Implement intersection function of line segments.

src/sig/Vector2.java

@@ -13,6 +13,33 @@ public class Vector2 {
         this.v=v;
         this.w=w;
     }
+    final public static int COLLINEAR=0;
+    final public static int CLOCKWISE=1;
+    final public static int COUNTER_CLOCKWISE=2;
+    public static int orientation(Vector2 v1,Vector2 v2,Vector2 v3) {
+        int val = (int)((v2.v-v1.v)*(v3.u-v2.u)-(v2.u-v1.u)*(v3.v-v2.v));
+        if (val==0) {return COLLINEAR;}
+        return (val>0)?CLOCKWISE:COUNTER_CLOCKWISE;
+    }
+    public static boolean onSegment(Vector2 v1,Vector2 v2,Vector2 v3) {
+        return v2.u<=Math.max(v1.u,v3.u)&&v2.u>=Math.min(v1.u,v3.u)&&
+            v2.v<=Math.max(v1.v,v3.v)&&v2.v>=Math.min(v1.v,v3.v);
+    }
+    public static boolean intersect(Vector2 p1,Vector2 p2,Vector2 p3,Vector2 p4) {
+        int o1=orientation(p1,p3,p2);
+        int o2=orientation(p1,p3,p4);
+        int o3=orientation(p2,p4,p1);
+        int o4=orientation(p2,p4,p3);
+
+        if (o1!=o2&&o3!=o4) {return true;}
+        if ((o1==COLLINEAR&&onSegment(p1,p2,p3))||
+            (o2==COLLINEAR&&onSegment(p1,p4,p3))||
+            (o3==COLLINEAR&&onSegment(p2,p1,p4))||
+            (o4==COLLINEAR&&onSegment(p2,p3,p4))) {
+            return true;
+        }
+        return false;
+    }
     @Override
     protected Object clone(){
         return new Vector2(u,v,w);