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#lang br
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(require racket/file rackunit graph racket/dict)
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(define (string->orbits str)
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(for/list ([ln (in-list (string-split str))])
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(map string->symbol (string-split ln ")"))))
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(define orbit-recs (string->orbits (file->string "06.rktd")))
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(define (count-orbits orbit-recs)
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(match/values (bfs (undirected-graph orbit-recs) 'COM)
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[(dists _) (apply + (hash-values dists))]))
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;; 1
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(check-eq? (count-orbits orbit-recs) 261306)
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(define (count-transfers orbit-recs)
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(match/values (bellman-ford (undirected-graph orbit-recs) 'YOU)
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[(dists _) (- (hash-ref dists 'SAN) 2)]))
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;; 2
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(check-eq? (count-transfers orbit-recs) 382)
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;; alternate solution, using only lists
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(define orbit-dict (for/list ([rec (in-list orbit-recs)])
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(cons (second rec) (first rec))))
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(define orbit-chains
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;; path from each object back to 'COM
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(for/list ([orbiter (in-dict-keys orbit-dict)])
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(let loop ([chain (list orbiter)])
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(match (assq (car chain) orbit-dict)
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[#false (reverse chain)]
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[(app cdr dest) (loop (cons dest chain))]))))
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;; 1
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;; orbit count is one less than the length of each chain
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(check-eq? (apply + (map sub1 (map length orbit-chains))) 261306)
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(define you-chain (assq 'YOU orbit-chains))
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(define san-chain (assq 'SAN orbit-chains))
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(define common-tail (take-common-prefix (reverse you-chain) (reverse san-chain)))
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;; 2
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;; answer path is two less than the length of path between YOU and SAN
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(check-eq?
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(- (+ (- (length you-chain) (length common-tail))
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(- (length san-chain) (length common-tail)))
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2)
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382)
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