In that case which 2 digits of 1997 you get should perhaps depend on the number justification. If left truncate off the right so you get 19. If right truncate off left to get 97.

Since numbers are usually right justified this is the typical behavior and I think what the ICU lib is assuming, patterns being most common in fixed length data.

In variable length data I agree with your analysis which is to never truncate most significant digits.

On Jul 9, 2012 2:00 PM, "Steve Hanson" <smh@uk.ibm.com> wrote:
The ICU web documentation says the following about formatting (unparsing) numbers, which has been copied into the DFDL specification:

• If the number of actual integer digits exceeds the maximum integer digits, then only the least significant digits are shown. For example, 1997 is formatted as "97" if the maximum integer digits is set to 2.
• If the number of actual integer digits is less than the minimum integer digits, then leading zeros are added. For example, 1997 is formatted as "01997" if the minimum integer digits is set to 5.
• If the number of actual fraction digits exceeds the maximum fraction digits, then rounding is performed to the maximum fraction digits. For example, 0.125 is formatted as "0.12" if the maximum fraction digits is 2. This behavior can be changed by specifying a rounding increment and/or a rounding mode.
• If the number of actual fraction digits is less than the minimum fraction digits, then trailing zeros are added. For example, 0.125 is formatted as "0.1250" if the mimimum fraction digits is set to 4.

·        The term maximum fraction digits is the total number of ‘0’ and ‘#’ characters in the fraction sub-pattern above.

·        The term minimum fraction digits is the total number of ‘0’ characters (only) in the fraction sub-pattern above.

·        The term maximum integer digits is the total number of ‘0’ and ‘#’ characters in the integer sub-pattern above.

·        The term minimum integer digits is the total number of ‘0’ characters (only) in the integer sub-pattern above.

That all looks to make sense, but on close reading the ICU behaviour of maximum integer digits appears to be undesirable, in that it will silently truncate oversize integer portions.  From above "For example, 1997 is formatted as "97" if the maximum integer digits is set to 2."

Interestingly, while ICU derives minimum integer digits, minimum fraction digits and maximum fraction digits from the pattern, ICU does not derive maximum integer digits from the pattern and instead uses a default of 309. There is an explicit ICU API call that you have to make to set it.  

Because of this inconsistent ICU behaviour, the IBM DFDL implementation has omitted to use this ICU API today, and so allows up to 309 digits to be formatted regardless of pattern. Eg, "#0" works for infoset values"1", "12", "123456789" without any loss. As well as avoiding the silent truncation, this is convenient for variable length text numbers, as a single textNumberPattern value such as "#0" can be set in scope and widely used, but means variable length text numbers do not have their integer digit length policed (fixed length text numbers are policed by length of element).  

I think it is worth ratifying that the spec words above are the true intended behaviour, and if so noting that an implementation should a) not set the ICU API, b) let the maximum integer digits default to 309, and c) implement maximum integer digits processing itself.

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
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