AoC 2018 Day 9: Marble Madness

Source: Marble Mania

Part 1: Place marbles in a circle such that each marble is placed by skipping one place except for marbles divisible by 23. For those, don’t place them, skip back 7 places, and remove that marble as well. Add these two marbles to your current score.

Given a specific player count and last marble, what’s the highest score?

Well. That’s a thing. What this problem really comes down to is a custom data structure problem. What we want is a circular list, where you can spin the list forward or backwards and efficiently add/remove the item at the first element of the list.

Since Racket doesn’t seem to have a data structure exactly like this, what I’m going to do is create my own by combining two lists. Essentially, we’ll have a list of items before (prev) to the current head of the list (stored in reverse order) and another for items after (next) the current head. As we move forward, we move items from next to prev and backwards is the reverse. If either list runs out, you can refresh it by applying a reverse to its partner list. reverse is expensive, but you only rarely need to do it (depending on performance) leading to a decent amortized runtime. Fun times.

#lang racket

(provide (all-defined-out))

(struct circular-list (prev next)
  #:transparent
  #:methods gen:custom-write
  [(define write-proc
     (λ (cls out depth)
       (fprintf out "#<circular-list ~a>" (circular-list->list cls))))])

; Create an empty circular list
(define (make-circular-list)
  (circular-list '() '()))

; Turn a list into a circular list
(define (list->circular-list ls)
  (circular-list '() ls))

; Turn a circular list back into a regular one at the current point
(define (circular-list->list cls)
  (match-define (circular-list prev next) cls)
  (append next (reverse prev)))

; Test if a circular list contains no elements
(define (circular-list-empty? cls)
  (match-define (circular-list prev next) cls)
  (and (null? prev) (null? next)))

; Get size of a circular-list
(define (circular-list-length cls)
  (match-define (circular-list prev next) cls)
  (+ (length prev) (length next)))

; Look at the current first item of a circular list
(define (circular-list-peek cls)
  (match-define (circular-list prev next) cls)
  (cond
    [(null? next) (last prev)]
    [else (first next)]))

; Add a new item to the head of a circular list
(define (circular-list-push cls value)
  (match-define (circular-list prev next) cls)
  (circular-list (list* value prev) next))

; Remove the current head of a circular list
(define (circular-list-pop cls)
  (match-define (circular-list prev next) cls)
  (cond
    [(null? next)
     (define next (reverse prev))
     (circular-list '() (rest next))]
    [else
     (circular-list prev (rest next))]))

; Rotate a circular list n positions
; Positive numbers rotate 'forward', negative 'backwards'
(define (circular-list-rotate cls [steps 1])
  (match-define (circular-list prev next) cls)
  (let loop ([prev (circular-list-prev cls)]
             [next (circular-list-next cls)]
             [steps steps])
    (cond
      [(zero? steps) (circular-list prev next)]
      [(negative? steps)
       (cond
         [(null? prev)
          (define prev (reverse next))
          (loop (rest prev) (list (first prev)) (add1 steps))]
         [else
          (loop (rest prev) (list* (first prev) next) (add1 steps))])]
      [else
       (cond
         [(null? next)
          (define next (reverse prev))
          (loop (list (first next)) (rest next) (sub1 steps))]
         [else
          (loop (list* (first next) prev) (rest next) (sub1 steps))])])))

; Rotate a circular list until the head matches the given prediate
(define (circular-list-rotate-until cls pred?)
  (let loop ([length (circular-list-length cls)]
             [cls cls])
    (cond
      [(or (zero? length)
           (pred? (circular-list-peek cls)))
       cls]
      [else
       (loop (sub1 length)
             (circular-list-rotate cls -1))])))

That’s a bit of code. circular-list-rotate is by far the most interesting code (since it has to handle reversing the lists), but hopefully still worth it.

Ironically, the code I had a bug in for the longest time? circular-list-peek. When all of the data was in prev, I was taking the first instead of last element… Oops. Works much better when you do it correctly.

