master
Matthew Butterick 2 years ago
parent a708f18669
commit 61e5bf7d9c

@ -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)

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