day 6

day-6/code.scm

@@ -0,0 +1,70 @@
+(import (srfi :43)) ; vector extensions
+
+; returns a list of numbers parsed from the first line of the file,
+; separated by commas
+(define (read-comma-separated-numbers file)
+  (let loop [(draws '()) (n 0)]
+    (let [(c (get-char file))]
+      (cond
+	[(char=? c #\,) (loop (cons n draws) 0)]
+	[(char-whitespace? c) (reverse (cons n draws))]
+	[else (loop draws (+ (* n 10) (string->number (string c))))]))))
+
+;; naive version, as per the example
+; simulate lanterfish growth and returns the number of fishes after given days
+;(define (simulate-growth fishes days)
+;  (let days-loop [(day 0) (fishes fishes)]
+;    (if (= day days)
+;      (length fishes)
+;      (days-loop
+;	(+ day 1)
+;	(let grow [(new '()) (old fishes)]
+;	  (if (null? old)
+;	    new
+;	    (if (= (car old) 0)
+;	      (grow (cons 6 (cons 8 new)) (cdr old))
+;	      (grow (cons (- (car old) 1) new) (cdr old)))))))))
+
+;; optimized version
+;; instead of keeping track of fishes
+;; we just keep track of how many are living at each age.
+;; so, the living vector is actually indexed by age
+(define (sum living)
+  (vector-fold (lambda (i sum value) (+ sum value)) 0 living))
+
+; returns a new vector with the new number of fishes at each age
+(define (grow living)
+  (let [(new (make-vector 9 0))]
+    (vector-for-each
+      (lambda (age count)
+	(if (= age 0)
+	  (begin
+	    (vector-set! new 8 count)
+	    (vector-set! new 6 count))
+	  (vector-set! new (- age 1)
+		       (+ count (vector-ref new (- age 1))))))
+      living)
+    new))
+
+; simulate lanterfish growth and returns the number of fishes after given days
+(define (simulate-growth fishes days)
+  (let [(living-per-day (make-vector 9 0))]
+    (let init-loop [(fishes fishes)]
+      (when (not (null? fishes))
+	(vector-set! living-per-day (car fishes)
+		     (+ 1 (vector-ref living-per-day (car fishes))))
+	(init-loop (cdr fishes))))
+    (let loop [(day 0)
+	       (living-per-day living-per-day)]
+      (if (= day days)
+	(sum living-per-day)
+	(loop (+ day 1) (grow living-per-day))))))
+
+(call-with-input-file
+  "input"
+  (lambda (file)
+    (let [(input (read-comma-separated-numbers file))]
+      (printf "part 1:~% count after 80 days: ~a~%"
+	      (simulate-growth input 80))
+      (printf "part 1:~% count after 256 days: ~a~%"
+	      (simulate-growth input 256)))))

day-6/input

@@ -0,0 +1 @@
+1,1,3,5,1,3,2,1,5,3,1,4,4,4,1,1,1,3,1,4,3,1,2,2,2,4,1,1,5,5,4,3,1,1,1,1,1,1,3,4,1,2,2,5,1,3,5,1,3,2,5,2,2,4,1,1,1,4,3,3,3,1,1,1,1,3,1,3,3,4,4,1,1,5,4,2,2,5,4,5,2,5,1,4,2,1,5,5,5,4,3,1,1,4,1,1,3,1,3,4,1,1,2,4,2,1,1,2,3,1,1,1,4,1,3,5,5,5,5,1,2,2,1,3,1,2,5,1,4,4,5,5,4,1,1,3,3,1,5,1,1,4,1,3,3,2,4,2,4,1,5,5,1,2,5,1,5,4,3,1,1,1,5,4,1,1,4,1,2,3,1,3,5,1,1,1,2,4,5,5,5,4,1,4,1,4,1,1,1,1,1,5,2,1,1,1,1,2,3,1,4,5,5,2,4,1,5,1,3,1,4,1,1,1,4,2,3,2,3,1,5,2,1,1,4,2,1,1,5,1,4,1,1,5,5,4,3,5,1,4,3,4,4,5,1,1,1,2,1,1,2,1,1,3,2,4,5,3,5,1,2,2,2,5,1,2,5,3,5,1,1,4,5,2,1,4,1,5,2,1,1,2,5,4,1,3,5,3,1,1,3,1,4,4,2,2,4,3,1,1