new file mode 100644
--- /dev/null
+++ b/src/filter.cc
@@ -0,0 +1,268 @@
+// f-filter.cc						-*- C++ -*-
+
+// Based on Tony Richardson's filter.m.
+//
+// Originally translated to C++ by KH (Kurt.Hornik@ci.tuwien.ac.at)
+// with help from Fritz Leisch and Andreas Weingessel on Oct 20, 1994.
+//
+// Rewritten to use templates to handle both real and complex cases by
+// jwe, Wed Nov  1 19:15:29 1995.
+
+#include "config.h"
+#include "defun-dld.h"
+#include "tree-const.h"
+#include "help.h"
+
+template <class T>
+Array<T>
+filter (Array<T>& b, Array<T>& a, Array<T>& x, Array<T>& si)
+{
+  Array<T> y;
+
+  int a_len  = a.length ();
+  int b_len  = b.length ();
+  int x_len  = x.length ();
+
+  int si_len = si.length ();
+
+  int ab_len = a_len > b_len ? a_len : b_len;
+
+  b.resize (ab_len, 0.0);
+
+  if (si.length () != ab_len - 1)
+    {
+      error ("filter: si must be a vector of length max (length (a), length (b)) - 1");
+      return y;
+    }
+
+  T norm = a.elem (0);
+
+  if (norm == 0.0)
+    {
+      error ("filter: the first element of a must be non-zero");
+      return y;
+    }
+
+  y.resize (x_len, 0.0);
+
+  if (norm != 1.0)
+    b = b / norm;
+
+  if (a_len > 1)
+    {
+      a.resize (ab_len, 0.0);
+
+      if (norm != 1.0)
+	a = a / norm;
+
+      for (int i = 0; i < x_len; i++)
+	{
+	  y.elem (i) = si.elem (0) + b.elem (0) * x.elem (i);
+
+	  if (si_len > 1)
+	    {
+	      for (int j = 0; j < si_len - 1; j++)
+		si.elem (j) = si.elem (j+1) - a.elem (j+1) * y.elem (i)
+		  + b.elem (j+1) * x.elem (i);
+
+	      si.elem (si_len-1) = b.elem (si_len) * x.elem (i)
+		- a.elem (si_len) * y.elem (i);
+	    }
+	  else
+	    si.elem (0) = b.elem (si_len) * x.elem (i)
+	      - a.elem (si_len) * y.elem (i);
+	}
+    }
+  else if (si_len > 0)
+    {
+      for (int i = 0; i < x_len; i++)
+	{
+	  y.elem (i) = si.elem (0) + b.elem (0) * x.elem (i);
+
+	  if (si_len > 1)
+	    {
+	      for (int j = 0; j < si_len - 1; j++)
+		si.elem (j) = si.elem (j+1) + b.elem (j+1) * x.elem (i);
+
+	      si.elem (si_len-1) = b.elem (si_len) * x.elem (i);
+	    }
+	  else
+	    si.elem (0) = b.elem (1) * x.elem (i);
+	}
+    }
+  else
+    y = b.elem (0) * x;
+
+  return y;
+}
+
+template <class T>
+Array<T>
+filter (Array<T>& b, Array<T>& a, Array<T>& x)
+{
+  int a_len = a.length ();
+  int b_len = b.length ();
+
+  int si_len = (a_len > b_len ? a_len : b_len) - 1;
+
+  Array<T> si (si_len, 0.0);
+
+  return filter (b, a, x, si);
+}
+
+extern Array<double>
+filter (Array<double>&, Array<double>&, Array<double>&, Array<double>&);
+
+extern Array<double>
+filter (Array<double>&, Array<double>&, Array<double>&);
+
+extern Array<Complex>
+filter (Array<Complex>&, Array<Complex>&, Array<Complex>&, Array<Complex>&);
+
+extern Array<Complex>
+filter (Array<Complex>&, Array<Complex>&, Array<Complex>&);
+
+DEFUN_DLD_BUILTIN ("filter", Ffilter, Sfilter, 10,
+  "usage: [y [, sf]] = filter (b, a, x [, si])\n\
+\n\
+y = filter (b, a, x) returns the solution to the following linear,\n\
+time-invariant difference equation:\n\
+\n\
+  a[1] y[n] + ... + a[la] y[n-la+1] = b[1] x[n] + ... + b[lb] x[n-lb+1],\n\
+where la = length (a) and lb = length (b).\n\
+\n\
+[y, sf] = filter (b, a, x, si) sets the initial state of the system, si,\n\
+and returns the final state, sf.  The state vector is a column vector\n\
+whose length is equal to the length of the longest coefficient vector\n\
+minus one.  If si is not set, the initial state vector is set to all\n\
+zeros.\n\
+\n\
+The particular algorithm employed is known as a transposed Direct Form II\n\
+implementation.")
