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Backwards era-0 years and add,roll

(Peter Edberg, 2012-05-23; updated 2012-06-07, 2012-06-13)

This is for #9226, Calendar::add year direction for era 0 depends on calendar
(and note, all behavior described here is the same in ICU4C and ICU4J)

There are three ICU calendars in which era-0 years go backwards (era-0 year values increase as one goes backwards in time); the current era is 1 for all of these:

  • gregorian
  • roc (based on gregorian)
  • coptic

This is not generally documented (although some references to gregorian behavior indicate this), and it not necessarily expected for all of these cases, since in other similar calendars era-0 years go forward:

  • japanese (based on gregorian), current era is 235
  • ethiopic (very similar to coptic), current era is 1
  • chinese lunar, current era is 78

Of course, in calendars with only one era, for which the current era is 0, years go forward in time, for example buddhist (based on gregorian), hebrew, islamic.

Note, as a separate issue, I think we need APIs that provide meta information about calendars, such as whether the era-0 years go backwards or forwards. That will be the subject of a separate bug...

Calendars in which era-0 years go backwards should never have a negative value in the year field. Calendars in which era-0 years go forwards use negative (or 0) values in the year field to represent years before the beginning of era-1 (as explicitly noted for the hebrew calendar in the Java class documentation for Calendar).

For calendars in which era-0 years go backwards, the add/roll methods for years in era-0 (and the ucal_add/ucal_roll functions) have behavior that can seem strange. And in addition, before we even address that:

  • Calling roll/ucal_roll for year can modify the era, contrary to the documented behavior of not modifying fields more significant than the one it is called to modify. For example, set a gregorian calendar to era 1 year 4, then call roll(YEAR, -5); the result is era 0 year 2 (i.e. 2 BCE).

But now on to the era-0 issues for gregorian, roc, coptic:

  • With the calendar set to era-0, calling add or roll for YEAR with a positive value does increase the value of the YEAR field, but that moves backwards in time. This is generally consistent with documentation for add and roll that says, e.g. -
    • “Add a specified signed amount to a particular field” (ucal_add, ucal_roll)
    • “Adds the specified (signed) amount of time to the given time field, based on the calendar's rules” (Calendar::add)
    • “add(f, delta) adds delta to field f” (ICU4J Calendar class documentation of rules for field manipulation methods)
  • However, it explicitly contradicts the documented behavior for ICU4C Calendar::roll -
    • “Rolling by a positive value always means rolling forward in time; e.g., rolling the year by +1 on '100 BC' will result in '99 BC', for Gregorian calendar”
  • Clients are also likely to assume that adding a positive year value moves forward in time (as with all other calendar fields besides year). In fact this assumption is the cause of the following bug in getFieldDifference -
    • #9227 Per Calendar::fieldDifference, 1911 to 1913 is 5879430 yrs (for roc)
  • This behavior can also cause infinite loops with a series of add functions. Set a gregorian calendar to era 1 year 2, then make the following series of calls:
    • add(YEAR, -3); the result is era 0 year 2 (2 CE)
    • add(YEAR, -3); the result is era 1 year 2 (2 BCE)
    • add(YEAR, -3); the result is era 0 year 2 (2 CE)
    • ...

For comparison, here is the JDK (1.6.0_31) behavior for Gregorian:

  • If the calendar is set to era 1 year 4, then:
    • add(year, -7) does cross into era 0 as expected (and as in ICU), resulting in era 0, year 4.
    • roll(year, -7) actually wraps around within era 1 to the just under the maximum value for year, resulting in era 1, year 292278991. This is actually consistent with the documentation, and with how roll behaves for other fields.
  • If the calendar is set to era 0, then add of a positive value moves forward in time (unlike current ICU behavior), crossing to era 1 if necessary, and add of a negative value moves backward in time. However, in era 0, the behavior of roll is inconsistent and strange; roll with a positive value moves backwards in time, and roll across the era boundary changes eras, with strange results. If the calendar is set to era 0 year 4 month January, then:
    • add(year, 7) results in era 1, year 4
    • roll(year, 7) results in era 0, year 11
    • roll(year, -7) results in era 1, year 292269057, month July
  • For era 0 the behavior of roll just seems erroneous and should not influence ICU behavior.

Proposal: add or roll for YEAR with a positive value should always move forward in time (as already explicitly documented for ICU Calendar::roll, and as add already behaves in the JDK), and with a negative value should always move backward in time (except when roll results in a wrap). Improve the documentation, e.g. to explain that the delta is not always just added to the value of the calendar field.

Additionally, when roll is called with a year value that would cross era boundaries: Wrap the year if the era has definite bounds at both ends, otherwise pin the year. This will change the ICU behavior.

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