有没有办法从输出例程中的主垂直列表中获取 \prevdepth 的值？

Question 1

@wipet 在他的回答中展示了如何解决这个问题，只要我们可以假设\prevdepth文档中所有材料的深度足够小（\maxdepth实际上它位于下方）。在这种情况下，我们可以使用框 255 的深度作为计算的度量，\prevdepth如果最近的贡献中有任何余数，则将进行计算，并使用它来调整新的深度以匹配该深度。如果没有余数，那么这实际上也不重要。事实上，Don Knuth 在 TeXbook 中已经提到了这个技巧，他讨论了一个输出例程，该例程在文本的随机位置添加索引标题（普通文本具有这种很好的特性，其深度小于\maxdepth...通常但不幸的是并非总是如此。

但是，如果不能保证这一点，这种方法就会失效，而一般情况正是我过去和现在所追求的。他还正确地指出，找出\prevdepth主垂直列表上的值实际上不会对我们有帮助，所以我上面的问题有部分错误：我们还需要知道最近的贡献中是否确实存在问题，如果存在，则处理它。

所以，在过去的几天里，我一直在想，这是否真的不可能在基本 TeX 中找到（或者它是否需要在 luatex 中进行扩展，或者已经在那里可用并在 ConTeXt 中得到解决……除了你所说的 @Martin 之外，了解它也会很有趣）。就目前而言，在 TeX 中毕竟是可能的。解决方案有点复杂，也许可以进一步简化，但它并不复杂到无法使用（希望不要太复杂以至于我忽略了一些情况）。

主要思想不是尝试直接确定情况，而是连续使用 2 个输出例程，以确保最近的贡献为空。在此过程中，我们收集了足够的信息，以便随后剖析收集到的材料并做我们想做的事情。

我最初的想法是\aftergroup重新获得控制权并进行查看，但这个想法行不通，因为这样插入的标记在输出例程结束时不会被执行。相反，TeX 立即再次调用“buildpage”程序，该程序会获取最近贡献的任何内容并将其移动到主垂直列表，并且只有发生这种情况后才会查看我插入的标记（换句话说，为时已晚）。

因此，更复杂的方法是，第一个 OR 将 box255 放回原处，但只是将其更改\vsize为最大可能尺寸。此外，它还用于\aftergroup让我们稍后再次获得控制权。随着我们进行更改，\vsize我们将把包括材料近期贡献在内的所有内容放到主垂直列表中，然后我们的控制令牌才会启动。最后，我们将输出例程更改为第二个，然后返回。

插入的令牌\aftergroup将发出强制惩罚（实际上它会做得更多，见下文），以便抓取所有内容并调用第二个 OR。

在 OR 中，我们现在处于更好的情况：

我们知道最近的贡献是空的（除了（\penalty 10000）
我们可以将收集到的东西存放在box255中
或者我们可以使用\vsplit等来操作它，例如，我们在第一个 OR 中得到的金额分割
我们可以使用它\aftergroup在手术结束后获得控制权。
后者允许我们\prevdepth在主垂直列表上进行更改以表示所需的任何内容

我相信这应该可以完全解决问题。

以下是（或多或少有文档化的代码，包括一些测试数据，用于演示各种场景）。它有点长，但主要是因为我试图正确记录最重要的方面和一些更微妙的点。享受：

\tracingoutput1
\showboxbreadth\maxdimen\showboxdepth\maxdimen
\tracingpages1
\tracingonline1

\vsize20\baselineskip

\lineskip=13pt  % for identifications

% this is our trial material used below. We will arrange things so
% that the first para will be longer than a page so that we will end
% up with some material on recent contributions. The OR is actually
% then triggered when the first ``p'' is seen. Alternatively one can
% uncomment the \vskip or the \penalty in which case the OR will be
% triggered by them or you could uncomment ``Line2 and3'' then the
% break happens somewhere in the middle of the first paragraph (in
% vmode inn that case)

\def\testmaterial{%
  Line 1 \hfil\break
%  Line 2 \hfil\break
%  Line 3 \hfil\break
  \vadjust{\penalty -333 }
  some text some text some text some text some text some text
  some text some text some text
   and some more text ggg \vrule depth 88pt

%\ vskip 17pt
% \penalty 15

  pppppppppppppppppppppppppp

  \showlists
}


% now this here is just to see the whole stuff being processed by the
% normal OR and see the \showlists result for it. One can then compare
% that to the showlists result we get later to check for differences

