On Mar 4, 2018, at 12:42 PM, Clem Cole <clemc@ccc.com> wrote:

On Sun, Mar 4, 2018 at 3:23 PM, Doug McIlroy <doug@cs.dartmouth.edu> wrote:

I hadn't realized that groff hyphenation had been taken from
Tex, not troff. Is that becuase Tex did a better job, or
because troff's was deemed proprietary?

Given the author, I would guess the later as he wanted to be FOSS and would not have looked at the ditroff source - but that guess is worth just that ;-)

I remembered reading about Knuth's line-breaking  algorithm in
Software Practice & Experience in early eighties and being quite
impressed with it. So may be that clear description of the algorithm
has something to do with it? Ah, here it is:

“Breaking Paragraphs into lines” by Donald Knuth & Plass,
SP&E, Volume 11, issue 11, Nov. 1981

(Download from Wiley is not free)