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Re: limits of the generic

• To: "Jeff Greif" <jgreif@a...>
• Subject: Re: limits of the generic
• From: Jeni Tennison <jeni@j...>
• Date: Mon, 30 Sep 2002 12:27:54 +0100
• Cc: xml-dev@l...
• In-reply-to: <01a901c26810$471b1d80$6400a8c0@JMG>
• Organization: Jeni Tennison Consulting Ltd
• References: <200209282248.SAA07986@m...><170201240678.20020929000004@j...><013901c267e6$fbd22ef0$6400a8c0@JMG><116272975257.20020929195541@j...><016501c267ef$c36b29a0$6400a8c0@JMG><30283490097.20020929225056@j...><01a901c26810$471b1d80$6400a8c0@JMG>
• Reply-to: Jeni Tennison <jeni@j...>

Hi Jeff,

> It seems there's something wrong with the pinning algorithm.
> Probably, there should be an attempt to do pinning only at the end
> of the computation (by analogy with rounding) -- the idea of pinning
> to a valid date after adding Y/M components, then later dealing with
> the day and time parts seems of dubious merit.

Agreed that it's something wrong with the pinning algorithm, but I
don't think that leaving the pinning until the end works either.
We're trying to find an algorithm where following is true:

  2004-01-29T12:00:00 + P1M  <

  2004-01-30T00:00:00 + P1M  <

  2004-01-30T12:00:00 + P1M  <
  
  2004-01-31T00:00:00 + P1M  <

  2004-01-31T12:00:00 + P1M  <
  
  2004-02-01T00:00:00 + P1M

It seems obvious that 2004-01-29T12:00:00 + P1M => 2004-02-29T12:00:00
and that 2004-02-01T00:00 => 2004-03-01T00:00:00. Which means that we
need to find something where the following is true:

  2004-02-29T12:00:00  <

  2004-01-30T00:00:00 + P1M  <

  2004-01-30T12:00:00 + P1M  <
  
  2004-01-31T00:00:00 + P1M  <

  2004-01-31T12:00:00 + P1M  <
  
  2004-03-01T00:00:00

And now things are difficult, to say the least. If the days are pinned
back then 2004-01-29T12:00:00 + P1M < 2004-01-30T00:00:00 + P1M
won't be true. If they're rounded up, then 2004-01-31T12:00:00 + P1M <
2004-02-01T00:00:00 + P1M won't be true. In either case, keeping the
relationship 2004-01-30T12:00:00 + P1M < 2004-01-31T00:00:00 + P1M is
going to be hard.

It's not just a matter of the times causing problems, or that it's a
leap year causing the difficulty. There are similar issues with:

  2002-01-28 + P1M  <

  2002-01-29 + P1M  <

  2002-01-30 + P1M  <

  2002-01-31 + P1M  <

  2002-02-01 + P1M

Hmm... The thing is that adding durations to date/times is a useful
thing to do, so I'd be loath to make it impossible in XPath 2.0. If it
were done through a function call instead, that would enable the +
operator to retain its usual semantics. What do you think? Would it
matter if:

  2002-01-28  <  2002-01-30

but not:

  add-yearMonthDuration-to-date(
    xs:date('2002-01-28'), xs:yearMonthDuration('P1M')) <
  add-yearMonthDuration-to-date(
    xs:date('2002-01-30'), xs:yearMonthDuration('P1M'))

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/

• Follow-Ups:
  • Re: limits of the generic
    • From: Joe English <jenglish@f...>
  • Re: limits of the generic
    • From: "Jeff Greif" <jgreif@a...>

• References:
  • Re: limits of the generic
    • From: John Cowan <jcowan@r...>
  • Re: limits of the generic
    • From: Jeni Tennison <jeni@j...>
  • Re: limits of the generic
    • From: "Jeff Greif" <jgreif@a...>
  • Re: limits of the generic
    • From: Jeni Tennison <jeni@j...>
  • Re: limits of the generic
    • From: "Jeff Greif" <jgreif@a...>
  • Re: limits of the generic
    • From: Jeni Tennison <jeni@j...>
  • Re: limits of the generic
    • From: "Jeff Greif" <jgreif@a...>

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