Advent of Code 2021 https://adventofcode.com/
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#lang racket
(define input
(let* ([lines (file->lines "day14.txt")]
[template (first lines)]
[rule-strings (drop lines 2)]
[rules (map (λ (rs) (string-split rs " -> ")) rule-strings)])
(list template rules)))
(define template (first input))
(define rules (second input))
(define (find-pairs template)
(reverse
(foldl (λ (el acc) (cons (substring template el (+ 2 el)) acc))
'()
(range 0 (sub1 (string-length template))))))
(define (find-inserts pairs rules)
(map (λ (p) (second (assoc p rules))) pairs))
(define (insert-pairs template inserts)
(foldl (λ (idx acc)
(let* ([x (string (string-ref template idx))]
[insert (list-ref (append inserts (list "")) idx)])
(string-append acc x insert)))
"" (range 0 (string-length template))))
(define (step template rules)
(let* ([pairs (find-pairs template)]
[inserts (find-inserts pairs rules)]
[next (insert-pairs template inserts)])
next))
(define (element-counts str)
(let* ([lst (map string (string->list str))]
[ht (make-hash)])
(for-each (λ (x) (hash-set! ht x (+ 1 (hash-ref ht x 0)))) lst)
ht))
(define part1
(let* ([final (foldl (λ (i acc) (step acc rules)) template (range 0 10))]
[counts (element-counts final)]
[values (hash-values counts)]
[minimum (apply min values)]
[maximum (apply max values)]
[diff (- maximum minimum)])
diff))
part1
; PART 2
; uhoh. start over
(define (inc-hash ht key amount)
(hash-set! ht key (+ amount (hash-ref! ht key 0))))
(define (dec-hash ht key amount)
(hash-set! ht key (- (hash-ref! ht key 0) amount)))
(define (step2 ht)
(define pairs (filter (λ (p) (> (cdr p) 0)) (hash->list ht)))
(for-each (λ (pair-and-count)
(let* ([pair (car pair-and-count)]
[count (cdr pair-and-count)]
[ins (second (assoc pair rules))]
[new1 (string-append (substring pair 0 1) ins)]
[new2 (string-append ins (substring pair 1 2))])
(inc-hash ht new1 count)
(inc-hash ht new2 count)
(dec-hash ht pair count)))
pairs)
ht)
(define (pairs->element-counts ht)
(let* ([el-ht (make-hash)]
[pair-counts (hash->list ht)])
(for-each (λ (p) (let* ([pair (car p)]
[el1 (substring pair 0 1)]
[el2 (substring pair 1 2)]
[count (cdr p)])
(inc-hash el-ht el1 count)
(inc-hash el-ht el2 count)))
pair-counts)
el-ht))
(define part2
(let* ([ht (make-hash)]
[pairs (find-pairs template)]) ; find the initial pairs in the template
(for-each (λ (pair) (inc-hash ht pair 1)) pairs) ; add them to the hash table
(let* ([res (foldl (λ (el acc) (step2 acc)) ht (range 0 40))] ; step!
[el-counts (pairs->element-counts res)]
[first-char (substring template 0 1)]
[last-char (substring template (sub1 (string-length template)))])
(inc-hash el-counts first-char 1) ; add the first..
(inc-hash el-counts last-char 1) ; and last chars twice..
(let* ([lst (hash-map el-counts (λ (key val) (list key (/ val 2))))] ; then divide everything by 2
[values (map second lst)]
[minimum (apply min values)]
[maximum (apply max values)]
[diff (- maximum minimum)])
diff))))
part2