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diff --git a/racket/aoc2020/day-10/day-10.rkt b/racket/aoc2020/day-10/day-10.rkt
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+#lang racket
+
+(require advent-of-code
+         threading
+         algorithms
+         memoize)
+
+;; part 1
+(define adapters
+  (~> (fetch-aoc-input (find-session) 2020 10)
+      (string-split "\n")
+      (map string->number _)
+      (sort <)
+      ((λ (xs) (flatten (list 0 xs (+ 3 (last xs))))))))
+
+(~>> adapters
+     (sliding _ 2)
+     (map (match-lambda
+            [(list b a) (- a b)]))
+     (group-by identity)
+     (map length)
+     (apply *))
+
+;; part 2
+(define subpaths
+  (for*/hash ([adapter (in-list adapters)])
+    (define predecessor-candidates (inclusive-range (+ 1 adapter) (+ 3 adapter)))
+    (values adapter (filter (λ (p) (member p adapters)) predecessor-candidates))))
+
+(define/memo (find-paths from to)
+             (define paths (hash-ref subpaths from 'failed))
+             (match paths
+               ['failed 0]
+               [(list-no-order (== to) _ ...) 1]
+               [ts (for/sum ([t (in-list ts)]) (find-paths t to))]))
+
+(find-paths (first adapters) (last adapters))