RNG algorithm is changed to Xorshift

RNG algorithm is changed from original one to Xorshift.
Xorshift uses Rand_state[0]-Rand_state[3] variables as RNG state,
therefore Rand_place and Rand_state[4]-Rand_state[63] variables are
unused now.

diff --git a/src/z-rand.c b/src/z-rand.c
index d1e82e6..680d1ca 100644 (file)
--- a/src/z-rand.c
+++ b/src/z-rand.c
@@ -73,7 +73,12 @@ u16b Rand_place;
 /*
  * Current "state" table for the "complex" RNG
  */
-u32b Rand_state[RAND_DEG];
+u32b Rand_state[RAND_DEG] = {
+       123456789,
+       362436069,
+       521288629,
+       88675123,
+};
 
 
 /*
@@ -81,26 +86,12 @@ u32b Rand_state[RAND_DEG];
  */
 void Rand_state_init(u32b seed)
 {
-       int i, j;
-
-       /* Seed the table */
-       Rand_state[0] = seed;
-
-       /* Propagate the seed */
-       for (i = 1; i < RAND_DEG; i++) Rand_state[i] = LCRNG(Rand_state[i-1]);
-
-       /* Cycle the table ten times per degree */
-       for (i = 0; i < RAND_DEG * 10; i++)
-       {
-               /* Acquire the next index */
-               j = Rand_place + 1;
-               if (j == RAND_DEG) j = 0;
+       int i;
 
-               /* Update the table, extract an entry */
-               Rand_state[j] += Rand_state[Rand_place];
-
-               /* Advance the index */
-               Rand_place = j;
+       /* Initialize Xorshift Algorithm RNG */
+       for (i = 1; i <= 4; ++ i) {
+               seed = 1812433253UL * (seed ^ (seed >> 30)) + i;
+               Rand_state[i-1] = seed;
        }
 }
 
@@ -123,12 +114,12 @@ s32b Rand_div(u32b m)
        /* Hack -- simple case */
        if (m <= 1) return (0);
 
-       /* Partition size */
-       n = (0x10000000 / m);
-
        /* Use a simple RNG */
        if (Rand_quick)
        {
+               /* Partition size */
+               n = (0x10000000 / m);
+
                /* Wait for it */
                while (1)
                {
@@ -146,27 +137,16 @@ s32b Rand_div(u32b m)
        /* Use a complex RNG */
        else
        {
-               /* Wait for it */
-               while (1)
-               {
-                       int j;
-
-                       /* Acquire the next index */
-                       j = Rand_place + 1;
-                       if (j == RAND_DEG) j = 0;
-
-                       /* Update the table, extract an entry */
-                       r = (Rand_state[j] += Rand_state[Rand_place]);
+               /* Xorshift Algorithm RNG */
+               u32b t = Rand_state[0] ^ (Rand_state[0] << 11);
 
-                       /* Hack -- extract a 28-bit "random" number */
-                       r = (r >> 4) / n;
+               Rand_state[0] = Rand_state[1];
+               Rand_state[1] = Rand_state[2];
+               Rand_state[2] = Rand_state[3];
 
-                       /* Advance the index */
-                       Rand_place = j;
+               Rand_state[3] = (Rand_state[3] ^ (Rand_state[3] >> 19)) ^ (t ^ (t >> 8));
 
-                       /* Done */
-                       if (r < m) break;
-               }
+               r = Rand_state[3] % m;
        }
 
        /* Use the value */