Re: PATCH: better pseudorandom numbers

[Peter Stephenson <pws@xxxxxxx>]

Peter Stephenson wrote:
> I'm not happy with the way the seed is stored as raw bytes.  It might be
> better to keep it as an array of three numbers and convert to and from
> string format each time.  With 64-bit zlong's, it will fit in a single
> integer parameter, but that's not portable.

I like this version better.  It stores the seed as 12 hex digits.  It's a
little extra work, but much more user-friendly for printing and setting.

Index: zshconfig.ac
===================================================================
RCS file: /cvsroot/zsh/zsh/zshconfig.ac,v
retrieving revision 1.20
diff -u -r1.20 zshconfig.ac
--- zshconfig.ac	2001/10/10 16:02:24	1.20
+++ zshconfig.ac	2001/11/15 11:57:10
@@ -938,7 +938,8 @@
 	       brk sbrk \
 	       pathconf sysconf \
 	       tgetent tigetflag tigetnum tigetstr setupterm \
-	       pcre_compile pcre_study pcre_exec)
+	       pcre_compile pcre_study pcre_exec \
+	       erand48)
 AC_FUNC_STRCOLL
 
 dnl  Check if tgetent accepts NULL (and will allocate its own termcap buffer)
Index: Doc/Zsh/mod_mathfunc.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/mod_mathfunc.yo,v
retrieving revision 1.2
diff -u -r1.2 mod_mathfunc.yo
--- Doc/Zsh/mod_mathfunc.yo	2000/05/11 00:01:03	1.2
+++ Doc/Zsh/mod_mathfunc.yo	2001/11/15 11:57:10
@@ -52,3 +52,36 @@
 
 Note that the C tt(pow) function is available in ordinary math evaluation
 as the `tt(**)' operator and is not provided here.
+
+The function tt(rand48) is available if your system's mathematical library
+has the function tt(erand48(3)).  It returns a pseudo-random floating point
+number between 0 and 1.  It takes a single string optional argument.
+
+If the argument is not present, the random number seed is initialised by
+three calls to the tt(rand(3)) function --- this produces the same random
+numbers as the next three values of tt($RANDOM).
+
+If the argument is present, it gives the name of a scalar parameter where
+the current random number seed will be stored.  On the first call, the
+value must contain at least twelve hexadecimal digits (the remainder of the
+string is ignored), or the seed will be initialised in the same manner as
+for a call to tt(rand48) with no argument.  Subsequent calls to
+tt(rand48)LPAR()var(param)RPAR() will then maintain the seed in the
+parameter var(param) as a string of twelve hexadecimal digits, with no base
+signifier.  The random number sequences for different parameters are
+completely independent, and are also independent from that used by calls to
+tt(rand48) with no argument.
+
+For example, consider
+
+example(print $(( rand48(seed) ))
+print $(( rand48() ))
+print $(( rand48(seed) )))
+
+Assuming tt($seed) does not exist, it will be initialised by the first
+call.  In the second call, the default seed is initialised; note, however,
+that because of the properties of tt(rand()) there is a correlation between
+the seeds used for the two initialisations, so for more secure uses, you
+should generate your own 12-byte seed.  The third call returns to the same
+sequence of random numbers used in the first call, unaffected by the
+intervening tt(rand48()).
Index: Src/Modules/mathfunc.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/Modules/mathfunc.c,v
retrieving revision 1.2
diff -u -r1.2 mathfunc.c
--- Src/Modules/mathfunc.c	2000/05/26 09:47:28	1.2
+++ Src/Modules/mathfunc.c	2001/11/15 11:57:10
@@ -82,6 +82,12 @@
 MF_YN
 };
 
+/* also functions taking a string argument */
+
+enum {
+MS_RAND48
+};
+
 /*
  * also to do, but differently argument or returned: abs (no type
  * conversion), atan2.
@@ -119,6 +125,12 @@
 
 
 static struct mathfunc mftab[] = {
+  /* Functions taking string arguments */
+#ifdef HAVE_ERAND48
+  /* here to avoid comma hassle */
+  STRMATHFUNC("rand48", math_string, MS_RAND48),
+#endif
+
   NUMMATHFUNC("abs", math_func, 1, 1, MF_ABS | BFLAG(BF_FRAC) |
 	      TFLAG(TF_NOCONV|TF_NOASS)),
   NUMMATHFUNC("acos", math_func, 1, 1, MF_ACOS | BFLAG(BF_FRAC)),
@@ -447,6 +459,100 @@
 
   return ret;
 }
+
+/**/
+static mnumber
+math_string(char *name, char *arg, int id)
+{
+    mnumber ret = zero_mnumber;
+    char *send;
+    /*
+     * Post-process the string argument, which is just passed verbatim.
+     * Not clear if any other functions that use math_string() will
+     * want this, but assume so for now.
+     */
+    while (iblank(*arg))
+	arg++;
+    send = arg + strlen(arg);
+    while (send > arg && iblank(send[-1]))
+	send--;
+    *send = '\0';
+
+    switch (id)
+    {
+#ifdef HAVE_ERAND48
+    case MS_RAND48:
+	{
+	    static unsigned short seedbuf[3];
+	    static int seedbuf_init;
+	    unsigned short tmp_seedbuf[3], *seedbufptr;
+	    int do_init = 1;
+
+	    if (*arg) {
+		/* Seed is contained in parameter named by arg */
+		char *seedstr;
+		seedbufptr = tmp_seedbuf;
+		if ((seedstr = getsparam(arg)) && strlen(seedstr) >= 12) {
+		    int i, j;
+		    do_init = 0;
+		    /*
+		     * Decode three sets of four hex digits corresponding
+		     * to each unsigned short.
+		     */
+		    for (i = 0; i < 3 && !do_init; i++) {
+			unsigned short *seedptr = seedbuf + i;
+			*seedptr = 0;
+			for (j = 0; j < 4; j++) {
+			    if (*seedstr >= '0' && *seedstr <= '9')
+				*seedptr += *seedstr - '0';
+			    else if (tolower(*seedstr) >= 'a' &&
+				     tolower(*seedstr) <= 'f')
+				*seedptr += tolower(*seedstr) - 'a' + 10;
+			    else {
+				do_init = 1;
+				break;
+			    }
+			    seedstr++;
+			    if (j < 3)
+				*seedptr *= 16;
+			}
+		    }
+		}
+		else if (errflag)
+		    break;
+	    }
+	    else
+	    {
+		/* Use default seed: must be initialised. */
+		seedbufptr = seedbuf;
+		if (!seedbuf_init)
+		    seedbuf_init = 1;
+		else
+		    do_init = 1;
+	    }
+	    if (do_init) {
+		seedbufptr[0] = (unsigned short)rand();
+		seedbufptr[1] = (unsigned short)rand();
+		seedbufptr[2] = (unsigned short)rand();
+	    }
+	    ret.type = MN_FLOAT;
+	    ret.u.d = erand48(seedbufptr);
+
+	    if (*arg)
+	    {
+		char outbuf[13];
+		sprintf(outbuf, "%04x%04x%04x", (int)seedbufptr[0],
+			(int)seedbufptr[1], (int)seedbufptr[2]);
+		setsparam(arg, ztrdup(outbuf));
+	    }
+	}
+	break;
+#endif
+    }
+
+    return ret;
+}
+
 
 /**/
 int

-- 
Peter Stephenson <pws@xxxxxxx>                  Software Engineer
CSR Ltd., Science Park, Milton Road,
Cambridge, CB4 0WH, UK                          Tel: +44 (0)1223 392070


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