toString: Fix bug in emitting exponential notation

intl.emu

@@ -152,7 +152,7 @@
         </dd>
       </dl>
       <emu-alg>
-        1. <ins>If _value_ has the [[Value]] and [[FractionDigits]] internal slots, let _primValue_ be RenderAmountValueWithFractionDigits(_value_.[[Value]], _value_.[[FractionDigits]]) else</ins><del>Let</del> <ins>let</ins _primValue_ be ? ToPrimitive(_value_, ~number~).
+        1. <ins>If _value_ has the [[Value]] and [[FractionDigits]] internal slots, let _primValue_ be MVtoDecimalString(_value_.[[Value]], _value_.[[FractionDigits]]) else</ins><del>Let</del> <ins>let</ins _primValue_ be ? ToPrimitive(_value_, ~number~).
         1. If _primValue_ is a BigInt, return ℝ(_primValue_).
         1. If _primValue_ is a String, then
           1. Let _str_ be _primValue_.

spec.emu

@@ -20,6 +20,92 @@ location: https://github.com/tc39/proposal-amount/
   </ul>
 </emu-intro>
 
+<emu-clause id="sec-numbers-and-dates">
+  <h1>Numbers and Dates</h1>
+  <emu-clause id="sec-number-objects">
+    <h1>Number Objects</h1>
+    <emu-clause id="sec-properties-of-the-number-prototype-object">
+      <h1>Properties of the Number Prototype Object</h1>
+    </emu-clause>
+      <emu-clause id="sec-number.prototype.tofixed">
+        <h1>Number.prototype.toFixed ( _fractionDigits_ )</h1>
+        <emu-note>
+          <p>This method returns a String containing this Number value represented in decimal fixed-point notation with _fractionDigits_ digits after the decimal point. If _fractionDigits_ is *undefined*, 0 is assumed.</p>
+        </emu-note>
+        <p>It performs the following steps when called:</p>
+        <emu-alg>
+          1. Let _x_ be ? ThisNumberValue(*this* value).
+          1. Let _f_ be ? ToIntegerOrInfinity(_fractionDigits_).
+          1. Assert: If _fractionDigits_ is *undefined*, then _f_ is 0.
+          1. If _f_ is not finite, throw a *RangeError* exception.
+          1. If _f_ &lt; 0 or _f_ > 100, throw a *RangeError* exception.
+          1. If _x_ is not finite, return Number::toString(_x_, 10).
+          1. <ins>Return MVtoDecimalString(ℝ(_x_), _f_).</ins>
+          1. <del>Set _x_ to ℝ(_x_).</del>
+          1. <del>Let _s_ be the empty String.</del>
+          1. <del>If _x_ &lt; 0, then</del>
+            1. <del>Set _s_ to *"-"*.</del>
+            1. <del>Set _x_ to -_x_.</del>
+          1. <del>If _x_ ≥ 10<sup>21</sup>, then</del>
+            1. <del>Let _m_ be ! ToString(𝔽(_x_)).</del>
+          1. <del>Else,</del>
+            1. <del>Let _n_ be an integer for which _n_ / 10<sup>_f_</sup> - _x_ is as close to zero as possible. If there are two such _n_, pick the larger _n_.</del>
+            1. <del>If _n_ = 0, let _m_ be *"0"*. Otherwise, let _m_ be the String value consisting of the digits of the decimal representation of _n_ (in order, with no leading zeroes).</del>
+            1. <del>If _f_ ≠ 0, then</del>
+              1. <del>Let _k_ be the length of _m_.</del>
+              1. <del>If _k_ ≤ _f_, then</del>
