1 files changed, 35 insertions, 0 deletions
diff --git a/racket/aoc2020/day-05/day-05.rkt b/racket/aoc2020/day-05/day-05.rkt
new file mode 100644
index 0000000..bd89ede
--- /dev/null
+++ b/racket/aoc2020/day-05/day-05.rkt
@@ -0,0 +1,35 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading)
+
+(define tickets
+  (for/list ([l (in-lines (open-day 5 2020))])
+    (~>> l (regexp-match #px"(.{7})(.{3})") rest)))
+
+(define (find-place str min-p max-p l r)
+  (if (string=? "" str)
+      min-p
+      (let ([p-range (/ (add1 (- max-p min-p)) 2)] [c (substring str 0 1)])
+        (cond
+          [(string=? c l) (find-place (substring str 1) min-p (- max-p p-range) l r)]
+          [(string=? c r) (find-place (substring str 1) (+ min-p p-range) max-p l r)]))))
+
+(define (find-row str)
+  (find-place str 0 127 "F" "B"))
+(define (find-col str)
+  (find-place str 0 7 "L" "R"))
+
+(define (ticket-id t)
+  (let ([row (find-row (first t))] [col (find-col (second t))]) (+ col (* 8 row))))
+
+;; part 1
+(define occupied-seats
+  (~>> (for/list ([t (in-list tickets)])
+         (ticket-id t))))
+
+(apply max occupied-seats)
+
+;; part 2
+(set-first (set-subtract
+            (list->set (inclusive-range (apply min occupied-seats) (apply max occupied-seats)))
+            (list->set occupied-seats)))