day 9

2020/09.rkt

@@ -0,0 +1,33 @@
+#lang br
+(require racket/file rackunit)
+
+(define vec (list->vector (map string->number (file->lines "09.rktd"))))
+
+(define preamble-length 25)
+
+(define (idx-valid? idx)
+  (define predecessors (for/list ([val (in-vector vec (- idx preamble-length) idx)])
+                         val))
+  (define target (vector-ref vec idx))
+  (for/or ([ints (in-combinations predecessors 2)])
+    (eq? (apply + ints) target)))
+
+(define invalid-number
+  (for/first ([idx (in-range (add1 preamble-length) (vector-length vec))]
+              #:unless (idx-valid? idx))
+    (vector-ref vec idx)))
+
+(check-equal? invalid-number 14144619)
+
+(check-equal?
+ (for/or ([lidx (in-range 0 (vector-length vec))])
+   (let/ec skip-to-next-lidx
+     (for/or ([ridx (in-range (add1 lidx) (vector-length vec))])
+       (define ints (for/list ([val (in-vector vec lidx (add1 ridx))])
+                      val))
+       (define sum (apply + ints))
+       (cond
+         [(> sum invalid-number) (skip-to-next-lidx #false)]
+         [(eq? sum invalid-number) (+ (apply min ints) (apply max ints))]
+         [else #false]))))
+ 1766397)