From f0d18d31ced0227be78733ba2e1240aa1fa43a00 Mon Sep 17 00:00:00 2001
From: MsK` <msk@lywenn.eu.org>
Date: Sat, 11 Dec 2021 10:49:04 +0100
Subject: [PATCH] customize default return value of matrix-get when peeking
 outside

---
 09-smoke-basin/code.scm | 12 ++++++------
 common.scm              | 23 ++++++++++++++++-------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/09-smoke-basin/code.scm b/09-smoke-basin/code.scm
index f791ab6..89e4f22 100644
--- a/09-smoke-basin/code.scm
+++ b/09-smoke-basin/code.scm
@@ -17,10 +17,10 @@
       (lambda (y line)
 	(vector-for-each
 	  (lambda (x altitude)
-	    (let [(left (matrix-get heightmap (- x 1) y))
-		  (right (matrix-get heightmap (+ x 1) y))
-		  (up (matrix-get heightmap x (- y 1)))
-		  (down (matrix-get heightmap x (+ y 1)))]
+	    (let [(left (matrix-get heightmap (- x 1) y 9))
+		  (right (matrix-get heightmap (+ x 1) y 9))
+		  (up (matrix-get heightmap x (- y 1) 9))
+		  (down (matrix-get heightmap x (+ y 1) 9))]
 	      (when (and (< altitude left)
 			 (< altitude right)
 			 (< altitude up)
@@ -49,7 +49,7 @@
       (let [(ofs (offset x y))]
 	(when (= (vector-ref visited ofs) 0)
 	  (vector-set! visited ofs 1)
-	  (when (< (matrix-get heightmap x y) 9)
+	  (when (< (matrix-get heightmap x y 9) 9)
 	    (closure x y)
 	    (when (> x 0)        (flood-fill (- x 1) y))
 	    (when (< x width-1)  (flood-fill (+ x 1) y))
@@ -103,7 +103,7 @@
 	(let y-loop [(y 0)]
 	  (let x-loop [(x 0)]
 	    (let [(ofs (offset x y))
-		  (gray-level (* (matrix-get heightmap x y) 20))]
+		  (gray-level (* (matrix-get heightmap x y 9) 20))]
 	      ; red channel
 	      (vector-set! data ofs gray-level)
 	      ; green channel
diff --git a/common.scm b/common.scm
index 32916a2..368c43b 100644
--- a/common.scm
+++ b/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 '())]