Editorial: DRY refactor of time formatting

This editorial commit removes redundant spec text dealing with time
formatting:
* Adds a new AO FormatTimeString, and calls it in TemporalTimeToString,
  TemporalDateTimeToString, and TimeString (the legacy date AO in 262).
* Removes FormatSecondsStringPart and replaces it with a new AO
  FormatFractionalSeconds, and call it in FormatTimeString and
  TemporalDurationToString. The text of this new AO is aligned with
  similar text in GetOffsetStringFor in #2607.
* Replaces sub-second formatting text in TemporalDurationToString
  with a call to FormatFractionalSeconds.
* Aligns polyfill code to these spec changes.
* Adjusts polyfill code in a few places to better match the spec.

Note that this commit doesn't touch spec text for formatting time zone
offsets because there are several in-flight PRs dealing with offsets
and I wanted to keep this PR merge-conflict-free. But once those
PRs land and the dust settles, then I'll send another editorial PR to
DRY offset string formatting too, by using FormatTimeString to replace
bespoke formatting text for offsets.

polyfill/lib/ecmascript.mjs

@@ -22,6 +22,7 @@ const StringFromCharCode = String.fromCharCode;
 const StringPrototypeCharCodeAt = String.prototype.charCodeAt;
 const StringPrototypeMatchAll = String.prototype.matchAll;
 const StringPrototypeReplace = String.prototype.replace;
+const StringPrototypeSlice = String.prototype.slice;
 
 import bigInt from 'big-integer';
 import callBound from 'call-bind/callBound';
@@ -2344,56 +2345,52 @@ export function ISOYearString(year) {
   if (year < 0 || year > 9999) {
     let sign = year < 0 ? '-' : '+';
     let yearNumber = MathAbs(year);
-    yearString = sign + `000000${yearNumber}`.slice(-6);
+    yearString = sign + ToZeroPaddedDecimalString(yearNumber, 6);
   } else {
-    yearString = `0000${year}`.slice(-4);
+    yearString = ToZeroPaddedDecimalString(year, 4);
   }
   return yearString;
 }
 
 export function ISODateTimePartString(part) {
-  return `00${part}`.slice(-2);
+  return ToZeroPaddedDecimalString(part, 2);
 }
 
-export function FormatSecondsStringPart(second, millisecond, microsecond, nanosecond, precision) {
-  if (precision === 'minute') return '';
-
-  const secs = `:${ISODateTimePartString(second)}`;
-  let fraction = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
-
+export function FormatFractionalSeconds(subSecondNanoseconds, precision) {
+  let fraction;
   if (precision === 'auto') {
-    if (fraction === 0) return secs;
-    fraction = `${fraction}`.padStart(9, '0');
-    while (fraction[fraction.length - 1] === '0') fraction = fraction.slice(0, -1);
+    if (subSecondNanoseconds === 0) return '';
+    const fractionFullPrecision = ToZeroPaddedDecimalString(subSecondNanoseconds, 9);
+    // now remove any trailing zeroes
+    fraction = Call(StringPrototypeReplace, fractionFullPrecision, [/0+$/, '']);
   } else {
-    if (precision === 0) return secs;
-    fraction = `${fraction}`.padStart(9, '0').slice(0, precision);
+    if (precision === 0) return '';
+    const fractionFullPrecision = ToZeroPaddedDecimalString(subSecondNanoseconds, 9);
+    fraction = Call(StringPrototypeSlice, fractionFullPrecision, [0, precision]);
   }
-  return `${secs}.${fraction}`;
+  return `.${fraction}`;
+}
+
+export function FormatTimeString(hour, minute, second, subSecondNanoseconds, precision) {
+  let result = `${ISODateTimePartString(hour)}:${ISODateTimePartString(minute)}`;
+  if (precision === 'minute') return result;
+
+  result += `:${ISODateTimePartString(second)}`;
+  result += FormatFractionalSeconds(subSecondNanoseconds, precision);
+  return result;
 }
 
