start day19

2016/day19/input.rkt

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+#lang reader "lang.rkt"
+3017957

2016/day19/lang.rkt

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+#lang br/quicklang ;; http://adventofcode.com/2016/day/18
+(provide read-syntax
+         (rename-out [mb #%module-begin]))
+
+(define (read-syntax path port)
+  (strip-bindings
+   #`(module mod "lang.rkt"
+       #,(string-trim (port->string port)))))
+
+(define-macro (mb NUM-STR)
+  #'(#%module-begin
+     (displayln (solve (string->number NUM-STR) #f))
+     (displayln (solve (string->number NUM-STR) #t))))
+
+(define (idx-after vec x)
+  (or
+   (for/first ([idx (in-range (modulo (add1 x) (vector-length vec)) (vector-length vec))]
+               #:when (vector-ref vec idx))
+     idx)
+   (idx-after vec 0)))
+
+(define (solve num [circle? #f])
+  (define elves (make-vector num #t))
+  (let loop ([taker 0][elves-left num])
+    (cond
+      [(= elves-left 2) (add1 taker)]
+      [else
+       (define giver (for/fold ([elf taker])
+                               ([i (in-range (if circle? (floor (/ num 2)) 1))])
+                       (idx-after elves elf)))
+       (vector-set! elves giver #f)
+       (loop (idx-after elves taker) (sub1 elves-left))])))
+

2016/day19/test.rkt

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+#lang reader "lang.rkt"
+5