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*) ⇒ true
(setq snap-shot (make-random-state))
;; The series from any given point is random,
;; but if you backtrack to that point, you get the same series.
(list (loop for i from 1 to 10 collect (random))
(let ((*random-state* snap-shot))
(loop for i from 1 to 10 collect (random)))
(loop for i from 1 to 10 collect (random))
(let ((*random-state* snap-shot))
(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))
Binding *random-state* to a different random state object correctly saves and restores the old random state object.