 export function TemporalInstantToString(instant, timeZone, precision) {
   let outputTimeZone = timeZone;
   if (outputTimeZone === undefined) outputTimeZone = 'UTC';
   const dateTime = GetPlainDateTimeFor(outputTimeZone, instant, 'iso8601');
-  const year = ISOYearString(GetSlot(dateTime, ISO_YEAR));
-  const month = ISODateTimePartString(GetSlot(dateTime, ISO_MONTH));
-  const day = ISODateTimePartString(GetSlot(dateTime, ISO_DAY));
-  const hour = ISODateTimePartString(GetSlot(dateTime, ISO_HOUR));
-  const minute = ISODateTimePartString(GetSlot(dateTime, ISO_MINUTE));
-  const seconds = FormatSecondsStringPart(
-    GetSlot(dateTime, ISO_SECOND),
-    GetSlot(dateTime, ISO_MILLISECOND),
-    GetSlot(dateTime, ISO_MICROSECOND),
-    GetSlot(dateTime, ISO_NANOSECOND),
-    precision
-  );
+  const dateTimeString = TemporalDateTimeToString(dateTime, precision, 'never');
   let timeZoneString = 'Z';
   if (timeZone !== undefined) {
     const offsetNs = GetOffsetNanosecondsFor(outputTimeZone, instant);
     timeZoneString = FormatDateTimeUTCOffsetRounded(offsetNs);
   }
-  return `${year}-${month}-${day}T${hour}:${minute}${seconds}${timeZoneString}`;
+  return `${dateTimeString}${timeZoneString}`;
 }
 
 function formatAsDecimalNumber(num) {
@@ -2439,20 +2436,11 @@ export function TemporalDurationToString(
     (years === 0 && months === 0 && weeks === 0 && days === 0 && hours === 0 && minutes === 0) ||
     precision !== 'auto'
   ) {
-    const fraction = MathAbs(ms.toJSNumber()) * 1e6 + MathAbs(µs.toJSNumber()) * 1e3 + MathAbs(ns.toJSNumber());
-    let decimalPart = ToZeroPaddedDecimalString(fraction, 9);
-    if (precision === 'auto') {
-      while (decimalPart[decimalPart.length - 1] === '0') {
-        decimalPart = decimalPart.slice(0, -1);
-      }
-    } else if (precision === 0) {
-      decimalPart = '';
-    } else {
-      decimalPart = decimalPart.slice(0, precision);
-    }
-    let secondsPart = seconds.abs().toString();
-    if (decimalPart) secondsPart += `.${decimalPart}`;
-    timePart += `${secondsPart}S`;
+    const secondsPart = formatAsDecimalNumber(seconds.abs());
+    const subSecondNanoseconds =
+      MathAbs(ms.toJSNumber()) * 1e6 + MathAbs(µs.toJSNumber()) * 1e3 + MathAbs(ns.toJSNumber());
+    const subSecondsPart = FormatFractionalSeconds(subSecondNanoseconds, precision);
+    timePart += `${secondsPart}${subSecondsPart}S`;
   }
   let result = `${sign < 0 ? '-' : ''}P${datePart}`;
   if (timePart) result = `${result}T${timePart}`;
@@ -2496,14 +2484,13 @@ export function TemporalDateTimeToString(dateTime, precision, showCalendar = 'au
     ));
   }
 
