Editorial: Combine chinese/dangi and other reference year clauses

After having fixed the assertion failures etc. in the previous commits,
these two clauses look a lot more alike. It's possible to combine them,
only special-casing two things for chinese/dangi: the maximum number 30
of days in month, and the extra lookup in the table to handle rare
month codes.

Closes: #123

Author: ptomato

spec.emu

@@ -1311,45 +1311,29 @@ contributors: Google, Ecma International
           1. Let _month_ be _monthsInYear_.
         1. Else,
           1. Let _month_ be _fields_.[[Month]].
+        1. If _fields_.[[Year]] is not ~unset~, then
+          1. [declared="isoDate"] If there exists no combination of inputs such that ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], ..., ...) would return an ISO Date Record _isoDate_ for which ISODateWithinLimits(_isoDate_) is *true*, throw a *RangeError* exception.
+          1. NOTE: The above step exists so as not to require calculating whether the month and day described in _fields_ exist in user-provided years arbitrarily far in the future or past.
+          1. If _monthCode_ is ~unset~, then
+            1. Let _fieldsISODate_ be ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], _month_, 1).
+            1. Set _monthCode_ to NonISOCalendarISOToDate(_calendar_, _fieldsISODate_).[[MonthCode]].
+          1. Else,
+            1. Set _monthCode_ to ? ConstrainMonthCode(_calendar_, _fields_.[[Year]], _monthCode_, _overflow_).
+          1. Let _daysInMonth_ be CalendarDaysInMonth(_calendar_, _fields_.[[Year]], _month_).
+        1. Else,
+          1. Assert: _monthCode_ is not ~unset~.
+          1. If _calendar_ is *"chinese"* or *"dangi"*, let _daysInMonth_ be 30; else, let _daysInMonth_ be the maximum number of days in the month described by _monthCode_ in any year.
+        1. If _fields_.[[Day]] > _daysInMonth_, then
+          1. If _overflow_ is ~reject~, throw a *RangeError* exception.
+          1. Let _day_ be _daysInMonth_.
+        1. Else,
+          1. Let _day_ be _fields_.[[Day]].
         1. If _calendar_ is *"chinese"* or *"dangi"*, then
           1. NOTE: This special case handles combinations of month and day that theoretically could occur but are not known to have occurred historically and cannot be accurately calculated to occur in the future, even if it may be possible to construct a PlainDate with such combinations due to inaccurate approximations. This is explicitly mentioned here because as time goes on, these dates may become known to have occurred historically, or may be more accurately calculated to occur in the future.
-          1. If _fields_.[[Year]] is not ~unset~, then
-            1. [declared="isoDate"] If there exists no combination of inputs such that ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], ..., ...) would return an ISO Date Record _isoDate_ for which ISODateWithinLimits(_isoDate_) is *true*, throw a *RangeError* exception.
-            1. NOTE: The above step exists so as not to require calculating whether the month and day described in _fields_ exist in user-provided years arbitrarily far in the future or past.
-            1. If _monthCode_ is ~unset~, then
-              1. Let _fieldsISODate_ be ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], _month_, 1).
-              1. Set _monthCode_ to NonISOCalendarISOToDate(_calendar_, _fieldsISODate_).[[MonthCode]].
-            1. Else,
-              1. Set _monthCode_ to ? ConstrainMonthCode(_calendar_, _fields_.[[Year]], _monthCode_, _overflow_).
-            1. Let _daysInMonth_ be CalendarDaysInMonth(_calendar_, _fields_.[[Year]], _month_).
-          1. Else,
-            1. Let _daysInMonth_ be 30.
-          1. If _fields_.[[Day]] > _daysInMonth_, then
-            1. If _overflow_ is ~reject~, throw a *RangeError* exception.
-            1. Let _day_ be _daysInMonth_.
-          1. Else,
-            1. Let _day_ be _fields_.[[Day]].
           1. Let _row_ be the row in <emu-xref href="#chinese-dangi-iso-reference-years"></emu-xref> with a value in the "Month Code" column matching _monthCode_.
           1. If the "Reference Year (Days 1-29)" column of _row_ is "—", or _day_ = 30 and the "Reference Year (<emu-not-ref>Day</emu-not-ref> 30)" column of _row_ is "—", then
             1. If _overflow_ is ~reject~, throw a *RangeError* exception.
             1. Set _monthCode_ to CreateMonthCode(! ParseMonthCode(_monthCode_).[[MonthNumber]], *false*).
-        1. Else,
-          1. If _fields_.[[Year]] is not ~unset~, then
-            1. If there exists no combination of inputs such that ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], ..., ...) would return an ISO Date Record _isoDate_ for which ISODateWithinLimits(_isoDate_) is *true*, throw a *RangeError* exception.
-            1. If _monthCode_ is ~unset~, then
-              1. Let _fieldsISODate_ be ! CalendarIntegersToISO(_calendar_, _fields_.[[Year]], _month_, 1).
-              1. Set _monthCode_ to NonISOCalendarISOToDate(_calendar_, _fieldsISODate_).[[MonthCode]].
-            1. Else,
-              1. Set _monthCode_ to ? ConstrainMonthCode(_calendar_, _fields_.[[Year]], _monthCode_, _overflow_).
-            1. Let _daysInMonth_ be CalendarDaysInMonth(_calendar_, _fields_.[[Year]], _month_).
-          1. Else,
-            1. Assert: _monthCode_ is not ~unset~.
-            1. Let _daysInMonth_ be the maximum number of days in the month described by _monthCode_ in any year.
-          1. If _fields_.[[Day]] > _daysInMonth_, then
-            1. If _overflow_ is ~reject~, throw a *RangeError* exception.
-            1. Let _day_ be _daysInMonth_.
-          1. Else,
-            1. Let _day_ be _fields_.[[Day]].
         1. Let _referenceYear_ be the ISO reference year for _monthCode_ and _day_ as described above. If _calendar_ is *"chinese"* or *"dangi"*, the reference years in <emu-xref href="#chinese-dangi-iso-reference-years"></emu-xref> are to be used.
         1. Return the latest possible ISO Date Record _isoDate_ such that _isoDate_.[[Year]] = _referenceYear_ and NonISOCalendarISOToDate(_calendar_, _isoDate_) returns a Calendar Date Record whose [[MonthCode]] and [[Day]] field values respectively equal _monthCode_ and _day_.
       </emu-alg>