d06 alternate solution

2019/06.rkt

@@ -1,5 +1,5 @@
 #lang br
-(require racket/file rackunit graph)
+(require racket/file rackunit graph racket/dict)
 
 (define (string->orbits str)
   (for/list ([ln (in-list (string-split str))])
@@ -19,4 +19,34 @@
                 [(dists _) (- (hash-ref dists 'SAN) 2)]))
 
 ;; 2
-(check-eq? (count-transfers orbit-recs) 382)
+(check-eq? (count-transfers orbit-recs) 382)
+
+
+;; alternate solution, using only lists
+
+(define orbit-dict (for/list ([rec (in-list orbit-recs)])
+                     (cons (second rec) (first rec))))
+
+(define orbit-chains
+  ;; path from each object back to 'COM
+  (for/list ([orbiter (in-dict-keys orbit-dict)])
+    (let loop ([chain (list orbiter)])
+      (match (assq (car chain) orbit-dict)
+        [#false (reverse chain)]
+        [(app cdr dest) (loop (cons dest chain))]))))
+
+;; 1
+;; orbit count is one less than the length of each chain
+(check-eq? (apply + (map sub1 (map length orbit-chains))) 261306)
+
+(define you-chain (assq 'YOU orbit-chains))
+(define san-chain (assq 'SAN orbit-chains))
+(define common-tail (take-common-prefix (reverse you-chain) (reverse san-chain)))
+
+;; 2
+;; answer path is two less than the length of path between YOU and SAN
+(check-eq?
+ (- (+ (- (length you-chain) (length common-tail))
+       (- (length san-chain) (length common-tail)))
+    2)
+ 382)