Add split_mix_64 utility

compile

@@ -6,12 +6,12 @@ CFLAGS="-g -Wall -Werror "
 XORSHIFT_SRC="xorshift.c ppm.c"
 XORSHIFT_OUT=xorshift
 
-echo "Clean outputs dir..."
+echo "Cleaning outputs dir..."
 rm -rf "./outputs/*"
 
-echo -e "\nBuild xorshift generators..."
+echo -e "\nBuilding xorshift generators..."
 (set -x ; $CC $CFLAGS $XORSHIFT_SRC -o $XORSHIFT_OUT)
 
-echo -e "\nTest xorshift generators..."
+echo -e "\nTesting xorshift generators..."
 ./xorshift
 echo "Generated"

xorshift.c

@@ -5,10 +5,14 @@
 #include <time.h>
 
 typedef struct {
-	u64 x;
-	u64 y;
-	u64 z;
-	u64 w;
+  u64 seed;
+} SplitMix64State;
+
+typedef struct {
+  u64 x;
+  u64 y;
+  u64 z;
+  u64 w;
 } XOR256State;
 
 typedef u64(RandGenFunc)(void *s);
@@ -17,9 +21,12 @@ void test_xorshift_256(Image *img);
 u64 xorshift_256_generate(void *s);
 void test_xoshiro_256ss(Image *img);
 u64 xoshiro_256ss_generate(void *s);
+void test_xoshiro_256p(Image *img);
+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};
@@ -39,10 +46,8 @@ int main(int argc, char *argv[]) {
   };
 
   test_xorshift_256(&img);
-  write_p6_ppm("./outputs/xorshift256.ppm", &img);
-
   test_xoshiro_256ss(&img);
-  write_p6_ppm("./outputs/xoshiro256ss.ppm", &img);
+  test_xoshiro_256p(&img);
 
   return EXIT_SUCCESS;
 }
@@ -50,6 +55,7 @@ int main(int argc, char *argv[]) {
 void test_xorshift_256(Image *img) {
   XOR256State state = init_xor_256_state();
   write_image_data(img, xorshift_256_generate, (void *)&state);
+  write_p6_ppm("./outputs/xorshift256.ppm", img);
 }
 
 u64 xorshift_256_generate(void *s) {
@@ -68,6 +74,7 @@ u64 xorshift_256_generate(void *s) {
 void test_xoshiro_256ss(Image *img) {
   XOR256State state = init_xor_256_state();
   write_image_data(img, xoshiro_256ss_generate, (void *)&state);
+  write_p6_ppm("./outputs/xoshiro256ss.ppm", img);
 }
 
 u64 xoshiro_256ss_generate(void *s) {
@@ -87,15 +94,40 @@ u64 xoshiro_256ss_generate(void *s) {
   return result;
 }
 
+void test_xoshiro_256p(Image *img) {
+  XOR256State state = init_xor_256_state();
+  write_image_data(img, xoshiro_256p_generate, (void *)&state);
+  write_p6_ppm("./outputs/xoshiro256p.ppm", img);
+}
+
+u64 xoshiro_256p_generate(void *s) {
+  XOR256State *state = (XOR256State *)s;
+
+  const u64 result = state->w + state->x;
+  const u64 t = state->z << 17;
+
+  state->y ^= state->w;
+  state->x ^= state->z;
+  state->z ^= state->y;
+  state->w ^= state->x;
+
+  state->y ^= t;
+  state->x = rol64(state->x, 45);
+
+  return result;
+}
+
 u64 rol64(u64 x, u64 bits) { return (x << bits) | (x >> (64 - bits)); }
 
 XOR256State init_xor_256_state() {
-	return (XOR256State){
-      .x = lrand48(),
-      .y = lrand48(),
-      .z = lrand48(),
-      .w = lrand48(),
-	};
+  SplitMix64State sm64 = {.seed = lrand48()};
+
+  return (XOR256State){
+      .x = split_mix_64(&sm64),
+      .y = split_mix_64(&sm64),
+      .z = split_mix_64(&sm64),
+      .w = split_mix_64(&sm64),
+  };
 }
 
 void write_image_data(Image *img, RandGenFunc *gen_rand, void *s) {
@@ -112,3 +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);
+}