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Re: [css3-multicol] updated editor's draft

From: Bert Bos <bert@w3.org>
Date: Wed, 22 May 2013 17:22:45 +0200
To: www-style@w3.org
Cc: Morten Stenshorne <mstensho@opera.com>, Håkon Wium Lie <howcome@opera.com>
Message-Id: <201305221722.45763.bert@w3.org>
On Wednesday 15 May 2013 16:57:47 Morten Stenshorne wrote:
> Håkon Wium Lie <howcome@opera.com> writes:
> > The multicol ED has been updated:
> >   http://dev.w3.org/csswg/css-multicol/
> > 
> > [...]
> > 
> > Revised the pseudo-algorithm, as per;
> > 
> >   http://lists.w3.org/Archives/Public/www-style/2013Apr/0347.html
> >   http://lists.w3.org/Archives/Public/www-style/2013Apr/att-0225/tm
> >   p.html
> 
> (reading Editor's Draft 10 May 2013)
> 
> There's really no difference between the expressions
> "(U - ((N - 1) * column_gap)) / N" on line 05
> and:
> "((U + column_gap) / N) - column-gap" on line 09 and 14
> 
> I suggest that one of them be picked and used consistently. The first
> one is probably slightly more human-friendly (one can see clearly
> what's going on; the total width of all columns is the block width
> minus gaps, and the column width is that number divided by the
> number of columns), while the second is reduced and more
> CPU-friendly. :) I think I vote for the second one, since a similar
> construct also can be found on current line 08 (where we resolve the
> count from width).

Yes, they are the same. Which one is easier to read I don't know.

> 
> I think the pseudo algorithm from line 03 can be simplified to
> something like this:
> 
> (03) if (column-width = auto)
> (04)   N := column-count;
> (05) else
> (06)   N := floor((U + column-gap) / (column-width + column-gap))
> (07)   if (column-count != auto)
> (08)     N := min(column-count, N)
> (09)   endif
> (10)   N := max(1, N)
> (11) endif
> (12) W := max(0, (U + column-gap) / N - column-gap)
> 
> This should give the exact same results, with no code duplication.

I have a preference for pure formulas, instead of variables that change 
value, because this might well be implemented in Lisp or Prolog instead 
of C. (Lines 12 & 13 were split only because the formula was too wide 
for my window...) How to optimize for speed, for size, or for something 
else, is up to the programmer.

Note, e.g., that in some cases it is not necessary to compute the max() 
in your last line, because the expression cannot be negative. Whether it 
is worth optimizing for that is another question...

Apart from this max(), the W is indeed the same in all three cases, so 
how about showing just the three cases for N, like in your version, but 
purely declaratively, and then the W separately:

    if column-width = auto then
      N := column-count
    else if column-count = auto then
      N := max(1, floor((U + column-gap)/(column-width + column-gap)))
    else
      N := min(column-count, max(1, 
        floor((U + column-gap)/(column-width + column-gap))))

  And:

    W := max(0, ((U + column-gap)/N - column-gap)


> 
> > I've kept:
> >   - the 'exit' statements
> 
> I had to change that in my suggestion above.
> 
> >   - these assumptions:
> >       -- that the block direction is unconstrained
> >       -- that no column breaks are added through style sheets
> 
> The used value of column-width isn't affected by breaks or block
> direction restrictions, but used column-count may be, but that
> depends on what "used" means in this case. The column-count property
> has the following phases:
> 
> - Specified/computed (whatever the stylesheet or style attributes
> says)
> 
> - After running the pseudo algorithm
> 
> - Actual layout
> 
> Consider this example:
> 
> <body style="width:600px;">
>   <div style="column-width:200px; column-gap:0; height:2em;
> line-height:1em;"> line<br>
>      line<br>
>      line<br>
>      line<br>
>      line<br>
>      line<br>
>      line<br>
>    </div>
> </body>
> 
> The computed value of column-count is auto.
> The pseudo algorithm resolves column-count to 3.
> Layout ends up with 4 columns, due to restricted height.
> 
> What if we define it so that *used* column-count in the example above
> is 3, while *actual* column-count is 4 [1]? If we do that, you can
> remove the assumptions.
> 
> [1] http://www.w3.org/TR/CSS21/cascade.html#value-stages

That sounds attractive. Does it influence anything else apart from the 
terminology in the spec?

> 
> Also, it should be mentioned that the pseudo algorithm needs to be
> run separately on each column row, since the available width in
> different rows may vary (different page sizes, regions, what not).

Isn't that already covered by the line "In paged media, user agents may 
perform this calculation on a per-page basis"? Or do you mean that the 
"may" should be a "must"?



Bert
-- 
  Bert Bos                                ( W 3 C ) http://www.w3.org/
  http://www.w3.org/people/bos                               W3C/ERCIM
  bert@w3.org                             2004 Rt des Lucioles / BP 93
  +33 (0)4 92 38 76 92            06902 Sophia Antipolis Cedex, France
Received on Wednesday, 22 May 2013 15:23:18 UTC

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