diff -N CircleFit.java
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ CircleFit.java	24 Mar 2006 02:48:46 -0000	1.1
@@ -0,0 +1,69 @@

+/*
+ * CircleFit.java
+ *
+ * Created on March 23, 2006, 3:42 PM
+ *
+ *
+ */
+
+package org.lcsim.fit.threepointcircle;
+
+/**
+ * A class which encapsulates the result of a circle fit.
+ * @author Norman Graf
+ */
+public class CircleFit
+{
+    private double _x0;
+    private double _y0;
+    private double _r;
+    /**
+     * Creates a new instance of CircleFit
+     * @param x0 the x coordinate of the center of the circle
+     * @param y0 the y coordinate of the center of the circle
+     * @param r  the radius of the circle
+     */
+    public CircleFit(double x0, double y0, double r)
+    {
+        _x0 = x0;
+        _y0 = y0;
+        _r = r;
+    }
+    
+    /**
+     * The x coordinate of the center of the circle.
+     * @return the x coordinate of the center of the circle
+     */
+    public double x0()
+    {
+        return _x0;
+    }
+    
+    /**
+     * The y coordinate of the center of the circle
+     * @return
+     */
+    public double y0()
+    {
+        return _y0;
+    }
+    
+    /**
+     * The radius of the circle
+     * @return the radius of the circle
+     */
+    public double radius()
+    {
+        return _r;
+    }
+    
+    /**
+     * String representation of this object
+     * @return String representation of this object
+     */
+    public String toString()
+    {
+        return "CircleFit: x0= "+_x0+" y0= "+_y0+" r= "+_r;
+    }
+    
+}

diff -N ThreePointCircleFitter.java
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ ThreePointCircleFitter.java	24 Mar 2006 02:48:47 -0000	1.1
@@ -0,0 +1,128 @@

+/*
+ * ThreePointCircleFitter.java
+ *
+ * Created on March 23, 2006, 3:41 PM
+ *
+ *
+ */
+
+package org.lcsim.fit.threepointcircle;
+
+import static java.lang.Math.pow;
+import static java.lang.Math.sqrt;
+/**
+ * A class to return the circle which passes through three points.
+ * @author Norman Graf
+ */
+public class ThreePointCircleFitter
+{
+    private CircleFit _fit;
+    private static double[][] a = {{0, 0, 0},{0, 0, 0},{0, 0, 0}};
+    private static double[][] m = {{0, 0, 0},{0, 0, 0},{0, 0, 0}};
+    
+    /** Creates a new instance of ThreePointCircleFitter */
+    public ThreePointCircleFitter()
+    {
+    }
+    
+    /**
+     * Find the circle which passes through three points
+     * @param p1 The first point.
+     * @param p2 The second point.
+     * @param p3 The third point.
+     * @return true if a circle can be found which passes through the points.
+     */
+    public boolean fit(double[] p1, double[] p2, double[] p3)
+    {
+        int i;
+        double r, m11, m12, m13, m14;
+
+        double[][] P = {p1, p2, p3};
+        
+        for (i = 0; i < 3; i++)                    // find minor 11
+        {
+            a[i][0] = P[i][0];
+            a[i][1] = P[i][1];
+            a[i][2] = 1;
+        }
+        m11 = determinant( a, 3 );
+        if(m11==0) 
+        {
+            _fit = null;
+            return false;
+        }
+        
+        for (i = 0; i < 3; i++)                    // find minor 12
+        {
+            a[i][0] = P[i][0]*P[i][0] + P[i][1]*P[i][1];
+            a[i][1] = P[i][1];
+            a[i][2] = 1;
+        }
+        m12 = determinant( a, 3 );
+        
+        for (i = 0; i < 3; i++)                    // find minor 13
+        {
+            a[i][0] = P[i][0]*P[i][0] + P[i][1]*P[i][1];
+            a[i][1] = P[i][0];
+            a[i][2] = 1;
+        }
+        m13 = determinant( a, 3 );
+        
+        for (i = 0; i < 3; i++)                    // find minor 14
+        {
+            a[i][0] = P[i][0]*P[i][0] + P[i][1]*P[i][1];
+            a[i][1] = P[i][0];
+            a[i][2] = P[i][1];
+        }
+        m14 = determinant( a, 3 );
+              
+        double x0 =  0.5 * m12 / m11;                 // center of circle
+        double y0 = -0.5 * m13 / m11;
+        r  = sqrt( x0*x0 + y0*y0 + m14/m11 );
+        
+        _fit = new CircleFit(x0, y0, r);
+        
+        return true;
+    }
+    
+    /**
+     * Get the result of the fit.
+     * @return The Circle which passes through the points.
+     */
+    public CircleFit getFit()
+    {
+        return _fit;
+    }
+    
+    // Recursive definition of determinant using expansion by minors.
+    private double determinant(double[][] a, int n)
+    {
+        int i, j, j1, j2;
+        double d = 0.;
+   
+        if (n == 2)                                // terminate recursion
+        {
+            d = a[0][0]*a[1][1] - a[1][0]*a[0][1];
+        }
+        else
+        {
+            d = 0;
+            for (j1 = 0; j1 < n; ++j1 )            // do each column
+            {
+                for (i = 1; i < n; ++i)            // create minor
+                {
+                    j2 = 0;
+                    for (j = 0; j < n; ++j)
+                    {
+                        if (j == j1) continue;
+                        m[i-1][j2] = a[i][j];
+                        j2++;
+                    }
+                }
+                // sum (+/-)cofactor * minor
+                d += pow(-1.0, j1)*a[0][j1]*determinant( m, n-1 );
+            }
+        }
+        return d;
+    }
+}

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