d05

2018/05-test.txt

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+dabAcCaCBAcCcaDA

2018/05.rkt

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+#lang debug br
+(require racket/file racket/set)
+
+(define fp (open-input-file "05.txt"))
+
+(define fpbs (port->bytes fp))
+
+(define (react fpbs)
+  (let/ec exit
+    (for/fold ([bs fpbs])
+              ([i (in-naturals)])
+      (or
+       (for/first ([(left lidx) (in-indexed bs)]
+                   [right (in-bytes (subbytes bs 1))]
+                   #:when (= 32 (abs (- left right))))
+         (bytes-append (subbytes bs 0 lidx) (subbytes bs (+ lidx 2))))
+       (exit (bytes-length bs))))))
+
+(define (★)
+  (react fpbs))
+
+(define possible-units
+  (map char->integer (remove-duplicates (map char-upcase (map integer->char (bytes->list fpbs))) char=?)))
+
+(define (remove-unit fbps unit)
+  (apply bytes
+         (for/list ([b (in-bytes fbps)]
+                    #:unless (or (= b unit)
+                                 (= b (+ unit 32))))
+           b)))
+
+;; very slow.
+(define (★★)
+  (argmin cdr
+          (for/list ([(unit idx) (in-indexed possible-units)])
+            (cons unit (react (remove-unit fpbs unit))))))
+
+(module+ test
+    (require rackunit)
+    (check-equal? (time (★)) 10564)
+    (check-equal? (time (★★)) 6336))