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52 lines
2.0 KiB
Plaintext
52 lines
2.0 KiB
Plaintext
9 years ago
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#lang scribble/lp2
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@(require scribble/manual aoc-racket/helper)
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@(require (for-label openssl/md5))
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@aoc-title[4]
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Our @link-rp["day4-input.txt"]{input} is a string of eight characters that represents part of a key for making an MD5 hash.
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@chunk[<day4>
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<setup>
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<q1>
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<q2>
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<test>]
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@section{What is the lowest-numbered MD5 hash starting with five zeroes?}
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We're asked to create an MD5 hash from an input key that consists of our eight-character input joined to a decimal number. The puzzle asks us to find the lowest decimal number that, when joined to our input, produces an MD5 hash that starts with five zeroes.
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Whether or not you already know what an MD5 hash is, you can search the Racket docs and will soon find the @racketmodname[openssl/md5] module and the @racket[md5] function. Then, this puzzle is easy: starting at @racket[0], make new input keys with each integer, and stop when we find one that results in the MD5 hash we want. (The approach is similar to the second part of @secref["day-1"].)
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@chunk[<setup>
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(require racket rackunit openssl/md5)
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]
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@chunk[<q1>
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(define (q1 str)
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(for/or ([i (in-naturals)])
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(define md5-key (string-append str (~a i)))
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(define md5-hash (md5 (open-input-string md5-key)))
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(and (string-prefix? md5-hash "00000") i)))
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]
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@section{How about six zeroes?}
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Exactly the same, except we test for a string of six zeroes. It is likely, however, to take quite a bit longer to run, as the sixth zero essentially makes the criterion 10 times more stringent.
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@chunk[<q2>
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(define (q2 str)
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(for/or ([i (in-naturals)])
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(define md5-key (string-append str (~a i)))
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(define md5-hash (md5 (open-input-string md5-key)))
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(and (string-prefix? md5-hash "000000") i)))]
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@section{Testing our input}
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@chunk[<test>
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(module+ test
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(define input-str (file->string "day4-input.txt"))
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(check-equal? (q1 input-str) 346386)
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(check-equal? (q2 input-str) 9958218))]
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