From c7ce1b20d2f37f97f9cfcd57cb4fb4c7cd66a3a0 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Thu, 23 Jun 2011 18:02:23 -0400
Subject: initial commit of prng implementation by Szabolcs Nagy

---
 src/prng/random.c | 111 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 107 insertions(+), 4 deletions(-)

(limited to 'src/prng/random.c')

diff --git a/src/prng/random.c b/src/prng/random.c
index e6b7fd1f..4b2daef2 100644
--- a/src/prng/random.c
+++ b/src/prng/random.c
@@ -1,8 +1,111 @@
+/*
+ * random.c - Copyright © 2011 Szabolcs Nagy
+ * Permission to use, copy, modify, and/or distribute this code
+ * for any purpose with or without fee is hereby granted.
+ * There is no warranty.
+*/
+
 #include <stdlib.h>
+#include <stdint.h>
+
+/*
+this code uses the same lagged fibonacci generator as the
+original bsd random implementation except for the seeding
+
+different seeds produce different sequences with long period
+(other libcs seed the state with a park-miller generator
+when seed=0 some fail to produce good random sequence
+others produce the same sequence as another seed)
+*/
+
+static uint32_t init[] = {
+0x00000000,0x5851f42d,0xc0b18ccf,0xcbb5f646,
+0xc7033129,0x30705b04,0x20fd5db4,0x9a8b7f78,
+0x502959d8,0xab894868,0x6c0356a7,0x88cdb7ff,
+0xb477d43f,0x70a3a52b,0xa8e4baf1,0xfd8341fc,
+0x8ae16fd9,0x742d2f7a,0x0d1f0796,0x76035e09,
+0x40f7702c,0x6fa72ca5,0xaaa84157,0x58a0df74,
+0xc74a0364,0xae533cc4,0x04185faf,0x6de3b115,
+0x0cab8628,0xf043bfa4,0x398150e9,0x37521657};
+
+static int n = 31;
+static int i = 3;
+static int j = 0;
+static uint32_t *x = init+1;
+
+static uint32_t lcg31(uint32_t x) {
+	return (1103515245*x + 12345) & 0x7fffffff;
+}
+
+static uint64_t lcg64(uint64_t x) {
+	return 6364136223846793005ull*x + 1;
+}
+
+static void *savestate() {
+	x[-1] = (n<<16)|(i<<8)|j;
+	return x-1;
+}
+
+static void loadstate(uint32_t *state) {
+	x = state+1;
+	n = x[-1]>>16;
+	i = (x[-1]>>8)&0xff;
+	j = x[-1]&0xff;
+}
+
+void srandom(unsigned seed) {
+	int k;
+	uint64_t s = seed;
+
+	if (n == 0) {
+		x[0] = s;
+		return;
+	}
+	i = n == 31 || n == 7 ? 3 : 1;
+	j = 0;
+	for (k = 0; k < n; k++) {
+		s = lcg64(s);
+		x[k] = s>>32;
+	}
+	/* make sure x contains at least one odd number */
+	x[0] |= 1;
+}
+
+char *initstate(unsigned seed, char *state, size_t size) {
+	void *old = savestate();
+	if (size < 8)
+		return 0;
+	else if (size < 32)
+		n = 0;
+	else if (size < 64)
+		n = 7;
+	else if (size < 128)
+		n = 15;
+	else if (size < 256)
+		n = 31;
+	else
+		n = 63;
+	x = (uint32_t*)state + 1;
+	srandom(seed);
+	return old;
+}
+
+char *setstate(char *state) {
+	void *old = savestate();
+	loadstate((uint32_t*)state);
+	return old;
+}
 
-/* FIXME */
+long random(void) {
+	long k;
 
-long random()
-{
-	return rand();
+	if (n == 0)
+		return x[0] = lcg31(x[0]);
+	x[i] += x[j];
+	k = x[i]>>1;
+	if (++i == n)
+		i = 0;
+	if (++j == n)
+		j = 0;
+	return k;
 }
-- 
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