1 files changed, 33 insertions, 0 deletions
diff --git a/Doc/Zsh/mod_mathfunc.yo b/Doc/Zsh/mod_mathfunc.yo
index 05ce9fe45..637c22d8f 100644
--- a/Doc/Zsh/mod_mathfunc.yo
+++ b/Doc/Zsh/mod_mathfunc.yo
@@ -52,3 +52,36 @@ a floating point or integer value (by truncation) respectively.
 
 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()).