You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
64 lines
2.4 KiB
Racket
64 lines
2.4 KiB
Racket
#lang scribble/lp2
|
|
@(require scribble/manual aoc-racket/helper)
|
|
|
|
@aoc-title[10]
|
|
|
|
@link["http://adventofcode.com/day/10"]{The puzzle}. Our @link-rp["day10-input.txt"]{input} is a short numeric key.
|
|
|
|
@chunk[<day10>
|
|
<day10-setup>
|
|
<day10-q1>
|
|
<day10-q2>
|
|
<day10-test>]
|
|
|
|
@section{What's the length of the sequence after 40 iterations?}
|
|
|
|
The puzzle asks us to compute the @italic{look and say} sequence invented by mathematician John Conway. Each iteration of the sequence is the description of the last step if you said it in numbers. So @racket[1] becomes ``one 1'', written @racket[11]; @racket[11] becomes ``two ones'', or @racket[21], then @racket[1211], @racket[111221], and so on.
|
|
|
|
As in @secref{Day_1}, this puzzle relies on cumulative state, so we'll loop using @racket[for/fold]. To generate the new string for each pass of the loop, we'll use @racket[regexp-match*] to find every contiguous run of digits. Each digit run will be converted into a list with the number of digits and then digit. Then all these lists of numbers are concatenated into a new string, and the process repeats.
|
|
|
|
The second part of the puzzle is just going to change the number of iterations. So we'll make one function that we can use for both parts.
|
|
|
|
@chunk[<day10-setup>
|
|
(require racket rackunit)
|
|
|
|
(define (look-and-say iterations input-key)
|
|
(for/fold ([start input-key])
|
|
([i (in-range iterations)])
|
|
(define digit-runs (regexp-match* #px"(\\d)\\1*" start))
|
|
(string-append*
|
|
(map ~a
|
|
(append-map (λ(digit-run)
|
|
(list (string-length digit-run)
|
|
(substring digit-run 0 1)))
|
|
digit-runs)))))
|
|
|
|
]
|
|
|
|
@chunk[<day10-q1>
|
|
|
|
(define (q1 input-key)
|
|
(string-length (look-and-say 40 input-key)))]
|
|
|
|
|
|
|
|
@section{After 50 iterations?}
|
|
|
|
We just run the same @racket[look-and-say] function 50 times rather than 40.
|
|
|
|
@chunk[<day10-q2>
|
|
|
|
(define (q2 input-key)
|
|
(string-length (look-and-say 50 input-key))) ]
|
|
|
|
|
|
@section{Testing Day 10}
|
|
|
|
@chunk[<day10-test>
|
|
(module+ test
|
|
(define input-key (file->string "day10-input.txt"))
|
|
(check-equal? (q1 input-key) 492982)
|
|
(check-equal? (q2 input-key) 6989950))]
|
|
|
|
|