-  year = ISOYearString(year);
-  month = ISODateTimePartString(month);
-  day = ISODateTimePartString(day);
-  hour = ISODateTimePartString(hour);
-  minute = ISODateTimePartString(minute);
-  const seconds = FormatSecondsStringPart(second, millisecond, microsecond, nanosecond, precision);
+  const yearString = ISOYearString(year);
+  const monthString = ISODateTimePartString(month);
+  const dayString = ISODateTimePartString(day);
+  const subSecondNanoseconds = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
+  const timeString = FormatTimeString(hour, minute, second, subSecondNanoseconds, precision);
   const calendar = MaybeFormatCalendarAnnotation(GetSlot(dateTime, CALENDAR), showCalendar);
-  return `${year}-${month}-${day}T${hour}:${minute}${seconds}${calendar}`;
+  return `${yearString}-${monthString}-${dayString}T${timeString}${calendar}`;
 }
 
 export function TemporalMonthDayToString(monthDay, showCalendar = 'auto') {
@@ -2555,31 +2542,18 @@ export function TemporalZonedDateTimeToString(
 
   const tz = GetSlot(zdt, TIME_ZONE);
   const dateTime = GetPlainDateTimeFor(tz, instant, 'iso8601');
-
-  const year = ISOYearString(GetSlot(dateTime, ISO_YEAR));
-  const month = ISODateTimePartString(GetSlot(dateTime, ISO_MONTH));
-  const day = ISODateTimePartString(GetSlot(dateTime, ISO_DAY));
-  const hour = ISODateTimePartString(GetSlot(dateTime, ISO_HOUR));
-  const minute = ISODateTimePartString(GetSlot(dateTime, ISO_MINUTE));
-  const seconds = FormatSecondsStringPart(
-    GetSlot(dateTime, ISO_SECOND),
-    GetSlot(dateTime, ISO_MILLISECOND),
-    GetSlot(dateTime, ISO_MICROSECOND),
-    GetSlot(dateTime, ISO_NANOSECOND),
-    precision
-  );
-  let result = `${year}-${month}-${day}T${hour}:${minute}${seconds}`;
+  let dateTimeString = TemporalDateTimeToString(dateTime, precision, 'never');
   if (showOffset !== 'never') {
     const offsetNs = GetOffsetNanosecondsFor(tz, instant);
-    result += FormatDateTimeUTCOffsetRounded(offsetNs);
+    dateTimeString += FormatDateTimeUTCOffsetRounded(offsetNs);
   }
   if (showTimeZone !== 'never') {
     const identifier = ToTemporalTimeZoneIdentifier(tz);
     const flag = showTimeZone === 'critical' ? '!' : '';
-    result += `[${flag}${identifier}]`;
+    dateTimeString += `[${flag}${identifier}]`;
   }
-  result += MaybeFormatCalendarAnnotation(GetSlot(zdt, CALENDAR), showCalendar);
-  return result;
+  dateTimeString += MaybeFormatCalendarAnnotation(GetSlot(zdt, CALENDAR), showCalendar);
+  return dateTimeString;
 }
 
 export function IsOffsetTimeZoneIdentifier(string) {

polyfill/lib/plaintime.mjs

@@ -47,10 +47,8 @@ function TemporalTimeToString(time, precision, options = undefined) {
     ));
   }
 
-  hour = ES.ISODateTimePartString(hour);
-  minute = ES.ISODateTimePartString(minute);
-  const seconds = ES.FormatSecondsStringPart(second, millisecond, microsecond, nanosecond, precision);
-  return `${hour}:${minute}${seconds}`;
+  const subSecondNanoseconds = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
+  return ES.FormatTimeString(hour, minute, second, subSecondNanoseconds, precision);
 }
 
