Re: rand() is broken

[Andrey A. Chernov]

On Sun, Feb 02, 2003 at 13:26:21 +0300, Andrey A. Chernov wrote:

> Workaround I find so far is something like that
> 
> #define MASK 123459876

I found nothing better. Here is fix for 0 problem I plan to commit:

--- stdlib/rand.c.old   Sat Jan  4 20:39:19 2003
+++ stdlib/rand.c       Sun Feb  2 14:43:42 2003
@@ -70,14 +70,18 @@
  * Park and Miller, Communications of the ACM, vol. 31, no. 10,
  * October 1988, p. 1195.
  */
+#define SHIFT_MASK 123459876
        long hi, lo, x;
 
-       hi = *ctx / 127773;
-       lo = *ctx % 127773;
+       /* Can't be initialized with 0, so use shifting mask. */
+       x = *ctx ^ SHIFT_MASK;
+       hi = x / 127773;
+       lo = x % 127773;
        x = 16807 * lo - 2836 * hi;
-       if (x <= 0)
+       if (x < 0)
                x += 0x7fffffff;
-       return ((*ctx = x) % ((u_long)RAND_MAX + 1));
+       *ctx = x ^ SHIFT_MASK;
+       return (x % ((u_long)RAND_MAX + 1));
 #endif  /* !USE_WEAK_SEEDING */
 }
 
@@ -86,8 +90,10 @@
 rand_r(unsigned int *ctx)
 {
        u_long val = (u_long) *ctx;
-       *ctx = do_rand(&val);
-       return (int) *ctx;
+       int r = do_rand(&val);
+
+       *ctx = (unsigned int) val;
+       return (r);
 }
 
 
--- stdlib/random.c.old Sun Mar 24 23:42:48 2002
+++ stdlib/random.c     Sun Feb  2 15:24:38 2003
@@ -142,6 +142,10 @@
  */
 #define        MAX_TYPES       5               /* max number of types above */
 
+#ifndef USE_WEAK_SEEDING
+#define SHIFT_MASK 123459876
+#endif
+
 static long degrees[MAX_TYPES] =       { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 };
 static long seps [MAX_TYPES] = { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 };
 
@@ -171,12 +175,12 @@
        0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b, 0xf5ad9d0e, 0x8999220b,
        0x27fb47b9,
 #else   /* !USE_WEAK_SEEDING */
-       0x991539b1, 0x16a5bce3, 0x6774a4cd, 0x3e01511e, 0x4e508aaa, 0x61048c05,
-       0xf5500617, 0x846b7115, 0x6a19892c, 0x896a97af, 0xdb48f936, 0x14898454,
-       0x37ffd106, 0xb58bff9c, 0x59e17104, 0xcf918a49, 0x09378c83, 0x52c7a471,
-       0x8d293ea9, 0x1f4fc301, 0xc3db71be, 0x39b44e1c, 0xf8a44ef9, 0x4c8b80b1,
-       0x19edc328, 0x87bf4bdd, 0xc9b240e5, 0xe9ee4b1b, 0x4382aee7, 0x535b6b41,
-       0xf3bec5da
+       0x52a59789, 0x43164b1c, 0x7be52a82, 0x748ef343, 0x642a8923, 0x6ade1cd8,
+       0x1ae76e27, 0x24b915ee, 0x2c42f326, 0x12ab3ee1, 0x4679af03, 0x876d19a0,
+       0xe9e535ba, 0xad2471c8, 0x710262f8, 0xe1c16494, 0x29224bcc, 0x9710c348,
+       0x7347f8e4, 0xe01ef1b4, 0x2030c33f, 0xd465e38,  0x925375aa, 0x6091d15d,
+       0x467ee7d7, 0x92713312, 0x32346127, 0x8350e834, 0x3dadc6ea, 0x391364f2,
+       0x8226561c,
 #endif  /* !USE_WEAK_SEEDING */
 };
 
@@ -236,12 +240,14 @@
  */
        long hi, lo;
 
+       /* Can't be initialized with 0, so use shifting mask. */
+       x ^= SHIFT_MASK;
        hi = x / 127773;
        lo = x % 127773;
        x = 16807 * lo - 2836 * hi;
-       if (x <= 0)
+       if (x < 0)
                x += 0x7fffffff;
-       return (x);
+       return (x ^ SHIFT_MASK); /* return state, not value */
 #endif  /* !USE_WEAK_SEEDING */
 }
 
@@ -473,7 +479,12 @@
 
        if (rand_type == TYPE_0) {
                i = state[0];
-               state[0] = i = (good_rand(i)) & 0x7fffffff;
+               state[0] = good_rand(i);
+#ifndef USE_WEAK_SEEDING
+               i = state[0] ^ SHIFT_MASK;
+#else
+               i = state[0] & 0x7fffffff;
+#endif
        } else {
                /*
                 * Use local variables rather than static variables for speed.

-- 
Andrey A. Chernov
http://ache.pp.ru/

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