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+;; Advent of Code 2020 - Day 6
+
+(require 'cl-lib)
+(require 'dash)
+(require 'f)
+(require 'ht)
+(require 's)
+
+(defvar day6/input (s-split "\n\n" (f-read "/tmp/aoc/day6.txt") t)
+  "Input, split into groups (with people in each group still distinct)")
+
+;; Puzzle 1
+
+(defun day6/count-answers (group-answers)
+  "I suspect doing it this way will be useful in puzzle 2."
+  (let ((table (ht-create)))
+    (-each group-answers
+      (lambda (answer)
+        (cl-loop for char across answer
+                 do (ht-set table char (+ 1 (or (ht-get table char)
+                                                0))))))
+    table))
+
+(message "Solution to day6/1: %s"
+         (cl-loop for group being the elements of day6/input
+                  sum (length
+                       (ht-keys
+                        (day6/count-answers (s-lines group))))))
+
+;; Puzzle 2
+
+(defun day6/count-unanimous-answers (answers)
+  (ht-reject (lambda (_key value) (not (= value (length answers))))
+             (day6/count-answers answers)))
+
+(message "Solution to day6/2: %s"
+         (cl-loop for group being the elements of day6/input
+                  sum (length
+                       (ht-keys
+                        (day6/count-unanimous-answers (s-split "\n" group t))))))