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From "Peter B. West" <pbw...@powerup.com.au>
Subject Re: String.intern() thoughts
Date Thu, 04 Dec 2003 12:36:53 GMT
Glen Mazza wrote:
...
> 
> I think the next thing to consider is the storage of
> specified vs. computed values.  Let's say we store
> pointers for many properties to the same
> {"property-name", "property-value"} pair.  A specified
> property value of "10%" would not make this a very
> helpful data structure if that percentage resolves to
> different computed values for each property sharing
> this pair.  I believe the goal for us then would be
> just store the computed value for each pair (meaning
> many more pairs), as long as we take into account the
> can't-resolve-everything-without-knowledge-of-layout
> issue.

alt.design makes no attempt to look for commonalities here.  It resolves 
every possible property value, and keeps a partly-resolved value for 
percentages.  Inheritance is (almost exclusively) of computed values.
<quote>
For a given inheritable property, if that property is present on a 
child, then that value of the property is used for that child (and its 
descendants until explicitly re-set in a lower descendant); otherwise, 
the specified value of that property on the child is the computed value 
of that property on the parent formatting object.
</quote> 5.1.4 Inheritance

There is an exception that comes to mind.  For "line-height" a value may 
be specified which, although not specified as a percentage, is a factor 
by which the font-size (from memory) is multiplied.  When such a value 
is inherited, it is the factor, not the computed value.

In general, the computed value of a percentage is inherited.  That still 
leaves a problem, because the computed value is unknown.  In alt.design, 
what is effectively a link back to the unresolved property is specified 
as the value.  When the parent property is resolved, that resolved value 
is then available to the inheriting descendants.

Yet-to-be-implemented is the handling of expressions involving 
percentages, as mentioned in an earlier post.

In essence, alt.design attempts to resolve every property to its 
computed value, and store the result only on nodes to which the property 
applies.  There is no attempt to reduce storage by procedures like 
interning strings.  Note, though, that the process of resolving 
properties does eliminate most strings.  My objective was that when 
areas were being laid out, the relevant properties would all be directly 
available.

Peter
-- 
Peter B. West <http://www.powerup.com.au/~pbwest/resume.html>


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