Unclear text in the UBA (UAX#9) of Unicode 6.3

Philippe Verdy verdy_p at wanadoo.fr
Mon Apr 21 11:18:09 CDT 2014

There are some cases where these rules will not be clear enough. Look at
the following where overlaps do occur; but directionality still matters:

"This is an [<<] example [>>] for demonstration only."

There are two parsings possible if you just consider a hierarchic layout
where overlaps are disabled:

1. "This is an [...] for demonstration only.", embedding "<<...>>", itself
embedding "] example [" (here the square brackets match externally)

2. "This is an [...] example [...] for demonstration only.", embedding two
spans for "<<" and ">>" separately (they also pair externally)

Now suppose that the term "example" is translated in Arabic: It is not very
clear how the UBA will work while preserving the correct pariing direction
of the 3 pairs (one pair is "<<...>>", there are two pairs for "[...]").
Still all 3 pairs have a coherent direction that Bidi-reordering or glyph
mirorring should not mix.

I see only one solution to tag such text so that it will behave correctly:
either the two pairs of square brackets or the pair or guillemets should be
encoded with isolated Bidi overrides. But then what is happening to the
ordering of the surrounding text?

There should be a stable way to encode this case so that UBA will still
work in preserving the correct reding order, and the expected semantics and
orientation of pairs and the fact that the guillemets are effectively not
really embedding the brackets, but the translated word "example".

There are several ways to use Bidi-override or Bidi-embedding controls; I
don't know which one is better but all of them should still work with UBA.
I just hope that the complex cases of the brackets in the middle ("]...[")
can be handled gracefully.

My opinion would require embedding and isolating the each square bracket,
they will no longer match together (externally they are treated as symbols
with transparent direction, but how we ensure that the sequence "[<<]" will
still occur before the RTL (Arabic) "example" word followed by the sequence
"[>>]" and that the rest of the sentence (for demonstration only) will still
occur in the correct order : we also have to embed/isolate the "example",
or the whole sequence "[<<] example [>>]" so that the main sentence "This is
an ... for demonstration only" will stil have a coherent reading direction.

Such cases are not so exceptional because they occur to represent two
distinct parallel readings of te same text, where in one reading for one
kind of pairs will simply treat the other pairs as ignored "transparently".

It should be an interesting case to investigate for validating UBA
algorithms in a conformance test case.

2014-04-21 16:32 GMT+02:00 Asmus Freytag <asmusf at ix.netcom.com>:

>  On 4/21/2014 1:33 AM, Eli Zaretskii wrote:
>  Date: Sun, 20 Apr 2014 23:03:20 -0700
> From: Asmus Freytag <asmusf at ix.netcom.com> <asmusf at ix.netcom.com>
> CC: Eli Zaretskii <eliz at gnu.org> <eliz at gnu.org>, unicode at unicode.org,
>  Kenneth Whistler <ken at unicode.org> <ken at unicode.org>
>          Note that the current embedding level is not changed by this rule.
>     What does this last sentence mean by "the current embedding level"?
>     The first bullet of X6 mandates that "the current character's
>     embedding level" _is_ changed by this rule, so what other "current
>     embedding level" is alluded to here?
>      I'm punting on that one - can someone else answer this?
> I assume "current embedding level" here meant "the embedding level of
> the last entry on the directional status stack". (This is a natural
> slip to make if you think in terms of an optimized implementation that
> stores each component of the top of the directional status stack in a
> variable, as suggested in 3.3.2.)
> James
>  In general, I heartily dislike "specifications" that just narrate a
> particular implementation...
>  I cannot agree more.
> In fact, my main gripe about the UBA additions in 6.3 are that some of
> their crucial parts are not formally defined, except by an algorithm
> that narrates a specific implementation.  The two worst examples of
> that are the "definitions" of the isolating run sequence and of the
> bracket pair.  I didn't ask about those because I succeeded to figure
> them out, but it took many readings of the corresponding parts of the
> document.  It is IMO a pity that the two main features added in 6.3
> are based on definitions that are so hard to penetrate, and which
> actually all but force you to use the specific implementation
> described by the document.
> My working definition that replaces BD13 is this:
>   An isolating run sequence is the maximal sequence of level runs of
>   the same embedding level that can be obtained by removing all the
>   characters between an isolate initiator and its matching PDI (or
>   paragraph end, if there is no matching PDI) within those level runs.
> As for bracket pair (BD16), I'm really amazed that a concept as easy
> and widely known/used as this would need such an obscure definition
> that must have an algorithm as its necessary part.  How about this
> instead:
>   A bracket pair is a pair of an opening paired bracket and a closing
>   paired bracket characters within the same isolating run sequence,
>   such that the Bidi_Paired_Bracket property value of the former
>   character or its canonical equivalent equals the latter character or
>   its canonical equivalent, and all the opening and closing bracket
>   characters in between these two are balanced.
> Then we could use the algorithm to explain what it means for brackets
> to be balanced (for those readers who somehow don't already know
> that).
> Again, thanks for clarifying these subtle issues.  I can now proceed
> to updating the Emacs bidirectional display with the changes in
> Unicode 6.3.
>  FWIW here is the restatement of BD16 that I used for myself (and that I
> put
> into the source comments of the sample Java implementation):
>     // The following is a restatement of BD 16 using non-algorithmic
> language.
>     //
>     // A bracket pair is a pair of characters consisting of an opening
>     // paired bracket and a closing paired bracket such that the
>     // Bidi_Paired_Bracket property value of the former equals the latter,
>     // subject to the following constraints.
>     // - both characters of a pair occur in the same isolating run sequence
>     // - the closing character of a pair follows the opening character
>     // - any bracket character can belong at most to one pair, the
> earliest possible one
>     // - any bracket character not part of a pair is treated like an
> ordinary character
>     // - pairs may nest properly, but their spans may not overlap otherwise
>     // Bracket characters with canonical decompositions are supposed to be
> treated
>     // as if they had been normalized, to allow normalized and
> non-normalized text
>     // to give the same result.
> Your language is more concise, but you may compare for differences.
> A./
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