random: bugfix: incorrect WELL512a.

* rand.c (rand32_bug): New static function formed by
renaming the original buggy rand32.
(rand32_good): Copy of rand32 with two bugfixes. The term
involving variable r2 must be only left shifted by 28 bits,
and not xor-ed with the original value. The order of
operations is wrong in the term that contains the & operation.
(rand32): New static function pointer variable, serving
as the rand32 function. Points to rand32_good by default.
(rand_compat_fixup): Test for 243 or lower compatibility,
under which rand32 is made point to rand32_bug.  This is done
before the call to make_random_state for replacing
*random-state*, which has to use the old function.

* txr.1: compat note added.

* tests/013/maze.expected: Updated.

1 files changed, 8 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 2088419c..067f9b80 100644
--- a/txr.1
+++ b/txr.1
@@ -75522,6 +75522,14 @@ of these version values, the described behaviors are provided if
 is given an argument which is equal or lower. For instance
 .code "-C 103"
 selects the behaviors described below for version 105, but not those for 102.
+.IP 243
+Two mistakes in the pseudo-random-number generator (PRNG) were discovered,
+affecting \*(TX 243 and older. Using this compatibility value, or lower, will
+restore the buggy behavior, allowing pseudo-random number sequences produced
+by those older versions can be reproduced. The PRNG is intended to be an
+implementation of the WELL-512 PRNG described by Panneton and L'Ecuyer.
+The coding mistakes, however, resulted in the PRNG being an implementation of
+something other than WELL-512.
 .IP 242
 In \*(TX 242 and older, the instantiation of an object whose type inherits
 from the same supertype more than once resulted in duplicate execution