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#lang racket
(require advent-of-code)
(define raw-grid (fetch-aoc-input (find-session) 2020 11))
(define/match (parse _)
[(#\L) 'empty]
[(#\#) 'occupied]
[(#\.) 'floor])
(define seat-grid
(for*/hash ([(row r) (in-indexed (in-list (string-split raw-grid)))]
[(col c) (in-indexed (in-string row))])
(values (cons r c) (parse col))))
(define (next-seat-state seat state h rule [max-occupy 4])
(define neighbor-states (rule seat h))
(match* (state (count (curry eq? 'occupied) neighbor-states))
[('empty 0) 'occupied]
[('occupied n)
#:when (>= n max-occupy)
'empty]
[(_ _) state]))
(define (stabilize h [i 1] #:rule rule #:max-occupy [max-occupy 4])
(define h*
(for/hash ([(seat state) (in-hash h)])
(cond
[(eq? state 'floor) (values seat state)]
[else (values seat (next-seat-state seat state h rule max-occupy))])))
(if (equal? h h*)
(count (curry equal? 'occupied) (hash-values h))
(stabilize h* (add1 i) #:rule rule #:max-occupy max-occupy)))
;; part 1
(define (find-nearest-neighbors p h)
(match-define (cons r c) p)
(for*/list ([r* (in-inclusive-range (sub1 r) (add1 r))]
[c* (in-inclusive-range (sub1 c) (add1 c))]
[p* (in-value (cons r* c*))]
#:unless (equal? p p*))
(hash-ref h p* 'out-of-bounds)))
(stabilize seat-grid #:rule find-nearest-neighbors)
;; part 2
(define (find-visible-neighbors p h)
(match-define (cons r c) p)
(define directions
(for*/list ([dr '(-1 0 1)] [dc '(-1 0 1)] #:unless (= 0 dr dc))
(cons dr dc)))
(for/list ([dir (in-list directions)] #:do [(match-define (cons dr dc) dir)])
(for*/first ([i (in-naturals 1)]
#:do [(define p* (cons (+ r (* i dr)) (+ c (* i dc))))
(define state (hash-ref h p* 'out-of-bounds))]
#:unless (equal? state 'floor))
state)))
(stabilize seat-grid #:rule find-visible-neighbors #:max-occupy 5)
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