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+;; Advent of Code 2020 - Day 5
+
+(require 'cl-lib)
+(require 'dash)
+(require 'f)
+(require 'ht)
+(require 's)
+(require 'seq)
+
+(defvar day5/input
+  (-filter (lambda (s) (not (seq-empty-p s)))
+           (s-lines (f-read "/tmp/aoc/day5.txt"))))
+
+(defun day5/lower (sequence)
+  (seq-subseq sequence 0 (/ (length sequence) 2)))
+
+(defun day5/upper (sequence)
+  (seq-subseq sequence (/ (length sequence) 2)))
+
+(defun day5/seat-id (column row)
+  (+ column (* 8 row)))
+
+(defun day5/find-seat (boarding-pass)
+  (let ((rows (number-sequence 0 127))
+        (columns (number-sequence 0 7)))
+    (cl-loop for char across boarding-pass
+             do (pcase char
+                  (?F (setq rows (day5/lower rows)))
+                  (?B (setq rows (day5/upper rows)))
+                  (?R (setq columns (day5/upper columns)))
+                  (?L (setq columns (day5/lower columns))))
+             finally return (day5/seat-id (car columns) (car rows)))))
+
+;; Puzzle 1
+
+(message "Solution to day5/1: %s"
+         (cl-loop for boarding-pass in day5/input
+                  maximize (day5/find-seat boarding-pass)))
+
+;; Puzzle 2
+
+(defun day5/all-seats-in (row)
+  (-map (lambda (column) (day5/seat-id column row))
+        (number-sequence 0 7)))
+
+(message "Solution to day5/2: %s"
+         (let ((all-seats (ht-create)))
+           (-each (-mapcat #'day5/all-seats-in (number-sequence 1 126))
+             (lambda (seat) (ht-set all-seats seat nil)))
+
+           (cl-loop for boarding-pass in day5/input
+                    do (ht-remove all-seats (day5/find-seat boarding-pass))
+
+                    ;; Remove seats that lack adjacent entries, those
+                    ;; are missing on the plane.
+                    finally return
+                    (car
+                     (-filter (lambda (seat)
+                                (and (not (ht-contains? all-seats (- seat 1)))
+                                     (not (ht-contains? all-seats (+ seat 1)))))
+                              (ht-keys all-seats))))))