Move ff_kbd_window_init() to a separate file

This function is not tightly coupled to mdct, and it's in the way
of making a fixed-point mdct implementation.

Signed-off-by: Mans Rullgard <mans@mansr.com>

1 files changed, 48 insertions, 0 deletions

diff --git a/libavcodec/kbdwin.c b/libavcodec/kbdwin.c
new file mode 100644
index 0000000000..c2c1c59040
--- /dev/null
+++ b/libavcodec/kbdwin.c
@@ -0,0 +1,48 @@
+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <assert.h>
+#include <libavutil/mathematics.h>
+#include "libavutil/attributes.h"
+#include "kbdwin.h"
+
+#define BESSEL_I0_ITER 50 // default: 50 iterations of Bessel I0 approximation
+
+av_cold void ff_kbd_window_init(float *window, float alpha, int n)
+{
+   int i, j;
+   double sum = 0.0, bessel, tmp;
+   double local_window[FF_KBD_WINDOW_MAX];
+   double alpha2 = (alpha * M_PI / n) * (alpha * M_PI / n);
+
+   assert(n <= FF_KBD_WINDOW_MAX);
+
+   for (i = 0; i < n; i++) {
+       tmp = i * (n - i) * alpha2;
+       bessel = 1.0;
+       for (j = BESSEL_I0_ITER; j > 0; j--)
+           bessel = bessel * tmp / (j * j) + 1;
+       sum += bessel;
+       local_window[i] = sum;
+   }
+
+   sum++;
+   for (i = 0; i < n; i++)
+       window[i] = sqrt(local_window[i] / sum);
+}
+