Re: [Nano-devel] Softwrap navigation overhaul

[Benno Schulenberg]

On Tue, Feb 7, 2017, at 21:13, David Ramsey wrote:
> On Tue, Feb 7, 2017 at 9:04 AM, Benno Schulenberg
> <address@hidden> wrote:
> > Is it possible for the chunk routines to get called with nrows = 0?
> 
> It probably shouldn't be.  Removed.

But the answer is yes.  There are three places where a chunks()
routine is called with editwinrows - 1, one place where it's called
with editwinrows - nrows, and once with editwinrows / 2.  They
all risk being zero.  The question then becomes: does the case
need special treatment?  It seems not: for nrows == zero, all
the ifs and fors just fall through and become noops.  Good.

+       for (i = nrows; i > 0; i--) {
+           size_t last_chunk = (strlenpt((*line)->data) / editwincols);
+
+           if (current_chunk < last_chunk) {
+               current_chunk++;
+               continue;
+           }
+ [...]

This is horrible.  For every chunk++ you recompute the line span
completely superfluously.  You have to duplicate the assignment
to last_chunk: once before the loop, and once at the tail of the
loop.  For a small optimization, you could conditionalize the latter
with an 'if (i > 0)', but for now just drive straight.

(Later on we will probably, a la Mark Majeres, want to simply store
the line span of each line in its linestruct (misnomed filestruct).
That should speed things up a bit.)

Also, I don't like the parenthesis around last_chunk computation.


Okay, just testing again, with the attached file, run:

    stty cols 44 && src/nano --soft +3 chinese.html

See how the cursor sits on the "<" of the <bottom> tag.
Now press <Up> ten times.  Notice how the cursor jumps
from left to right now and then.  Weird!  After ten presses
the cursor sits on the "<" of the <top> tag.  Now try pressing
<Down> ten times.  After the second press, nothing further
happens -- the cursor is stuck at the tail of the first chunk.

Now press and hold <Right>.  The cursor happily runs through
all of the chunks.  But after a while it gets stuck at the tail
of the last chunk.  Now press <Down>.  The cursor is after the
">" of the <bottom> tag.

Also notice how some of the chunks seem to start with two
blank columns.  But if you go there and select them, it selects
the two columns as a whole.  If you cut it and then paste it at
the top of the file, a character appears.  The same character
that suddenly appears when you type x instead of cutting it.

Now run:

    stty rows 1 && stty cols 44 && src/nano --soft +3 chinese.html

Press M-\ <Down> <Down> xxxxx  <Left> <Left> <Left> <Right>
<Right> <Right>...  The cursor seems to be completely stuck.

For a still more frustrating experience, run:

    stty rows 4 && stty cols 44 && src/nano --soft +3 chinese.html

Try <Up> <Up> <Up>...  Try all other directions...  No movement.


Well... this is complicated.  It has to do with double-width
characters sitting on the boundary between two chunks.  When
this happens, this character should not be included in the
lefthand chunk, and the righthand chunk should start a column
earlier -- I think.  But that makes the calculation of the number
of chunks wildly complicated.  Don't know how to solve that.

Maybe a "solution" would be to not use the last column of the
screen, so that a double-width character in the penultimate
column can "overflow" into this column instead of disappearing?

(Problem is related to https://savannah.gnu.org/bugs/?49440.)

Benno

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