Initial commit

2021/day03/day03.scm

@@ -0,0 +1,47 @@
+(import (chicken io)
+	(chicken bitwise)
+	srfi-1
+	srfi-13)
+
+(define (binary-string->list s)
+  (map (lambda (c) (- (char->integer c) (char->integer #\0)))
+       (string->list s)))
+
+(define (list->binary-string l)
+  (string-concatenate (map number->string l)))
+
+(define (read-input #!optional (port (current-input-port)))
+  (map binary-string->list (read-lines port)))
+
+(define (part1 l)
+  (let* ((n (/ (length l) 2))
+	 (counts (apply map + l))
+	 (most-common (map (lambda (x) (if (> x n) 1 0)) counts))
+	 (gamma (string->number (list->binary-string most-common) 2))
+	 (epsilon (string->number (list->binary-string (map (lambda (d) (- 1 d)) most-common)) 2)))
+    (* gamma epsilon)))
+
+(define (o2-rating numbers i)
+  (if (= 1 (length numbers))
+      (car numbers)
+      (let* ((digits (map (lambda (l) (list-ref l i)) numbers))
+	     (most-common (if (>= (apply + digits) (/ (length numbers) 2)) 1 0))
+	     (new-numbers (filter (lambda (n) (= (list-ref n i) most-common)) numbers)))
+	(o2-rating new-numbers (+ 1 i)))))
+
+(define (co2-rating numbers i)
+  (if (= 1 (length numbers))
+      (car numbers)
+      (let* ((digits (map (lambda (l) (list-ref l i)) numbers))
+	     (least-common (if (>= (apply + digits) (/ (length numbers) 2)) 0 1))
+	     (new-numbers (filter (lambda (n) (= (list-ref n i) least-common)) numbers)))
+	(co2-rating new-numbers (+ 1 i)))))
+
+(define (part2 l)
+  (let ((o2 (string->number (list->binary-string (o2-rating l 0)) 2))
+	(co2 (string->number (list->binary-string (co2-rating l 0)) 2)))
+    (* o2 co2)))
+
+(let ((in (read-input)))
+  (print (part1 in))
+  (print (part2 in)))