Day 20. Will replace hash with set.

input/20.txt

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src/20.rkt

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+#lang curly-fn racket
+
+(require "../lib.rkt")
+
+(match-define (list input1 input2)
+  (problem-input-grouped 20))
+
+(define algorithm
+  (list->vector (string->list input1)))
+
+(define image
+  (for/fold ([img (hash)])
+            ([line (string-lines input2)]
+             [row (range (length (string-lines input2)))])
+    (for/fold ([img img])
+              ([char (string->list line)]
+               [col (range (string-length line))])
+      (hash-set img (cons row col) char))))
+
+(define (square row col)
+  (for*/list ([r (range* (sub1 row) (add1 row))]
+              [c (range* (sub1 col) (add1 col))])
+    (cons r c)))
+
+(define (square->number pixels)
+  (~>> (map #{match % [#\. "0"] [#\# "1"]} pixels)
+       (apply string-append)
+       string->binary))
+
+(define (image-bounds img)
+  (define keys (hash-keys img))
+  (define rows (map car keys))
+  (define cols (map cdr keys))
+  (values (minimum rows) (maximum rows)
+          (minimum cols) (maximum cols)))
+
+(define (show img)
+  (define-values (min-row max-row min-col max-col)
+    (image-bounds img))
+  (for ([row (range* min-row max-row)])
+    (for ([col (range* min-col max-col)])
+      (display (hash-ref img (cons row col) #\.)))
+    (newline)))
+
+(define (enhance default img)
+  (define-values (min-row max-row min-col max-col)
+    (image-bounds img))
+  (for*/fold ([img* (hash)])
+             ([row (range* (sub1 min-row) (add1 max-row))]
+              [col (range* (sub1 min-col) (add1 max-col))])
+    (~>> (square row col)
+         (map #{hash-ref img % default})
+         square->number
+         (vector-ref algorithm)
+         (hash-set img* (cons row col)))))
+
+(define part1
+  (let ([img (enhance #\# (enhance #\. image))])
+    (count #{char=? % #\#} (hash-values img))))
+
+(define part2
+  (for/fold ([img image]
+             [default #\.]
+             #:result (count #{char=? % #\#} (hash-values img)))
+            ([_ (range 50)])
+    (values (enhance default img)
+            (match default [#\. #\#] [#\# #\.]))))
+
+(show-solution part1 part2)