xC: make fancy-prompt.lua alignment more reliable

And generally clean up that script.

plugins/xC/fancy-prompt.lua

@@ -1,7 +1,7 @@
 --
 -- fancy-prompt.lua: the fancy multiline prompt you probably want
 --
--- Copyright (c) 2016, Přemysl Eric Janouch <p@janouch.name>
+-- Copyright (c) 2016 - 2022, Přemysl Eric Janouch <p@janouch.name>
 --
 -- Permission to use, copy, modify, and/or distribute this software for any
 -- purpose with or without fee is hereby granted.
@@ -40,7 +40,7 @@ xC.hook_prompt (function (hook)
 		if buffer == current then
 			current_n = i
 		elseif buffer.new_messages_count ~= buffer.new_unimportant_count then
-			if active ~= "" then active = active .. "," end
+			active = active .. ","
 			if buffer.highlighted then
 				active = active .. "!"
 				bg_color = "224"
@@ -48,7 +48,6 @@ xC.hook_prompt (function (hook)
 			active = active .. i
 		end
 	end
-	if active ~= "" then active = "(" .. active .. ")" end
 	local x = current_n .. ":" .. current.name
 	if chan and chan.users_len ~= 0 then
 		local params = ""
@@ -56,25 +55,34 @@ xC.hook_prompt (function (hook)
 			params = params .. " +" .. mode .. " " .. param
 		end
 		local modes = chan.no_param_modes .. params:sub (3)
-		if modes ~= "" then x = x .. "(+" .. modes .. ")" end
+		if modes ~= "" then
+			x = x .. "(+" .. modes .. ")"
+		end
 		x = x .. "{" .. chan.users_len .. "}"
 	end
-	if current.hide_unimportant then x = x .. "<H>" end
+	if current.hide_unimportant then
-
+		x = x .. "<H>"
-	local lines, cols = xC.get_screen_size ()
+	end
-	x = x .. " " .. active .. string.rep (" ", cols)
+	if active ~= "" then
+		x = x .. " (" .. active:sub (2) .. ")"
+	end
 
 	-- Readline 7.0.003 seems to be broken and completely corrupts the prompt.
 	-- However 8.0.004 seems to be fine with these, as is libedit 20191231-3.1.
 	--x = x:gsub("[\128-\255]", "?")
 
-	-- Cut off extra characters and apply formatting, including the hack.
+	-- Align to the terminal's width and apply formatting, including the hack.
-	-- FIXME: this doesn't count with full-width or zero-width characters.
+	local lines, cols = xC.get_screen_size ()
-	--   We might want to export wcwidth() above term_from_utf8 somehow.
+	local trailing, width = " ", xC.measure (x)
-	local overflow = utf8.offset (x, cols - 1)
+	while cols > 0 and width >= cols do
-	if overflow then x = x:sub (1, overflow) end
+		x = x:sub (1, utf8.offset (x, -1) - 1)
+		trailing, width = ">", xC.measure (x)
+	end
+
 	x = "\x01\x1b[0;4;1;38;5;16m\x1b[48;5;" .. bg_color .. "m\x02" ..
-		x ..  "\x01\x1b[0;4;1;7;38;5;" .. bg_color .. "m\x02 \x01\x1b[0;1m\x02"
+		x .. string.rep (" ", cols - width - 1) ..
+		"\x01\x1b[0;4;1;7;38;5;" .. bg_color .. "m\x02" ..
+		trailing .. "\x01\x1b[0;1m\x02"
 
 	local user_prefix = function (chan, user)
 		for i, chan_user in ipairs (chan.users) do

xC.c

@@ -3776,7 +3776,7 @@ explode_text (struct exploder *self, const char *text)
 		if (!strchr ("\a\b\x0e\x0f\x1b" /* BEL BS SO SI ESC */, *p))
 			str_append_c (&filtered, *p);
 
-	size_t term_len = 0;
+	size_t term_len = 0, processed = 0, len;
 	char *term = iconv_xstrdup (self->ctx->term_from_utf8,
 		filtered.str, filtered.len + 1, &term_len);
 	str_free (&filtered);
@@ -3785,11 +3785,10 @@ explode_text (struct exploder *self, const char *text)
 	memset (&ps, 0, sizeof ps);
 
 	wchar_t wch;
-	size_t len, processed = 0;
 	while ((len = mbrtowc (&wch, term + processed, term_len - processed, &ps)))
 	{
 		hard_assert (len != (size_t) -2 && len != (size_t) -1);
-		processed += len;
+		hard_assert ((processed += len) <= term_len);
 
 		struct line_char *c = line_char_new (wch);
 		c->attrs = self->attrs;
@@ -10527,6 +10526,33 @@ lua_plugin_get_screen_size (lua_State *L)
 	return 2;
 }
 
+static int
+lua_plugin_measure (lua_State *L)
+{
+	struct lua_plugin *plugin = lua_touserdata (L, lua_upvalueindex (1));
+	const char *line = lua_plugin_check_utf8 (L, 1);
+
+	size_t term_len = 0, processed = 0, width = 0, len;
+	char *term = iconv_xstrdup (plugin->ctx->term_from_utf8,
+		(char *) line, strlen (line) + 1, &term_len);
+
+	mbstate_t ps;
+	memset (&ps, 0, sizeof ps);
+
+	wchar_t wch;
+	while ((len = mbrtowc (&wch, term + processed, term_len - processed, &ps)))
+	{
+		hard_assert (len != (size_t) -2 && len != (size_t) -1);
+		hard_assert ((processed += len) <= term_len);
+
+		int wch_width = wcwidth (wch);
+		width += MAX (0, wch_width);
+	}
+	free (term);
+	lua_pushinteger (L, width);
+	return 1;
+}
+
 static int
 lua_ctx_gc (lua_State *L)
 {
@@ -10547,6 +10573,7 @@ static luaL_Reg lua_plugin_library[] =
 	// And these are methods:
 
 	{ "get_screen_size", lua_plugin_get_screen_size },
+	{ "measure",         lua_plugin_measure         },
 	{ "__gc",            lua_ctx_gc                 },
 	{ NULL,              NULL                       },
 };