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(defstruct polym ()
input
rewrites
(memo (hash)))
(defun read-input (: (name "input"))
(let ((po (new polym)))
(each ((line (file-get-lines name)))
(match-case line
(`@a -> @b` (push `@a@b` po.rewrites))
(`@{a 1}@b` (set po.input `@a@b`))))
po))
(defmeth polym rec1 (po pair depth : (leftmost t))
(placelet ((memo [po.memo ^(,pair ,depth ,leftmost)]))
(condlet
(((re memo))
re)
(((rw (and (plusp (pdec depth))
[find pair po.rewrites starts-with])))
(match `@{x 1}@{y 1}@{z 1}` rw
(let ((lhist po.(rec1 `@x@z` depth leftmost))
(rhist po.(rec1 `@z@y` depth nil)))
(set memo [hash-uni lhist rhist +]))))
(leftmost
(set memo (hash-zip pair '(1 1))))
(t
(set memo (hash-zip (rest pair) '(1)))))))
(defmeth polym rec (po pairs depth : (leftmost t))
(let ((hist (hash)))
(each ((p pairs)
(c 0))
(let ((rhist po.(rec1 p depth (zerop c))))
(set hist [hash-uni hist rhist +])))
hist))
(defun solve (: (name "input") (depth 10))
(let* ((po (read-input name))
(hist po.(rec (tuples* 2 po.input) depth)))
(- (cdr [find-max hist : cdr])
(cdr [find-min hist : cdr]))))
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