initial implementation of xorshift128 and xorshift128plus

xorshift.c

@@ -0,0 +1,54 @@
+#include "xorshift.h"
+
+#include <assert.h>
+#include <string.h>
+
+static uint32_t xorshift128_seed[4]     = {1, 1, 1, 1};
+static uint64_t xorshift128plus_seed[2] = {1, 1};
+
+void set_xorshift128_seed(uint32_t seed[4])
+{
+    for (size_t i = 0; i < 4; ++i) {
+
+        assert(seed[i] != (uint32_t)0);
+        xorshift128_seed[i] = seed[i];
+    }
+}
+
+uint32_t xorshift128()
+{
+    uint32_t t = xorshift128_seed[0];
+
+    t ^= t << 11;
+    t ^= t >> 8;
+
+    xorshift128_seed[0] = xorshift128_seed[1];
+    xorshift128_seed[1] = xorshift128_seed[2];
+    xorshift128_seed[2] = xorshift128_seed[3];
+
+    xorshift128_seed[3] ^= xorshift128_seed[3] >> 19;
+    xorshift128_seed[3] ^= t;
+    return xorshift128_seed[3];
+}
+
+
+void set_xorshift128plus_seed(uint64_t seed[2])
+{
+    for (size_t i = 0; i < 2; ++i) {
+
+        assert(seed[i] != (uint64_t)0);
+        xorshift128plus_seed[i] = seed[i];
+    }
+}
+
+uint64_t xorshift128plus()
+{
+    uint64_t x = xorshift128plus_seed[0];
+    uint64_t const y = xorshift128plus_seed[1];
+
+    xorshift128plus_seed[0] = y;
+    x ^= x << 23;
+    xorshift128plus_seed[1] = x ^ y ^ (x >> 17) ^ (y >> 26);
+
+    return xorshift128plus_seed[1] + y;
+}