day10

aoc-racket.scrbl

@@ -18,3 +18,4 @@
 @include-section[(submod "day7.scrbl" doc)]
 @include-section[(submod "day8.scrbl" doc)]
 @include-section[(submod "day9.scrbl" doc)]
+@include-section[(submod "day10.scrbl" doc)]

day10-input.txt

@@ -0,0 +1 @@
+1321131112

day10.scrbl

@@ -0,0 +1,63 @@
+#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))]
+
+

day7.scrbl

@@ -38,7 +38,7 @@ becomes
 
 (@racket[wire-value-cache] is just a performance enhancement, so that wire values don't have to be computed multiple times.)
 
-One gotcha when using syntax transformers is that newly defined identifiers can mask others. For instance, one of the wires in our input is named @tt{if}. When our syntax transformer defines the @tt{if} function, it will override the usual meaning of @racket[if]. There are plenty of elegant ways to prevent these name collisions. (The most important of which is called @italic{syntax hygiene}, and permeates the design of Racket's syntax-transformation system.) But because this is a puzzle, we'll take the cheap way out: we won't use @racket[if] elsewhere in our code, and instead use @racket[cond].
+One gotcha when using syntax transformers is that identifiers introduced by a transformer can silently override others (in the same way that identifiers defined inside a @racket[let] will override those with the same name outside the @racket[let]). For instance, one of the wires in our input is named @tt{if}. When our syntax transformer defines the @tt{if} function, it will override the usual meaning of @racket[if]. There are plenty of elegant ways to prevent these name collisions. (The most important of which is called @italic{syntax hygiene}, and permeates the design of Racket's syntax-transformation system.) But because this is a puzzle, we'll take the cheap way out: we won't use @racket[if] elsewhere in our code, and instead use @racket[cond].
 
 @chunk[<day7-setup>
        (require racket rackunit