% language=uk

\startcomponent onandon-execute

\environment onandon-environment

\startchapter[title={Executing \TEX}]

Much of the \LUA\ code in \CONTEXT\ originates from experiments. When it survives
in the source code it is probably used, waiting to be used or kept for
educational purposes. The functionality that we describe here has already been
present for a while in \CONTEXT, but improved a little starting with \LUATEX\
1.08 due to an extra helper. The code shown here is generic and not used in
\CONTEXT\ as such.

Say that we have this code:

\startbuffer
for i=1,10000 do
    tex.sprint("1")
    tex.sprint("2")
    for i=1,3 do
        tex.sprint("3")
        tex.sprint("4")
        tex.sprint("5")
    end
    tex.sprint("\\space")
end
\stopbuffer

\typebuffer

% \ctxluabuffer

When we call \type {\directlua} with this snippet we get some 30 pages of \type
{12345345345}. The printed text is saved till the end of the \LUA\ call, so
basically we pipe some 170.000 characters to \TEX\ that get interpreted as one
paragraph.

Now imagine this:

\startbuffer
\setbox0\hbox{xxxxxxxxxxx} \number\wd0
\stopbuffer

\typebuffer

which gives \getbuffer. If we check the box in \LUA, with:

\startbuffer
tex.sprint(tex.box[0].width)
tex.sprint("\\enspace")
tex.sprint("\\setbox0\\hbox{!}")
tex.sprint(tex.box[0].width)
\stopbuffer

\typebuffer

the result is {\tttf \ctxluabuffer}, which is not what you would expect at first
sight. However, if you consider that we just pipe to a \TEX\ buffer that gets
parsed after the \LUA\ call, it will be clear that the reported width is the
width that we started with. It will work all right if we say:

\startbuffer
tex.sprint(tex.box[0].width)
tex.sprint("\\enspace")
tex.sprint("\\setbox0\\hbox{!}")
tex.sprint("\\directlua{tex.sprint(tex.box[0].width)}")
\stopbuffer

\typebuffer

because now we get: {\tttf\ctxluabuffer}. It's not that complex to write some
support code that makes this more convenient. This can work out quite well but
there is a drawback. If we use this code:

\startbuffer
print(status.input_ptr)
tex.sprint(tex.box[0].width)
tex.sprint("\\enspace")
tex.sprint("\\setbox0\\hbox{!}")
tex.sprint("\\directlua{print(status.input_ptr)\
    tex.sprint(tex.box[0].width)}")
\stopbuffer

\typebuffer

Here we get \type {6} and \type {7} reported. You can imagine that when a lot of
nested \type {\directlua} calls happen, we can get an overflow of the input level
or (depending on what we do) the input stack size. Ideally we want to do a \LUA\
call, temporarily go to \TEX, return to \LUA, etc.\ without needing to worry
about nesting and possible crashes due to \LUA\ itself running into problems. One
charming solution is to use so|-|called coroutines: independent \LUA\ threads
that one can switch between --- you jump out from the current routine to another
and from there back to the current one. However, when we use \type {\directlua}
for that, we still have this nesting issue and what is worse, we keep nesting
function calls too. This can be compared to:

\starttyping
\def\whatever{\ifdone\whatever\fi}
\stoptyping

where at some point \type {\ifdone} is false so we quit. But we keep nesting when
the condition is met, so eventually we can end up with some nesting related
overflow. The following:

\starttyping
\def\whatever{\ifdone\expandafter\whatever\fi}
\stoptyping

is less likely to overflow because there we have tail recursion which basically
boils down to not nesting but continuing. Do we have something similar in
\LUATEX\ for \LUA ? Yes, we do. We can register a function, for instance:

\starttyping
lua.get_functions_table()[1] = function() print("Hi there!") end
\stoptyping

and call that one with:

\starttyping
\luafunction 1
\stoptyping

This is a bit faster than calling a function like:

