[DFDL-WG] Action 283: Provision for fallback mappings

Tue Aug 25 10:56:36 EDT 2015

Would an IBM-specific property, to be proposed for future inclusion in
DFDL. E.g., something like

ibmdfdl:encodingErrorFallbackPolicy="never" or "fallback" with other enums
reserved for the future.

I would like to pave a path for these sorts of proposed features. It would
be good to see if this alone is sufficient to meet your customer's needs
who are asking for this, or whether they will need even a bit more control
than this.

It looks like we just missed some unparse behavior in
dfdl:encodingErrorPolicy="replace", as clearly when a Unicode character has
no mapping, and the target encoding is SBCS and ascii-derived, then the
0x1A character is the right thing.

However, I know what will happen in Daffodil is what the standard ICU
library does, with its default mapping definitions, and I don't know that
this 0x1A substitution character is properly used in those mappings.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are
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On Tue, Aug 25, 2015 at 9:29 AM, Steve Hanson <smh at uk.ibm.com> wrote:

> Today the DFDL 1.0 spec has property dfdl:encodingErrorPolicy to control
> what happens when an unmappable or malformed character is encountered -
> 'error' or 'replace'. When 'replace' the appropriate substitution character
> is used.
>
> There is also the orthogonal question of fallback mappings, which are
> mappings specified by an encoding which is not a normal round-trip
> mapping.  DFDL does not currently provide for switching on fallback
> mappings. Here's what ICU says about this at
> http://userguide.icu-project.org/conversion/data.
>
> *In the CHARMAP section of a .ucm file, each line contains a Unicode code
> point (like <U(1-6 hexadecimal digits for the code point)> ), a codepage
> character byte sequence (each byte like \xhh (2 hexadecimal digits} ), and
> an optional "precision" or "fallback" indicator.*
>
> *The precision indicator either must be present in all mappings or in none
> of them. The indicator is a pipe symbol ‘|’ followed by a 0, 1, 2, 3, or 4
> that has the following meaning:*
>
>    - *|0 - A "normal", roundtrip mapping from a Unicode code point and
>    back.*
>    - *|1 - A "fallback" mapping only from Unicode to the codepage, but
>    not back.*
>    - *|2 – A subchar1 mapping. The code point is unmappable, and if a
>    substitution is performed, then the subchar1 should be used rather than the
>    subchar. Otherwise, such mappings are ignored.*
>    - *|3 - A "reverse fallback" mapping only from the codepage to
>    Unicode, but not back to the codepage.*
>    - *|4 - A "good one-way" mapping only from Unicode to the codepage,
>    but not back.*
>
> *Fallback mappings from Unicode typically do not map codes for the same
> character, but for "similar" ones. This mapping is sometimes done if a
> character exists in Unicode but not in the codepage. To replace it, ICU
> maps a codepage code to a similar-looking code for human-readable output.
> This mapping feature is not useful for text data transmission especially in
> markup languages where a Unicode code point can be escaped with its code
> point value. The ICU application programming interface (API) *
> *ucnv_setFallback()** controls this fallback behavior.*
>
> *"Reverse fallbacks" are technically similar, but the same Unicode
> character can be encoded twice in the codepage. ICU always uses reverse
> fallbacks at runtime.*
>
> *A subset of the fallback mappings from Unicode is always used at runtime:
> Those that map private-use Unicode code points. Fallbacks from private-use
> code points are often introduced as replacements for previous roundtrip
> mappings for the same pair of codes. These replacements are used when a
> Unicode version assigns a new character that was previously mapped to that
> private-use code point. The mapping table is then changed to map the same
> codepage byte sequence to the new Unicode code point (as a new roundtrip)
> and the mapping from the old private-use code point to the same codepage
> code is preserved as a fallback.*
>
> *A "good one-way" mapping is like a fallback, but ICU always uses "good
> one-way" mappings at runtime, regardless of the fallback API flag.*
>
> *The idea is that fallbacks normally lose information, such as mapping
> from a compatibility variant of a letter to the ASCII version; however,
> fallbacks from PUA and reverse fallbacks are assumed to be for "the same
> character", just an older code for it.*
>
> So the default behaviour for ICU is to use "good one-way" mappings,
> "reverse fallback" mappings, and "fallback" mappings from private-use-area
> code points, but only to use normal "fallback" mappings if the setFallback
> API has been used.
>
> IBM customers have requested the ability to use normal "fallback"
> mappings. At the current time, the only solution open to them is to change
> the .ucm file (or create a variant) and change the "|1" mappings to "|4" so
> that "fallback" mappings become "good one-way" mappings.
>
> A proposal to support fallbacks was submitted a few years ago by Mike.
> https://www.ogf.org/pipermail/dfdl-wg/2011-November/001631.html. It
> proposed adding new DFDL annotations to allow replacement characters and
> fallback mappings to be specified.  This was rejected as ICU already
> provides this via the .ucm file. But no simpler alternative materialised,
> and the resulting erratum only added dfdl:encodingErrorPolicy, which does
> not handle fallbacks.
>
> Given a) the precedent of existing IBM DFDL and Daffodil behaviour which
> (should) match the ICU default, b) the orthogonality of substitition
> characters (an error has occurred) and fallbacks (defined mappings for a
> purpose), and b) an IBM recommendation not to switch on fallbacks by
> default, it feels like we need a new property eg: *dfdl:useEncodingFallbacks
> 'yes' | 'no'*.  Alternatives welcome. The names
> dfdl:encodingFallbackPolicy or dfdl:encodingPrecisionPolicy are better, but
> then comes the problem of finding meaningful enum values...
>
> Also noted: The woridng for dfdl:encodingErrorPolicy 'replace' says: *If
> 'replace' then any error when decoding characters results in the insertion
> of the Unicode Replacement Character (U+FFFD) as the replacement for that
> error.* That is not strictly true, as the same ICU page says:
>
>    - *Conversion from a codepage to Unicode occurs and an unassigned
>    codepoint is found*
>    *1.        **If the input sequence is of length 1 and a subchar1 byte
>    is specified for the codepage *[in the .ucm file]*, output U+001A*
>    *2.        **Otherwise output U+FFFD*
>
>
> There is then the question of how do the two properties interact.
> Specifically, if fallbacks are not being used, does encountering a code
> point with a fallback result dfdl:encodingErrorPolicy coming in to play?  I
> suspect so but needs verifying.
>
> Regards
>
> Steve Hanson
> Architect, *IBM DFDL*
> <http://www.ibm.com/developerworks/library/se-dfdl/index.html>
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK
> *smh at uk.ibm.com* <smh at uk.ibm.com>
> tel:+44-1962-815848
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> IBM United Kingdom Limited - Registered in England and Wales with number
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>
>
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