queue: implement song "priorities"

Sorts remaining songs by priority.  This can be used for the
much-demanded "queue feature".

src/queue.c

@@ -21,6 +21,8 @@
 #include "queue.h"
 #include "song.h"
 
+#include <stdlib.h>
+
 /**
  * Generate a non-existing id number.
  */
@@ -104,6 +106,7 @@ queue_append(struct queue *queue, struct song *song)
 		.song = song,
 		.id = id,
 		.version = queue->version,
+		.priority = 0,
 	};
 
 	queue->order[queue->length] = queue->length;
@@ -220,6 +223,30 @@ queue_move_range(struct queue *queue, unsigned start, unsigned end, unsigned to)
 	}
 }
 
+/**
+ * Moves a song to a new position in the "order" list.
+ */
+static void
+queue_move_order(struct queue *queue, unsigned from_order, unsigned to_order)
+{
+	assert(queue != NULL);
+	assert(from_order < queue->length);
+	assert(to_order <= queue->length);
+
+	const unsigned from_position =
+		queue_order_to_position(queue, from_order);
+
+	if (from_order < to_order) {
+		for (unsigned i = from_order; i < to_order; ++i)
+			queue->order[i] = queue->order[i + 1];
+	} else {
+		for (unsigned i = from_order; i > to_order; --i)
+			queue->order[i] = queue->order[i - 1];
+	}
+
+	queue->order[to_order] = from_position;
+}
+
 void
 queue_delete(struct queue *queue, unsigned position)
 {
@@ -308,15 +335,123 @@ queue_finish(struct queue *queue)
 	g_rand_free(queue->rand);
 }
 
+static const struct queue_item *
+queue_get_order_item_const(const struct queue *queue, unsigned order)
+{
+	assert(queue != NULL);
+	assert(order < queue->length);
+
+	return &queue->items[queue->order[order]];
+}
+
+static uint8_t
+queue_get_order_priority(const struct queue *queue, unsigned order)
+{
+	return queue_get_order_item_const(queue, order)->priority;
+}
+
+static gint
+queue_item_compare_order_priority(gconstpointer av, gconstpointer bv,
+				  gpointer user_data)
+{
+	const struct queue *queue = user_data;
+	const unsigned *const ap = av;
+	const unsigned *const bp = bv;
+	assert(ap >= queue->order && ap < queue->order + queue->length);
+	assert(bp >= queue->order && bp < queue->order + queue->length);
+	uint8_t a = queue->items[*ap].priority;
+	uint8_t b = queue->items[*bp].priority;
+
+	if (G_LIKELY(a == b))
+		return 0;
+	else if (a > b)
+		return -1;
+	else
+		return 1;
+}
+
+static void
+queue_sort_order_by_priority(struct queue *queue, unsigned start, unsigned end)
+{
+	assert(queue != NULL);
+	assert(queue->random);
+	assert(start <= end);
+	assert(end <= queue->length);
+
+	g_qsort_with_data(&queue->order[start], end - start,
+			  sizeof(queue->order[0]),
+			  queue_item_compare_order_priority,
+			  queue);
+}
+
+/**
+ * Shuffle the order of items in the specified range, ignoring their
+ * priorities.
+ */
+static void
+queue_shuffle_order_range(struct queue *queue, unsigned start, unsigned end)
+{
+	assert(queue != NULL);
+	assert(queue->random);
+	assert(start <= end);
+	assert(end <= queue->length);
+
+	for (unsigned i = start; i < end; ++i)
+		queue_swap_order(queue, i,
+				 g_rand_int_range(queue->rand, i, end));
+}
+
+/**
+ * Sort the "order" of items by priority, and then shuffle each
+ * priority group.
+ */
+void
+queue_shuffle_order_range_with_priority(struct queue *queue,
+					unsigned start, unsigned end)
+{
+	assert(queue != NULL);
+	assert(queue->random);
+	assert(start <= end);
+	assert(end <= queue->length);
+
+	if (start == end)
+		return;
+
+	/* first group the range by priority */
+	queue_sort_order_by_priority(queue, start, end);
+
+	/* now shuffle each priority group */
+	unsigned group_start = start;
+	uint8_t group_priority = queue_get_order_priority(queue, start);
+
+	for (unsigned i = start + 1; i < end; ++i) {
+		uint8_t priority = queue_get_order_priority(queue, i);
+		assert(priority <= group_priority);
+
+		if (priority != group_priority) {
+			/* start of a new group - shuffle the one that
+			   has just ended */
+			queue_shuffle_order_range(queue, group_start, i);
+			group_start = i;
+			group_priority = priority;
+		}
+	}
+
+	/* shuffle the last group */
+	queue_shuffle_order_range(queue, group_start, end);
+}
+
 void
 queue_shuffle_order(struct queue *queue)
 {
-	assert(queue->random);
+	queue_shuffle_order_range_with_priority(queue, 0, queue->length);
+}
 
