Allow relative level argument in /level command

mods/lzr_level_select/init.lua

@@ -404,15 +404,48 @@ minetest.register_chatcommand("level", {
 		if lzr_gamestate.is_loading() then
 			return false, S("Can’t start a level while loading!")
 		end
-		local pack, level = string.match(param, "([a-zA-Z0-9_]+) ([0-9]+)")
+		-- Parse arguments. level argument may be relative (tilde notation)
-		if not pack then
+		local pack, level = string.match(param, "([a-zA-Z0-9_]+) (~?-?[0-9]*)")
-			pack = "__core"
+
-			level = tonumber(param)
+		local current_data = lzr_levels.get_current_level_data()
+		local current_pack
+		if current_data then
+			current_pack = current_data.name
+		end
+		-- Select level pack. Defaults to the current level pack
+		-- if playing one or __core if playing no level pack
+		-- (singleton levels don't count)
+		if not pack then
+			if current_pack then
+				pack = current_pack
+			else
+				pack = "__core"
+			end
+			level = string.match(param, "(~?-?[0-9]*)")
 		end
-		level = tonumber(level)
 		if not level then
 			return false
 		end
+		-- Handle relative level argument (e.g. "~1" to move to the next level)
+		local is_relative_level = string.sub(level, 1, 1) == "~"
+		if is_relative_level then
+			local current_level = lzr_levels.get_current_level()
+			-- Relative level argument is only allowed if playing in a level
+			-- and the chosen level pack is the same as the current one.
+			if not current_level then
+				return false, S("Level argument can’t be relative when not playing in a level.")
+			end
+			if current_pack ~= pack then
+				return false, S("Level argument can’t be relative when choosing a different level pack.")
+			end
+			level = minetest.parse_relative_number(level, current_level)
+		else
+			level = tonumber(level)
+		end
+		if not level then
+			return false
+		end
+
 		local level_data = lzr_levels.get_level_pack(pack)
 		if not level_data then
 			return false, S("Level pack “@1” doesn’t exist.", pack)