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+;; Advent of Code 2020 - Day 4
+
+(require 'cl-lib)
+(require 's)
+(require 'dash)
+(require 'f)
+
+(cl-defstruct day4/passport
+  byr ;; Birth Year
+  iyr ;; Issue Year
+  eyr ;; Expiration Year
+  hgt ;; Height
+  hcl ;; Hair Color
+  ecl ;; Eye Color
+  pid ;; Passport ID
+  cid ;; Country ID
+  )
+
+(defun day4/parse-passport (input)
+  (let* ((pairs (s-split " " (s-replace "\n" " " input) t))
+         (slots
+          (-map
+           (lambda (pair)
+             (pcase-let ((`(,key ,value) (s-split ":" (s-trim pair))))
+               (list (intern (format ":%s" key)) value)))
+           pairs)))
+    (apply #'make-day4/passport (-flatten slots))))
+
+(defun day4/parse-passports (input)
+  (-map #'day4/parse-passport (s-split "\n\n" input t)))
+
+(setq day4/input (day4/parse-passports (f-read "/tmp/aoc/day4.txt")))
+
+;; Puzzle 1
+
+(defun day4/validate (passport)
+  "Check that all fields except CID are present."
+  (cl-check-type passport day4/passport)
+  (and (day4/passport-byr passport)
+       (day4/passport-iyr passport)
+       (day4/passport-eyr passport)
+       (day4/passport-hgt passport)
+       (day4/passport-hcl passport)
+       (day4/passport-ecl passport)
+       (day4/passport-pid passport)))
+
+(message "Solution to day4/1: %s" (cl-loop for passport being the elements of day4/input
+                                           count (day4/validate passport)))
+
+;; Puzzle 2
+
+(defun day4/year-bound (min max value)
+  (and
+   (s-matches? (rx (= 4 digit)) value)
+   (<= min (string-to-number value) max)))
+
+(defun day4/check-unit (unit min max value)
+  (and
+   (string-match (rx (group (+? digit)) (literal unit)) value)
+   (<= min (string-to-number (match-string 1 value)) max)))
+
+(defun day4/properly-validate (passport)
+  "Opting for readable rather than clever here."
+  (and
+   (day4/validate passport)
+
+   ;; byr (Birth Year) - four digits; at least 1920 and at most 2002.
+   (day4/year-bound 1920 2002 (day4/passport-byr passport))
+
+   ;; iyr (Issue Year) - four digits; at least 2010 and at most 2020.
+   (day4/year-bound 2010 2020 (day4/passport-iyr passport))
+
+   ;; eyr (Expiration Year) - four digits; at least 2020 and at most 2030.
+   (day4/year-bound 2020 2030 (day4/passport-eyr passport))
+
+   ;; hgt (Height) - a number followed by either cm or in:
+   ;; If cm, the number must be at least 150 and at most 193.
+   ;; If in, the number must be at least 59 and at most 76.
+   (or (day4/check-unit "cm" 150 193 (day4/passport-hgt passport))
+       (day4/check-unit "in" 59 76 (day4/passport-hgt passport)))
+
+   ;; hcl (Hair Color) - a # followed by exactly six characters 0-9 or a-f.
+   (s-matches? (rx ?# (= 6 hex)) (day4/passport-hcl passport))
+
+   ;; ecl (Eye Color) - exactly one of: amb blu brn gry grn hzl oth.
+   (-contains? '("amb" "blu" "brn" "gry" "grn" "hzl" "oth")
+               (day4/passport-ecl passport))
+
+   ;; pid (Passport ID) - a nine-digit number, including leading zeroes.
+   (s-matches? (rx line-start (= 9 digit) line-end)
+               (day4/passport-pid passport))
+
+   ;; cid (Country ID) - ignored, missing or not.
+   ))
+
+(message "Solution to day4/2: %s"
+         (cl-loop for passport being the elements of day4/input
+                  count (day4/properly-validate passport)))