tag/Set: move code to ApplyTagWithFallback()

Makefile.am

@@ -977,6 +977,7 @@ libtag_a_SOURCES =\
 	src/tag/TagItem.hxx \
 	src/tag/TagHandler.cxx src/tag/TagHandler.hxx \
 	src/tag/Mask.hxx \
+	src/tag/Fallback.hxx \
 	src/tag/Settings.cxx src/tag/Settings.hxx \
 	src/tag/TagConfig.cxx src/tag/TagConfig.hxx \
 	src/tag/TagNames.c \

src/tag/Fallback.hxx

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2003-2018 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.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_TAG_FALLBACK_HXX
+#define MPD_TAG_FALLBACK_HXX
+
+#include <utility>
+
+template<typename F>
+bool
+ApplyTagFallback(TagType type, F &&f) noexcept
+{
+	if (type == TAG_ALBUM_ARTIST)
+		/* fall back to "Artist" if no "AlbumArtist" was found */
+		return f(TAG_ARTIST);
+
+	return false;
+}
+
+template<typename F>
+bool
+ApplyTagWithFallback(TagType type, F &&f) noexcept
+{
+	return f(type) || ApplyTagFallback(type, std::forward<F>(f));
+}
+
+#endif

src/tag/Set.cxx

@@ -18,16 +18,19 @@
  */
 
 #include "Set.hxx"
+#include "Fallback.hxx"
 #include "TagBuilder.hxx"
 #include "util/StringView.hxx"
 
+#include <functional>
+
 #include <assert.h>
 
 /**
  * Copy all tag items of the specified type.
  */
 static bool
-CopyTagItem(TagBuilder &dest, TagType dest_type,
+CopyTagItem2(TagBuilder &dest, TagType dest_type,
 	     const Tag &src, TagType src_type)
 {
 	bool found = false;
@@ -49,9 +52,9 @@ CopyTagItem(TagBuilder &dest, TagType dest_type,
 static void
 CopyTagItem(TagBuilder &dest, const Tag &src, TagType type)
 {
-	if (!CopyTagItem(dest, type, src, type) &&
-	    type == TAG_ALBUM_ARTIST)
-		CopyTagItem(dest, type, src, TAG_ARTIST);
+	ApplyTagWithFallback(type,
+			     std::bind(CopyTagItem2, std::ref(dest), type,
+				       std::cref(src), std::placeholders::_1));
 }
 
 /**
@@ -105,9 +108,10 @@ TagSet::InsertUnique(const Tag &tag,
 
 	assert((group_mask & (tag_mask_t(1) << unsigned(type))) == 0);
 
-	if (!CheckUnique(type, tag, type, group_mask) &&
-	    (type != TAG_ALBUM_ARTIST ||
-	     /* fall back to "Artist" if no "AlbumArtist" was found */
-	     !CheckUnique(type, tag, TAG_ARTIST, group_mask)))
+	if (!ApplyTagWithFallback(type,
+				  std::bind(&TagSet::CheckUnique, this,
+					    type, std::cref(tag),
+					    std::placeholders::_1,
+					    group_mask)))
 		InsertUnique(tag, type, "", group_mask);
 }