It's obviously less disruptive to the DFDL spec to add extra enums to dfdl:encodingErrorPolicy.  My concern in doing that is the orthogonality of substitition characters (an error has occurred) and fallbacks (defined mappings for a purpose). So let's look at the scenarios we need to support and see if that can generate a set of reasonably natural enums:
1) Error unmappable characters; fallbacks not required => "error"
2) Replace unmappable characters; fallbacks not required => "replace"
3) Error unmappable characters; fallbacks required => "fallback"
4) Replace unmappable characters; fallbacks required => "fallbackOrReplace"

I think two new enums are needed as one IBM product that uses IBM DFDL said it wanted fallback but not substitution.

Regards
 
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848



From:        Mike Beckerle <mbeckerle.dfdl@gmail.com>
To:        Steve Hanson/UK/IBM@IBMGB
Cc:        DFDL-WG <dfdl-wg@ogf.org>
Date:        26/08/2015 14:32
Subject:        Re: [DFDL-WG] Action 283: Provision for fallback mappings



Or... perhaps dfdl:encodingErrorPolicy="replaceOrFallback", that is, perhaps we can just add another enum value to reflect this policy rather than adding more properties.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy


On Tue, Aug 25, 2015 at 10:56 AM, Mike Beckerle <mbeckerle.dfdl@gmail.com> wrote:
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 subject to the OGF Intellectual Property Policy


On Tue, Aug 25, 2015 at 9:29 AM, Steve Hanson <smh@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: 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:


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
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
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IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU


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