d04

2019/04.rkt

@@ -0,0 +1,48 @@
+#lang br
+(require racket/file rackunit)
+
+(match-define (list low high)
+  (map string->number (string-split (file->string "04.rktd") "-")))
+
+(define (double-digit? num)
+  (define numstr (number->string num))
+  (match (for/list ([c0 (in-string numstr)]
+                    [c1 (in-string numstr 1)]
+                    #:when (char=? c0 c1))
+           (string->number (string c0)))
+    [(? null?) #false]
+    [nums nums]))
+
+(define (num->digits num)
+  (for/list ([c (in-string (number->string num))])
+    (string->number (string c))))
+
+(define (monotonic? num)
+  (apply <= (num->digits num)))
+
+;; 1
+(check-eq?
+ (for/sum ([num (in-range low (add1 high))]
+           #:when (and (double-digit? num) (monotonic? num)))
+   1)
+ 1079)
+
+(define (triple-digit? num digit)
+  (define numstr (number->string num))
+  (define digitc (car (string->list (number->string digit))))
+  (for/or ([c0 (in-string numstr)]
+           [c1 (in-string numstr 1)]
+           [c2 (in-string numstr 2)])
+    (char=? c0 c1 c2 digitc)))
+
+;;2
+(check-eq?
+ (for/sum ([num (in-range low (add1 high))]
+           #:when (and (for/or ([digits (in-value (double-digit? num))]
+                                #:when digits
+                                [digit (in-list digits)])
+                         (not (triple-digit? num digit)))
+                       (monotonic? num)))
+   1)
+ 699)
+

2019/04.rktd

@@ -0,0 +1 @@
+245318-765747