Racket solutions & explanations for the Advent of Code puzzles
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aoc-racket/day05.rkt

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#lang scribble/lp2
@(require scribble/manual aoc-racket/helper)
@aoc-title[5]
@defmodule[aoc-racket/day05]
@link["http://adventofcode.com/day/5"]{The puzzle}. Our @link-rp["day05-input.txt"]{input} is a list of random-looking but not really random text strings.
@chunk[<day05>
<day05-setup>
<day05-q1>
<day05-q2>
<day05-test>]
@isection{How many strings are ``nice''?}
A string is ``nice'' if it meets certain criteria:
@itemlist[
@item{Contains three vowels (= @litchar{aeiou}).}
@item{Contains a double letter.}
@item{Does not contain @litchar{ab}, @litchar{cd}, @litchar{pq}, or @litchar{xy}.}
]
This is a job for @iracket[regexp-match]. There's nothing tricky here (except for remembering that certain matching functions require the @iracket[pregexp] pattern prefix rather than @racket[regexp]).
@chunk[<day05-setup>
(require racket rackunit)
(provide (all-defined-out))
]
@chunk[<day05-q1>
(define (nice? str)
(define (three-vowels? str)
(>= (length (regexp-match* #rx"[aeiou]" str)) 3))
(define (double-letter? str)
(regexp-match #px"(.)\\1" str))
(define (no-kapu? str)
(not (regexp-match #rx"ab|cd|pq|xy" str)))
(and (three-vowels? str)
(double-letter? str)
(no-kapu? str)))
(define (q1 words)
(length (filter nice? words)))
]
@section{How many strings are ``nice'' under new rules?}
This time a string is ``nice`` if it:
@itemlist[
@item{Contains a pair of two letters that appears twice without overlapping}
@item{Contains a letter that repeats with at least one letter in between}
]
Again, a test of your regexp-writing skills.
@chunk[<day05-q2>
(define (nicer? str)
(define (nonoverlapping-pair? str)
(regexp-match #px"(..).*\\1" str))
(define (separated-repeater? str)
(regexp-match #px"(.).\\1" str))
(and (nonoverlapping-pair? str)
(separated-repeater? str) #t))
(define (q2 words)
(length (filter nicer? words)))]
@section{Testing Day 5}
@chunk[<day05-test>
(module+ test
(define input-str (file->lines "day05-input.txt"))
(check-equal? (q1 input-str) 238)
(check-equal? (q2 input-str) 69))]