組み合わせパズルなんかを解くときに楽。かも
permutations-any,permutations*-any。util.combinationsに追加って感じで*1
(define (but-kth lis k)
(receive (head tail) (split-at lis k)
(append! head (cdr tail))))
(define (any-with-index pred lis1 . lists)
(let1 index -1
(apply any
(lambda (item1 . items)
(apply pred `(,(inc! index) ,item1 . ,items)))
lis1 lists)))
(define (permutations-any pred set)
(cond ((null? set) (pred set))
((null? (cdr set)) (pred set))
(else
(any-with-index
(lambda (ind elt)
(permutations-any
(lambda (subperm) (pred (cons elt subperm)))
(but-kth set ind)))
set))))
(define (permutations*-any pred set . maybe-eq)
(cond
((null? set) (pred set))
((null? (cdr set)) (pred set))
(else
(let1 eq (get-optional maybe-eq eqv?)
(let loop ((i 0) (seen '()) (p set))
(cond
((null? p) #f)
((member (car p) seen eq) (loop (+ i 1) seen (cdr p)))
(else
(or (permutations*-any
(lambda (subperm) (pred (cons (car p) subperm))) (but-kth set i) eq)
(loop (+ i 1) (cons (car p) seen) (cdr p))))))))))まーちゃんと動くかどうかは検証してないんだけれども。
(permutations-any (pa$ apply >) '(10 9 8 7 6 5 4 3 2 1))
はすぐに#tを返すけど、
(any (pa$ apply >) (permutations '(10 9 8 7 6 5 4 3 2 1)))
はまず10!個のリストを生成するんで終わんない。
-everyも作れよ！ってな！
*1:てかソースがモロパクり
