C++ UUID generator

UUID.hpp

#ifndef UUID_H
#define UUID_H

#include <string>
#include <cstdlib>

//*Adapted from https://gist.github.com/ne-sachirou/882192
//*std::rand() can be replaced with other algorithms as Xorshift for better perfomance
//*Random seed must be initialized by user

namespace MathUtils{
const std::string CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

std::string generateUUID(){
  std::string uuid = std::string(36,' ');
  int rnd = 0;
  int r = 0;
  
  uuid[8] = '-';
  uuid[13] = '-';
  uuid[18] = '-';
  uuid[23] = '-';

  uuid[14] = '4';

  for(int i=0;i<36;i++){
    if (i != 8 && i != 13 && i != 18 && i != 14 && i != 23) {
      if (rnd <= 0x02) {
          rnd = 0x2000000 + (std::rand() * 0x1000000) | 0;
      }
      rnd >>= 4;
      uuid[i] = CHARS[(i == 19) ? ((rnd & 0xf) & 0x3) | 0x8 : rnd & 0xf];
    }
  }
  return uuid;
}
#endif

I generated 1 lakh UUIds using this methodology. at 6k it lost its uniqueness and started producing duplicates