1 files changed, 44 insertions, 0 deletions
diff --git a/xorshift.c b/xorshift.c
new file mode 100644
index 0000000..b0c6e0a
--- /dev/null
+++ b/xorshift.c
@@ -0,0 +1,44 @@
+/*	“An experimental exploration of Marsaglia’s xorshift generators,
+ *	scrambled”, Sebastiano Vigna */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <assert.h>
+#include <unistd.h>
+
+#include "xorshift.h"
+
+uint64_t xorshift_step (randctx * const st) {
+	uint64_t s0 = st->s[ st->p ];
+	uint64_t s1 = st->s[ st->p = ( st->p + 1 ) & (XORSHIFT_S-1) ];
+	s1 ^= s1 << 31; // a
+	s1 ^= s1 >> 11; // b
+	s0 ^= s0 >> 30; // c
+	return ( st->s[ st->p ] = s0 ^ s1 ) * 1181783497276652981LL;
+}
+
+static uint64_t rand64 () {
+	uint64_t rand;
+	while (!__builtin_ia32_rdrand64_step (&rand));
+	return rand;
+}
+
+void xorshift_seed (randctx * const st) {
+	/* seed with high-quality randomness */
+	for (unsigned char i = 0; i < XORSHIFT_S; i++) {
+		st->s[i] = rand64 ();
+	}
+}
+
+#if 0
+uint64_t xorshift_step (randctx * const st) {
+	uint64_t x = st->s[0];
+	x ^= x >> 12; // a
+	x ^= x << 25; // b
+	x ^= x >> 27; // c
+	st->s[0] = x;
+	return x * 2685821657736338717LL;
+}
+#endif
+