The master luatex svn sources are currently not at a publicly accessible location, I understand that this is an essentially temporary situation due to a change of hosting arrangements however the luatex developers push regularly to the texlive svn so that is usually only days or at most weeks behind. Texlive svn is browsable at https://www.tug.org/svn/texlive/trunk/Build/source/texk/web2c/luatexdir/
starts with the comment
@ We come now to what is probably the most interesting algorithm of \TeX: the mechanism for choosing the ``best possible'' breakpoints
that yield the individual lines of a paragraph. \TeX's line-breaking
algorithm takes a given horizontal list and converts it to a sequence
of boxes that are appended to the current vertical list. In the course
of doing this, it creates a special data structure containing three
kinds of records that are not used elsewhere in \TeX. Such nodes are
created while a paragraph is being processed, and they are destroyed
afterwards; thus, the other parts of \TeX\ do not need to know
anything about how line-breaking is done.
D Knuth's text, from the original tex.web sources.