+(asdf:load-system :adventofcode2020)
+(in-package #:adventofcode2020)
+
+(defun calc-seat (seat-spec)
+ (let ((row (string-right-trim "RL" seat-spec))
+ (col (string-left-trim "FB" seat-spec))
+ (convert-to-int
+ (lambda (str one)
+ (loop with len = (1- (length str))
+ for c across str
+ for i downfrom len
+ summing (if (char= c one)
+ (ash 1 i) 0)))))
+ (mapcar convert-to-int (list row col) '(#\B #\R))))
+
+(defun calc-seat-id (row-col)
+ (destructuring-bind (row col) row-col
+ (+ (* row 8) col)))
+
+(day 05 input
+ (let* ((lines (list-from input))
+ (seat-ids (-<>> lines
+ (mapcar (compose #'calc-seat-id #'calc-seat))
+ (sort <> #'>))))
+ (part1 (first seat-ids))
+ (part2 (loop for big in seat-ids
+ for small in (cdr seat-ids)
+ when (= small (- big 2))
+ return (1+ small)))))
+
+(def-suite day05)
+(in-suite day05)
+
+(test calc-seats
+ (is (equal
+ '((44 5) (70 7) (14 7) (102 4))
+ (mapcar #'calc-seat '("FBFBBFFRLR" "BFFFBBFRRR"
+ "FFFBBBFRRR" "BBFFBBFRLL")))))
+
+(run! 'day05)