+                1. <del>Let _z_ be the String value consisting of _f_ + 1 - _k_ occurrences of the code unit 0x0030 (DIGIT ZERO).</del>
+                1. <del>Set _m_ to the string-concatenation of _z_ and _m_.</del>
+                1. <del>Set _k_ to _f_ + 1.</del>
+              1. <del>Let _a_ be the first _k_ - _f_ code units of _m_.</del>
+              1. <del>Let _b_ be the other _f_ code units of _m_.</del>
+              1. <del>Set _m_ to the string-concatenation of _a_, *"."*, and _b_.</del>
+          1. <del>Return the string-concatenation of _s_ and _m_.</del>
+        </emu-alg>
+        <emu-note>
+          <p>The output of `toFixed` may be more precise than `toString` for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,</p>
+          <p>
+            `(1000000000000000128).toString()` returns *"1000000000000000100"*, while<br>
+            `(1000000000000000128).toFixed(0)` returns *"1000000000000000128"*.
+          </p>
+        </emu-note>
+        <emu-clause id="sec-mvtodecimalstring" type="abstract operation">
+          <h1>
+            MVtoDecimalString (
+              _x_: a mathematical value,
+              _fractionDigits_ : a non-negative integer
+            ): a String
+          </h1>
+          <dl class="header">
+            <dt>description</dt>
+            <dd>It renders the given Intl mathematical value with a given number of fractional digits</dd>
+          </dl>
+          <emu-alg>
+            1. Let _s_ be the empty String.
+            1. If _x_ &lt; 0, then
+              1. Set _s_ to *"-"*.
+              1. Set _x_ to -_x_.
+            1. If _x_ ≥ 10<sup>21</sup>, then
+              1. Let _m_ be ! ToString(𝔽(_x_)).
+            1. Else,
+              1. Let _n_ be an integer for which _n_ / 10<sup>_fractionDigits_</sup> - _x_ is as close to zero as possible. If there are two such _n_, pick the larger _n_.
+              1. If _n_ = 0, let _m_ be *"0"*. Otherwise, let _m_ be the String value consisting of the digits of the decimal representation of _n_ (in order, with no leading zeroes).
+              1. If _fractionDigits_ ≠ 0, then
+                1. Let _k_ be the length of _m_.
+                1. If _k_ ≤ _fractionDigits_, then
+                  1. Let _z_ be the String value consisting of _fractionDigits_ + 1 - _k_ occurrences of the code unit 0x0030 (DIGIT ZERO).
+                  1. Set _m_ to the string-concatenation of _z_ and _m_.
+                  1. Set _k_ to _fractionDigits_ + 1.
+                1. Let _a_ be the first _k_ - _fractionDigits_ code units of _m_.
+                1. Let _b_ be the other _fractionDigits_ code units of _m_.
+                1. Set _m_ to the string-concatenation of _a_, *"."*, and _b_.
+            1. Return the string-concatenation of _s_ and _m_.
+          </emu-alg>
+        </emu-clause>
+      </emu-clause>
+  </emu-clause>
+</emu-clause>
+
 <emu-clause id="sec-the-amount-object">
   <h1>The Amount Object</h1>
   <emu-intro id="sec-amount-intro">
@@ -262,79 +348,33 @@ location: https://github.com/tc39/proposal-amount/
         </emu-alg>
       </emu-clause>
 
