a

libc/lrand.c

@@ -0,0 +1,83 @@
+#include	<u.h>
+#include	<libc.h>
+
+/*
+ *	algorithm by
+ *	D. P. Mitchell & J. A. Reeds
+ */
+
+#define	LEN	607
+#define	TAP	273
+#define	MASK	0x7fffffffL
+#define	A	48271
+#define	M	2147483647
+#define	Q	44488
+#define	R	3399
+#define	NORM	(1.0/(1.0+MASK))
+
+static	ulong	rng_vec[LEN];
+static	ulong*	rng_tap = rng_vec;
+static	ulong*	rng_feed = 0;
+static	Lock	lk;
+
+static void
+isrand(long seed)
+{
+	long lo, hi, x;
+	int i;
+
+	rng_tap = rng_vec;
+	rng_feed = rng_vec+LEN-TAP;
+	seed = seed%M;
+	if(seed < 0)
+		seed += M;
+	if(seed == 0)
+		seed = 89482311;
+	x = seed;
+	/*
+	 *	Initialize by x[n+1] = 48271 * x[n] mod (2**31 - 1)
+	 */
+	for(i = -20; i < LEN; i++) {
+		hi = x / Q;
+		lo = x % Q;
+		x = A*lo - R*hi;
+		if(x < 0)
+			x += M;
+		if(i >= 0)
+			rng_vec[i] = x;
+	}
+}
+
+void
+srand(long seed)
+{
+	lock(&lk);
+	isrand(seed);
+	unlock(&lk);
+}
+
+long
+lrand(void)
+{
+	ulong x;
+
+	lock(&lk);
+
+	rng_tap--;
+	if(rng_tap < rng_vec) {
+		if(rng_feed == 0) {
+			isrand(1);
+			rng_tap--;
+		}
+		rng_tap += LEN;
+	}
+	rng_feed--;
+	if(rng_feed < rng_vec)
+		rng_feed += LEN;
+	x = (*rng_feed + *rng_tap) & MASK;
+	*rng_feed = x;
+
+	unlock(&lk);
+
+	return x;
+}