s, uint64_t n) {
+uint64_t mj_hashString(const char* s, uint64_t n) {
   uint64_t h = 5381;
   int c;
   while ((c = *s++)) {
-    h  = ((h << 5) + h) + c;
+    h = ((h << 5) + h) ^ c;
   }
   return h % n;
 }
@@ -1070,7 +1070,7 @@ int mj_name2id(const mjModel* m, int type, const char* name) {
 
   // search
   if (num) {    // look up at hash address
-    uint64_t hash = mj_hashdjb2(name, num);
+    uint64_t hash = mj_hashString(name, num);
     uint64_t i = hash;
 
     do {

src/engine/engine_support.h

@@ -114,7 +114,7 @@ MJAPI void mj_angmomMat(const mjModel* m, mjData* d, mjtNum* mat, int body);
 //-------------------------- name functions --------------------------------------------------------
 
 // get string hash, see http://www.cse.yorku.ca/~oz/hash.html
-uint64_t mj_hashdjb2(const char* s, uint64_t n);
+uint64_t mj_hashString(const char* s, uint64_t n);
 
 // get id of object with the specified mjtObj type and name, returns -1 if id not found
 MJAPI int mj_name2id(const mjModel* m, int type, const char* name);

src/user/user_model.cc

@@ -1492,7 +1492,7 @@ static int namelist(vector<T*>& list, int adr, int* name_adr, char* names, int*
       continue;
     }
 
-    uint64_t j = mj_hashdjb2(list[i]->name.c_str(), map_size);
+    uint64_t j = mj_hashString(list[i]->name.c_str(), map_size);
 
     // find first empty slot using linear probing
     for (; map[j]!=-1; j=(j+1) % map_size) {}