diff -N HelixSwimmer.java
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ HelixSwimmer.java	20 Aug 2005 01:37:17 -0000	1.1
@@ -0,0 +1,52 @@

+package org.lcsim.util.swim;
+
+import hep.physics.vec.Hep3Vector;
+
+// For more info on swimming see: http://www.phys.ufl.edu/~avery/fitting/transport.pdf
+
+/**
+ * A simple helix smimmer for use in org.lcsim. Uses standard lcsim units
+ * Tesla, mm, GeV.
+ * @author tonyj
+ * @version $Id: HelixSwimmer.java,v 1.1 2005/08/20 01:37:17 tonyj Exp $
+ */
+public class HelixSwimmer
+{
+   private static final double fieldConversion = 0.299792458 / 1000; // mm
+   private double field;
+   private Helix helix;
+   
+   /** Creates a new instance of HelixSwimmer
+    * @param B field strength in Tesla; uniform, solenoidal, directed along z-axis
+    */
+   public HelixSwimmer(double b)
+   {
+      field = b*fieldConversion;
+   }
+    /**
+     * Sets parameters for helix swimmmer.
+     * 
+     * @param p 3-momentum (px,py,pz)
+     * @param r0 initial position(x0,y0,z0)
+     * @param iq charge iq = q/|e| = +1/0/-1
+     */
+    public void setTrack(Hep3Vector p, Hep3Vector r0, int iq) 
+    {
+       if (iq != 0 && field != 0)
+       {
+          double temp = p.x()*p.x()+p.y()*p.y();
+          double pmom = Math.sqrt(temp + p.z()*p.z());
+          double sin_lambda = p.z()/pmom;
+          double pt = Math.sqrt(temp);
+          double radius = iq * pt/field;
+          double phi = Math.atan2(p.y(),p.x());
+          double lambda = Math.asin(sin_lambda);
+          helix = new Helix(r0,radius,phi,lambda);
+       }
+       
+    } 
+    public Hep3Vector getPointAtDistance(double alpha) 
+    {
+        return helix.getPointAtDistance(alpha);
+    }
+}

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