Advent of Code, Day 5, Part II

advent-05.rkt

#lang racket
(define in "...")

(define ss
  (sort (for/list ([l (in-list (with-input-from-string in port->lines))]) 
          (string->number
           (string-append "#b"
                          (regexp-replaces l '(("F" "0")
                                               ("B" "1")
                                               ("L" "0")
                                               ("R" "1"))))))
        <))

(for/first ([s (in-list ss)]
            [s+1 (in-list (rest ss))]
            #:when (= (+ s 2) s+1))
  (add1 s))