aoc 2021 day02 part2HEADmaster

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+;; https://adventofcode.com/2021/day/2
+
+; optional -- 'sum' is more visually indicative than 'apply +'
+(func sum (nums) (apply + nums))
+
+(func update-vertical-coords (current instruction)
+    (def aim
+        (if (eq? "up" (first instruction))
+            (- (last current) (last instruction))
+            (+ (last current) (last instruction))))
+    (list
+        (first current) ; horiz unchanged
+        (first (rest current)) ; depth unchanged
+        aim))
+
+(func update-coords (current instruction)
+    ; expecting (horiz depth aim)
+    (if (eq? "forward" (first instruction))
+        (list
+            (+ (first current) (last instruction))  ; increase horizontal
+            (+ (first (rest current)) (* (last current) (last instruction))) ; increase depth by aim * x
+            (last current)) ; aim unchanged
+        (update-vertical-coords current instruction)))
+
+(func do-the-thing (inp)
+
+    ; since order matters now (both multiplication and addition),
+    ; go through each and update one by one
+    ; this *could* be recursive, but i don't know if that would
+    ; be any clearer
+    (def current (list 0 0 0))
+    (for-each inp
+        (redef current (update-coords current _item_)))
+
+    (print (concat "horiz: " (->string (first current))))
+    (print (concat "depth: " (->string (first (rest current)))))
+    (print (concat "answer: " (->string (apply * (most current))))))
+
+(def lines 
+    (map
+        (lambda (x) (list (first x) (string->int (first (rest x))))) ; convert second values to int
+        (map 
+            (lambda (x) (split x " "))
+            (map strip
+                (read-lines "input.txt")))))
+
+(do-the-thing lines)