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+#lang racket
+(require advent-of-code
+         threading
+         algorithms)
+
+(define motion-data
+  (~> (open-aoc-input (find-session) 2021 2 #:cache (string->path "./cache"))
+      (port->list read _)
+      (chunks-of _ 2)))
+
+;; part 1
+(for/fold ([depth 0] [position 0] #:result (* depth position)) ([motion (in-list motion-data)])
+  (match motion
+    [(list 'forward x) (values depth (+ position x))]
+    [(list 'up x) (values (- depth x) position)]
+    [(list 'down x) (values (+ depth x) position)]))
+
+;; part 2
+(for/fold ([aim 0] [depth 0] [position 0] #:result (* depth position))
+          ([motion (in-list motion-data)])
+  (match motion
+    [(list 'forward x) (values aim (+ depth (* aim x)) (+ position x))]
+    [(list 'up x) (values (- aim x) depth position)]
+    [(list 'down x) (values (+ aim x) depth position)]))