From: Pat Thoyts <patthoyts@users.sourceforge.net>
Date: Tue, 16 Dec 2014 13:04:13 +0000 (+0000)
Subject: Use 16 bit galois field based PRNG.
X-Git-Url: https://privyetmir.co.uk/gitweb.cgi?a=commitdiff_plain;h=b321441f72bff1b839f7c9782ea5ebe4102ce7e8;p=vetinari_clock

Use 16 bit galois field based PRNG.

This has much better distribution in the low bits over its period.
---

diff --git a/main.c b/main.c
index 34c5e8e..0ab0b8d 100644
--- a/main.c
+++ b/main.c
@@ -19,16 +19,19 @@
 #define CLKP BIT6 /* P1.6 */
 #define CLKN BIT7 /* P1.7 */
 
-static unsigned long next = 0;
-static void my_srand(unsigned long seed)
+static uint16_t prng_state = 0xACE1u;
+static void my_srand(uint16_t seed)
 {
-    next = seed;
+    prng_state = seed;
 }
-static unsigned int
+static uint16_t
 my_rand()
 {
-    next = next * 1103515245 + 12345;
-    return (unsigned int)next;
+    unsigned lsb = prng_state & 1;
+    prng_state >>= 1;
+    if (lsb == 1)
+        prng_state ^= 0xB400u;
+    return prng_state;
 }
 
 static void
@@ -53,11 +56,10 @@ main(void)
     WDTCTL = WDTPW + WDTHOLD;
 
     /* external 32.768kHz crystal */
-    BCSCTL1 |= DIVA_3; /* ACLK/8 */
+    BCSCTL1 |= DIVA_3; /* ACLK/8: 4096 clock interrupts per second */
     BCSCTL3 |= XCAP_3; /* enable 12.5pF internal capacitance */
 
-    srand(2);
-    my_srand(0xdeadbeef);
+    my_srand(0xACE1u); /* FIX ME: get a physically derived seed */
     setup();
 
     /* enable global interrupts */