There are two goals here:
- don't break after a short word that immediately follows punctuation,
- don't break before a short word that immediately precedes punctuation,
subject to regular constraints of good line breaking.
One simple solution is to declare punctuation as particularly good places to break (a negative penalty, sufficiently large in magnitude). This will let TeX trade off trying to break at punctuation with its other line-breaking considerations (badness, demerits, other penalties), but will not guarantee that there are absolutely no breaks of that kind.
Here's a before-and-after, to illustrate:
As you can see,
- In the first paragraph, the
, it at the end of the third line has moved to the next line after the change.
- In the second paragraph, the
el. at the beginning of the fourth line and the at, at the beginning of the sixth line have moved to the previous line after the change.
- The third paragraph has been included to show that this trick isn't a guarantee: the
it. at the beginning of the fourth line remains there, because there's simply no way to fit it on the previous line.
This was achieved with:
\catcode`.=\active \def.{\char`.\penalty -200\relax}
\catcode`,=\active \def,{\char`,\penalty -200\relax}
in the following document:
\documentclass{article}
\begin{document}
\frenchspacing % Makes it easier
\hsize=20em
\parskip=10pt
% First, three paragraphs with the default settings
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut blandit placerat justo, sed dictum sem. Donec erat elit, tincidunt non, it vel, tincidunt vehicula velit. Etiam pharetra ante at porta elementum. In nulla purus, faucibus non accumsan non, consequat eget.
Natis nulla blandit luctus tellus, sit amet posuere lacus maxius quis. In sit amet mattis est, a vehiula velit. Nam interum solicitudin el. In faucibus vulputate purus nec consectelur crass metus ipsum, blandit iln ullamcorpert at, portitor vita dolor. Duis sed mauris i inset inculis malesuada. Quisque laoret eu dui eget sage melittis corpum verborum.
Volutpat libero ac auctor. Donec semper, as id ultrices rhoncus, lectus nulla consequat nisi, ac sagitis risus lectus vel felis. Ut gravida it. Nam malesuada ante turpis eget. Ipsum factum verbum verdit.
\pagebreak
% Now the same text, with the meanings of . and , changed.
\catcode`.=\active \def.{\char`.\penalty -200\relax}
\catcode`,=\active \def,{\char`,\penalty -200\relax}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut blandit placerat justo, sed dictum sem. Donec erat elit, tincidunt non, it vel, tincidunt vehicula velit. Etiam pharetra ante at porta elementum. In nulla purus, faucibus non accumsan non, consequat eget.
Natis nulla blandit luctus tellus, sit amet posuere lacus maxius quis. In sit amet mattis est, a vehiula velit. Nam interum solicitudin el. In faucibus vulputate purus nec consectelur crass metus ipsum, blandit iln ullamcorpert at, portitor vita dolor. Duis sed mauris i inset inculis malesuada. Quisque laoret eu dui eget sage melittis corpum verborum.
Volutpat libero ac auctor. Donec semper, as id ultrices rhoncus, lectus nulla consequat nisi, ac sagitis risus lectus vel felis. Ut gravida it. Nam malesuada ante turpis eget. Ipsum factum verbum verdit.
% Change it back
\catcode`.=12 \catcode`,=12
\pagebreak
% Same text again, to show that nothing's permanently changed.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut blandit placerat justo, sed dictum sem. Donec erat elit, tincidunt non, it vel, tincidunt vehicula velit. Etiam pharetra ante at porta elementum. In nulla purus, faucibus non accumsan non, consequat eget.
Natis nulla blandit luctus tellus, sit amet posuere lacus maxius quis. In sit amet mattis est, a vehiula velit. Nam interum solicitudin el. In faucibus vulputate purus nec consectelur crass metus ipsum, blandit iln ullamcorpert at, portitor vita dolor. Duis sed mauris i inset inculis malesuada. Quisque laoret eu dui eget sage melittis corpum verborum.
Volutpat libero ac auctor. Donec semper, as id ultrices rhoncus, lectus nulla consequat nisi, ac sagitis risus lectus vel felis. Ut gravida it. Nam malesuada ante turpis eget. Ipsum factum verbum verdit.
\end{document}
Notes:
- I wouldn't be surprised if changing the meanings of
. and , like this breaks something. (In fact I was surprised that nothing got messed up in this example, then I realized that catcode changes don't apply to tokens that have already been read in.)
- You can tweak the penalties: I used -200 just as an example, but anything from -1 to -9999 will have some effect. (In this example the threshold for all these changes to take effect seems to be -175, though one change happens even at -100.) A penalty that is ≤ -10000 forces a line break, which is not what you want.
- You can do the same for more punctuation characters (
?!:;) or have different penalties for different punctuation characters.
- Things are a bit harder with
\nonfrenchspacing (the default), where spaces are larger after punctuation. It may be doable but coming up with these examples was a lot of work so I haven't pursued it. Left as an exercise :-)
- With LuaTeX you can even change the line-breaking algorithm, which would be a cool way to guarantee no short words at line edges (if that's what you need).
Edit: I couldn't resist implementing the “guaranteed” solution in LuaTeX. This version should work with both \frenchspacing and \nonfrenchspacing. What it does is to detect certain sequences and insert infinite (10000) penalties to prevent a break:
(punct, space, short_word, space) -> (punct, space, short_word, penalty, space)
and
(space, short_word, punct) -> (penalty, space, short_word, punct)
For the example above, this produces:
Note the overfull box in the last paragraph because the constraints are quite strict, but that's what we asked for. (Anyway, you probably won't have overfull boxes with wider and longer paragraphs, and you can fix them in the usual ways, of rewriting or adding \emergencystretch and so on.)