 export class PlainTime {

spec/abstractops.html

@@ -680,22 +680,60 @@ <h1>
     </emu-alg>
   </emu-clause>
 
-  <emu-clause id="sec-temporal-formatsecondsstringpart" aoid="FormatSecondsStringPart">
-    <h1>FormatSecondsStringPart ( _second_, _millisecond_, _microsecond_, _nanosecond_, _precision_ )</h1>
+  <emu-clause id="sec-temporal-formatfractionalseconds" type="abstract operation">
+    <h1>
+      FormatFractionalSeconds (
+        _subSecondNanoseconds_: an integer,
+        _precision_: either an integer in the inclusive range 0 to 9 or *"auto"*
+      ): a String
+    </h1>
+    <dl class="header">
+      <dt>description</dt>
+      <dd>
+        If _precision_ is zero, or if _precision_ is *"auto"* and _subSecondNanoseconds_ is zero, then an empty String will be returned.
+        Otherwise, the output will be a decimal point followed by a sequence of fractional seconds digits, either with no trailing zeroes if _precision_ is *"auto"* or truncated to _precision_ digits.
+      </dd>
+    </dl>
     <emu-alg>
-      1. Assert: _second_, _millisecond_, _microsecond_, and _nanosecond_ are integers.
-      1. If _precision_ is *"minute"*, return *""*.
-      1. Let _secondsString_ be the string-concatenation of the code unit 0x003A (COLON) and ToZeroPaddedDecimalString(_second_, 2).
-      1. Let _fraction_ be _millisecond_ &times; 10<sup>6</sup> + _microsecond_ &times; 10<sup>3</sup> + _nanosecond_.
       1. If _precision_ is *"auto"*, then
-        1. If _fraction_ is 0, return _secondsString_.
-        1. Set _fraction_ to ToZeroPaddedDecimalString(_fraction_, 9).
-        1. Set _fraction_ to the longest possible substring of _fraction_ starting at position 0 and not ending with the code unit 0x0030 (DIGIT ZERO).
+        1. If _subSecondNanoseconds_ is 0, return the empty String.
+        1. Let _fractionString_ be ToZeroPaddedDecimalString(_subSecondNanoseconds_, 9).
+        1. Set _fractionString_ to the longest prefix of _fractionString_ ending with a code unit other than 0x0030 (DIGIT ZERO).
       1. Else,
-        1. If _precision_ is 0, return _secondsString_.
-        1. Set _fraction_ to ToZeroPaddedDecimalString(_fraction_, 9).
-        1. Set _fraction_ to the substring of _fraction_ from 0 to _precision_.
-      1. Return the string-concatenation of _secondsString_, the code unit 0x002E (FULL STOP), and _fraction_.
+        1. If _precision_ is 0, return the empty String.
+        1. Let _fractionString_ be ToZeroPaddedDecimalString(_subSecondNanoseconds_, 9).
+        1. Set _fractionString_ to the substring of _fractionString_ from 0 to _precision_.
+      1. Return the string-concatenation of the code unit 0x002E (FULL STOP), and _fractionString_.
+    </emu-alg>
+  </emu-clause>
+
+  <emu-clause id="sec-temporal-formattimestring" type="abstract operation">
+    <h1>
+      FormatTimeString (
+        _hour_: an integer,
+        _minute_: an integer,
+        _second_: an integer,
+        _subSecondNanoseconds_: an integer,
+        _precision_: an integer in the inclusive range 0 to 9, *"minute"*, or *"auto"*
+      ): a String
+    </h1>
+    <dl class="header">
+      <dt>description</dt>
+      <dd>
+        The output will be formatted like ±HH:MM if _precision_ is *"minute"*.
+        Otherwise, the output will be formatted like ±HH:MM:SS if _precision_ is zero, or if _subSecondNanoseconds_ is zero and _precision is *"auto"*.
+        Otherwise, the output will be formatted like ±HH:MM:SS.fff where "fff" is a sequence of fractional seconds digits, either with no trailing zeroes if _precision_ is *"auto"* or truncated to _precision_ digits.
+      </dd>
+    </dl>
+    <emu-alg>
+      1. Let _hh_ be ToZeroPaddedDecimalString(_hour_, 2).
+      1. Let _mm_ be ToZeroPaddedDecimalString(_minute_, 2).
+      1. Let _result_ be the string-concatenation of _hh_, the code unit 0x003A (COLON), and _mm_.
+      1. If _precision_ is *"minute"*, return _result_.
+      1. Let _ss_ be ToZeroPaddedDecimalString(_second_, 2).
+      1. Let _subSecondsPart_ be FormatFractionalSeconds(_subSecondNanoseconds_, _precision_).
+      1. Set _result_ to the string-concatenation of _result_, the code unit 0x003A (COLON), _ss_, and _subSecondsPart_.
+      1. Return _result_.
     </emu-alg>
   </emu-clause>
 

