Advent of Code 2025 Discussions

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Nice, indeed, I ended up using a set for the first part and then refactoring to a hash table for the second part.

Spoilers 
#lang racket/base

(require
  racket/set)

(define tachyon-manifold
  (with-input-from-file "input.txt"
    (lambda ()
      (for/list ([line (in-lines)])
        (string->list line)))))

(define manifold-edge
  (sub1 (length (car tachyon-manifold))))

(define (beam-split beam)
  (define row (car beam))
  (define col (cdr beam))
  (let* ([fork* (if (<= 0 (- col 1) manifold-edge)
                    (list (cons row (- col 1)))
                    (list))]
         [fork* (if (<= 0 (+ col 1) manifold-edge)
                    (cons (cons row (+ col 1)) fork*)
                    fork*)])
    fork*))

(define (beam-update beams beam from)
  (hash-update
   beams beam (lambda (count) (+ count (hash-ref beams from))) 0))

(define (beams-update beams from fork*)
  (for/fold ([beams beams])
            ([beam (in-list fork*)])
    (beam-update beams beam from)))

(define (beams-total beams)
  (define last (sub1 (length tachyon-manifold)))
  (for/sum ([(posn count) (in-immutable-hash beams)]
            #:when (= last (car posn)))
    count))

(define (propagate-beams manifold)
  (for/fold ([beams (hash)]
             [split 0]
             #:result (values (beams-total beams) split))
            ([(cells row) (in-indexed (in-list manifold))])
    (for/fold ([beams beams]
               [split split]
               #:result (values beams split))
              ([(cell col) (in-indexed (in-list cells))])
      (define from (cons (- row 1) col))
      (define beam (cons row col))
      (case cell
        [(#\S)
         (values (hash-set beams beam 1) split)]
        [(#\.)
         (if (hash-has-key? beams from)
             (values (beam-update beams beam from) split)
             (values beams split))]
        [(#\^)
         (cond
           [(hash-has-key? beams from)
            (define -<* (beam-split beam))
            (define -<N (if (null? -<*) 0 1))
            (values (beams-update beams from -<*) (+ split -<N))]
           [else
            (values beams split)])]))))
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