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+;; Advent of Code 2020 - Day 7
+
+(require 'cl-lib)
+(require 'dash)
+(require 'f)
+(require 's)
+(require 'ht)
+
+(defvar day7/input
+  (s-lines (s-chomp (f-read "/tmp/aoc/day7.txt"))))
+
+(defun day7/parse-bag (input)
+  (string-match (rx line-start
+                    (group (one-or-more (or letter space)))
+                    "s contain "
+                    (group (one-or-more anything))
+                    "." line-end)
+                input)
+  (cons (match-string 1 input)
+        (-map
+         (lambda (content)
+           (unless (equal content "no other bags")
+             (progn
+               (string-match
+                (rx (group (one-or-more digit))
+                    space
+                    (group (one-or-more anything) "bag"))
+                content)
+               (cons (match-string 2 content)
+                     (string-to-number (match-string 1 content))))))
+         (s-split ", " (match-string 2 input)))))
+
+(defun day7/id-or-next (table bag-type)
+  (unless (ht-contains? table bag-type)
+    (ht-set table bag-type (length (ht-keys table))))
+  (ht-get table bag-type))
+
+(defun day7/build-graph (input &optional flip)
+  "Represent graph mappings directionally using an adjacency
+  matrix, because that's probably easiest.
+
+  By default an edge means 'contains', with optional argument
+  FLIP edges are inverted and mean 'contained by'."
+
+  (let ((bag-mapping (ht-create))
+        (graph (let ((length (length input)))
+                 (apply #'vector
+                        (-map (lambda (_) (make-vector length 0)) input)))))
+    (cl-loop for bag in (-map #'day7/parse-bag input)
+             for bag-id = (day7/id-or-next bag-mapping (car bag))
+             do (-each (-filter #'identity (cdr bag))
+                  (pcase-lambda (`(,contained-type . ,count))
+                    (let ((contained-id (day7/id-or-next bag-mapping contained-type)))
+                      (if flip
+                          (aset (aref graph contained-id) bag-id count)
+                        (aset (aref graph bag-id) contained-id count))))))
+    (cons bag-mapping graph)))
+
+;; Puzzle 1
+
+(defun day7/find-ancestors (visited graph start)
+  (ht-set visited start t)
+  (cl-loop for bag-count being the elements of (aref graph start)
+           using (index bag-id)
+           when (and (> bag-count 0)
+                     (not (ht-contains? visited bag-id)))
+           do (day7/find-ancestors visited graph bag-id)))
+
+(message
+ "Solution to day7/1: %s"
+ (pcase-let* ((`(,mapping . ,graph) (day7/build-graph day7/input t))
+              (shiny-gold-id (ht-get mapping "shiny gold bag"))
+              (visited (ht-create)))
+   (day7/find-ancestors visited graph shiny-gold-id)
+   (- (length (ht-keys visited)) 1)))
+
+;; Puzzle 2
+
+(defun ht-find-by-value (table value)
+  (ht-find (lambda (_key item-value) (equal item-value value)) table))
+
+(defun day7/count-contained-bags (mapping graph start)
+  (cl-loop for bag-count being the elements of (aref graph start)
+           using (index bag-id)
+           when (> bag-count 0)
+           sum (+ bag-count
+                  (* bag-count (day7/count-contained-bags mapping graph bag-id)))))
+
+(message "Solution to day7/2: %s"
+         (pcase-let* ((`(,mapping . ,graph) (day7/build-graph day7/input))
+                      (shiny-gold-id (ht-get mapping "shiny gold bag")))
+           (day7/count-contained-bags mapping graph shiny-gold-id)))