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#lang racket
(require threading
"../../jj-aoc.rkt")
(define sea-floor-data
(for/vector ([l (in-lines (open-day 9))]
#:unless (equal? l ""))
(~>> l
string->list
(map (λ~>> ~a string->number))
list->vector)))
(define max-rows (vector-length sea-floor-data))
(define max-cols (vector-length (vector-ref sea-floor-data 0)))
(define-values (min-rows min-cols) (values 0 0))
(define (vector2d-ref vec coord)
(match-define `(,r ,c) coord)
(~> vec
(vector-ref r)
(vector-ref c)))
(define (adjacent coords)
(match-define `(,r ,c) coords)
`((,(add1 r) ,c)
(,(sub1 r) ,c)
(,r ,(add1 c))
(,r ,(sub1 c))))
(define (valid-coordinate coord)
(match-define `(,r ,c) coord)
(and (>= r min-rows)
(< r max-rows)
(>= c min-cols)
(< c max-cols)))
;; part 1
(define (lowest-point? vec coord)
(for*/and ([neighbor (in-list (adjacent coord))]
#:when (valid-coordinate neighbor))
(< (vector2d-ref vec coord) (vector2d-ref vec neighbor))))
(for*/sum ([r (in-range min-rows max-rows)]
[c (in-range min-cols max-cols)]
[coord (in-value `(,r ,c))]
#:when (lowest-point? sea-floor-data coord))
(add1 (vector2d-ref sea-floor-data coord)))
;; part 2
;; all the basins are bordered by the edges or by ridges of elevation 9,
;; so it's not really necessary to start at a low point
(define walked (make-hash))
(define (walkable? vec coord)
(and (< (vector2d-ref vec coord) 9)
(not (hash-has-key? walked coord))))
(define (walk-the-basin vec coord)
(cond
[(walkable? vec coord)
(hash-set! walked coord 'visited)
(add1 (for/sum [(neighbor (in-list (adjacent coord)))
#:when (valid-coordinate neighbor)]
(walk-the-basin vec neighbor)))]
[else 0]))
(define basins
(for*/list ([r (in-range min-rows max-rows)]
[c (in-range min-cols max-cols)]
[coord (in-value `(,r ,c))]
#:when (walkable? sea-floor-data coord))
(walk-the-basin sea-floor-data coord)))
(~> basins
(sort >)
(take 3)
(apply * _))
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