Re: Differences in `ne` and `bp` line-breaking behavior

[G. Branden Robinson]

At 2024-12-04T00:08:23+0100, onf wrote:
> On Tue Dec 3, 2024 at 11:56 PM CET, Tadziu Hoffmann wrote:
> > I was thinking of something similar to
> >
> >   .ne d    Advance drawing position to the next vertical
> >            position trap and spring the trap, if it is
> >            nearer than distance d (default scaling unit v).
[...]
>
> Reversing their order might make the intended meaning more obvious:
>   .ne d    If the next vertical position trap is nearer than
>            distance d (default scaling unit v), advance drawing
>            position to it and spring it.

The phrase "and spring it" occurred to me as well.  I think I'll go with
something much (exactly?) like this.

I might as well cross reference register .trunc while I'm there.

While we're on the subject, I was careful to say "vertical position
trap" instead of "page location trap", because `ne` interacts with
diversion traps as well.

I am sorely tempted to describe `ne` as "not a conditional `bp`, but a
conditional `sp`".

`sp` has a noteworthy limitation.

       .sp dist  Break and move the next text baseline down by dist, or
                 until springing a page location trap (default scaling
                 unit v).  A negative dist will not reduce the position
                 of the text baseline below zero.  Prefixing dist with
                 the | operator moves to a position relative to the page
                 top for positive N, and the bottom if N is negative; in
                 all cases, one line height (vee) is added to dist.
                 dist is ignored inside a diversion.

Check the final sentence.  `ne` inherits this limitation.  In a
diversion, you can ask `ne` for tons of space, but you'll only get 1v at
a time.  Even if you invoke it more than once.

Input:

$ groff -ww <<EOF
.de trap
. tm BOING (.d=\\n[.d]u)
..
.di div
.tm JUST GOT STARTED (.d=\n[.d]u)
.dt 5v trap
.nf
1
.tm FIRST LINE OUT (.d=\n[.d]u)
2
.ne 3v
3
4
.tm ALMOST DONE (.d=\n[.d]u)
.ne
.tm JUST A BIT MORE (.d=\n[.d]u)
.ne
.tm I KEEP MASHING THIS THING BUT NOTHING HAPPENS (.d=\n[.d]u)
5
.tm DONE (.d=\n[.d]u)
.di
EOF

Output:

JUST GOT STARTED (.d=0u)
FIRST LINE OUT (.d=12000u)
ALMOST DONE (.d=48000u)
JUST A BIT MORE (.d=48000u)
I KEEP MASHING THIS THING BUT NOTHING HAPPENS (.d=48000u)
BOING (.d=-1u)
DONE (.d=60000u)

Regards,
Branden

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