\starttyping
\directlua{HiThere()}
\stoptyping

which can also be achieved by

\starttyping
\directlua{print("Hi there!")}
\stoptyping

which sometimes can be more convenient. Anyway, a function call is what we can
use for our purpose as it doesn't involve interpretation and effectively behaves
like a tail call. The following snippet shows what we have in mind:

\startbuffer[code]
local stepper = nil
local stack   = { }
local fid     = 0xFFFFFF
local goback  = "\\luafunction" .. fid .. "\\relax"

function tex.resume()
    if coroutine.status(stepper) == "dead" then
        stepper = table.remove(stack)
    end
    if stepper then
        coroutine.resume(stepper)
    end
end

lua.get_functions_table()[fid] = tex.resume

function tex.yield()
    tex.sprint(goback)
    coroutine.yield()
    texio.closeinput()
end

function tex.routine(f)
    table.insert(stack,stepper)
    stepper = coroutine.create(f)
    tex.sprint(goback)
end
\stopbuffer

\ctxluabuffer[code]

\startbuffer[demo]
tex.routine(function()
    tex.sprint(tex.box[0].width)
    tex.sprint("\\enspace")
    tex.sprint("\\setbox0\\hbox{!}")
    tex.yield()
    tex.sprint(tex.box[0].width)
end)
\stopbuffer

\typebuffer[demo]
We start a routine, jump out to \TEX\ in the middle, come back when we're done
and continue. This gives us: \ctxluabuffer [demo], which is what we expect.

\setbox0\hbox{xxxxxxxxxxx}

\ctxluabuffer[demo]

This mechanism permits efficient (nested) loops like:

\startbuffer[demo]
tex.routine(function()
    for i=1,10000 do
        tex.sprint("1")
        tex.yield()
        tex.sprint("2")
        tex.routine(function()
            for i=1,3 do
                tex.sprint("3")
                tex.yield()
                tex.sprint("4")
                tex.yield()
                tex.sprint("5")
            end
        end)
        tex.sprint("\\space")
        tex.yield()
    end
end)
\stopbuffer

\typebuffer[demo]

We do create coroutines, go back and forwards between \LUA\ and \TEX, but avoid
memory being filled up with printed content. If we flush paragraphs (instead of
e.g.\ the space) then the main difference is that instead of a small delay due to
the loop unfolding in a large set of prints and accumulated content, we now get a
steady flushing and processing.

However, we can still have an overflow of input buffers because we still nest
them: the limitation at the \TEX\ end has moved to a limitation at the \LUA\ end.
How come? Here is the code that we use:

\typebuffer[code]

The \type {routine} creates a coroutine, and \type {yield} gives control to \TEX.
The \type {resume} is done at the \TEX\ end when we're finished there. In
practice this works fine and when you permit enough nesting and levels in \TEX\
then you will not easily overflow.

When I picked up this side project and wondered how to get around it, it suddenly
struck me that if we could just quit the current input level then nesting would
not be a problem. Adding a simple helper to the engine made that possible (of
course figuring it out took a while):

\startbuffer[code]
local stepper = nil
local stack   = { }
local fid     = 0xFFFFFF
local goback  = "\\luafunction" .. fid .. "\\relax"

function tex.resume()
    if coroutine.status(stepper) == "dead" then
        stepper = table.remove(stack)
    end
    if stepper then
        coroutine.resume(stepper)
    end
end

lua.get_functions_table()[fid] = tex.resume

if texio.closeinput then
    function tex.yield()
        tex.sprint(goback)
        coroutine.yield()
        texio.closeinput()
    end
else
    function tex.yield()
        tex.sprint(goback)
        coroutine.yield()
    end
end

function tex.routine(f)
    table.insert(stack,stepper)
    stepper = coroutine.create(f)
    tex.sprint(goback)
end
\stopbuffer

\ctxluabuffer[code]

\typebuffer[code]

The trick is in \type {texio.closeinput}, a recent helper and one that should be
used with care. We assume that the user knows what she or he is doing. On an old
laptop with a i7-3840 processor running \WINDOWS\ 10 the following snippet takes
less than 0.35 seconds with \LUATEX\ and 0.26 seconds with \LUAJITTEX.

