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diff --git a/racket/aoc2021/day-05/day-05.rkt b/racket/aoc2021/day-05/day-05.rkt
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+++ b/racket/aoc2021/day-05/day-05.rkt
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+#lang racket
+(require advent-of-code
+         threading)
+
+(define data
+  (for/list ([l (in-lines (open-aoc-input (find-session) 2021 5 #:cache (string->path "./cache")))])
+    (~> l (string-replace " -> " ",") (string-split ",") (map string->number _))))
+
+(define (trace-line! x y vec)
+  (define linear-coord (+ y (* 1000 x)))
+  (vector-set! vec linear-coord (+ 1 (vector-ref vec linear-coord))))
+
+(define/match (orthogonal? coord-pair)
+  [((or (list n _ n _) (list _ n _ n))) #t]
+  [(_) #f])
+
+(define-values (orthogonal-lines diagonal-lines) (partition orthogonal? data))
+
+(define (dir a b)
+  (if (< a b) 1 -1))
+
+(define (trace-orthogonal! coord-pairs tracer result)
+  (for ([coord-pair (in-list coord-pairs)])
+    (match coord-pair
+      [(list x y1 x y2)
+       (for ([y (inclusive-range y1 y2 (dir y1 y2))])
+         (tracer x y result))]
+      [(list x1 y x2 y)
+       (for ([x (inclusive-range x1 x2 (dir x1 x2))])
+         (tracer x y result))])))
+
+(define (trace-diagonal! coord-pairs tracer result)
+  (for ([coord-pair (in-list coord-pairs)])
+    (match-define (list x1 y1 x2 y2) coord-pair)
+    (for ([x (inclusive-range x1 x2 (dir x1 x2))] [y (inclusive-range y1 y2 (dir y1 y2))])
+      (tracer x y result))))
+
+;; part 1
+(define sea-floor (make-vector 1000000))
+(trace-orthogonal! orthogonal-lines trace-line! sea-floor)
+(vector-count (curry <= 2) sea-floor)
+
+;; part 2
+;; since the orthogonal lines have already been traced,
+;; all I need to do is add the diagonal ones to the existing vector
+(trace-diagonal! diagonal-lines trace-line! sea-floor)
+(vector-count (curry <= 2) sea-floor)
+
+;; alternate sparse representation
+(define (trace-line-sparse! x y dict)
+  (hash-update! dict (list x y) (curry + 1) 0))
+
+(define sea-floor-dict (make-hash))
+(trace-orthogonal! orthogonal-lines trace-line-sparse! sea-floor-dict)
+(count (curry <= 2) (hash-values sea-floor-dict))
+(trace-diagonal! diagonal-lines trace-line-sparse! sea-floor-dict)
+(count (curry <= 2) (hash-values sea-floor-dict))