Lua: Implement function to resolve filename w.r.t. another

lua.lyx

@@ -120,8 +120,8 @@ Expanded to string token containing path and filename of this Lua script.
 
 \begin_layout Standard
 In practicular, this is suitable to be passed as base argument of various
- functions like loadfile, dofile, gui.bitmap_load, gui.bitmap_load_png and
- gui.bitmap_load_pal.
+ functions like loadfile, dofile, resolve_filename, gui.bitmap_load, gui.bitmap_lo
+ad_png and gui.bitmap_load_pal.
 \end_layout
 
 \begin_layout Section
@@ -388,6 +388,18 @@ Execute lua script from <filename>, resolved relative to <base> (if empty,
  current directory) and return all return values.
 \end_layout
 
+\begin_layout Subsection
+resolve_filename: Resolve name of file relative to another
+\end_layout
+
+\begin_layout Itemize
+Syntax: string resolve_file(string filename, string base)
+\end_layout
+
+\begin_layout Standard
+Resolve name of file <filename> relative to <base> and return the result.
+\end_layout
+
 \begin_layout Standard
 \begin_inset Newpage pagebreak
 \end_inset

src/lua/loadfile.cpp

@@ -199,4 +199,15 @@ namespace
 		int new_sp = lua_gettop(L.handle());
 		return new_sp - (old_sp - 1);
 	});
+
+	function_ptr_luafun resolvefile(LS, "resolve_filename", [](lua_state& L, const std::string& fname)
+	{
+		std::string file2;
+		std::string file1 = L.get_string(1, fname.c_str());
+		if(L.type(2) != LUA_TNIL && L.type(2) != LUA_TNONE)
+			file2 = L.get_string(2, fname.c_str());
+		std::string absfilename = resolve_file_relative(file1, file2);
+		L.pushlstring(absfilename);
+		return 1;
+	});
 }