Add split_mix_64 utility

xorshift.c

@@ -4,6 +4,10 @@
 #include <stdlib.h>
 #include <time.h>
 
+typedef struct {
+  u64 seed;
+} SplitMix64State;
+
 typedef struct {
   u64 x;
   u64 y;
@@ -22,6 +26,7 @@ u64 xoshiro_256p_generate(void *s);
 u64 rol64(u64 x, u64 bits);
 XOR256State init_xor_256_state();
 void write_image_data(Image *img, RandGenFunc *gen_rand, void *s);
+u64 split_mix_64(SplitMix64State *state);
 
 int main(int argc, char *argv[]) {
   struct timespec ts = {0};
@@ -115,11 +120,13 @@ u64 xoshiro_256p_generate(void *s) {
 u64 rol64(u64 x, u64 bits) { return (x << bits) | (x >> (64 - bits)); }
 
 XOR256State init_xor_256_state() {
+  SplitMix64State sm64 = {.seed = lrand48()};
+
   return (XOR256State){
-      .x = lrand48(),
-      .y = lrand48(),
-      .z = lrand48(),
-      .w = lrand48(),
+      .x = split_mix_64(&sm64),
+      .y = split_mix_64(&sm64),
+      .z = split_mix_64(&sm64),
+      .w = split_mix_64(&sm64),
   };
 }
 
@@ -137,3 +144,13 @@ void write_image_data(Image *img, RandGenFunc *gen_rand, void *s) {
     }
   }
 }
+
+u64 split_mix_64(SplitMix64State *state) {
+  state->seed += 0x9E3779B97f4A7C15;
+
+  u64 result = state->seed;
+  result = (result ^ (result >> 30)) * 0xBF58476D1CE4E5B9;
+  result = (result ^ (result >> 27)) * 0x94D049BB133111EB;
+
+  return result ^ (result >> 31);
+}