beginning to fold in HOD code with jeremy tinker's approval

c_tools/hod/least_squares.c

@@ -0,0 +1,22 @@
+#include "header.h"
+
+/* Just a little ditty to do a least squares fit to the arrays given
+ * (assuming no errors).
+ */
+void least_squares(double *x, double *y, int n, double *a, double *b)
+{
+  int i,j;
+  double delta,sx=0,sy=0,sxx=0,sxy=0,syy=0;
+  
+  for(i=0;i<n;++i)
+    {
+      sx+=x[i];
+      sy+=y[i];
+      sxx+=x[i]*x[i];
+      syy+=y[i]*y[i];
+      sxy+=x[i]*y[i];
+    }
+  delta=n*sxx-sx*sx;
+  *a=(sxx*sy-sx*sxy)/delta;
+  *b=(n*sxy-sx*sy)/delta;
+}