How to rigidly chop long lines in plain TeX

Question

I am using plain TeX and want to typeset an appendix that contains several hundred URLs. When a URL goes past the right margin, I would like TeX to stop printing (i.e. not carry into the margin, not print a warning black rectangle) and then pick up the rest of the URL on a new (non-indented) line where it left off.

Something like this, where 1, 2, 3 are akin to footnote numbers:

1: http://www.aaa.com/rem
ainder 2: http://www.bbb.
com/remainder_again  3: h
ttp://www.ccc.com/ending

Is this possible?

I am using a Python script to generate the .tex files, so, if worst comes to worst, I can have Python estimate the content of each line. Unfortunately, I am not using a mono-spaced font, so that could turn pretty ugly.

Thanks in advance.

Answer 1

Since you're generating the TeX code with Python, one option is to separate every character with \hskip0pt. For example:

\def\a{a\hskip0pt}
\def\b{\a\a\a\a\a\a\a\a\a\a}
\def\c{\b\b\b\b\b\b\b\b\b\b}
\def\d{\c\c\c\c\c\c\c\c\c\c}

\d

Answer 2

The LaTeX url package can be used with plain TeX using miniltx (or you could copy the minimal code, of course). For example

\input miniltx %
\input url.sty %
\urlstyle{rm}
\def\UrlBreaks{\do\/\do\a\do\b\do\c\do\d\do\e\do\f\do\g\do\h\do\i\do\j\do\k\do\l\do\m\do\n\do\o\do\p\do\q\do\r\do\s\do\t\do\u\do\v\do\w\do\x\do\y\do\z\do\A\do\B\do\C\do\D\do\E\do\F\do\G\do\H\do\I\do\J\do\K\do\L\do\M\do\N\do\O\do\P\do\Q\do\R\do\S\do\T\do\U\do\V\do\W\do\X\do\Y\do\Z}
\hsize 4.4cm %
\noindent
1.~\url{http://www.aaa.com/remainder}
2.~\url{http://www.bbb.com/remainder_again}
3.~\url{http://www.ccc.com/ending}
\bye

will give you breaking at any character using the method suggested in the url manual and Forcing linebreaks in \url. Things are dependent on line-width: with very few 'normal' spaces in the example above avoiding overfull boxes is hard, and the equivalent of LaTeX's \sloppy may also be required, e.g.

\tolerance 9999 %
\emergencystretch 3em %

I'm assuming we have to allow breaks everywhere: that is normally not such a great plan if it can be avoided.

To understand what is happening, notice that url (ab)uses math mode to allow breaking 'anywhere', making any breakable URL character into a mathbin character inside the URL.