d06

2018/06-test.txt

@@ -0,0 +1,6 @@
+1, 1
+1, 6
+8, 3
+3, 4
+5, 5
+8, 9

2018/06.rkt

@@ -0,0 +1,65 @@
+#lang debug br
+
+#|
+Start with the area of the grid bounded by the locations.
+Calculate the neighborhoods within this grid.
+Any location that has a grid neighborhood that includes an edge piece
+will grow to infinity (because adjacent pieces outside the grid will also be closest to that location)
+So ignore those. And search the others for the answer.
+|#
+
+(define locs (for/list ([ln (in-lines (open-input-file "06.txt"))])
+               (match (map string->number (string-split ln ", "))
+                 [(list x y) (cons x y)])))
+
+(define (bounding-box locs)
+  (match locs
+    [(list (cons xs ys) ...)
+     (list (cons (apply min xs) (apply max ys))
+           (cons (apply max xs) (apply min ys)))]))
+
+(define bbox (bounding-box locs))
+
+(define (manhattan-dist loca locb)
+  (match (list loca locb)
+    [(list (cons xa ya) (cons xb yb))
+     (+ (abs (- xa xb)) (abs (- ya yb)))]))
+
+(define (closest-to pt locs)
+  (match (sort (map (λ (loc) (cons (manhattan-dist pt loc) loc)) locs) < #:key car)
+    [(list* (cons score _) (cons score2 _) _) #:when (= score score2) #false] ; because tie 
+    [(cons (cons _ loc) _) loc]))
+
+(define (finite-neighborhoods locs)
+  (define closest-table (make-hash))
+  (match-define (list (cons xmin ymax) (cons xmax ymin)) bbox)
+  (for* ([x (in-range xmin (add1 xmax))]
+         [y (in-range ymin (add1 ymax))])
+    (define pt (cons x y))
+    (define loc (closest-to pt locs))
+    (when loc
+      (if (or (= x xmin) (= x xmax) (= y ymin) (= y ymax)) ; on edge, therefore infinite
+          (hash-set! closest-table loc #f) ; invalidate loc
+          (hash-update! closest-table loc (λ (val) (and val (cons pt val))) null))))
+  (filter values (hash-values closest-table)))
+
+(define (★)
+  (apply max (map length (finite-neighborhoods locs))))
+
+(define (region locs target-dist)
+  (match-define (list (cons xmin ymax) (cons xmax ymin)) bbox)
+  (for*/list ([x (in-range xmin (add1 xmax))]
+              [y (in-range ymin (add1 ymax))]
+              [pt (in-value (cons x y))]
+              [total-dist (in-value (for/sum ([loc (in-list locs)])
+                                      (manhattan-dist pt loc)))]
+              #:when (< total-dist target-dist))
+    pt))
+
+(define (★★)
+  (length (region locs 10000)))
+
+(module+ test
+  (require rackunit)
+  (check-equal? (time (★)) 6047)
+  (check-equal? (time (★★)) 46320))

2018/06.txt

@@ -0,0 +1,50 @@
+165, 169
+334, 217
+330, 227
+317, 72
+304, 232
+115, 225
+323, 344
+161, 204
+316, 259
+63, 250
+280, 205
+84, 282
+271, 158
+190, 296
+106, 349
+171, 178
+203, 108
+89, 271
+193, 254
+111, 210
+341, 343
+349, 311
+143, 172
+170, 307
+128, 157
+183, 315
+211, 297
+74, 281
+119, 164
+266, 345
+184, 62
+96, 142
+134, 61
+117, 52
+318, 72
+338, 287
+61, 215
+323, 255
+93, 171
+325, 249
+183, 171
+71, 235
+329, 306
+322, 219
+151, 298
+180, 255
+336, 291
+72, 300
+223, 286
+179, 257

2018/stub.rkt

@@ -2,9 +2,11 @@
 
 #;(define (★)
   )
+#;(★)
 
 #;(define (★★)
   )
+#;(★★)
 
 #;(module+ test
   (require rackunit)