diff --git a/src/window.c b/src/window.c
new file mode 100644
index 0000000..536bede
--- /dev/null
+++ b/src/window.c
@@ -0,0 +1,149 @@
+/*
+** Copyright (C) 2007-2009 Erik de Castro Lopo <erikd@mega-nerd.com>
+**
+** This program is free software: you can redistribute it and/or modify
+** it under the terms of the GNU General Public License as published by
+** the Free Software Foundation, either version 2 or version 3 of the
+** License.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <math.h>
+
+#include "window.h"
+
+#define ARRAY_LEN(x)		((int) (sizeof (x) / sizeof (x [0])))
+
+
+static double besseli0 (double x) ;
+static double factorial (int k) ;
+
+void
+calc_kaiser_window (double * data, int datalen, double beta)
+{
+	/*
+	**          besseli0 (beta * sqrt (1- (2*x/N).^2))
+	** w (x) =  --------------------------------------,  -N/2 <= x <= N/2
+	**                 besseli0 (beta)
+	*/
+
+	double two_n_on_N, denom ;
+	int k ;
+
+	denom = besseli0 (beta) ;
+
+	if (! isfinite (denom))
+	{	printf ("besseli0 (%f) : %f\nExiting\n", beta, denom) ;
+		exit (1) ;
+		} ;
+
+	for (k = 0 ; k < datalen ; k++)
+	{	double n = k + 0.5 - 0.5 * datalen ;
+		two_n_on_N = (2.0 * n) / datalen ;
+		data [k] = besseli0 (beta * sqrt (1.0 - two_n_on_N * two_n_on_N)) / denom ;
+		} ;
+
+	return ;
+} /* calc_kaiser_window */
+
+void
+calc_nuttall_window (double * data, int datalen)
+{
+    const double a [4] = { 0.355768, 0.487396, 0.144232, 0.012604 } ;
+	int k ;
+
+	/*
+	**	Nuttall window function from :
+	**
+	**	http://en.wikipedia.org/wiki/Window_function
+	*/
+
+	for (k = 0 ; k < datalen ; k++)
+	{	double scale ;
+
+		scale = M_PI * k / (datalen - 1) ;
+
+		data [k] = a [0] - a [1] * cos (2.0 * scale) + a [2] * cos (4.0 * scale) - a [3] * cos (6.0 * scale) ;
+		} ;
+
+	return ;
+} /* calc_nuttall_window */
+
+/*==============================================================================
+*/
+
+static double
+besseli0 (double x)
+{	int k ;
+	double result = 0.0 ;
+
+	for (k = 1 ; k < 25 ; k++)
+	{	double temp ;
+
+		temp = pow (0.5 * x, k) / factorial (k) ;
+		result += temp * temp ;
+		} ;
+
+	return 1.0 + result ;
+} /* besseli0 */
+
+static double
+factorial (int val)
+{	static double memory [64] = { 1.0 } ;
+	static int have_entry = 0 ;
+
+	int k ;
+
+	if (val < 0)
+	{	printf ("Oops : val < 0.\n") ;
+		exit (1) ;
+		} ;
+
+	if (val > ARRAY_LEN (memory))
+	{	printf ("Oops : val > ARRAY_LEN (memory).\n") ;
+		exit (1) ;
+		} ;
+
+	if (val < have_entry)
+		return memory [val] ;
+
+	for (k = have_entry + 1 ; k <= val ; k++)
+		memory [k] = k * memory [k - 1] ;
+
+	return memory [val] ;
+} /* factorial */
+
+/*==============================================================================
+*/
+
+static void init_test (void) __attribute__ ((constructor)) ;
+
+static void
+init_test (void)
+{
+	/* puts (__func__) ;*/
+
+	assert (factorial (0) == 1.0) ;
+	assert (factorial (2) == 2.0) ;
+	assert (factorial (0) == 1.0) ;
+	assert (factorial (5) == 120.0) ;
+	assert (factorial (8) == 40320.0) ;
+
+	assert (fabs (besseli0 (0.0) - 1.0) < 1e-8) ;
+	assert (fabs (besseli0 (0.5) - 1.06348337074132) < 1e-8) ;
+	assert (fabs (besseli0 (1.0) - 1.26606587775201) < 1e-14) ;
+	assert (fabs (besseli0 (2.0) - 2.27958530233607) < 1e-14) ;
+	assert (fabs (besseli0 (3.5) - 7.37820343222548) < 1e-14) ;
+
+} /* init_test */
+
diff --git a/src/window.h b/src/window.h
new file mode 100644
index 0000000..c1ceab0
--- /dev/null
+++ b/src/window.h
@@ -0,0 +1,20 @@
+/*
+** Copyright (C) 2007 Erik de Castro Lopo <erikd@mega-nerd.com>
+**
+** This program is free software: you can redistribute it and/or modify
+** it under the terms of the GNU General Public License as published by
+** the Free Software Foundation, either version 2 or version 3 of the
+** License.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+extern void calc_kaiser_window (double * data, int datalen, double beta) ;
+
+extern void calc_nuttall_window (double * data, int datalen) ;