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diff --git a/src/FilterPlugin.hxx b/src/FilterPlugin.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
+ *
+ * This header declares the filter_plugin class.  It describes a
+ * plugin API for objects which filter raw PCM data.
+ */
+
+#ifndef MPD_FILTER_PLUGIN_HXX
+#define MPD_FILTER_PLUGIN_HXX
+
+#include "gerror.h"
+
+#include <stddef.h>
+
+struct config_param;
+struct filter;
+
+struct filter_plugin {
+	const char *name;
+
+	/**
+         * Allocates and configures a filter.
+	 */
+	struct filter *(*init)(const struct config_param *param,
+			       GError **error_r);
+
+	/**
+	 * Free instance data.
+         */
+	void (*finish)(struct filter *filter);
+
+	/**
+	 * Opens a filter.
+	 *
+	 * @param audio_format the audio format of incoming data; the
+	 * plugin may modify the object to enforce another input
+	 * format
+	 */
+	const struct audio_format *
+	(*open)(struct filter *filter,
+		struct audio_format *audio_format,
+		GError **error_r);
+
+	/**
+	 * Closes a filter.
+	 */
+	void (*close)(struct filter *filter);
+
+	/**
+	 * Filters a block of PCM data.
+	 */
+	const void *(*filter)(struct filter *filter,
+			      const void *src, size_t src_size,
+			      size_t *dest_buffer_r,
+			      GError **error_r);
+};
+
+/**
+ * Creates a new instance of the specified filter plugin.
+ *
+ * @param plugin the filter plugin
+ * @param param optional configuration section
+ * @param error location to store the error occurring, or NULL to
+ * ignore errors.
+ * @return a new filter object, or NULL on error
+ */
+struct filter *
+filter_new(const struct filter_plugin *plugin,
+	   const struct config_param *param, GError **error_r);
+
+/**
+ * Creates a new filter, loads configuration and the plugin name from
+ * the specified configuration section.
+ *
+ * @param param the configuration section
+ * @param error location to store the error occurring, or NULL to
+ * ignore errors.
+ * @return a new filter object, or NULL on error
+ */
+struct filter *
+filter_configured_new(const struct config_param *param, GError **error_r);
+
+/**
+ * Deletes a filter.  It must be closed prior to calling this
+ * function, see filter_close().
+ *
+ * @param filter the filter object
+ */
+void
+filter_free(struct filter *filter);
+
+/**
+ * Opens the filter, preparing it for filter_filter().
+ *
+ * @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
+ */
+const struct audio_format *
+filter_open(struct filter *filter, struct audio_format *audio_format,
+	    GError **error_r);
+
+/**
+ * Closes the filter.  After that, you may call filter_open() again.
+ *
+ * @param filter the filter object
+ */
+void
+filter_close(struct filter *filter);
+
+/**
+ * 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
+ */
+const void *
+filter_filter(struct filter *filter, const void *src, size_t src_size,
+	      size_t *dest_size_r,
+	      GError **error_r);
+
+#endif