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