day13

day13.rkt

@@ -0,0 +1,52 @@
+#lang racket
+
+(define (map-map f lst) ; map . map
+  (map (λ (x) (map (λ (y) (f y)) x)) lst))
+
+(define data
+  (let* ([input (file->lines "day13.txt")]
+         [point-lines (filter (λ (l) (string-contains? l ",")) input)]
+         [points (map-map string->number (map (λ (p) (string-split p ",")) point-lines))] ; e.g. '(6 10)
+         [fold-lines (filter (λ (l) (string-contains? l "fold")) input)]
+         [folds (map (λ (l) (let* ([matches (regexp-match #rx"(.)=(.+)" l)]) ; e.g. '("y" 7)
+                              (list (second matches) (string->number (third matches))))) fold-lines)])
+    (list points folds)))
+
+(define points (first data))
+(define folds (second data))
+
+(define (draw points)
+  (let* ([width (apply max (map first points))] [height (apply max (map second points))])
+    (for-each (λ (row) (begin
+                (for-each (λ (col) (printf "~a" (if (member (list col row) points) "#" "."))) (range 0 (+ 1 width)))
+                (printf "~n"))) (range 0 (+ 1 height)))))
+
+(define (move-point point dir loc)
+  (let* ([which (if (equal? "x" dir) first second)] ; which coordinate to change. x -> fold left -> change the first digit, etc.
+         [diff (* 2 (- (which point) loc))] ; calculate the amount the point should move left/up
+         [diffX (if (equal? "x" dir) (λ (x) (- x diff)) identity)]
+         [diffY (if (equal? "y" dir) (λ (x) (- x diff)) identity)])
+    (if (< (which point) loc) ; if it is below the fold
+        point ; just return it
+        (list (diffX (first point)) (diffY (second point)))))) ; otherwise, move the point
+
+(define (fold points f) ; should have picked a different name..
+  (let* ([direction (first f)]
+         [location (second f)]
+         [new-points (map (λ (point) (move-point point direction location)) points)])
+   (remove-duplicates new-points)))
+
+(define part1
+  (let* ([first-fold (first folds)]
+         [next (fold points first-fold)])
+    (length next)))
+
+part1
+
+; PART 2
+
+(define part2
+  (draw
+   (foldl (λ (f sheet) (fold sheet f)) points folds)))
+
+part2

day13.test.txt

@@ -0,0 +1,21 @@
+6,10
+0,14
+9,10
+0,3
+10,4
+4,11
+6,0
+6,12
+4,1
+0,13
+10,12
+3,4
+3,0
+8,4
+1,10
+2,14
+8,10
+9,0
+
+fold along y=7
+fold along x=5

day13.txt

@@ -0,0 +1,840 @@
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+
+fold along x=655
+fold along y=447
+fold along x=327
+fold along y=223
+fold along x=163
+fold along y=111
+fold along x=81
+fold along y=55
+fold along x=40
+fold along y=27
+fold along y=13
+fold along y=6