+{
+  Octave_object retval;
+
+  int nargin  = args.length ();
+
+  if (nargin < 3 || nargin > 4)
+    {
+      print_usage ("filter");
+      return retval;
+    }
+
+  const char *errmsg = "filter: arguments must be vectors";
+
+  int x_is_vector = (args(2).rows () == 1 || args(2).columns () == 1);
+
+  int si_is_vector = (nargin == 4
+		      && (args(3).rows () == 1 || args(3).columns () == 1));
+
+  if (args(0).is_complex_type ()
+      || args(1).is_complex_type ()
+      || args(2).is_complex_type ()
+      || (nargin == 4 && args(3).is_complex_type ()))
+    {
+      ComplexColumnVector b = args(0).complex_vector_value ();
+      ComplexColumnVector a = args(1).complex_vector_value ();
+      ComplexColumnVector x = args(2).complex_vector_value ();
+
+      if (! error_state)
+	{
+	  if (nargin == 3)
+	    {
+	      ComplexColumnVector y (filter (b, a, x));
+
+	      if (x_is_vector)
+		retval (0) = tree_constant (y, (args(2).columns () == 1));
+	      else
+		retval (0) = y;
+	    }
+	  else
+	    {
+	      ComplexColumnVector si = args(3).complex_vector_value ();
+
+	      if (! error_state)
+		{
+		  ComplexColumnVector y (filter (b, a, x, si));
+
+		  if (si_is_vector)
+		    retval (1) = tree_constant (si, (args(3).columns () == 1));
+		  else
+		    retval (1) = si;
+
+		  if (x_is_vector)
+		    retval (0) = tree_constant (y, (args(2).columns () == 1));
+		  else
+		    retval (0) = y;
+		}
+	      else
+		error (errmsg);
+	    }
+	}
+      else
+	error (errmsg);
+    }
+  else
+    {
+      ColumnVector b = args(0).vector_value ();
+      ColumnVector a = args(1).vector_value ();
+      ColumnVector x = args(2).vector_value ();
+
+      if (! error_state)
+	{
+	  if (nargin == 3)
+	    {
+	      ColumnVector y (filter (b, a, x));
+
+	      if (x_is_vector)
+		retval (0) = tree_constant (y, (args(2).columns () == 1));
+	      else
+		retval (0) = y;
+	    }
+	  else
+	    {
+	      ColumnVector si = args(3).vector_value ();
+
+	      if (! error_state)
+		{
+		  ColumnVector y (filter (b, a, x, si));
+
+		  if (si_is_vector)
+		    retval (1) = tree_constant (si, (args(3).columns () == 1));
+		  else
+		    retval (1) = si;
+
+		  if (x_is_vector)
+		    retval (0) = tree_constant (y, (args(2).columns () == 1));
+		  else
+		    retval (0) = y;
+		}
+	      else
+		error (errmsg);
+	    }
+	}
+      else
+	error (errmsg);
+    }
+
+  return retval;
+}
+      
+template Array<double>
+filter (Array<double>&, Array<double>&, Array<double>&, Array<double>&);
+
+template Array<double>
+filter (Array<double>&, Array<double>&, Array<double>&);
+
+template Array<Complex>
+filter (Array<Complex>&, Array<Complex>&, Array<Complex>&, Array<Complex>&);
+
+template Array<Complex>
+filter (Array<Complex>&, Array<Complex>&, Array<Complex>&);
+
+/*
+;;; Local Variables: ***
+;;; mode: C++ ***
+;;; page-delimiter: "^/\\*" ***
+;;; End: ***
+*/