1 files changed, 44 insertions, 0 deletions
diff --git a/2020/day-07/day-07.rkt b/2020/day-07/day-07.rkt
new file mode 100644
index 0000000..6ad63e5
--- /dev/null
+++ b/2020/day-07/day-07.rkt
@@ -0,0 +1,44 @@
+#lang racket
+(require "../../jj-aoc.rkt"
+         threading
+         rebellion/collection/entry
+         rebellion/collection/multidict)
+
+(define raw-rules (~> (open-day 7 2020) (port->string) (string-split "\n")))
+
+(define (split-rule r)
+  (match-define (list head tail) (string-split (string-trim r #px" bags?.") " bags contain "))
+  (~>> tail
+       (regexp-split #px"( bags?,\\s)" _)
+       (map (λ~> (regexp-match* #px"(\\d+) (\\w+ \\w+)" _ #:match-select rest)))
+       append*
+       (map (λ~> (match _
+                   [(list n c) (list (string->symbol c) (string->number n))]
+                   ['() '()])))
+       (cons (string->symbol head) _)))
+
+(define rules-multidict
+  (for*/multidict ([ln (in-list raw-rules)] #:do [(match-define (list* from tos) (split-rule ln))]
+                                            [to (in-list tos)])
+    (entry from to)))
+
+;; part 1
+
+(define (bags-that-eventually-contain target)
+  (for/fold ([holders (set)]) ([rule (in-multidict-entries rules-multidict)])
+    (match rule
+      [(entry outside (list (== target) _))
+       (set-union (set-add holders outside) (bags-that-eventually-contain outside))]
+      [_ holders])))
+
+(time (set-count (bags-that-eventually-contain '|shiny gold|)))
+
+;; part 2
+
+(define (bags-that-are-contained-by target)
+  (for/sum ([holding (in-multidict-entries rules-multidict)])
+           (match holding
+             [(entry (== target) (list held n)) (+ n (* n (bags-that-are-contained-by held)))]
+             [_ 0])))
+
+(time (bags-that-are-contained-by '|shiny gold|))