I created small package which enables Patrick's code also with plain luaTeX. After some discussions with other Czech users, I found that the code doesn't work for cases when the single letter is on the beginning of brackets etc. There was also requirement to process only certain letters, not all. So I modified the code slightly:
prevent-single.lua:
-- Module prevent-single
-- code originally created by Patrick Gundlach
-- http://tex.stackexchange.com/q/27780/2891
-- The code was adapted for plain TeX and added some more features
-- 1. It is possible to turn this functionality only for some letters
-- 2. Code now works even for single letters after brackets etc.
--
local M = {}
local utf_match = unicode.utf8.match
local utf_char = unicode.utf8.char
local match_char = function(x) return utf_match(x,"%a") end
local match_table = function(x, chars)local chars=chars or {}; return chars[x] end
local singlechars = nil-- {a=true,i=true,z=true, v=true, u=true, o = true}
local debug = false
-- Enable processing only for certain letters
-- must be table in the {char = true, char2=true} form
local set_singlechars= function(c)
--print("Set single chars lua")
--for k,_ in pairs(c) do print(k) end
singlechars = c
end
local set_debug= function(x)
debug = x
end
local prevent_single_letter = function (head)
local singlechars = singlechars
local test_fn = singlechars and match_table or match_char
local space = true
while head do
local id = head.id
if id == 10 then
space=true
elseif space==true and id == 37 and utf_match(utf_char(head.char), "%a") then -- a letter
if test_fn(utf_char(head.char), singlechars ) and head.next.id == 10 then
-- only if we are at a one letter word
local p = node.new("penalty")
p.penalty = 10000
if debug then
local w = node.new("whatsit","pdf_literal")
w.data = "q 1 0 1 RG 1 0 1 rg 0 0 m 0 5 l 2 5 l 2 0 l b Q"
node.insert_after(head,head,w)
node.insert_after(head,w,p)
else
node.insert_after(head,head,p)
end
end
space = false
end
head = head.next
end
return true
end
M.preventsingle = prevent_single_letter
M.singlechars = set_singlechars
M.debug = set_debug
return M
prevent-single.tex
\input luatexbase.sty
% Modify pre_linebreak_filter callback so the spaces can be inserted
\directlua{%
preventsingle = require "prevent-single"
luatexbase.add_to_callback("pre_linebreak_filter", preventsingle.preventsingle,"~")
% Process string and make table of enabled single letters
% By default, spaces for all single letters are inserted
% This can be modified with \singlechars macro
set_singlechars = function(chars)
local utf_gmatch = unicode.utf8.gmatch
local chars = chars or ""
local singlechars = {}
for char in utf_gmatch(chars,"(\%a)") do
singlechars[char] = true
end
preventsingle.singlechars(singlechars)
end
}
% Set letters which are prevented from breaking
\def\singlechars#1{%
\directlua{set_singlechars("#1")}
}
% Enable inserting of visual marks for debugging
\def\preventsingledebugon{%
\directlua{preventsingle.debug(true)}
}
% Disable inserting of visual marks for dewbugging
\def\preventsingledebugoff{%
\directlua{preventsingle.debug(false)}
}
Sample document:
\input ucode
\uselanguage{czech}
\input prevent-single
\singlechars{aAzZiIvVuUoO}
\preventsingledebugon
\input luaotfload.sty
\font\hello={name:Linux Libertine O:+rlig;+clig;+liga;+tlig} at 12pt
\hsize=3in
\hello
Příliš žluťoučký kůň úpěl ďábelské ódy.
Text s krátkými souhláskami a samohláskami i dalšími jevy z nabídky možností (v textu možnými).
Grafika, ffinance -- pomlčka a tak.
I začátek odstavce je třeba řešit, i když výskyt zalomení není pravděpodobný.
Co třeba í znaky š diakritikou?
\preventsingledebugoff
Různé možnosti [v závorkách <a jiných znacích
\bye
And the result:
The code is also on github and I will post it to CTAN.
texdoc enctex, but it's not easy. The best is to get the habit of inserting ties~. – egreg Sep 07 '11 at 17:40