New script to delete unused tags on startup

official/delete_unused_tags.lua

@@ -0,0 +1,46 @@
+--[[
+    This file is part of darktable,
+    copyright (c) 2018 Tobias Ellinghaus
+
+    darktable 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 3 of the License, or
+    (at your option) any later version.
+
+    darktable 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 darktable.  If not, see <http://www.gnu.org/licenses/>.
+]]
+--[[
+DELETE UNUSED TAGS
+A simple script that will automatically delete all tags that are not attached to any images
+
+USAGE
+* require this script from your main lua file
+* restart darktable
+
+all tags that are not used will be automatically deleted at every restart
+]]
+
+local dt = require "darktable"
+
+dt.configuration.check_version(...,{5,0,0})
+
+-- deleting while iterating the tags list seems to break the iterator!
+local unused_tags = {}
+
+for _, t in ipairs(dt.tags) do
+  if #t == 0 then
+    table.insert(unused_tags, t.name)
+  end
+end
+
+for _,name in pairs(unused_tags) do
+  print("deleting tag `" .. name .. "'")
+  tag = dt.tags.find(name)
+  tag:delete()
+end