\testmaterial

\vfill

\eject


%==============================================

\newdimen\savedvsize
\newbox\savedORbox


% now for a set of special output routines:
% the first one does the following
%
% - save away current \vsize and set it to \maxdimen
% - then unbox 255 and readd the output penalty (unless it is 10000)
% - set up a new output routine for the next time
% - finally install control with \aftergroup\addendpenalty
%

% The point here is that the \aftergroup token is not actually
% directly executed the moment the OR ends. If TeX ends an OR it looks
% at the recent contribution and if they are not empty it will call
% ``buildpage'' and move them to the main vertical list. And only if
% this has happened the \aftergroup token gets executed.  Now given
% that we set \vsize to the largest possible dimen this means that all
% the remainder that was not used first time around will now end up on
% the main vertial list and only then \addendpenalty kicks in.


\output{%
  \global\savedvsize\vsize
  \global\vsize\maxdimen
%--- tracing --------------
  \showthe\outputpenalty
  \showlists
%--------------------------
  \unvbox255
%
% above I claimed we put \outputpenalty back (which we should) but to
% make things more visible I put back a special penalty that can be
% easily recognised in \showlists
%
%  \penalty \ifnum\outputenalty=10000 0 \else \outputpenalty \fi
  \penalty-777
  \global\output{\ORtwo}%
  \aftergroup\addendpenalty
}

% The macro \addendpenalty is used with \aftergroup from the output
% routine to gain control again. It adds a penalty to trigger the next
% output routine. However, we are quite likely in horizontal mode when
% the OR returns (just have seen the start of a paragraph) so we check
% for this. If true we remove the indentation box and end the
% paragraph. As a result all that get contributed is \parskip but no
% box (so \prevdepth is not touched). We signal with the penalty value
% whether or not we have seen hmode as we will have to remove that
% extra parskip in the next OR.

\def\addendpenalty{%
 \ifhmode
   \setbox0\lastbox\par\penalty-10010
 \else
   \penalty-10011 
 \fi}


% the second Or now should receive everything that was on the main
% vertical list with the recent contributions being empty (well empty
% except for a \penalty10000 that TeX puts in the place where it
% triggered the OR).
%
% What we have to do now is to remove the surplus \parskip at the
% bottom of 255 if we have been in hmode before. This is something we
% can determine by looking at the \outputpenalty that should be -10010
% in that case (otherwise -10011)
%
% then we save all of 255 in a spare box and return from the OR. To
% gain control afterwards we issue \aftergroup\XXX

\def\ORtwo{%
%--- tracing --------------
  \showthe\outputpenalty
  \showbox255\showlists
%--------------------------
  \ifnum\outputpenalty=-10010
    \setbox255=\vbox
      {\unvbox255
       \unskip  % this gets rid of the \parskip from hmode
      }
  \fi
  \global\setbox\savedORbox\box255
  \aftergroup\XXX
}

% Note that now the macro \XXX is immediately called when the OR ends
% as the recent contributions are empty now. Thus this macro now is
% getting us in a good shape:
%
% - it can access \prevdepth and \prevgraf (which is in fact having
% the same issue) and it can change them as necessary.
%
% - it has the complete main vertical list at its disposal (saved
% in \savedORbox)
%
%  - there is nothing left in recent contributions so anything
%  following is new material, so we can now arrange everything to our
%  liking and reprocess


\def\XXX{%
%--- tracing --------------
  \showthe\prevdepth  % this is finally the outer one and we could
                      % change it if needed
  \showlists          % nothing on it not even the penalty remains
                      % only prevdepth and prevgraf set (incorrectly)
%--------------------------
%  \global\vsize\savedvsize
  \global\vsize20\baselineskip
  \global\output{\plainoutput}%
  \unvbox\savedORbox
}

% what we do above is set the vsize back to 20 baselines set up the
% plainoutput routine again and reprocess and we get 100% the same as
% in the initial test (well, in one place there is penalty 777 but
% that was just mark that spot, normally we would have \outputpenalty
% there which was 0. In the original there was nothing in this space
% only glue but that is equivalent


% and here is now the real test: we set a very short vsize so the
% first OR is triggered with \testmaterial and some part of it ends up
% in recent contributions.

\vsize=3\baselineskip

\testmaterial


\bye

Answer