customize default return value of matrix-get when peeking outside

common.scm

@@ -13,19 +13,28 @@
   (- (char->integer c) 48)) ; 48 is ASCII's number zero
 
 (define-record-type matrix (fields width height data))
-(define (matrix-get matrix x y)
-  (if (or (< x 0)
-	  (< y 0)
-	  (>= x (matrix-width matrix))
-	  (>= y (matrix-height matrix)))
-    9
-    (vector-ref (vector-ref (matrix-data matrix) y) x)))
+
 (define (matrix-from-data data)
   (make-matrix
     (vector-length (vector-ref data 0))
     (vector-length data)
     data))
 
+(define (matrix-get matrix x y default)
+  (if (or (< x 0)
+	  (< y 0)
+	  (>= x (matrix-width matrix))
+	  (>= y (matrix-height matrix)))
+    default
+    (vector-ref (vector-ref (matrix-data matrix) y) x)))
+
+(define (matrix-set! matrix x y value)
+  (when (and (>= x 0)
+	     (>= y 0)
+	     (< x (matrix-width matrix))
+	     (< y (matrix-height matrix)))
+    (vector-set! (vector-ref (matrix-data matrix) y) x value)))
+
 ; parse 0-9 numerical data from file and return a matrix
 (define (load-matrix file)
   (let y-loop [(heightmap '())]