This is a collection of PRNGs translated into vimscript because...
well, it all started out of dissatisfaction with "random colorscheme" scripts:
I patched one of them to use /dev/urandom, then I thought that it
wouldn't work on windows, so I translated rand() from unix v7
and got hooked by the clumsiness of the language.
I've written most of these individually, usually while waiting for $BoringComputerActivity to complete (fixing friend's computer, ...); this means at least that there are some repetitions, such as comments explaining vim's integers.
Most of the generators assume that exists('*xor'); as of
20130408, both mingw and Debian stable don't qualify.
UPDATE(20130512): Debian wheezy got released last week.
Some generators assume an int is exactly 32 bits and :throw otherwise.
Most of the generators test themselves against the original C
implementation if $DEBUG is set.
Quoting from vim/src/structs.h:
#if SIZEOF_INT <= 3 /* use long if int is smaller than 32 bits */
typedef long varnumber_T;
#else
typedef int varnumber_T;
#endif
Iow: vim uses a signed integer that's at least 32 bits wide; I have yet to find a machine where it's not exactly 32 bits.