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(ql:quickload '(fiveam str arrows))
(defparameter damaged #\#)
(defparameter operational #\.)
(defparameter unknown #\?)
(defparameter eg-input "???.### 1,1,3
.??..??...?##. 1,1,3
?#?#?#?#?#?#?#? 1,3,1,6
????.#...#... 4,1,1
????.######..#####. 1,6,5
?###???????? 3,2,1")
(destructuring-bind (row . damage-list)
(cl-ppcre:split "[\\s,]" "???.###. 1,1,3")
(setf damage-list (mapcar #'parse-integer damage-list))
(labels ((options (row-idx damage-group)
(cond
;; commited to all options
((= damage-group (length damage-list))
;; If still damaged springs are left, then
;; it was a wrong arrangement, otherwise counts
(if (find damaged (subseq row row-idx)) 0 1))
;; Not enough space to fit the current damaged-group
((<= (length row) (+ row-idx (elt damage-list damage-group))) 0)
;; Probe options. Either commit to a damage-group at row-idx or skip position
(t
(let* ((damaged-count (elt damage-list damage-group))
(next-slot (+ row-idx damaged-count)))
(+ (if (not (or (find operational (subseq row row-idx next-slot))
(char-equal damaged (aref row next-slot))))
;; commits to the damage group, because in condition it has space
;; Skips one slot, because it checked that it wasn't damaged
(options (1+ next-slot) (1+ damage-group))
0)
(if (not (char-equal damaged (aref row row-idx)))
(options (1+ row-idx) damage-group)
0)))))))
(options 0 0)))
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