util.stanza: Simplify and make pretty-printing look nicer

I've had this color theme in a local debug module for some time and I
quite like it. The colors are from the XMPP logo.

Removes extra XML serialization implementation in favor of the standard
one. Also removes recursive str=str..more string building.

The new two-level gsub has the accumulator in C space so shouldn't be
too bad. The inner gsub calls use no callback, so should be fast and
not create all that much garbage.

No serious benchmarking has been done, but who cares if it looks nice?

util/stanza.lua

@@ -11,7 +11,6 @@ local error         =         error;
 local t_insert      =  table.insert;
 local t_remove      =  table.remove;
 local t_concat      =  table.concat;
-local s_format      = string.format;
 local s_match       =  string.match;
 local tostring      =      tostring;
 local setmetatable  =  setmetatable;
@@ -491,38 +490,47 @@ end
 
 if do_pretty_printing then
 	local getstyle, getstring = termcolours.getstyle, termcolours.getstring;
-	local style_attrk = getstyle("yellow");
-	local style_attrv = getstyle("red");
-	local style_tagname = getstyle("red");
-	local style_punc = getstyle("magenta");
-
-	local attr_format = " "..getstring(style_attrk, "%s")..getstring(style_punc, "=")..getstring(style_attrv, "'%s'");
-	local top_tag_format = getstring(style_punc, "<")..getstring(style_tagname, "%s").."%s"..getstring(style_punc, ">");
-	--local tag_format = getstring(style_punc, "<")..getstring(style_tagname, "%s").."%s"..getstring(style_punc, ">").."%s"..getstring(style_punc, "</")..getstring(style_tagname, "%s")..getstring(style_punc, ">");
-	local tag_format = top_tag_format.."%s"..getstring(style_punc, "</")..getstring(style_tagname, "%s")..getstring(style_punc, ">");
-	function stanza_mt.pretty_print(t)
-		local children_text = "";
-		for _, child in ipairs(t) do
-			if type(child) == "string" then
-				children_text = children_text .. xml_escape(child);
-			else
-				children_text = children_text .. child:pretty_print();
-			end
-		end
 
-		local attr_string = "";
-		if t.attr then
-			for k, v in pairs(t.attr) do if type(k) == "string" then attr_string = attr_string .. s_format(attr_format, k, tostring(v)); end end
-		end
-		return s_format(tag_format, t.name, attr_string, children_text, t.name);
+	local blue1 = getstyle("1b3967");
+	local blue2 = getstyle("13b5ea");
+	local green1 = getstyle("439639");
+	local green2 = getstyle("a0ce67");
+	local orange1 = getstyle("d9541e");
+	local orange2 = getstyle("e96d1f");
+
+	local attr_replace = (
+		getstring(green2, "%1") .. -- attr name
+		getstring(green1, "%2") .. -- equal
+		getstring(orange1, "%3") .. -- quote
+		getstring(orange2, "%4") .. -- attr value
+		getstring(orange1, "%5") -- quote
+	);
+
+	local text_replace = (
+		getstring(green1, "%1") .. -- &
+		getstring(green2, "%2") .. -- amp
+		getstring(green1, "%3") -- ;
+	);
+
+	local function pretty(s)
+		-- Tag soup color
+		-- Outer gsub call takes each <tag>, applies colour to the brackets, the
+		-- tag name, then applies one inner gsub call to colour the attributes and
+		-- another for any text content.
+		return (s:gsub("(</?)([^ >]*)(.-)([?/]?>)([^<]*)", function(opening_bracket, tag_name, attrs, closing_bracket, content)
+			return getstring(blue1, opening_bracket)..getstring(blue2, tag_name)..
+				attrs:gsub("([^=]+)(=)([\"'])(.-)([\"'])", attr_replace) ..
+			getstring(blue1, closing_bracket) ..
+			content:gsub("(&#?)(%w+)(;)", text_replace);
+		end, 100));
+	end
+
+	function stanza_mt.pretty_print(t)
+		return pretty(tostring(t));
 	end
 
 	function stanza_mt.pretty_top_tag(t)
-		local attr_string = "";
-		if t.attr then
-			for k, v in pairs(t.attr) do if type(k) == "string" then attr_string = attr_string .. s_format(attr_format, k, tostring(v)); end end
-		end
-		return s_format(top_tag_format, t.name, attr_string);
+		return pretty(t:top_tag());
 	end
 else
 	-- Sorry, fresh out of colours for you guys ;)