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diff --git a/src/FilterInternal.hxx b/src/FilterInternal.hxx
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+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * 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 of the License, or
+ * (at your option) any later version.
+ *
+ * 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, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/** \file
+ *
+ * Internal stuff for the filter core and filter plugins.
+ */
+
+#ifndef MPD_FILTER_INTERNAL_HXX
+#define MPD_FILTER_INTERNAL_HXX
+
+struct audio_format;
+
+class Filter {
+public:
+	virtual ~Filter() {}
+
+	/**
+	 * Opens the filter, preparing it for FilterPCM().
+	 *
+	 * @param filter the filter object
+	 * @param audio_format the audio format of incoming data; the
+	 * plugin may modify the object to enforce another input
+	 * format
+	 * @param error location to store the error occurring, or NULL
+	 * to ignore errors.
+	 * @return the format of outgoing data
+	 */
+	virtual const audio_format *Open(audio_format &af,
+					 GError **error_r) = 0;
+
+	/**
+	 * Closes the filter.  After that, you may call Open() again.
+	 */
+	virtual void Close() = 0;
+
+	/**
+	 * Filters a block of PCM data.
+	 *
+	 * @param filter the filter object
+	 * @param src the input buffer
+	 * @param src_size the size of #src_buffer in bytes
+	 * @param dest_size_r the size of the returned buffer
+	 * @param error location to store the error occurring, or NULL
+	 * to ignore errors.
+	 * @return the destination buffer on success (will be
+	 * invalidated by filter_close() or filter_filter()), NULL on
+	 * error
+	 */
+	virtual const void *FilterPCM(const void *src, size_t src_size,
+				      size_t *dest_size_r,
+				      GError **error_r) = 0;
+};
+
+#endif