spec/duration.html

@@ -1936,18 +1936,10 @@ <h1>
         1. Let _zeroMinutesAndHigher_ be *false*.
         1. If _years_ = 0, and _months_ = 0, and _weeks_ = 0, and _days_ = 0, and _hours_ = 0, and _minutes_ = 0, set _zeroMinutesAndHigher_ to *true*.
         1. If _nonzeroSecondsAndLower_ is *true*, or _zeroMinutesAndHigher_ is *true*, or _precision_ is not *"auto"*, then
-          1. Let _fraction_ be abs(_milliseconds_) &times; 10<sup>6</sup> + abs(_microseconds_) &times; 10<sup>3</sup> + abs(_nanoseconds_).
-          1. Let _decimalPart_ be ToZeroPaddedDecimalString(_fraction_, 9).
-          1. If _precision_ is *"auto"*, then
-            1. Set _decimalPart_ to the longest possible substring of _decimalPart_ starting at position 0 and not ending with the code unit 0x0030 (DIGIT ZERO).
-          1. Else if _precision_ = 0, then
-            1. Set _decimalPart_ to *""*.
-          1. Else,
-            1. Set _decimalPart_ to the substring of _decimalPart_ from 0 to _precision_.
           1. Let _secondsPart_ be abs(_seconds_) formatted as a decimal number.
-          1. If _decimalPart_ is not *""*, then
-            1. Set _secondsPart_ to the string-concatenation of _secondsPart_, the code unit 0x002E (FULL STOP), and _decimalPart_.
-          1. Set _timePart_ to the string concatenation of _timePart_, _secondsPart_, and the code unit 0x0053 (LATIN CAPITAL LETTER S).
+          1. Let _subSecondNanoseconds_ be abs(_milliseconds_) &times; 10<sup>6</sup> + abs(_microseconds_) &times; 10<sup>3</sup> + abs(_nanoseconds_).
+          1. Let _subSecondsPart_ be FormatFractionalSeconds(_subSecondNanoseconds_, _precision).
+          1. Set _timePart_ to the string concatenation of _timePart_, _secondsPart_, _subSecondsPart_, and the code unit 0x0053 (LATIN CAPITAL LETTER S).
         1. Let _signPart_ be the code unit 0x002D (HYPHEN-MINUS) if _sign_ &lt; 0, and otherwise the empty String.
         1. Let _result_ be the string concatenation of _signPart_, the code unit 0x0050 (LATIN CAPITAL LETTER P) and _datePart_.
         1. If _timePart_ is not *""*, then

spec/mainadditions.html

@@ -415,6 +415,25 @@ <h1>
 
     <p>[...]</p>
 
+    <emu-clause id="sec-timestring" type="abstract operation">
+      <h1>
+        TimeString (
+          _tv_: a Number, but not *NaN*,
+        ): a String
+      </h1>
+      <dl class="header">
+      </dl>
+      <emu-alg>
+        1. <del>Let _hour_ be ToZeroPaddedDecimalString(ℝ(HourFromTime(_tv_)), 2).</del>
+        1. <del>Let _minute_ be ToZeroPaddedDecimalString(ℝ(MinFromTime(_tv_)), 2).</del>
+        1. <del>Let _second_ be ToZeroPaddedDecimalString(ℝ(SecFromTime(_tv_)), 2).</del>
+        1. <ins>Let _timeString_ be FormatTimeString(ℝ(HourFromTime(_tv_)), ℝ(MinFromTime(_tv_)), ℝ(SecFromTime(_tv_)), 0, 0).</ins>
+        1. Return the string-concatenation of _timeString_, the code unit 0x0020 (SPACE), and *"GMT"*.
+      </emu-alg>
+    </emu-clause>
+
+    <p>[...]</p>
+
     <emu-clause id="sec-timezoneestring" type="abstract operation">
       <h1>
         TimeZoneString (

