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(ql:quickload '(fiveam str arrows))
(defparameter eg-input "LLR
AAA = (BBB, BBB)
BBB = (AAA, ZZZ)
ZZZ = (ZZZ, ZZZ)")
(defun parse-input (lines)
(let ((graph (make-hash-table :test #'equal)))
(destructuring-bind (instructions blank . nodes) lines
(declare (ignorable blank))
(dolist (line nodes)
(destructuring-bind (node . leaves)
(remove-if #'str:emptyp
(cl-ppcre:split "[=,() ]" line))
(setf (gethash node graph) leaves)))
(values instructions graph))))
(defun traverse (instructions graph location terminatep)
(labels ((rec (steps)
(if (funcall terminatep location)
(values steps location)
(rec (+ steps (steps-needed)))))
(steps-needed ()
(loop
for direction across instructions
do (arrows:-<>
(ecase direction
(#\L #'first)
(#\R #'second))
(funcall <> (gethash location graph))
(setf location <>))
count direction until (funcall terminatep location))))
(rec 0)))
(defun solver1 (lines)
(multiple-value-bind (instructions graph)
(parse-input lines)
(traverse instructions graph "AAA" (lambda (p) (equal "ZZZ" p)))))
(defparameter eg-input2 "LR
11A = (11B, XXX)
11B = (XXX, 11Z)
11Z = (11B, XXX)
22A = (22B, XXX)
22B = (22C, 22C)
22C = (22Z, 22Z)
22Z = (22B, 22B)
XXX = (XXX, XXX)")
(defun solver2 (lines)
(multiple-value-bind (instructions graph)
(parse-input lines)
(arrows:->>
(remove-if-not (lambda (s) (str:ends-with-p "A" s))
(alexandria:hash-table-keys graph))
(mapcar (lambda (start)
(traverse instructions graph start (lambda (p) (str:ends-with-p "Z" p)))))
(apply #'lcm))))
(fiveam:test solutions
(fiveam:is (= 6 (solver1 (uiop:split-string eg-input :separator '(#\Newline)))))
(fiveam:is (= 6 (solver2 (uiop:split-string eg-input2 :separator '(#\Newline)))))
(fiveam:is (= 19199 (solver1 (uiop:read-file-lines "input"))))
(fiveam:is (= 13663968099527(solver2 (uiop:read-file-lines "input")))))
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