Day 4.

src/04.rkt

@@ -0,0 +1,40 @@
+#lang racket
+
+(require "../lib.rkt")
+
+(define-values (start end) (values 235741 706948))
+
+(define (neq? b1 b2) (not (eq? b1 b2)))
+
+(define (two-adjacent-digits? cs)
+  (if (< (length cs) 2) #f
+      (or (eq? (first cs)
+               (second cs))
+          (two-adjacent-digits? (cdr cs)))))
+
+(define (digits-never-decrease? cs)
+  (if (< (length cs) 2) #t
+      (and (char<=? (first cs)
+                    (second cs))
+           (digits-never-decrease? (cdr cs)))))
+
+(define (has-group-of-two? cs)
+  (cond [(< (length cs) 2) #f]
+        [(= (length cs) 2)
+         (eq? (first cs) (second cs))]
+        [else
+         (or (and (eq? (first cs)
+                       (second cs))
+                  (neq? (second cs)
+                        (third cs)))
+             (has-group-of-two? (dropf cs (λ (c) (eq? c (first cs))))))]))
+
+(define-values (part1 part2)
+  (let* ([passwords (map (compose string->list number->string) (range start (add1 end)))]
+         [passwords (filter two-adjacent-digits? passwords)]
+         [passwords (filter digits-never-decrease? passwords)]
+         [passwords* (filter has-group-of-two? passwords)])
+    (values (length passwords)
+            (length passwords*))))
+
+(show-solution part1 part2)