spec/plaindatetime.html

@@ -993,14 +993,13 @@ <h1>
       <h1>TemporalDateTimeToString ( _isoYear_, _isoMonth_, _isoDay_, _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_, _calendar_, _precision_, _showCalendar_ )</h1>
       <emu-alg>
         1. Assert: _isoYear_, _isoMonth_, _isoDay_, _hour_, _minute_, _second_, _millisecond_, _microsecond_, and _nanosecond_ are integers.
-        1. Let _year_ be ! PadISOYear(_isoYear_).
-        1. Let _month_ be ToZeroPaddedDecimalString(_isoMonth_, 2).
-        1. Let _day_ be ToZeroPaddedDecimalString(_isoDay_, 2).
-        1. Let _hour_ be ToZeroPaddedDecimalString(_hour_, 2).
-        1. Let _minute_ be ToZeroPaddedDecimalString(_minute_, 2).
-        1. Let _seconds_ be ! FormatSecondsStringPart(_second_, _millisecond_, _microsecond_, _nanosecond_, _precision_).
+        1. Let _yearString_ be ! PadISOYear(_isoYear_).
+        1. Let _monthString_ be ToZeroPaddedDecimalString(_isoMonth_, 2).
+        1. Let _dayString_ be ToZeroPaddedDecimalString(_isoDay_, 2).
+        1. Let _subSecondNanoseconds_ be _millisecond_ &times; 10<sup>6</sup> + _microsecond_ &times; 10<sup>3</sup> + _nanosecond_.
+        1. Let _timeString_ be FormatTimeString(_hour_, _minute_, _second_, _subSecondNanoseconds_, _precision_).
         1. Let _calendarString_ be ? MaybeFormatCalendarAnnotation(_calendar_, _showCalendar_).
-        1. Return the string-concatenation of _year_, the code unit 0x002D (HYPHEN-MINUS), _month_, the code unit 0x002D (HYPHEN-MINUS), _day_, 0x0054 (LATIN CAPITAL LETTER T), _hour_, the code unit 0x003A (COLON), _minute_, _seconds_, and _calendarString_.
+        1. Return the string-concatenation of _yearString_, the code unit 0x002D (HYPHEN-MINUS), _monthString_, the code unit 0x002D (HYPHEN-MINUS), _dayString_, 0x0054 (LATIN CAPITAL LETTER T), _timeString_, and _calendarString_.
       </emu-alg>
     </emu-clause>
 

spec/plaintime.html

@@ -813,10 +813,8 @@ <h1>
       <h1>TemporalTimeToString ( _hour_, _minute_, _second_, _millisecond_, _microsecond_, _nanosecond_, _precision_ )</h1>
       <emu-alg>
         1. Assert: _hour_, _minute_, _second_, _millisecond_, _microsecond_ and _nanosecond_ are integers.
-        1. Let _hour_ be ToZeroPaddedDecimalString(_hour_, 2).
-        1. Let _minute_ be ToZeroPaddedDecimalString(_minute_, 2).
-        1. Let _seconds_ be ! FormatSecondsStringPart(_second_, _millisecond_, _microsecond_, _nanosecond_, _precision_).
-        1. Return the string-concatenation of _hour_, the code unit 0x003A (COLON), _minute_, and _seconds_.
+        1. Let _subSecondNanoseconds_ be _millisecond_ &times; 10<sup>6</sup> + _microsecond_ &times; 10<sup>3</sup> + _nanosecond_.
+        1. Return FormatTimeString(_hour_, _minute_, _second_, _subSecondNanoseconds_, _precision_).
       </emu-alg>
     </emu-clause>