Hi, After some more time to investigate, I found out that the LCG used in PicoLisp was not the Haynes one, but the one referred as «Newlib» here: http://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use More auto-answers and other questions below.
On Wed, Apr 22, 2015 at 10:50 PM, Christophe Gragnic <christophegrag...@gmail.com> wrote: > Now the question. I found discrepancies between PicoLisp: > : (rand 1 1000) > -> 1 > : (rand 1 1000) > -> 934 > : (bye) > > And Ersatz > : (rand 1 1000) > -> 1 > : (rand 1 1000) > -> 967 > : (bye) > > I tried to inspect the source for getting some explanation with no luck. > PicoLisp > https://code.google.com/p/picolisp/source/browse/src/big.c#1213 > Ersatz: > https://code.google.com/p/picolisp/source/browse/ersatz/fun.src#3325 > Is there a reason for those different behaviours ? Maybe I was too tired. The difference is 33, and we see in the Ersatz src: n = (int)(Seed >>> 33) % n; But it is a coincidence since a bit shift should not have this effect (confirmed when trying with the args 0 and 1000): PicoLisp: : (rand 0 1000) -> 0 : (rand 0 1000) -> 648 : (rand 0 1000) -> 760 Ersatz: : (rand 0 1000) -> 0 : (rand 0 1000) -> 824 : (rand 0 1000) -> 880 Then I had to find the shift in the C code, which is hidden in the `hi` macro (for high bits): https://code.google.com/p/picolisp/source/browse/src/pico.h#161 #define hi(w) num((w)>>BITS) Where: #define WORD ((int)sizeof(long)) #define BITS (8*WORD) What I don't understand is why Ersatz shifts by 33, and PicoLisp by BITS, (which should IMHO be 32). This connects to my next remarks/questions: * The Wikipedia page mentions taking «bits 63...32». I guess that they start at bit 0, thus should the shift be 32 ? * Is 33 used because of the sign bit ? * In the PicoLisp src, why use LL and not ULL to define 6364136223846793005? chri -- http://profgra.org/lycee/ (site pro) http://delicious.com/profgraorg (liens, favoris) https://twitter.com/profgraorg http://microalg.info -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
