diff --git a/05-hydrothermal-venture/code.scm b/05-hydrothermal-venture/code.scm
index 04ae979..d37761a 100644
--- a/05-hydrothermal-venture/code.scm
+++ b/05-hydrothermal-venture/code.scm
@@ -1,15 +1,6 @@
 (import (srfi :43)) ; vector extensions
 (load "../common.scm")
 
-; reads a numnber from input
-; also consumes the next non-number character in the file!
-(define (get-number file)
-  (let loop [(n 0)]
-    (let [(c (get-char file))]
-      (if (char-numeric? c)
-	[loop (+ (char->number c) (* n 10))]
-	n))))
-
 ; reads pairs of coordinates
 ; returns a list of the form (((x1 y1) (x2 y2))...)
 (define (read-lines file)
diff --git a/11-dumbo-octopus/code.scm b/11-dumbo-octopus/code.scm
index 1bfde15..a555ec5 100644
--- a/11-dumbo-octopus/code.scm
+++ b/11-dumbo-octopus/code.scm
@@ -22,7 +22,7 @@
 (define (step! matrix)
   (let [(flashes '())]
     ; increase energy of all octopuses
-    (visit-matrix! 
+    (update-matrix! 
       matrix
       (lambda (x y energy)
 	(when (= energy 9)
@@ -35,7 +35,7 @@
 		      (flash! matrix (vec2i-x (car flash))
 			      (vec2i-y (car flash)))))))
     ; drain all octopuses that flashed
-    (visit-matrix! 
+    (update-matrix! 
       matrix
       (lambda (x y energy)
 	(if (< energy 10) energy 0)))
diff --git a/common.scm b/common.scm
index 8b6e854..68b1c51 100644
--- a/common.scm
+++ b/common.scm
@@ -7,6 +7,21 @@
       lines
       (loop (cons (get-line file) lines)))))
 
+; reads a numnber from input
+; also consumes the next non-number character in the file!
+(define (get-number file)
+  (let [(sign 1)]
+    (let loop [(index 0) (n 0)]
+      (let [(c (get-char file))]
+	(cond
+	  [(char-numeric? c)
+	   (loop (+ index 1) (+ (char->number c) (* n 10)))]
+	  [(and (char=? c #\-)
+		(= index 0))
+	   (set! sign -1)
+	   (loop 1 n)]
+	  [else (* sign n)])))))
+
 ; returns a list of numbers parsed from the first line of the file,
 ; separated by commas
 (define (read-comma-separated-numbers file)
@@ -15,7 +30,7 @@
       (cond
 	[(char=? c #\,) (loop (cons n draws) 0)]
 	[(char-whitespace? c) (reverse (cons n draws))]
-	[else (loop draws (+ (* n 10) (string->number (string c))))]))))
+	[else (loop draws (+ (* n 10) (char->number c)))]))))
 
 ; returns numbers 0 to 9 from ascii character
 (define (char->number c)
@@ -31,6 +46,13 @@
     (vector-length data)
     data))
 
+; returns a matrix object of given dimensions full of zeroes
+(define (filled-matrix width height filler)
+  (matrix-from-data
+    (do [(i 0 (+ i 1))
+	 (v '() (cons (make-vector width filler) v))]
+      ((>= i height) (list->vector v)) '())))
+
 ; returns value at x,y coordinates in matrix
 ; if coordinate is out of bounds, returns default
 (define (matrix-get matrix x y default)
@@ -74,7 +96,7 @@
 ; matrix visitor
 ; calls function with all coordinates of matrix and value at corresponding point
 ; value returned by function is set at the visiting coordinate
-(define (visit-matrix! matrix function)
+(define (update-matrix! matrix function)
   (let [(data (matrix-data matrix))
 	(width-1 (- (matrix-width matrix) 1))
 	(height-1 (- (matrix-height matrix) 1))]