-	for (unsigned i = 0; i < queue->length; i++)
-		queue_swap_order(queue, i,
-				 g_rand_int_range(queue->rand, i,
-						  queue->length));
+static void
+queue_shuffle_order_first(struct queue *queue, unsigned start, unsigned end)
+{
+	queue_swap_order(queue, start,
+			 g_rand_int_range(queue->rand, start, end));
 }
 
 void
@@ -337,3 +472,132 @@ queue_shuffle_range(struct queue *queue, unsigned start, unsigned end)
 		queue_swap(queue, i, ri);
 	}
 }
+
+/**
+ * Find the first item that has this specified priority or higher.
+ */
+G_GNUC_PURE
+static unsigned
+queue_find_priority_order(const struct queue *queue, unsigned start_order,
+			  uint8_t priority, unsigned exclude_order)
+{
+	assert(queue != NULL);
+	assert(queue->random);
+	assert(start_order <= queue->length);
+
+	for (unsigned order = start_order; order < queue->length; ++order) {
+		const unsigned position = queue_order_to_position(queue, order);
+		const struct queue_item *item = &queue->items[position];
+		if (item->priority <= priority && order != exclude_order)
+			return order;
+	}
+
+	return queue->length;
+}
+
+G_GNUC_PURE
+static unsigned
+queue_count_same_priority(const struct queue *queue, unsigned start_order,
+			  uint8_t priority)
+{
+	assert(queue != NULL);
+	assert(queue->random);
+	assert(start_order <= queue->length);
+
+	for (unsigned order = start_order; order < queue->length; ++order) {
+		const unsigned position = queue_order_to_position(queue, order);
+		const struct queue_item *item = &queue->items[position];
+		if (item->priority != priority)
+			return order - start_order;
+	}
+
+	return queue->length - start_order;
+}
+
+bool
+queue_set_priority(struct queue *queue, unsigned position, uint8_t priority,
+		   int after_order)
+{
+	assert(queue != NULL);
+	assert(position < queue->length);
+
+	struct queue_item *item = &queue->items[position];
+	uint8_t old_priority = item->priority;
+	if (old_priority == priority)
+		return false;
+
+	item->version = queue->version;
+	item->priority = priority;
+
+	if (!queue->random)
+		/* don't reorder if not in random mode */
+		return true;
+
+	unsigned order = queue_position_to_order(queue, position);
+	if (after_order >= 0) {
+		if (order == (unsigned)after_order)
+			/* don't reorder the current song */
+			return true;
+
+		if (order < (unsigned)after_order) {
+			/* the specified song has been played already
+			   - enqueue it only if its priority has just
+			   become bigger than the current one's */
+
+			const unsigned after_position =
+				queue_order_to_position(queue, after_order);
+			const struct queue_item *after_item =
+				&queue->items[after_position];
+			if (old_priority > after_item->priority ||
+			    priority <= after_item->priority)
+				/* priority hasn't become bigger */
+				return true;
+		}
+	}
+
+	/* move the item to the beginning of the priority group (or
+	   create a new priority group) */
+
+	const unsigned before_order =
+		queue_find_priority_order(queue, after_order + 1, priority,
+					  order);
+	const unsigned new_order = before_order > order
+		? before_order - 1
+		: before_order;
+	queue_move_order(queue, order, new_order);
+
+	/* shuffle the song within that priority group */
+
+	const unsigned priority_count =
+		queue_count_same_priority(queue, new_order, priority);
+	assert(priority_count >= 1);
+	queue_shuffle_order_first(queue, new_order,
+				  new_order + priority_count);
+
+	return true;
+}
+
+bool
+queue_set_priority_range(struct queue *queue,
+			 unsigned start_position, unsigned end_position,
+			 uint8_t priority, int after_order)
+{
+	assert(queue != NULL);
+	assert(start_position <= end_position);
+	assert(end_position <= queue->length);
+
+	bool modified = false;
+	int after_position = after_order >= 0
+		? (int)queue_order_to_position(queue, after_order)
+		: -1;
+	for (unsigned i = start_position; i < end_position; ++i) {
+		after_order = after_position >= 0
+			? (int)queue_position_to_order(queue, after_position)
+			: -1;
+
+		modified |= queue_set_priority(queue, i, priority,
+					       after_order);
+	}
+
+	return modified;
+}