a random state.
The current random state, which is used, for example, by the function random when a random state is not explicitly supplied.
(random-state-p *random-state*)
→
In the example below, the series from any given point is random, but if you backtrack to that point, you get the same series.
(defparameter *snapshot* (make-random-state))
→*SNAPSHOT*
(list (loop for i from 1 to 10 collect (random)) (let ((*random-state* *snapshot*)) (loop for i from 1 to 10 collect (random))) (loop for i from 1 to 10 collect (random)) (let ((*random-state* *snapshot*)) (loop for i from 1 to 10 collect (random))))
→((19 16 44 19 96 15 76 96 13 61) (19 16 44 19 96 15 76 96 13 61) (16 67 0 43 70 79 58 5 63 50) (16 67 0 43 70 79 58 5 63 50))
The implementation.
Binding *random-state* to a different random state object correctly saves and restores the old random state object.