Editorial: Simplify DifferenceISODate

Similar to DaysUntil, here we use the already-existing MakeDay operation
in ECMA-262 to convert ISO year/month/day to days since epoch, which can
be subtracted directly.

The algorithm is essentially doing what it already did: calculating a
number of days, and converting to weeks if necessary. However, it now does
so without having to swap the order of the dates and negate the result if
the second is earlier than the first.

(Unfortunately, the first half of DifferenceISODate, to compute the
difference in months, is still hard-to-read. I'll keep looking for a way
to simplify this.)

Author: ptomato

spec/calendar.html

@@ -890,7 +890,7 @@ <h1>Temporal.Calendar.prototype.dateUntil ( _one_, _two_ [ , _options_ ] )</h1>
         1. Set _two_ to ? ToTemporalDate(_two_).
         1. Set _options_ to ? GetOptionsObject(_options_).
         1. Let _largestUnit_ be ? ToLargestTemporalUnit(_options_, « *"hour"*, *"minute"*, *"second"*, *"millisecond"*, *"microsecond"*, *"nanosecond"* », *"auto"*, *"day"*).
-        1. Let _result_ be ! DifferenceISODate(_one_.[[ISOYear]], _one_.[[ISOMonth]], _one_.[[ISODay]], _two_.[[ISOYear]], _two_.[[ISOMonth]], _two_.[[ISODay]], _largestUnit_).
+        1. Let _result_ be DifferenceISODate(_one_.[[ISOYear]], _one_.[[ISOMonth]], _one_.[[ISODay]], _two_.[[ISOYear]], _two_.[[ISOMonth]], _two_.[[ISODay]], _largestUnit_).
         1. Return ! CreateTemporalDuration(_result_.[[Years]], _result_.[[Months]], _result_.[[Weeks]], _result_.[[Days]], 0, 0, 0, 0, 0, 0).
       </emu-alg>
     </emu-clause>

spec/intl.html

@@ -1713,7 +1713,7 @@ <h1>Temporal.Calendar.prototype.dateUntil ( _one_, _two_ [ , _options_ ] )</h1>
             1. Set _options_ to ? GetOptionsObject(_options_).
             1. Let _largestUnit_ be ? ToLargestTemporalUnit(_options_, « *"hour"*, *"minute"*, *"second"*, *"millisecond"*, *"microsecond"*, *"nanosecond"* », *"auto"*, *"day"*).
             1. If _calendar_.[[Identifier]] is *"iso8601"*, then
-              1. Let _result_ be ! DifferenceISODate(_one_.[[ISOYear]], _one_.[[ISOMonth]], _one_.[[ISODay]], _two_.[[ISOYear]], _two_.[[ISOMonth]], _two_.[[ISODay]], _largestUnit_).
+              1. Let _result_ be DifferenceISODate(_one_.[[ISOYear]], _one_.[[ISOMonth]], _one_.[[ISODay]], _two_.[[ISOYear]], _two_.[[ISOMonth]], _two_.[[ISODay]], _largestUnit_).
             1. Else,
               1. Let _result_ be ! CalendarDateDifference(_calendar_.[[Identifier]], _one_, _two_, _largestUnit_).
               1. Assert: ! IsValidDuration(_result_.[[Years]], _result_.[[Months]], _result_.[[Weeks]], _result_.[[Days]], 0, 0, 0, 0, 0, 0) is *true*.

spec/plaindate.html

@@ -707,18 +707,17 @@ <h1>
           _y2_: an integer,
           _m2_: an integer,
           _d2_: an integer,
-          _largestUnit_: a String,
-        )
+          _largestUnit_: *"year"*, *"month"*, *"week"*, or *"day"*,
+        ): a Date Duration Record
       </h1>
       <dl class="header">
         <dt>description</dt>
         <dd>
-          It returns a Date Duration Record with the elapsed duration from a first date until a second date, according to the reckoning of the ISO 8601 calendar.
+          The return value is the elapsed duration from a first date until a second date, according to the reckoning of the ISO 8601 calendar.
           No fields larger than _largestUnit_ will be non-zero in the resulting Date Duration Record.
         </dd>
       </dl>
       <emu-alg>
-        1. Assert: _largestUnit_ is one of *"year"*, *"month"*, *"week"*, or *"day"*.
         1. If _largestUnit_ is *"year"* or *"month"*, then
           1. Let _sign_ be -(! CompareISODate(_y1_, _m1_, _d1_, _y2_, _m2_, _d2_)).
           1. If _sign_ is 0, return ! CreateDateDurationRecord(0, 0, 0, 0).
@@ -756,24 +755,16 @@ <h1>
             1. Set _years_ to 0.
           1. Return ! CreateDateDurationRecord(_years_, _months_, 0, _days_).
         1. If _largestUnit_ is *"day"* or *"week"*, then
-          1. If ! CompareISODate(_y1_, _m1_, _d1_, _y2_, _m2_, _d2_) &lt; 0, then
-            1. Let _smaller_ be the Record { [[Year]]: _y1_, [[Month]]: _m1_, [[Day]]: _d1_ }.
-            1. Let _greater_ be the Record { [[Year]]: _y2_, [[Month]]: _m2_, [[Day]]: _d2_ }.
-            1. Let _sign_ be 1.
-          1. Else,
-            1. Let _smaller_ be the Record { [[Year]]: _y2_, [[Month]]: _m2_, [[Day]]: _d2_ }.
-            1. Let _greater_ be the Record { [[Year]]: _y1_, [[Month]]: _m1_, [[Day]]: _d1_ }.
-            1. Let _sign_ be -1.
-          1. Let _days_ be ! ToISODayOfYear(_greater_.[[Year]], _greater_.[[Month]], _greater_.[[Day]]) - ! ToISODayOfYear(_smaller_.[[Year]], _smaller_.[[Month]], _smaller_.[[Day]]).
-          1. Let _year_ be _smaller_.[[Year]].
-          1. Repeat, while _year_ &lt; _greater_.[[Year]],
-            1. Set _days_ to _days_ + ! ISODaysInYear(_year_).
-            1. Set _year_ to _year_ + 1.
+          1. Let _epochDays1_ be MakeDay(𝔽(_y1_), 𝔽(_m1_ - 1), 𝔽(_d1_)).
+          1. Assert: _epochDays1_ is finite.
+          1. Let _epochDays2_ be MakeDay(𝔽(_y2_), 𝔽(_m2_ - 1), 𝔽(_d2_)).
+          1. Assert: _epochDays2_ is finite.
+          1. Let _days_ be ℝ(_epochDays2_) - ℝ(_epochDays1_).
           1. Let _weeks_ be 0.
           1. If _largestUnit_ is *"week"*, then
-            1. Set _weeks_ to floor(_days_ / 7).
-            1. Set _days_ to _days_ modulo 7.
-          1. Return ! CreateDateDurationRecord(0, 0, _weeks_ &times; _sign_, _days_ &times; _sign_).
+            1. Set _weeks_ to RoundTowardsZero(_days_ / 7).
+            1. Set _days_ to remainder(_days_, 7).
+          1. Return ! CreateDateDurationRecord(0, 0, _weeks_, _days_).
       </emu-alg>
     </emu-clause>