testing for line intersections... this works in at least a reasonably trivial case - it will

receive more extensive testing shortly


git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@4619 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/Line.as

@@ -22,12 +22,17 @@
 package com.threerings.util {
 
 import flash.geom.Point;
+import flash.geom.Matrix;
 
 /**
  * Merely a typed container for two Points.
  */
 public class Line
 {
+    public static const INTERSECTION_NORTH :int = 1;
+    public static const INTERSECTION_SOUTH :int = 2;
+    public static const DOES_NOT_INTERSECT :int = 3;
+
     public var start :Point;
     public var stop :Point;
 
@@ -36,5 +41,44 @@ public class Line
         this.start = start;
         this.stop = stop;
     }
+
+    /**
+     * Tests if the given line intersects this line.  If thats all you need to know, testing for
+     * intersects() != DOES_NOT_INTERSECT is enough.  This method rotates both lines so that the 
+     * start point is on the left.  If the lines do intersect, it then returns INTERSECTION_NORTH
+     * if the end point of <code>line</code> is north of this line, INTERSECTION_SOUTH otherwise.
+     * 
+     * Intersections are inclusive.  If one or both points lands on this line, interects will not
+     * return DOES_NOT_INTERSECT.
+     */
+    public function intersects (line :Line) :int 
+    {
+        // rotate so that this line is horizontal, with the start on the left, at (0, 0)
+        var trans :Matrix = new Matrix();
+        trans.translate(-start.x, -start.y);
+        trans.rotate(-Math.atan2(stop.y - start.y, stop.x - start.x));
+        var thisLineStop :Point = trans.transformPoint(stop);
+        var thatLineStart :Point = trans.transformPoint(line.start);
+        var thatLineStop :Point = trans.transformPoint(line.stop);
+        if (thatLineStart.y >= 0 && thatLineStop.y <= 0) {
+            var interp :Point = Point.interpolate(thatLineStart, thatLineStop, thatLineStop.y / 
+                (thatLineStop.y + (-thatLineStart.y)));
+            if (interp.x >= 0 && interp.x <= thisLineStop.x) {
+                return INTERSECTION_NORTH;
+            } else {
+                return DOES_NOT_INTERSECT;
+            }
+        } else if (thatLineStart.y <= 0 && thatLineStop.y >= 0) {
+            interp = Point.interpolate(thatLineStop, thatLineStart, thatLineStart.y / 
+                (thatLineStart.y + (-thatLineStop.y)));
+            if (interp.x >= 0 && interp.x <= thisLineStop.x) {
+                return INTERSECTION_SOUTH;
+            } else {
+                return DOES_NOT_INTERSECT;
+            }
+        } else {
+            return DOES_NOT_INTERSECT;
+        }
+    }
 }
 }