songvec: sort songs by disc and track number

Sorting songs by file name does not make much sense.  Most of the
time, users want to add songs in track order to the playlist.

src/songvec.c

@@ -1,5 +1,6 @@
 #include "songvec.h"
 #include "song.h"
+#include "tag.h"
 
 #include <glib.h>
 
@@ -9,11 +10,56 @@
 
 static GMutex *nr_lock = NULL;
 
+/**
+ * Compare two tag values which should contain an integer value
+ * (e.g. disc or track number).  Either one may be NULL.
+ */
+static int
+compare_number_string(const char *a, const char *b)
+{
+	long ai = a == NULL ? 0 : strtol(a, NULL, 10);
+	long bi = b == NULL ? 0 : strtol(b, NULL, 10);
+
+	if (ai <= 0)
+		return bi <= 0 ? 0 : -1;
+
+	if (bi <= 0)
+		return 1;
+
+	return ai - bi;
+}
+
+static int
+compare_tag_item(const struct tag *a, const struct tag *b, enum tag_type type)
+{
+	if (a == NULL)
+		return b == NULL ? 0 : -1;
+
+	if (b == NULL)
+		return 1;
+
+	return compare_number_string(tag_get_value(a, type),
+				     tag_get_value(b, type));
+}
+
 /* Only used for sorting/searchin a songvec, not general purpose compares */
 static int songvec_cmp(const void *s1, const void *s2)
 {
 	const struct song *a = ((const struct song * const *)s1)[0];
 	const struct song *b = ((const struct song * const *)s2)[0];
+	int ret;
+
+	/* first sort by disc */
+	ret = compare_tag_item(a->tag, b->tag, TAG_ITEM_DISC);
+	if (ret != 0)
+		return ret;
+
+	/* then by track number */
+	ret = compare_tag_item(a->tag, b->tag, TAG_ITEM_TRACK);
+	if (ret != 0)
+		return ret;
+
+	/* still no difference?  compare file name */
 	return g_utf8_collate(a->url, b->url);
 }