Subject: Recursion depth limit for IDC's
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From jenkins@apple.com Tue Jun  1 16:53:41 1999
Date: Tue, 01 Jun 1999 16:51:50 -0700
Subject: Re: Brief note on length of ideograph descriptions
To: book@unicode.org
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> John,
> This sounds good and passes the reasonableness test.
> Are you going to propose language for a position paper
> for UTC to discuss on this?
> Or are are you suggesting that we simply add a paragraph
> to the book describing our estimate of the recursion
> level one might be expected to find in the worst case
> (for applications which choose to intepret IDC's as
> other than dingbats)?
Mark has pointed out that unless there is a formal limit on the complexity
of recursive ideographic description sequences, things could get out of
hand.  If you enter a text stream at a random point in the middle of
ideographs, you would have to back-track all the way to the beginning of the
document before you *know* whether or not you're at the beginning of an
ideographic description sequence.
Even if you treat the IDC's as visual blobs for rendering purposes, you may
choose to take them into account for other operations (cursor movement, say,
or line-breaking), and I think that this latter awareness is quite likely.
We've already got it on the docket at Apple, for example, to implement.
Given that, we should have a formal limitation on the depth of the
recursion.  E.g., we should say that an ideographic description sequence
more than eight levels deep is invalid.
I'm willing to draft language to that effect (in fact, I think I just did)
and have the UTC approve it so we can stick it in the book.  Mark can
present the issue at the meeting; neither Rick nor I can attend.
John H. Jenkins

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