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(define-module (day11)
#:use-module (ice-9 rdelim))
(define (part1 port)
(define line (read-line port))
(define nums (map string->number (string-split line #\space)))
(define (blink lst acc)
(if (null? lst)
(reverse acc)
(blink (cdr lst)
(if (zero? (car lst))
(cons 1 acc)
(let ((s (number->string (car lst))))
(if (even? (string-length s))
(cons (string->number (substring s (/ (string-length s) 2)))
(cons (string->number (substring s 0 (/ (string-length s) 2)))
acc))
(cons (* (car lst) 2024) acc)))))))
(define (iter lst left)
(if (zero? left)
(length lst)
(iter (blink lst '()) (- left 1))))
(iter nums 25))
(define memo (make-hash-table))
(define (next0 num)
(if (zero? num)
(list 1)
(let* ((s (number->string num))
(len (string-length s)))
(if (even? len)
(list (string->number (substring s 0 (/ len 2)))
(string->number (substring s (/ len 2))))
(list (* num 2024))))))
(define (next num)
(define val (hashv-ref memo num))
(if val
val
(let ((comp (next0 num)))
(hashv-set! memo num comp)
comp)))
(define (step ht)
(define new (make-hash-table))
(hash-for-each
(lambda (num amt)
(for-each
(lambda (v)
(hashv-set! new v
(+ (hashv-ref new v 0) amt)))
(next num)))
ht)
new)
(define (part2 port)
(define line (read-line port))
(define nums (map string->number (string-split line #\space)))
(define ht (make-hash-table))
(for-each (lambda (n)
(hashv-set! ht n
(+ 1 (hashv-ref ht n 0))))
nums)
(define (iter ht left)
(if (zero? left)
(let ((cnt 0))
(hash-for-each (lambda (k v) (set! cnt (+ cnt v)))
ht)
cnt)
(iter (step ht) (- left 1))))
(iter ht 75))
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