\startbuffer[code]
tex.routine(function()
    for i=1,10000 do
        tex.sprint("\\setbox0\\hpack{x}")
        tex.yield()
        tex.sprint(tex.box[0].width)
        tex.routine(function()
            for i=1,3 do
                tex.sprint("\\setbox0\\hpack{xx}")
                tex.yield()
                tex.sprint(tex.box[0].width)
            end
        end)
    end
end)
\stopbuffer

\typebuffer[code]

% \testfeatureonce {1} {\setbox0\hpack{\ctxluabuffer[code]}} \elapsedtime

Say that we run the bad snippet:

\startbuffer[code]
for i=1,10000 do
    tex.sprint("\\setbox0\\hpack{x}")
    tex.sprint(tex.box[0].width)
    for i=1,3 do
        tex.sprint("\\setbox0\\hpack{xx}")
        tex.sprint(tex.box[0].width)
    end
end
\stopbuffer

\typebuffer[code]

% \testfeatureonce {1} {\setbox0\hpack{\ctxluabuffer[code]}} \elapsedtime

This time we need 0.12 seconds in both engines. So what if we run this:

\startbuffer[code]
\dorecurse{10000}{%
    \setbox0\hpack{x}
    \number\wd0
    \dorecurse{3}{%
        \setbox0\hpack{xx}
        \number\wd0
    }%
}
\stopbuffer

\typebuffer[code]

% \testfeatureonce {1} {\setbox0\hpack{\getbuffer[code]}} \elapsedtime

Pure \TEX\ needs 0.30 seconds for both engines but there we lose 0.13 seconds on
the loop code. In the \LUA\ example where we yield, the loop code takes hardly
any time. As we need only 0.05 seconds more it demonstrates that when we use the
power of \LUA\ the performance hit of the switch is quite small: we yield 40.000
times! In general, such differences are far exceeded by the overhead: the time
needed to typeset the content (which \type {\hpack} doesn't do), breaking
paragraphs into lines, constructing pages and other overhead involved in the run.
In \CONTEXT\ we use a slightly different variant which has 0.30 seconds more
overhead, but that is probably true for all \LUA\ usage in \CONTEXT, but again,
it disappears in other runtime.

Here is another example:

\startbuffer[code]
\def\TestWord#1%
  {\directlua{
     tex.routine(function()
       tex.sprint("\\setbox0\\hbox{\\tttf #1}")
       tex.yield()
       tex.sprint(math.round(100 * tex.box[0].width/tex.hsize))
       tex.sprint(" percent of the hsize: ")
       tex.sprint("\\box0")
     end)
  }}
\stopbuffer

\typebuffer[code] \getbuffer[code]

\startbuffer
The width of next word is \TestWord {inline}!
\stopbuffer

\typebuffer \getbuffer

Now, in order to stay realistic, this macro can also be defined as:

\startbuffer[code]
\def\TestWord#1%
  {\setbox0\hbox{\tttf #1}%
   \directlua{
      tex.sprint(math.round(100 * tex.box[0].width/tex.hsize))
   } %
   percent of the hsize: \box0\relax}
\stopbuffer

\typebuffer[code]

We get the same result: \quotation {\getbuffer}.

We have been using a \LUA|-|\TEX\ mix for over a decade now in \CONTEXT, and have
never really needed this mixed model. There are a few places where we could
(have) benefitted from it and we might use it in a few places, but so far we have
done fine without it. In fact, in most cases typesetting can be done fine at the
\TEX\ end. It's all a matter of imagination.

\stopchapter

\stopcomponent
