[GRASS-SVN] r52520 - grass/trunk/lib/vector/diglib

[svn_grass at osgeo.org]

Author: mmetz
Date: 2012-08-04 11:24:40 -0700 (Sat, 04 Aug 2012)
New Revision: 52520

Modified:
   grass/trunk/lib/vector/diglib/inside.c
Log:
diglib: optimize dig_x_intersect

Modified: grass/trunk/lib/vector/diglib/inside.c
===================================================================
--- grass/trunk/lib/vector/diglib/inside.c	2012-08-04 18:21:34 UTC (rev 52519)
+++ grass/trunk/lib/vector/diglib/inside.c	2012-08-04 18:24:40 UTC (rev 52520)
@@ -21,9 +21,18 @@
 dig_x_intersect(double beg_x,
 		double end_x, double beg_y, double end_y, double Y)
 {
-    double b, a;
+    double b;
+    
+    /* solve simple linear equation to get X = a + b * Y
+     * with
+     * b = (end_x - beg_x) / (end_y - beg_y)
+     * a = beg_x - b * beg_y
+     * 
+     * simplify a + b * Y: 
+     * a + b * Y = beg_x - b * beg_y + b * Y
+     * a + b * Y = beg_x + b * (Y - beg_y) */
 
     b = (end_x - beg_x) / (end_y - beg_y);
-    a = beg_x - b * beg_y;
-    return (a + b * Y);
+
+    return beg_x + b * (Y - beg_y);
 }