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47 lines
1.5 KiB
Racket
47 lines
1.5 KiB
Racket
#lang debug br
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(require graph)
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#|
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Track the progress through the steps with a dag,
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but track the state of the prerequisites with another dag (in the reverse direction)
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|#
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(define dag (directed-graph null))
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(define prereqs (weighted-graph/directed null))
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(for ([ln (in-lines (open-input-file "07.txt"))])
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(match-define (list left right) (regexp-match* #rx"(?<=[Ss]tep )." ln))
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(add-directed-edge! dag left right)
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(add-directed-edge! prereqs right left +inf.0))
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(define (activate-prereq! v1 v2)
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;; a prereq is "met" if the edge weight is zero
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(add-directed-edge! prereqs v2 v1 0))
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(define (prereqs-met? v)
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;; check if all v's edges in the prereq graph are zero
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(andmap zero? (map (λ (n) (edge-weight prereqs v n)) (get-neighbors prereqs v))))
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(define (find-available g)
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(filter prereqs-met? (get-vertices g)))
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(define (★)
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(let loop ([vs-available (find-available prereqs)] [visited null])
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(cond
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[(= (length visited) (length (get-vertices dag)))
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(apply string-append (reverse visited))]
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[else
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(match (sort vs-available string<?)
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[(list this-v others ...)
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(for-each (λ (n) (activate-prereq! this-v n)) (get-neighbors dag this-v))
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(define this-visited (cons this-v visited))
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(loop (filter-not (λ (v) (member v this-visited)) (find-available prereqs))
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this-visited)])])))
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#;(define (★★)
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)
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#;(★★)
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(module+ test
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(require rackunit)
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(check-equal? (time (★)) "GNJOCHKSWTFMXLYDZABIREPVUQ")
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#;(check-equal? (time (★★)) 566)) |