day 3

day-3.scm

@@ -0,0 +1,75 @@
+(import (srfi :43))
+
+; returns a vector of 12 elements where each one counts the number
+; of times the n-th bit was zero in the input vector
+(define (count-zeroes-for-each-bit numbers)
+  (let [(zeroes (make-vector 12 0))]
+    (vector-for-each
+      (lambda (index number)
+	(let iterate [(bit 0)]
+	  (when (< bit 12)
+	    (when (= 0 (bitwise-and number 
+				  (bitwise-arithmetic-shift-left 1 bit)))
+	      (vector-set! zeroes bit 
+			   (+ 1 (vector-ref zeroes bit))))
+	    (iterate (+ 1 bit)))))
+      numbers)
+    zeroes))
+
+(define (part-1 numbers)
+  (let* [(zeroes (count-zeroes-for-each-bit numbers))]
+    (let iter-loop [(i 0) (gamma 0)]
+      (if (< i 12)
+	; We don't need to compute the number of ones
+	; as it is what's left when the zeroes are removed.
+	; So that's just the number of elements minus the number of zeroes.
+	(if (> (vector-ref zeroes i) (/ (vector-length numbers) 2))
+	  (iter-loop (+ i 1) (+ gamma (bitwise-arithmetic-shift-left 1 i)))
+	  (iter-loop (+ i 1) gamma))
+	(let [(epsilon (bitwise-xor gamma #b111111111111))]
+	  (printf "part 1: ~% gamma: ~a~% epsilon: ~a~% power consumption: ~a~%"
+		  gamma epsilon (* gamma epsilon)))))))
+
+; recursively filters the numbers checking for the n-th bit with the comparator
+; with <= as the comparator, it will check for bits that are set
+; with the opposite > comparator, it will check for bits that are not
+(define (determine-rating numbers comparator)
+  (let loop [(bit 11)
+	     (numbers (vector-map (lambda (index number) number) numbers))]
+    (if (= 1 (vector-length numbers))
+      (vector-ref numbers 0)
+      (let* [(zeroes (count-zeroes-for-each-bit numbers))
+	     (mask (bitwise-arithmetic-shift-left 1 bit))
+	     (target
+	       (if (apply comparator
+			  (list (vector-ref zeroes bit) (/ (vector-length numbers) 2)))
+		     0
+		     mask))]
+	;(printf "dataset: ~a~% zeroes: ~a~% bit: ~a~% mask: (#b~b)~%" numbers zeroes bit mask)
+	(loop
+	  (- bit 1)
+	  (vector-fold
+	    (lambda (index result number)
+	      (if (= target (bitwise-and number mask))
+		(vector-append result (vector number))
+		result))
+	    '#() numbers))))))
+
+(define (part-2 numbers)
+  (let [(oxygen-gen (determine-rating numbers <=))
+	(co2-scrub (determine-rating numbers >))]
+    (printf "part 2:~% O2-gen rating: ~a~% CO2-scrub rating: ~a~% life support: ~a~%"
+	    oxygen-gen co2-scrub (* oxygen-gen co2-scrub))))
+
+(call-with-input-file
+  "day-3.input"
+  (lambda (file)
+    (let [(numbers '#())]
+      (let read-loop ()
+	(let [(line (get-line file))]
+	  (when (not (eof-object? line))
+	    (set! numbers 
+	      (vector-append numbers (vector (string->number line 2))))
+	    (read-loop))))
+      (part-1 numbers)
+      (part-2 numbers))))