-      <emu-clause id="sec-amount-rendermvwithfractiondigits" type="abstract operation">
-        <h1>RenderAmountValueWithFractionDigits (
-          _v_: an Intl mathematical value,
-          _numDigits_: an integer,
+      <emu-clause id="sec-amount-roundtosignificantdigits" type="abstract operation">
+        <h1>RoundToSignificantDigits (
+          _v_: a mathematical value,
+          _n_: a non-negative integer,
           optional _roundingMode_: a rounding mode
-        ): a String
+        ): a mathematical value
         </h1>
         <dl class="header">
           <dt>description</dt>
-          <dd>It renders the given Intl mathematical value with a given number of fractional digits, rounding, if necessary, using the given rounding mode, which, if missing, is *"halfEven"*.</dd>
+          <dd>It computes the closest approximation to a given mathematical value that has at most the given number of significant digits, rounding (if necessary) according to the given rounding mode.</dd>
         </dl>
         <emu-alg>
-          1. If _v_ is ~not-a-number~, return *"NaN"*.
-          1. If _v_ is ~negative-infinity~, return *"-Infinity"*.
-          1. If _v_ is ~positive-infinity~, return *"Infinity"*.
-          1. If _v_ is ~minus-zero~, then
-            1. If _numDigits_ < 0, then
-              1. Let _exp_ be the unique string representation of -_numDigits_ in base 10 without duplicate leading zeros.
-              1. Return the string-concatenation of *"0"*, *"e"*, and _exp_.
-            1. Otherwise:
-              1. Let _trailingZeroes_ be *"0"* repeated _numDigits_ times.
-              1. Let _z_ be *"-0"*.
-              1. If _numDigits_ = 0, return _z_.
-              1. Otherwise, return the string-concatenation of _z_, *"."* and _trailingZeroes_.
           1. If _roundingMode_ is *undefined*, set _roundingMode_ to *"halfEven"*.
-          1. If _v_ < 0, let _prefix_ be *"-"*, else let _prefix_ be "".
-          1. If _v_ < 0, then
-            1. Set _v_ to -_v_.
-            1. Set _roundingMode_ to ReverseRoundingMode(_roundingMode_).
-          1. Let _rounded_ be ApplyRoundingModeToPositive(_v_ × 10<sup>_numDigits_</sup>, _roundingMode_).
-          1. Let _e_ be the smallest non-negative integer such that _rounded_ × 10<sup>-_numDigits_</sup> is an integer.
-          1. Let _s_ be the unique decimal string representation of _rounded_ without duplicate leading zeroes.
-          1. If _numDigits_ > _e_, then
-            1. If _v_ is an integer, return the string-concatenation of _prefix, _s_, *"."*, and *"0"* repeated _numDigits_ times.
-            1. Otherwise, return the string-concatenation of _prefix_, _s_ and *"0"* repeated _numDigits_ - _e_ times.
-          1. If 0 < _numDigits_, then
-            1. Return the string-concatenation of _prefix, _s_ and *"0"* repeated _e_ - _numDigits_ times.
-          1. Otherwise:
-            1. Assert: _rounded_ is an integer.
-            1. Let _firstDigit_ be the substring of _s_ from 0 to 1.
-            1. Let _p_ be the unique integer such that 1 ≤ _rounded_ × 10<sup>_p_</sup> < 10.
-            1. Let _n_ be _p_ + _numDigits_.
-            1. Assert: _n_ ≥ 0.
-            1. Let _exp_ be the unique string representation of _n_ in base-10 without duplicate leading zeros.
-            1. If _n_ = 0, then
-              1. Return the string-concatenation of _s_, *"e"*, and _exp_.
-            1. Otherwise:
-              1. Let _remainingDigits_ be the substring of _s_ from 1 to _n_.
-              1. Return the string-concatenation of _firstDigit_, *"."*, _remainingDigits_, *"e"*, and _exp_.
-        </emu-alg>
-      </emu-clause>
-
-      <emu-clause id="sec-amount-FractionToSignificantDigits" type="abstract operation">
-        <h1>FractionToSignificantDigits (
-          _value_: an Intl mathematical value,
-          _fractionDigits_: an integer
-        ): a non-negative integer
-        </h1>
-        <dl class="header"></dl>
-        <emu-alg>
-          1. If _value_ is one of ~not-a-number~, ~positive-infinity~, or ~negative-infinity~, then
-            1. Return 0.
-          1. If _value_ is one of 0 or ~negative-zero~, then
-            1. If _fractionDigits_ < 0, return 1.
-            1. Let _integerDigits_ be 1.
-          1. Else,
-            1. Let _integerDigits_ be the smallest integer such that 10<sup>_integerDigits_</sup> > abs(_value_).
-          1. Assert: _integerDigits_ + _fractionDigits_ > 0.
-          1. Return _integerDigits_ + _fractionDigits_.
+          1. If _v_ = 0, return 0.
+          1. If v &lt; 0, then
+            1. Let _reverseRoundingMode_ be ReverseRoundingMode(_roundingMode_).
+            1. Let _d_ be RoundToSignificantDigits(–_v_, _n_, _reverseRoundingMode_).
+            1. Return –_d_.
+          1. Let _e_ be the unique integer such that 10<sup>_e_</sup> ≤ _v_ < 10<sup>_e_+1</sup>.
+          1. Let _pow_ be _e_ - _n_.
+          1. Let _m_ be _v_ × 10<sup>–_pow_</sup>.
+          1. Let _rounded_ be ApplyRoundingModeToPositive(_m_, _roundingMode_).
+          1. Return _rounded_ × 10<sup>_n_ + _e_</sup>.
         </emu-alg>
       </emu-clause>
 
-      <emu-clause id="sec-amount-SignificantToFractionDigits" type="abstract operation">
+      <emu-clause id="sec-amount-significanttofractiondigits" type="abstract operation">
         <h1>SignificantToFractionDigits (
           _value_: an Intl mathematical value,
           _significantDigits_: a positive integer
@@ -481,7 +521,7 @@ location: https://github.com/tc39/proposal-amount/
       1. Let _u_ be _O_.[[Unit]].
       1. Let _v_ be _O_.[[Value]].
       1. Let _fractionDigits_ be _O_.[[FractionDigits]].
-      1. Let _s_ be RenderAmountValueWithFractionDigits(_v_, _fractionDigits_).
+      1. Let _s_ be MVtoDecimalString(_v_, _fractionDigits_).
       1. If _displayUnit_ is *"never"*, return _s_.
       1. If _u_ is *undefined*, then
         1. If _displayUnit_ is *"always"*, return the string-concatenation of _s_ and *"[1]"*.