[ion] Simulator/Emscripten/Blackbox: correct random implementation

ion/src/shared/random.cpp

@@ -3,10 +3,9 @@
 
 uint32_t Ion::random() {
   /* rand() returns a pseudo-random integral number in the range between 0 and
-   * RAND_MAX which is garantueed to be at least 32767.*/
-  uint32_t result = 0;
-  for (int i = 0; i < 131077; i++) {
-    result += rand();
-  }
-  return result;
+   * RAND_MAX. We assert that RAND_MAX is at least 8 bit, this way we can merge
+   * four rand() calls to create a single 32 bit value. Merging is done using
+   * XOR, whose output is as likely to be a 1 as a 0. */
+  static_assert(RAND_MAX >= (1<<8), "RAND_MAX is too small");
+  return (rand() << 24) ^ (rand() << 16) ^ (rand() << 8) ^ rand();
 }