Day 20: Part 1.

input/20.txt

@@ -0,0 +1,129 @@
+                                               A U       K     C   I         L   Q           W                                         
+                                               V H       F     D   F         Z   S           E                                         
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+                                                 A       L     N         E       A E E                                                 
+                                                 Q       S     T         R       A M T                                                 

src/16.rkt

@@ -38,8 +38,8 @@
   (second
    (foldr (λ (v acc)
             (match-let ([(list sum lst) acc])
-              (let ([sum (+ v sum)])
+              (let ([sum (% 10 (+ v sum))])
-                (list sum (cons (% 10 (abs sum)) lst)))))
+                (list sum (cons (abs sum) lst)))))
           (list 0 '(0)) ns)))
 
 (define (part2)

src/20.rkt

@@ -0,0 +1,105 @@
+#lang racket
+
+(require graph
+         (except-in "../lib.rkt" transpose))
+
+(define input
+  (problem-input 20))
+
+(define list-grid
+  (map string->list input))
+
+(define width
+  (length (first list-grid)))
+(define height
+  (length list-grid))
+
+(define vector-grid
+  (lists->vectors list-grid))
+
+(define (get-char coord)
+  (match-let ([(list x y) coord])
+    (vector-ref (vector-ref vector-grid y) x)))
+
+(define (neighbours coord)
+  (if (nchar=? #\. (get-char coord)) '()
+      (match-let* ([(list x y) coord]
+                   [ncoords (list (list (add1 x) y)
+                                  (list (sub1 x) y)
+                                  (list x (add1 y))
+                                  (list x (sub1 y)))])
+        (filter (∘ (∂ char=? #\.) (∂ get-char)) ncoords))))
+
+(define graph-grid
+  (let ([graph (unweighted-graph/undirected '())]
+        [coords (cartesian-product (range 2 (- width 2))
+                                   (range 2 (- height 2)))])
+    (for ([coord coords])
+      (for ([ncoord (neighbours coord)])
+        (add-edge! graph coord ncoord)))
+    graph))
+
+(define (add-coord-pairs hash coord-pairs side)
+  (for ([coord-pair coord-pairs])
+    (match-let* ([(list c1 c2) coord-pair]
+                 [char1 (get-char c1)]
+                 [char2 (get-char c2)]
+                 [(list x y) (match side
+                               [(or 'bottom 'right) c1]
+                               [(or 'top 'left) c2])]
+                 [coord (match side
+                          ['top    (list x (add1 y))]
+                          ['bottom (list x (sub1 y))]
+                          ['left   (list (add1 x) y)]
+                          ['right  (list (sub1 x) y)])])
+      (when (and (not (member char1 '(#\# #\. #\ )))
+                 (not (member char2 '(#\# #\. #\ ))))
+        (hash-set! hash
+                   (string->symbol (list->string (list char1 char2)))
+                   coord)))))
+
+(define (make-portals top-coords bottom-coords left-coords right-coords)
+  (let ([hash (make-hash)]
+        [top-coord-pairs
+         (map list
+              (cartesian-product (range 0 width) (list (first  top-coords)))
+              (cartesian-product (range 0 width) (list (second top-coords))))]
+        [bottom-coord-pairs
+         (map list
+              (cartesian-product (range 0 width) (list (first  bottom-coords)))
+              (cartesian-product (range 0 width) (list (second bottom-coords))))]
+        [left-coord-pairs
+         (map list
+              (cartesian-product (list (first  left-coords)) (range 0 height))
+              (cartesian-product (list (second left-coords)) (range 0 height)))]
+        [right-coord-pairs
+         (map list
+              (cartesian-product (list (first  right-coords)) (range 0 height))
+              (cartesian-product (list (second right-coords)) (range 0 height)))])
+    (add-coord-pairs hash top-coord-pairs    'top)
+    (add-coord-pairs hash bottom-coord-pairs 'bottom)
+    (add-coord-pairs hash left-coord-pairs   'left)
+    (add-coord-pairs hash right-coord-pairs  'right)
+    hash))
+
+(define outer-portals
+  (make-portals (list 0 1)
+                (list (- height 2) (- height 1))
+                (list 0 1)
+                (list (- width 2)  (- width 1))))
+
+(define inner-portals
+  (make-portals (list 90 91)
+                (list 37 38)
+                (list 96 97)
+                (list 37 38)))
+
+(for ([portal (hash-keys inner-portals)])
+  (add-edge! graph-grid (hash-ref outer-portals portal) (hash-ref inner-portals portal)))
+
+(define part1
+  (sub1 (length (fewest-vertices-path graph-grid
+                                      (hash-ref outer-portals 'AA)
+                                      (hash-ref outer-portals 'ZZ)))))
+
+(show-solution part1 #f)