Return a pseudo-random Number generated with an xoshiro128** algorithm using seed {expr}. The returned number is 32 bits, also on 64 bits systems, for consistency. {expr} can be initialized by srand() and will be updated by rand(). If {expr} is omitted, an internal seed value is used and updated. Returns -1 if {expr} is invalid.

Examples:

:echo rand()
:let seed = srand()
:echo rand(seed)
:echo rand(seed) % 16  " random number 0 - 15