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+;; Advent of Code 2020 - Day 3
+
+(require 'cl-lib)
+(require 'dash)
+(require 'f)
+(require 's)
+(require 'seq)
+
+(setq day3/input
+      (-filter (lambda (s) (not (seq-empty-p s)))
+         (s-lines (f-read "/tmp/aoc/day3.txt"))))
+
+(setq day3/input-width (length (elt day3/input 0)))
+(setq day3/input-height (length day3/input))
+
+(defun day3/thing-at-point (x y)
+  "Pun intentional."
+  (when (>= day3/input-height y)
+    (let ((x-repeated (mod (- x 1) day3/input-width)))
+      (elt (elt day3/input (- y 1)) x-repeated))))
+
+(defun day3/slope (x-steps y-steps)
+  "Produce the objects encountered through this slope until the
+  bottom of the map."
+  (cl-loop for x from 1 by x-steps
+           for y from 1 to day3/input-height by y-steps
+           collect (day3/thing-at-point x y)))
+
+;; Puzzle 1
+
+(defun day3/count-trees (x-steps y-steps)
+  (cl-loop for thing being the elements of (day3/slope x-steps y-steps)
+           count (= thing ?#)))
+
+(message "Solution to day3/1: One encounters %s trees" (day3/count-trees 3 1))
+
+;; Puzzle 2
+
+(message "Solution to day3/2 %s" (* (day3/count-trees 1 1)
+                                    (day3/count-trees 3 1)
+                                    (day3/count-trees 5 1)
+                                    (day3/count-trees 7 1)
+                                    (day3/count-trees 1 2)))