Re: [gforth] how to build a really working random seed ?

[mhx]

James Gere <address@hidden> writes Re: how to build a really working 
random seed ?

> Dear Marcel,
>                        did you try 5 or 6?
> James Gere

No, I did not, thanks!

The generator is even more puzzling than I thought
(unless I made an conversion mistake from 32 -> 64 bits)
-- see the test below.

To short circuit the test, enter
1073741825 xtest .

Apparently this generator has a cycle of 0 for the initial seed
1073741825.
The test finds other initial values that result in very short cycles.
There might be an initial value that results in a maximum length
sequence > 1685616737, but this sequence may have 'holes.'

-marcel

-----------------------------------------------------

variable (rnd)
0 VALUE minimum

: rnd ( -- n )
         (rnd) 32B@
         dup #13   lshift xor $FFFFFFFF and
         dup #17   rshift xor $FFFFFFFF and
         dup DUP 5 lshift xor (rnd) 32B! ;

: (xtest) ( maxcycle x -- cycle )
         DUP (rnd) !
         SWAP 0 ?DO  rnd over = IF  CR ." start = " .
                                    #20 HTAB ." -- short cycle of " I DEC.
                                    I UNLOOP EXIT
                             ENDIF
               LOOP
         DROP $7FFFFFFF ;

: xtest ( start -- )
        $7FFFFFFF TO minimum
        CR ." Searching for the shortest cycle."
        ( start )
        BEGIN
          minimum OVER (xtest)
          ( -- start u ) minimum MIN TO minimum
          ( start ) 1+
          minimum 0=
        UNTIL ( start ) DROP ;

DOC
(*
        Searching for the shortest cycle.
        start = 1          -- short cycle of 1685616737
        start = 2          -- short cycle of 1198653427
        start = 3          -- short cycle of 950033464
        start = 4          -- short cycle of 683909353
        start = 7          -- short cycle of 446874342
        start = 29         -- short cycle of 395322450
        start = 49         -- short cycle of 233610872
        start = 68         -- short cycle of 160881270
        start = 169        -- short cycle of 58748379
        start = 266        -- short cycle of 7897721
        start = 276        -- short cycle of 5522514
        start = 1040       -- short cycle of 1268127
        start = 2403       -- short cycle of 465414
        start = 10032      -- short cycle of 228244
        start = 70110      -- short cycle of 176576
        start = 71298      -- short cycle of 84339
        start = 123235     -- short cycle of 14938
        start = 672951     -- short cycle of 2835
        start = 1048315    -- short cycle of 1558
        start = 3457816    -- short cycle of 189
        start = 9516354    -- short cycle of 143
        start = 22220153   -- short cycle of 116
        start = 145584897  -- short cycle of 106
        start = 310366040  -- short cycle of 64
        start = 552122674  -- short cycle of 30
        start = 729376685  -- short cycle of 3
        start = 1073741825 -- short cycle of 0
*)
ENDDOC