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1 files changed, 39 insertions, 0 deletions

diff --git a/racket/aoc2019/day-04/day-04.rkt b/racket/aoc2019/day-04/day-04.rkt
new file mode 100644
index 0000000..9518779
--- /dev/null
+++ b/racket/aoc2019/day-04/day-04.rkt
@@ -0,0 +1,39 @@
+#lang racket
+
+(define (number->digits n [acc '()])
+  (cond
+    [(< n 10) (cons n acc)]
+    [else (number->digits (quotient n 10) (cons (remainder n 10) acc))]))
+
+(define (always-increasing? xs)
+  (match xs
+    [(list _) #t]
+    [(list* a b _) #:when (<= a b) (always-increasing? (cdr xs))]
+    [_ #f]))
+
+(define (adjacent-pair? xs)
+  (match xs
+    [(list _) #f]
+    [(list* a a _) a]
+    [_ (adjacent-pair? (cdr xs))]))
+
+;; part 1
+(for/sum ([password (inclusive-range 125730 579381)]
+          #:do [(define digits (number->digits password))]
+          #:when ((conjoin always-increasing? adjacent-pair?) digits))
+  1)
+
+;; part 2
+(define (not-in-adjacent-triplet? xs)
+  (match xs
+    [(list a a c _ _ _) #:when (not (= a c)) #t]
+    [(list b a a c _ _) #:when (not (or (= a b) (= a c))) #t]
+    [(list _ b a a c _) #:when (not (or (= a b) (= a c))) #t]
+    [(list _ _ b a a c) #:when (not (or (= a b) (= a c))) #t]
+    [(list _ _ _ b a a) #:when (not (= a b)) #t]
+    [_ #f]))
+
+(for/sum ([password (inclusive-range 125730 579381)]
+          #:do [(define digits (number->digits password))]
+          #:when ((conjoin always-increasing? adjacent-pair? not-in-adjacent-triplet?) digits))
+  1)
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