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#lang racket
(require "../../jj-aoc.rkt"
threading
memoize
algorithms
list-utils)
(define data (port->lines (open-day 14 2021)))
(define (starting-polymer d)
(~> d first string->list (sliding 2 1)))
(define (first-char d)
(first (first (starting-polymer d))))
(define (starting-counts d)
(~> d first frequencies hash->list make-hash))
(define (starting-pairs d)
(~>> d (drop _ 2) (map (λ~> (substring 0 2) string->list))))
(define (new-pairs d)
(~>> d
(drop _ 2)
(map (λ~> (string-replace " -> " "")
string->list
((match-lambda
[(list a b c) (list a c b)])
_)
(sliding 2 1)))))
(define (transform d)
(~>> (map list (starting-pairs d) (new-pairs d)) (append*) (apply hash)))
(define transformation (transform data))
(define/memo (get-count polymer times)
(match times
[0
(for/fold ([counts (hash)]) ([pair (in-list polymer)])
(hash-update counts (second pair) add1 0))]
[_
(for*/fold ([counts (hash)])
([pair (in-list polymer)]
[(c n) (in-hash (get-count (hash-ref transformation pair) (sub1 times)))])
(hash-update counts c (λ~> (+ n)) 0))]))
;; part 1
(define (process-polymer d n)
(~> d
starting-polymer
(get-count _ n)
(hash-update _ (first-char d) add1 0)
hash-values
(sort >)
((λ (l) (- (first l) (last l))))))
(process-polymer data 10)
;; part 2
(process-polymer data 40)
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