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diff --git a/src/queue.h b/src/queue.h
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+/*
+ * Copyright (C) 2003-2009 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., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifndef QUEUE_H
+#define QUEUE_H
+
+#include <glib.h>
+
+#include <assert.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+enum {
+	/**
+	 * reserve max_length * QUEUE_HASH_MULT elements in the id
+	 * number space
+	 */
+	QUEUE_HASH_MULT = 4,
+};
+
+/**
+ * A queue of songs.  This is the backend of the playlist: it contains
+ * an ordered list of songs.
+ *
+ * Songs can be addressed in three possible ways:
+ *
+ * - the position in the queue
+ * - the unique id (which stays the same, regardless of moves)
+ * - the order number (which only differs from "position" in random mode)
+ */
+struct queue {
+	/** configured maximum length of the queue */
+	unsigned max_length;
+
+	/** number of songs in the queue */
+	unsigned length;
+
+	/** the current version number */
+	uint32_t version;
+
+	/** all songs in "position" order */
+	struct song **songs;
+
+	/** holds version a song was modified on */
+	uint32_t *songMod;
+
+	/** map order numbers to positions */
+	unsigned *order;
+
+	/** map positions to song ids */
+	unsigned *positionToId;
+
+	/** map song ids to posiitons */
+	int *idToPosition;
+
+	/** repeat playback when the end of the queue has been
+	    reached? */
+	bool repeat;
+
+	/** play back songs in random order? */
+	bool random;
+
+	/** random number generator for shuffle and random mode */
+	GRand *rand;
+};
+
+static inline unsigned
+queue_length(const struct queue *queue)
+{
+	assert(queue->length <= queue->max_length);
+
+	return queue->length;
+}
+
+/**
+ * Determine if the queue is empty, i.e. there are no songs.
+ */
+static inline bool
+queue_is_empty(const struct queue *queue)
+{
+	return queue->length == 0;
+}
+
+/**
+ * Determine if the maximum number of songs has been reached.
+ */
+static inline bool
+queue_is_full(const struct queue *queue)
+{
+	assert(queue->length <= queue->max_length);
+
+	return queue->length >= queue->max_length;
+}
+
+/**
+ * Is that a valid position number?
+ */
+static inline bool
+queue_valid_position(const struct queue *queue, unsigned position)
+{
+	return position < queue->length;
+}
+
+/**
+ * Is that a valid order number?
+ */
+static inline bool
+queue_valid_order(const struct queue *queue, unsigned order)
+{
+	return order < queue->length;
+}
+
+static inline int
+queue_id_to_position(const struct queue *queue, unsigned id)
+{
+	if (id >= queue->max_length * QUEUE_HASH_MULT)
+		return -1;
+
+	assert(queue->idToPosition[id] >= -1);
+	assert(queue->idToPosition[id] < (int)queue->length);
+
+	return queue->idToPosition[id];
+}
+
+static inline int
+queue_position_to_id(const struct queue *queue, unsigned position)
+{
+	assert(position < queue->length);
+
+	return queue->positionToId[position];
+}
+
+static inline unsigned
+queue_order_to_position(const struct queue *queue, unsigned order)
+{
+	assert(order < queue->length);
+
+	return queue->order[order];
+}
+
+static inline unsigned
+queue_position_to_order(const struct queue *queue, unsigned position)
+{
+	assert(position < queue->length);
+
+	for (unsigned i = 0;; ++i) {
+		assert(i < queue->length);
+
+		if (queue->order[i] == position)
+			return i;
+	}
+}
+
+/**
+ * Returns the song at the specified position.
+ */
+static inline struct song *
+queue_get(const struct queue *queue, unsigned position)
+{
+	assert(position < queue->length);
+
+	return queue->songs[position];
+}
+
+/**
+ * Returns the song at the specified order number.
+ */
+static inline struct song *
+queue_get_order(const struct queue *queue, unsigned order)
+{
+	return queue_get(queue, queue_order_to_position(queue, order));
+}
+
+/**
+ * Is the song at the specified position newer than the specified
+ * version?
+ */
+static inline bool
+queue_song_newer(const struct queue *queue, unsigned position,
+		 uint32_t version)
+{
+	assert(position < queue->length);
+
+	return version > queue->version ||
+		queue->songMod[position] >= version ||
+		queue->songMod[position] == 0;
+}
+
+/**
+ * Initialize a queue object.
+ */
+void
+queue_init(struct queue *queue, unsigned max_length);
+
+/**
+ * Deinitializes a queue object.  It does not free the queue pointer
+ * itself.
+ */
+void
+queue_finish(struct queue *queue);
+
+/**
+ * Generate a non-existing id number.
+ */
+unsigned
+queue_generate_id(const struct queue *queue);
+
+/**
+ * Returns the order number following the specified one.  This takes
+ * end of queue and "repeat" mode into account.
+ *
+ * @return the next order number, or -1 to stop playback
+ */
+int
+queue_next_order(const struct queue *queue, unsigned order);
+
+/**
+ * Increments the queue's version number.  This handles integer
+ * overflow well.
+ */
+void
+queue_increment_version(struct queue *queue);
+
+/**
+ * Marks the specified song as "modified" and increments the version
+ * number.
+ */
+void
+queue_modify(struct queue *queue, unsigned order);
+
+/**
+ * Marks all songs as "modified" and increments the version number.
+ */
+void
+queue_modify_all(struct queue *queue);
+
+/**
+ * Appends a song to the queue and returns its position.  Prior to
+ * that, the caller must check if the queue is already full.
+ *
+ * If a song is not in the database (determined by
+ * song_in_database()), it is freed when removed from the queue.
+ */
+unsigned
+queue_append(struct queue *queue, struct song *song);
+
+/**
+ * Swaps two songs, addressed by their position.
+ */
+void
+queue_swap(struct queue *queue, unsigned position1, unsigned position2);
+
+/**
+ * Swaps two songs, addressed by their order number.
+ */
+static inline void
+queue_swap_order(struct queue *queue, unsigned order1, unsigned order2)
+{
+	unsigned tmp = queue->order[order1];
+	queue->order[order1] = queue->order[order2];
+	queue->order[order2] = tmp;
+}
+
+/**
+ * Moves a song to a new position.
+ */
+void
+queue_move(struct queue *queue, unsigned from, unsigned to);
+
+/**
+ * Removes a song from the playlist.
+ */
+void
+queue_delete(struct queue *queue, unsigned position);
+
+/**
+ * Removes all songs from the playlist.
+ */
+void
+queue_clear(struct queue *queue);
+
+/**
+ * Initializes the "order" array, and restores "normal" order.
+ */
+static inline void
+queue_restore_order(struct queue *queue)
+{
+	for (unsigned i = 0; i < queue->length; ++i)
+		queue->order[i] = i;
+}
+
+/**
+ * Shuffles a (position) range in the queue.  The songs are physically
+ * shuffled, not by using the "order" mapping.
+ */
+void
+queue_shuffle_range(struct queue *queue, unsigned start, unsigned end);
+
+#endif