From 9d9681d6a8b7d022bde993e0e51aed13174ad0c8 Mon Sep 17 00:00:00 2001
From: Nicholas Noll <nbnoll@eml.cc>
Date: Sun, 3 May 2020 20:34:22 -0700
Subject: added simple random number generator: NOT CRYTOGRAPHIC

---
 sys/libn/random.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 sys/libn/random.c

diff --git a/sys/libn/random.c b/sys/libn/random.c
new file mode 100644
index 0000000..1e2c2d5
--- /dev/null
+++ b/sys/libn/random.c
@@ -0,0 +1,90 @@
+#include <u.h>
+
+// ----------------------------------------------------------------------------
+// Internal structure
+
+uint64 
+rol64(uint64 x, int k)
+{
+	return (x << k) | (x >> (64 - k));
+}
+
+typedef struct Rng {
+	uint64 s[4];
+} Rng;
+
+uint64
+xoshiro256ss(Rng *state)
+{
+	uint64 *s     = state->s;
+	uint64 result = rol64(s[1] * 5, 7) * 9;
+	uint64 t      = s[1] << 17;
+
+	s[2] ^= s[0];
+	s[3] ^= s[1];
+	s[1] ^= s[2];
+	s[0] ^= s[3];
+
+	s[2] ^= t;
+	s[3] = rol64(s[3], 45);
+
+	return result;
+}
+
+typedef struct Mix
+{
+	uint64 s;
+} Mix;
+
+uint64 
+splitmix64(struct Mix *state) {
+	uint64 result = state->s;
+
+	state->s = result + 0x9E3779B97f4A7C15;
+	result = (result ^ (result >> 30)) * 0xBF58476D1CE4E5B9;
+	result = (result ^ (result >> 27)) * 0x94D049BB133111EB;
+	return result ^ (result >> 31);
+}
+
+static Rng RNG;
+
+// ----------------------------------------------------------------------------
+// Exported functions
+
+/* Initializes the global RNG */
+
+error
+rng·init(uint64 seed)
+{
+	Mix smstate = {seed};
+
+    for (int i=0; i < 4; i++) 
+        RNG.s[i] = splitmix64(&smstate);
+
+    return 0;
+}
+
+/* Returns a random float64 between 0 and 1 */
+double
+rng·random()
+{
+    uint64 r = xoshiro256ss(&RNG); 
+    return (double)r / UINT64_MAX;
+}
+
+/* Returns true or false on success of trial */
+bool
+rng·bernoulli(double f)
+{
+    return rng·random() < f;
+}
+
+/* Returns a random integer between 0 and max
+ * TODO: Modulo throws off uniformity 
+ */
+uint64
+rng·randi(int max)
+{
+    uint64 r = xoshiro256ss(&RNG); 
+    return r % max;
+}
-- 
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