With all that code, the solution is hopefully not nearly as complicated:

(let loop ([players (list->circular-list (range 1 (add1 (players))))]
           [scores (hash)]
           [table (list->circular-list '(0))]
           [marble 1])
  (cond
    ; Used up the last marble, output scores
    [(= marble (add1 (last-marble)))
     (apply max (hash-values scores))]
    ; Marbles divisible by 23 scores 23 + removes the marble 7 ago
    [(= 0 (remainder marble 23))
     (let* ([table (circular-list-rotate table -8)]
            [score (+ (circular-list-peek table) marble)]
            [table (circular-list-pop table)]
            [table (circular-list-rotate table 1)])
       (loop (circular-list-rotate players)
             (hash-update scores (circular-list-peek players) (curry + score) 0)
             table
             (add1 marble)))]
    ; All other marbles skip 1 and insert the marble
    [else
     (loop (circular-list-rotate players)
           scores
           (circular-list-push (circular-list-rotate table) marble)
           (add1 marble))]))

Well look at that.

$ racket marble-madness.rkt --players 477 --stop-at 70851

374690

I did create a print function that prints more or less the same thing in the problem statement to help debug:

(let loop ([players (list->circular-list (range 1 (add1 (players))))]
           [scores (hash)]
           [table (list->circular-list '(0))]
           [marble 1])

  (when (debug)
    (printf "[~a] [head:~a] ~a\n"
            (circular-list-peek players)
            (circular-list-peek (circular-list-rotate table))
            (circular-list-rotate-until table zero?)))

  ...)

Pretty handy:

$ racket marble-madness.rkt --players 9 --stop-at 26 --debug

[1] [head:0] #<circular-list (0)>
[2] [head:1] #<circular-list (0 1)>
[3] [head:0] #<circular-list (0 2 1)>
[4] [head:2] #<circular-list (0 2 1 3)>
[5] [head:1] #<circular-list (0 4 2 1 3)>
[6] [head:3] #<circular-list (0 4 2 5 1 3)>
[7] [head:0] #<circular-list (0 4 2 5 1 6 3)>
[8] [head:4] #<circular-list (0 4 2 5 1 6 3 7)>
[9] [head:2] #<circular-list (0 8 4 2 5 1 6 3 7)>
[1] [head:5] #<circular-list (0 8 4 9 2 5 1 6 3 7)>
[2] [head:1] #<circular-list (0 8 4 9 2 10 5 1 6 3 7)>
[3] [head:6] #<circular-list (0 8 4 9 2 10 5 11 1 6 3 7)>
[4] [head:3] #<circular-list (0 8 4 9 2 10 5 11 1 12 6 3 7)>
[5] [head:7] #<circular-list (0 8 4 9 2 10 5 11 1 12 6 13 3 7)>
[6] [head:0] #<circular-list (0 8 4 9 2 10 5 11 1 12 6 13 3 14 7)>
[7] [head:8] #<circular-list (0 8 4 9 2 10 5 11 1 12 6 13 3 14 7 15)>
[8] [head:4] #<circular-list (0 16 8 4 9 2 10 5 11 1 12 6 13 3 14 7 15)>
[9] [head:9] #<circular-list (0 16 8 17 4 9 2 10 5 11 1 12 6 13 3 14 7 15)>
[1] [head:2] #<circular-list (0 16 8 17 4 18 9 2 10 5 11 1 12 6 13 3 14 7 15)>
[2] [head:10] #<circular-list (0 16 8 17 4 18 9 19 2 10 5 11 1 12 6 13 3 14 7 15)>
[3] [head:5] #<circular-list (0 16 8 17 4 18 9 19 2 20 10 5 11 1 12 6 13 3 14 7 15)>
[4] [head:11] #<circular-list (0 16 8 17 4 18 9 19 2 20 10 21 5 11 1 12 6 13 3 14 7 15)>
[5] [head:1] #<circular-list (0 16 8 17 4 18 9 19 2 20 10 21 5 22 11 1 12 6 13 3 14 7 15)>
[6] [head:20] #<circular-list (0 16 8 17 4 18 19 2 20 10 21 5 22 11 1 12 6 13 3 14 7 15)>
[7] [head:10] #<circular-list (0 16 8 17 4 18 19 2 24 20 10 21 5 22 11 1 12 6 13 3 14 7 15)>
[8] [head:21] #<circular-list (0 16 8 17 4 18 19 2 24 20 25 10 21 5 22 11 1 12 6 13 3 14 7 15)>
[9] [head:5] #<circular-list (0 16 8 17 4 18 19 2 24 20 25 10 26 21 5 22 11 1 12 6 13 3 14 7 15)>
32

Being able to visualize a problem helps a lot…

Part 2: Try again with 100x as many marbles.

If part 1 was solved efficiently (which is why I spent a while on circular-list in the first place…), this is just a matter of running it:

$ racket marble-madness.rkt --players 477 --stop-at 7085100

3009951158
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