The code that produced the above (and even the idea) quite possibly has bugs in it that may even cause your LuaTeX compilation to crash, but here it is:
\documentclass{article}
\directlua{dofile("strict.lua")}
\begin{document}
\frenchspacing % Keeping same example as before
\hsize=20em
\parskip=10pt
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut blandit placerat justo, sed dictum sem. Donec erat elit, tincidunt non, it vel, tincidunt vehicula velit. Etiam pharetra ante at porta elementum. In nulla purus, faucibus non accumsan non, consequat eget.
Natis nulla blandit luctus tellus, sit amet posuere lacus maxius quis. In sit amet mattis est, a vehiula velit. Nam interum solicitudin el. In faucibus vulputate purus nec consectelur crass metus ipsum, blandit iln ullamcorpert at, portitor vita dolor. Duis sed mauris i inset inculis malesuada. Quisque laoret eu dui eget sage melittis corpum verborum.
Volutpat libero ac auctor. Donec semper, as id ultrices rhoncus, lectus nulla consequat nisi, ac sagitis risus lectus vel felis. Ut gravida it. Nam malesuada ante turpis eget. Ipsum factum verbum verdit.
\end{document}
where strict.lua is:
function is_punct(n)
if node.type(n.id) ~= 'glyph' then return false end
if n.char > 127 then return false end
c = string.char(n.char)
if c == '.' or c =='?' or c == '!' or c == ':' or c == ';' or c == ',' then
return true
end
return false
end
function no_punct_short_word_eol(head)
-- Prevents having a line that ends like "<punctuation><space><short_word>"
-- How we do this:
-- (1) detect such short words (punct, space, short_word, space)
-- (2) insert a penalty of 10000 between the short_word and the following space.
-- More concretely:
-- * A punctuation is one of .?!:;, which are the ones affected by \frenchspacing
-- * A space is any glue node.
-- * A short_word is a sequence of only glyph and kern nodes.
-- So we maintain a state machine: default -> seen_punct -> seen_space -> seen_word
-- where in the last state we maintain length. If we're in seen_word state and we see
-- a glue, and length is less than threshold, insert a penalty before the glue.
state = 'default'
root = head
while head do
if state == 'default' then
if is_punct(head) then
state = 'seen_punct'
end
elseif state == 'seen_punct' then
if node.type(head.id) == 'glue' then
state = 'seen_space'
else
state = 'default'
end
elseif state == 'seen_space' then
if node.type(head.id) == 'glyph' then
state = 'seen_word'
length = 1
elseif is_punct(head) then
state = 'seen_punct'
else
state = 'default'
end
elseif state == 'seen_word' then
if node.type(head.id) == 'glue' and length <= 2 then
-- Moment of truth
penalty = node.new('penalty')
penalty.penalty = 10000
root, new = node.insert_before(root, head, penalty)
-- TODO: Is 'head' invalidated now? Docs don't say anything...
state = 'default'
elseif node.type(head.id) == 'glyph' or node.type(head.id) == 'kern' then
if node.type(head.id) == 'glyph' then length = length + 1 end
else
state = 'default'
end
else
assert(false, string.format('Impossible state %s', state))
end
head = head.next
end
return root
end
luatexbase.add_to_callback('pre_linebreak_filter', no_punct_short_word_eol, 'Prevent short words after punctuation at end of sentence')
function no_bol_short_word_punct(head)
-- Prevents having a line that starts like "<short_word><punctuation>"
-- How we do this:
-- (1) detect such short words (space, short_word, punct)
-- (2) insert a penalty of 10000 between the space and the following short_word.
-- More concretely:
-- * A punctuation is one of .?!:;, which are the ones affected by \frenchspacing
-- * A space is any glue node.
-- * A short_word is a sequence of only glyph and kern nodes.
-- So we maintain a state machine: default -> seen_space -> seen_word
-- where in the last state we maintain length. If we're in seen_word state and we see
-- a punct, and length is less than threshold, insert a penalty before the glue.
-- Note that for this to work, we need to maintain a pointer to where we saw the glue.
state = 'default'
root = head
before_space = nil
while head do
if state == 'default' then
if node.type(head.id) == 'glue' then
state = 'seen_space'
before_space = head.prev
end
elseif state == 'seen_space' then
if node.type(head.id) == 'glyph' then
state = 'seen_word'
length = 1
else
state = 'default'
end
elseif state == 'seen_word' then
if is_punct(head) and length <= 2 then
-- Moment of truth
penalty = node.new('penalty')
penalty.penalty = 10000
root, new = node.insert_after(root, before_space, penalty)
-- TODO: Is 'head' invalidated now? Docs don't say anything...
state = 'default'
elseif node.type(head.id) == 'glyph' or node.type(head.id) == 'kern' then
if node.type(head.id) == 'glyph' then length = length + 1 end
elseif node.type(head.id) == 'glue' then
state = 'seen_space'
before_space = head.prev
else
state = 'default'
end
else
assert(false, string.format('Impossible state %s', state))
end
head = head.next
end
return root
end
luatexbase.add_to_callback('pre_linebreak_filter', no_bol_short_word_punct, 'Prevent short words at beginning of sentence before punctuation')
~instead of a space wherever I feel like it.