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5.lisp
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5.lisp
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(in-package #:cl-user)
(defpackage #:aoc-2020-5
(:use #:cl
#:alexandria
#:aoc-2020
#:arrows
#:cl-ppcre))
(in-package #:aoc-2020-5)
(defstruct boarding-pass
raw
row
column
seat-id)
(defun aoc-5a (&optional (input (read-file-into-string "5")))
(with-input-from-string (in input)
(loop :for pass :in (read-boarding-passes in)
:maximize (boarding-pass-seat-id pass))))
(defun read-boarding-passes (stream)
(loop :for line := (read-line stream nil)
:while line
:collect (read-boarding-pass line)))
(defun read-boarding-pass (string)
(let ((row (read-binary (subseq string 0 7)
#\F #\B))
(column (read-binary (subseq string 7)
#\L #\R)))
(make-boarding-pass :raw string
:row row
:column column
:seat-id (+ (* row 8)
column))))
(defun read-binary (string zero one)
(-<>> string
(map 'string (lambda (c)
(cond ((char= c zero) #\0)
((char= c one) #\1)
(t (error "unknown char ~s" c)))))
(parse-integer <> :radix 2)))
(defun aoc-5b (&optional (input (read-file-into-string "5")))
(with-input-from-string (in input)
(let* ((passes (read-boarding-passes in))
(seat-ids (sort (mapcar #'boarding-pass-seat-id passes)
#'<)))
(loop :for (a b) :on seat-ids
:thereis (when (= (- b a) 2)
(1+ a))))))