"Hardening" the bad prand

prand.c

@@ -2,14 +2,13 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <time.h>
+#include <unistd.h>
 
-#define BUILD_SEED                                                                                                     \
+#define PRAND_BUILD_SEED                                                                                               \
     ((uint64_t)(__TIME__[0]) * (uint64_t)(__TIME__[1]) * (uint64_t)(__TIME__[3]) * (uint64_t)(__TIME__[4]) *           \
      (uint64_t)(__TIME__[6]) * (uint64_t)(__TIME__[7]))
 
-#define PRNG_SAVE_INTERVAL 50 // Save every 1000 calls to prand()
+static uint64_t seed = PRAND_BUILD_SEED * 6364136223846793005ULL + 1;
-
-static uint64_t seed = BUILD_SEED;
 
 uint64_t prand() {
     seed = seed * 6364136223846793005ULL + 1;
@@ -17,31 +16,44 @@ uint64_t prand() {
 }
 
 inline uint64_t prand_range(uint64_t min, uint64_t max) {
-    return min + (prand() % (max - min + 1));
+    uint64_t range = max - min + 1;
+    uint64_t x;
+    uint64_t limit = UINT64_MAX - (UINT64_MAX % range);
+
+    do {
+        x = prand();
+    } while (x > limit);
+
+    return min + (x % range);
 }
 
 void sprand(uint64_t s) {
     if (s) {
-        seed = s;
+        seed ^= (s * 0x9e3779b97f4a7c15ULL) + (seed << 6) + (seed >> 2);
     } else {
-        rand_reseed();
+        seed ^= PRAND_BUILD_SEED;
     }
 }
 
-void rand_reseed() {
+#undef PRAND_BUILD_SEED
-    seed = BUILD_SEED;
-}
 
 #define PRAND_MAIN
 #ifdef PRAND_MAIN
 int main() {
     // time(NULL) provides a somewhat unique seed at runtime
     sprand(time(NULL));
+    sprand(getpid());
+    sprand(0);
 
     uint64_t rn1 = prand();
     uint64_t rn2 = prand();
 
     printf("rn1 = %lu\n", rn1);
     printf("rn2 = %lu\n", rn2);
+
+    for (int i = 0; i < 40; i++) {
+        uint64_t random = prand_range(0, 1000);
+        printf("random = %lu\n", random);
+    }
 }
 #endif