core/database: Add support for defragmenting databases

Removing items from the database leaves a NULL pointer "hole" that is
never filled in.  This doesn't affect correctness, but it could be
wasteful as items are added and removed.  This patch adds a function to
defragment the database without changing the order of items.

Implements #66: Add support for rebalancing databases
Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

TODO

@@ -12,15 +12,6 @@ Future work:
 
 	- Save a user's playlist as a group:
 
-	- Library defragment:
-		Ocarina 6.0 will leave holes in the library when tracks are
-		deleted, potentially leading to fragmentation and larger-than-
-		needed file sizes.  A "defragment" utility can be created to
-		clean up unused slots.
-
-		To help with fixing groups, a mapping of (old values) ->
-		(new values) should be kept.
-
 	- Fix track durations:
 		Some tracks in my library are tagged with the wrong duration,
 		so fix them as they are played.

core/database.c

@@ -165,6 +165,31 @@ void db_remove(struct database *db, struct db_entry *item)
 	db_autosave(db);
 }
 
+bool db_defrag(struct database *db)
+{
+	struct db_entry *dbe;
+	unsigned int i, cur;
+
+	if (db->db_size == db_actual_size(db))
+		return false;
+
+	for (cur = 0, i = 1; cur < db->db_size; cur++) {
+		if (DB_ENTRY_AT(db, cur))
+			continue;
+		while (i <= cur || !DB_ENTRY_AT(db, i))
+			i++;
+
+		dbe = DB_ENTRY_AT(db, i);
+		g_ptr_array_index(db->db_entries,   i) = NULL;
+		g_ptr_array_index(db->db_entries, cur) = dbe;
+		dbe->dbe_index = cur;
+	}
+
+	g_ptr_array_set_size(db->db_entries, db->db_size);
+	db_autosave(db);
+	return true;
+}
+
 unsigned int db_actual_size(const struct database *db)
 {
 	if (db->db_entries)

include/core/database.h

@@ -123,6 +123,12 @@ struct db_entry *db_insert(struct database *, const gchar *);
 /* Called to remove an item from the database. */
 void db_remove(struct database *, struct db_entry *);
 
+/*
+ * Called to shrink the database by removing any NULL pointers without
+ * changing the order of items in the database.
+ * Returns true if the database has been modified.
+ */
+bool db_defrag(struct database *);
 
 /* Returns the database item at the requested index. */
 struct db_entry *db_at(const struct database *, unsigned int);

tests/core/database.c

@@ -219,6 +219,19 @@ static void test_database(gconstpointer arg)
 	}
 	g_assert_cmpuint(i, ==, N + 1);
 
+	/* db_defrag(): Remove all NULL pointers */
+	i = 0;
+	g_assert_true(db_defrag(&db));
+	g_assert_cmpuint(db.db_size, ==, N / 2);
+	g_assert_cmpuint(db_actual_size(&db), ==, N / 2);
+	db_for_each(dbe, next, &db) {
+		g_assert_nonnull(dbe);
+		g_assert_cmpuint(INT_ENTRY(dbe)->ie_val, ==, (i * 2) + 1);
+		g_assert(dbe == db_at(&db, i));
+		i++;
+	}
+	g_assert_false(db_defrag(&db));
+
 	db_deinit(&db);
 	g_assert_cmpuint(db.db_size, ==, 0);
 	g_assert_cmpuint(db_actual_size(&db), ==, 0);