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67 lines
2.5 KiB
Racket
67 lines
2.5 KiB
Racket
#lang scribble/lp2
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@(require scribble/manual aoc-racket/helper)
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@aoc-title[10]
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@defmodule[aoc-racket/day10]
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@link["http://adventofcode.com/day/10"]{The puzzle}. Our @link-rp["day10-input.txt"]{input} is a short numeric key.
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@chunk[<day10>
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<day10-setup>
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<day10-q1>
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<day10-q2>
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<day10-test>]
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@section{What's the length of the sequence after 40 iterations?}
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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.
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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 is converted into a list with the number of digits and the digit itself. Then all these lists are concatenated into a new string, and the loop repeats.
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The second part of the puzzle is just going to change the number of iterations. So we'll make one function that can be used for both parts.
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@chunk[<day10-setup>
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(require racket rackunit)
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(provide (all-defined-out))
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(define (look-and-say iterations input-key)
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(for/fold ([start input-key])
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([i (in-range iterations)])
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(define digit-runs (regexp-match* #px"(\\d)\\1*" start))
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(string-append*
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(map ~a
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(append-map (λ(digit-run)
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(list (string-length digit-run)
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(substring digit-run 0 1)))
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digit-runs)))))
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]
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@chunk[<day10-q1>
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(define (q1 input-key)
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(string-length (look-and-say 40 input-key)))]
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@section{After 50 iterations?}
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We use the same @racket[look-and-say] function, but with an iteration argument of @racket[50] rather than @racket[40].
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@chunk[<day10-q2>
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(define (q2 input-key)
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(string-length (look-and-say 50 input-key))) ]
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@section{Testing Day 10}
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@chunk[<day10-test>
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(module+ test
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(define input-key (file->string "day10-input.txt"))
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(check-equal? (q1 input-key) 492982)
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(check-equal? (q2 input-key) 6989950))]
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