Day f o u r

4.rkt

@@ -0,0 +1,54 @@
+#lang racket
+
+(define range-start 356261)
+(define range-end 846303)
+
+;; garbage in garbage out!!!
+(define (nth-digit x n)
+  (modulo (floor (/ x (expt 10 n))) 10))
+
+(define (has-double? pwd)
+  (for/or ([i (in-range 5)])
+    (= (nth-digit pwd i)
+       (nth-digit pwd (add1 i)))))
+
+(define (increasing? pwd)
+  (for/and ([i (in-range 5)])
+    (>= (nth-digit pwd i)
+        (nth-digit pwd (add1 i)))))
+
+(define (pwd-valid? pwd)
+  (and (has-double? pwd)
+       (increasing? pwd)))
+
+;; Part 1
+(displayln (for/sum ([i (in-range range-start (add1 range-end))])
+             (cond [(pwd-valid? i) 1]
+                   [else 0])))
+
+;; okay we're going to do this the spaghetti way
+(define (has-exact-double? pwd)
+  (for/or ([i (in-range 5)])
+    (and
+     ;; ith digit is the same is (i+1)th digit
+     (= (nth-digit pwd i)
+        (nth-digit pwd (add1 i)))
+     ;; ith digit is not the same as (i+2)th digit, if exists
+     (not (= (nth-digit pwd i)
+             (cond
+               [(> i 3) -1]
+               [else (nth-digit pwd (+ i 2))])))
+     ;; ith digit is also not the same as (i-1)th digit, if exists
+     (not (= (nth-digit pwd i)
+             (cond
+               [(zero? i) -1]
+               [else (nth-digit pwd (sub1 i))]))))))
+
+(define (pwd-valid2? pwd)
+  (and (has-exact-double? pwd)
+       (increasing? pwd)))
+
+;; Part 2
+(displayln (for/sum ([i (in-range range-start (add1 range-end))])
+             (cond [(pwd-valid2? i) 1]
+                   [else 0])))