How to sort in Lua using locale?

[Mojca Miklavec <mojca.miklavec.lists@gmail.com>]

Hello,

I would like to create a document using some functionality that is
pretty close to creating an index, but the built-in index creation
code doesn't suffice. My main question is: how do I sort strings using
the built-in sorting rules for the current language?

Here is an example with a table of names:

names = {
    {first="Hans", last="Hagen"},
    {first="Hartmut", last="Henkel"},
    {first="Taco", last="Hoekwater"},
    {first="Caron", last="Haček"}
    {first="Ančka", last="Car"}
    {first="Ana", last="Car"}
    {first="Anita", last="Car"}
    {first="Matija", last="Čop"}
}

and I want to print out the following:

\section{C}
\person{Car}{Ana}
\person{Car}{Ančka}
\person{Car}{Anita}

\section{Č}
\person{Čop}{Matija}

\section{H}
\person{Haček}{Caron}
\person{Hagen}{Hans}
\person{Henkel}{Hartmut}
\person{Hoekwater}{Taco}

The following code works for English and needs a fix to respect local
sorting rules. I would be grateful for any hint before reinventing the
wheel (the hint can also be a pointer to the right line of ConTeXt
source code that does the sorting; I was a bit lost in the code).

\mainlanguage[sl]
\setuphead[section][style=\bf,before=\blank,after=,number=no]
\def\person#1#2{\hbox{{\sc #1}, #2}}

\starttext

Code that needs fixing:

\startluacode

names = {
	{first="Hans",    last="Hagen"},
	{first="Hartmut", last="Henkel"},
	{first="Taco",    last="Hoekwater"},
	{first="Caron",   last="Haček"},
	{first="Ančka",   last="Car"},
	{first="Ana",     last="Car"},
	{first="Anita",   last="Car"},
	{first="Matija",  last="Čop"},
}

-- this should sort with local sorting rules; how?
function sort_names(a,b)
	if (a.last == b.last) then
		return a.first<b.first
	else
		return a.last<b.last
	end
end

-- this is extremely ugly and needs to be fixed
-- with some UTF-8 library
function first_letter(str)
	if (string.byte(str,1)>127) then
		return string.sub(str,1,2)
	else
		return string.sub(str,1,1)
	end
end

table.sort(names, sort_names)

for k,v in pairs(names) do
	-- nested tables would also be an option; but are not required
	if (k==1) or (k>1 and (first_letter(names[k-1].last) ~=
first_letter(v.last))) then
		tex.print(string.format('\\section{%s}', first_letter(v.last)))
	end
	tex.print(string.format('\\person{%s}{%s}', v.last, v.first))
end

\stopluacode

\crlf\crlf

Desired result:

\section{C}
\person{Car}{Ana}
\person{Car}{Ančka}
\person{Car}{Anita}

\section{Č}
\person{Čop}{Matija}

\section{H}
\person{Haček}{Caron}
\person{Hagen}{Hans}
\person{Henkel}{Hartmut}
\person{Hoekwater}{Taco}

\stoptext

Thanks a lot,
   Mojca
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