command/Database: add "sort" parameter to "find" and "search"

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

src/db/DatabasePrint.cxx

@@ -22,6 +22,7 @@
 #include "Selection.hxx"
 #include "SongFilter.hxx"
 #include "SongPrint.hxx"
+#include "DetachedSong.hxx"
 #include "TimePrint.hxx"
 #include "TagPrint.hxx"
 #include "client/Response.hxx"
@@ -139,10 +140,36 @@ PrintPlaylistFull(Response &r, bool base,
 		time_print(r, "Last-Modified", playlist.mtime);
 }
 
+static bool
+CompareNumeric(const char *a, const char *b)
+{
+	long a_value = strtol(a, nullptr, 10);
+	long b_value = strtol(b, nullptr, 10);
+
+	return a_value < b_value;
+}
+
+static bool
+CompareTags(TagType type, const Tag &a, const Tag &b)
+{
+	const char *a_value = a.GetSortValue(type);
+	const char *b_value = b.GetSortValue(type);
+
+	switch (type) {
+	case TAG_DISC:
+	case TAG_TRACK:
+		return CompareNumeric(a_value, b_value);
+
+	default:
+		return strcmp(a_value, b_value) < 0;
+	}
+}
+
 void
 db_selection_print(Response &r, Partition &partition,
 		   const DatabaseSelection &selection,
 		   bool full, bool base,
+		   TagType sort,
 		   unsigned window_start, unsigned window_end)
 {
 	const Database &db = partition.GetDatabaseOrThrow();
@@ -161,16 +188,53 @@ db_selection_print(Response &r, Partition &partition,
 			    std::ref(r), base, _1, _2)
 		: VisitPlaylist();
 
-	if (window_start > 0 ||
-	    window_end < (unsigned)std::numeric_limits<int>::max())
-		s = [s, window_start, window_end, &i](const LightSong &song){
-			const bool in_window = i >= window_start && i < window_end;
-			++i;
-			if (in_window)
-				s(song);
-		};
+	if (sort == TAG_NUM_OF_ITEM_TYPES) {
+		if (window_start > 0 ||
+		    window_end < (unsigned)std::numeric_limits<int>::max())
+			s = [s, window_start, window_end, &i](const LightSong &song){
+				const bool in_window = i >= window_start && i < window_end;
+				++i;
+				if (in_window)
+					s(song);
+			};
 
-	db.Visit(selection, d, s, p);
+		db.Visit(selection, d, s, p);
+	} else {
+		// TODO: allow the database plugin to sort internally
+
+		/* the client has asked us to sort the result; this is
+		   pretty expensive, because instead of streaming the
+		   result to the client, we need to copy it all into
+		   this std::vector, and then sort it */
+		std::vector<DetachedSong> songs;
+
+		{
+			auto collect_songs = [&songs](const LightSong &song){
+				songs.emplace_back(song);
+			};
+
+			db.Visit(selection, d, collect_songs, p);
+		}
+
+		std::stable_sort(songs.begin(), songs.end(),
+				 [sort](const DetachedSong &a, const DetachedSong &b){
+					 return CompareTags(sort, a.GetTag(),
+							    b.GetTag());
+				 });
+
+		if (window_end < songs.size())
+			songs.erase(std::next(songs.begin(), window_end),
+				    songs.end());
+
+		if (window_start >= songs.size())
+			return;
+
+		songs.erase(songs.begin(),
+			    std::next(songs.begin(), window_start));
+
+		for (const auto &song : songs)
+			s((LightSong)song);
+	}
 }
 
 void
@@ -179,6 +243,7 @@ db_selection_print(Response &r, Partition &partition,
 		   bool full, bool base)
 {
 	db_selection_print(r, partition, selection, full, base,
+			   TAG_NUM_OF_ITEM_TYPES,
 			   0, std::numeric_limits<int>::max());
 }