day 9

day9.rkt

@@ -0,0 +1,84 @@
+#lang racket
+
+(define (map-map f lst) ; map . map
+  (map (λ (x) (map (λ (y) (f y)) x)) lst))
+
+(define (char->num chr) ; i hate this
+  (second (assoc chr (list '(#\0 0) '(#\1 1) '(#\2 2) '(#\3 3) '(#\4 4) '(#\5 5) '(#\6 6) '(#\7 7) '(#\8 8) '(#\9 9)))))
+
+(define grid ; file -> list of lists of numbers
+  (map-map char->num
+           (map string->list
+                (file->lines "day9.txt"))))
+
+(define (grid-value grid row col) ; value of grid at (row, col)
+  (if (or (< row 0) (> row (sub1 (length grid))) (< col 0) (> col (sub1 (length (first grid))))) ; if out of bounds
+      #f
+      (list-ref (list-ref grid row) col)))
+
+(define (neighbors grid row col) ; list of values of neighbors to (row, col)
+  (filter identity ; filter out #f
+          (list (grid-value grid row (sub1 col))
+                (grid-value grid row (add1 col))
+                (grid-value grid (sub1 row) col)
+                (grid-value grid (add1 row) col))))
+  
+(define (is-lowest? grid row col) ; is (row, col) a low point
+  (let ([ns (neighbors grid row col)]
+        [val (grid-value grid row col)])
+    (foldl (λ (n acc) (and (< val n) acc)) #t ns)))
+
+(define (low-point-coords grid) ; list of all low point coordinates in the grid
+  (let ([width (length (first grid))]
+        [height (length grid)])
+    (apply append ; flatten the list one level
+     (for/list ([row (range 0 height)]) ; using a for. i caved
+       (filter identity ; filter out #f
+               (for/list ([col (range 0 width)])
+                 (if (is-lowest? grid row col)
+                     (list row col)
+                     #f)))))))
+
+(define (low-point-values grid) ; the values of all of the low points in a grid
+  (let* ([points (low-point-coords grid)])
+    (map (λ (p) (grid-value grid (first p) (second p))) points)))
+
+(define part1
+  (let* ([points (low-point-values grid)]
+         [risk-levels (map add1 points)])
+    (foldl + 0 risk-levels)))
+
+part1
+
+; PART 2
+
+(define (neighbors-coords row col) ; list of _coordinates_ of neighbors to (row, col)
+  (list
+   (list row (sub1 col))
+   (list row (add1 col))
+   (list (sub1 row) col)
+   (list (add1 row) col)))
+
+(define (find-basin-around-point grid row col)
+  (define visited (make-hash))
+  (let loop ([r row] [c col])
+    (let* ([ns (neighbors-coords r c)] ; get all the neighboring coordinates
+           [ns-to-visit (filter (λ (n) (and
+                                   (grid-value grid (first n) (second n)) ; filter out out-of-bounds
+                                   (not (equal? (grid-value grid (first n) (second n)) 9)) ; and 9s
+                                   (not (hash-ref visited n #f)))) ns)]) ; and the ones we've seen
+      (hash-set! visited (list r c) (grid-value grid r c)) ; add this to the list of coordinates we've visited
+      (for-each (λ (n) (loop (first n) (second n))) ns-to-visit) ; recurse over the neighbors
+      (hash->list visited))))
+             
+(define (find-all-basins grid)
+  (let* ([points (low-point-coords grid)])
+    (map (λ (p) (find-basin-around-point grid (first p) (second p))) points)))
+
+(define part2
+  (let* ([basins (find-all-basins grid)]
+         [top3 (take (sort basins (λ (x y) (> (length x) (length y)))) 3)]
+         [lengths (map length top3)])
+   (foldl * 1 lengths)))
+
+part2

day9.test.txt

@@ -0,0 +1,5 @@
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+3987894921
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day9.txt

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