In a future commit, the "part" view returned from FormatDateTimePattern
may be a view into a string that goes out of scope. Ensure the AO only
returns valid views. A similar approach is used in Intl.NumberFormat.
There are a few FIXMEs that will need to be addressed, but this
implements most of the prototype method. The FIXMEs are mostly related
to range formatting, which has been entirely ignored so far. But other
than that, the following will need to be addressed:
* Determining flexible day periods must be made locale-aware.
* DST will need to be determined and acted upon.
* Time zones other than UTC and calendars other than Gregorian are
ignored.
* Some of our results differ from other engines as they have some
format patterns we do not. For example, they seem to have a lonely
{dayPeriod} pattern, whereas our closest pattern is
"{hour} {dayPeriod}".
Unlike the locale, the data locale has Unicode locale extensions removed
(e.g. the data locale for "en-US-u-ca-gregory" is just "en-US"). Cache
the data locale for LibUnicode lookups during formatting.
This was an oversight in e42d954743.
These fields should always follow the locale preference in the CLDR.
Overriding these fields would permit formats like "h:mm:ss" to result in
strings like "1:2:3" instead of "1:02:03".
TR-35's Matching Skeleton algorithm dictates how user requests including
fractional second digits should be handled when the CLDR format pattern
does not include that field. When the format pattern contains {second},
but does not contain {fractionalSecondDigits}, generate a second pattern
which appends "{decimal}{fractionalSecondDigits}" to the {second} field.
ECMA-402 doesn't explicitly handle a note in the TR-35 spec related to
expanding field lengths based on user-provided options. Instead, it
assumes the "implementation defined" locale data includes the possible
values.
LibUnicode does not generate every possible combination of field lengths
in its implementation of TR-35's "Missing Skeleton Fields", because the
number of generated patterns would grow out of control. Instead, it's
much simpler to handle this difference at runtime.
Other implementations unconditionally initialize [[pattern12]] from
[[pattern]] regardless of whether [[pattern]] has an hour pattern of h11
or h12. LibUnicode does not do this. So when InitializeDateTimeFormat
defaults the hour cycle to the locale's preferred hour cycle, if the
best format didn't have an equivalent hour pattern, [[pattern12]] will
be empty.
This adds plumbing for the Intl.DateTimeFormat object, constructor, and
prototype.
Note that unlike other Intl objects, the Intl.DateTimeFormat object has
a LibUnicode structure as a base. This is to prevent wild amounts of
code duplication between LibUnicode, Intl.DateTimeFormat, and other
not-yet-defined Intl structures, because there's 12 fields shared
between them.
