From 7cfe929c3692e74f4e6073960eaf0e08a9989811 Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@musicpd.org>
Date: Mon, 22 Oct 2018 12:42:18 +0200
Subject: [PATCH] db/Count: print empty group if song without grouped tag
 exists

Be consistent with "list" responses.
---
 NEWS                      | 1 +
 doc/protocol.xml          | 5 +++++
 src/db/Count.cxx          | 8 ++++----
 src/tag/VisitFallback.hxx | 8 ++++++++
 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/NEWS b/NEWS
index 9507057e7..5b8a3b48c 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,7 @@
 ver 0.20.22 (not yet released)
 * protocol
   - add tag fallbacks for AlbumArtistSort, ArtistSort
+  - "count group ..." can print an empty group
 * storage
   - curl: URL-encode paths
 * Android
diff --git a/doc/protocol.xml b/doc/protocol.xml
index 4b9a4c95a..75cdb5478 100644
--- a/doc/protocol.xml
+++ b/doc/protocol.xml
@@ -1599,6 +1599,11 @@ OK
               per-artist counts:
             </para>
             <programlisting>count group artist</programlisting>
+            <para>
+              A group with an empty value contains counts of matching
+              song which don't this group tag.  It exists only if at
+              least one such song is found.
+            </para>
           </listitem>
         </varlistentry>
 
diff --git a/src/db/Count.cxx b/src/db/Count.cxx
index ba0d443e7..dccb3e8c7 100644
--- a/src/db/Count.cxx
+++ b/src/db/Count.cxx
@@ -90,10 +90,10 @@ GroupCountVisitor(TagCountMap &map, TagType group, const LightSong &song)
 	assert(song.tag != nullptr);
 
 	const Tag &tag = *song.tag;
-	VisitTagWithFallback(tag, group,
-			     std::bind(CollectGroupCounts, std::ref(map),
-				       std::cref(tag),
-				       std::placeholders::_1));
+	VisitTagWithFallbackOrEmpty(tag, group,
+				    std::bind(CollectGroupCounts, std::ref(map),
+					      std::cref(tag),
+					      std::placeholders::_1));
 }
 
 void
diff --git a/src/tag/VisitFallback.hxx b/src/tag/VisitFallback.hxx
index 3ad486f5f..23992d44b 100644
--- a/src/tag/VisitFallback.hxx
+++ b/src/tag/VisitFallback.hxx
@@ -49,4 +49,12 @@ VisitTagWithFallback(const Tag &tag, TagType type, F &&f) noexcept
 				    });
 }
 
+template<typename F>
+void
+VisitTagWithFallbackOrEmpty(const Tag &tag, TagType type, F &&f) noexcept
+{
+	if (!VisitTagWithFallback(tag, type, f))
+		f("");
+}
+
 #endif