Fix up toPrecision

Author: jessealama

spec.emu

@@ -1019,12 +1019,14 @@ location: https://github.com/tc39/proposal-decimal/
       1. Let _O_ be the *this* value.
       1. Perform ? RequireInternalSlot(_O_, [[Decimal128Data]]).
       1. Let _precision_ be 0.
+      1. Let _roundingMode_ be *undefined*.
       1. If _options_ is not *undefined*, then
         1. If _options_ is not an Object, throw a *TypeError* exception.
         1. Let _optPrecision_ be ? Get(_options_, *"precision"*).
         1. If _optPrecision_ is neither *undefined* nor a Number, throw a *TypeError* exception.
         1. If _optPrecision_ is *undefined*, return Decimal128ToDecimalString(_O_).
         1. If _precision_ is *NaN*<sub>𝔽</sub>, *+∞*<sub>𝔽</sub> or *+∞*<sub>𝔽</sub>, throw a *RangeError* exception.
+        1. Set _roundingMode_ to ? Get(_options_, *"roundingMode"*).
         1. Set _precision_ to ℝ(_precision_).
       1. If _precision_ is not an integer, throw a *RangeError* exception.
       1. If _precision_ < 1, throw a *RangeError* exception.
@@ -1034,33 +1036,33 @@ location: https://github.com/tc39/proposal-decimal/
       1. If _d_ is *-∞*<sub>𝔻</sub>, return *"-Infinity"*.
       1. If sign(_d_) = 1, let _signPrefix_ be *""*, else let _signPrefix_ be *"-"*.
       1. If _d_ is *+0*<sub>𝔻</sub> or *-0*<sub>𝔻</sub>, then
-        1. If _precision_ = 0, then
-          1. Return *"0"*.
+        1. If _precision_ = 1, then
+          1. Return the string concatenation of _signPrefix_ and *"0"*.
+        1. Else,
+          1. Let _additionalZeroes_ be the string *"0"* repeated _precision_ times.
+          1. Return the string concatenation of *"0"*, *"."*, and _additionalZeroes_.
+      1. Let _q_ be the largest integer such that _d_ × 10<sup>−_q_</sup> is an integer.
+      1. Let _n_ be _d_ × 10<sup>−_q_</sup>.
+      1. Let _coefficient_ be the unique decimal representation of _n_ without leading zeroes.
+      1. Let _numDigits_ be the string length of _coefficient_.
+      1. If _numDigits_ < _precision_, then
+        1. Let _s_ be Decimal128ToDecimalString(_d_).
+        1. If _d_ is an integer, then
+          1. Let _additionalZeroes_ be the string *"0"* repeated _precision_ - _numDigits_ times.
+          1. Return the string concatenationof _coefficient_, *"."*, and _additionalZeroes_.
         1. Otherwise:
-          1. Let _extraZeroes_ be the string *"0"* repeated _precision_ times.
-          1. Return the string concatenation of *"0"*, *"."*, and _extraZeroes_.
-      1. Set _d_ to abs(_d_).
-      1. Let _coefficient_ be _d_ × 10<sup>-q</sup>.
-      1. Assert: _coefficient_ is an integer.
-      1. Let _coefficientStr_ be the unique decimal string representation of _coefficient_ without leading zeroes.
-      1. Let _firstDigit_ be the substring of _coefficientStr_ from 0 to 1.
-      1. Let _remainingDigits_ be the substring of _coefficientStr_ from 1.
-      1. Let _numCoefficientDigits_ be the length of _coefficientStr_.
-      1. Let _m_ be the mantissa of _d_.
-      1. Let _scaledM_ be _m_ × 10<sup>_numCoefficientDigits_</sup>.
-      1. Let _fractionalPart_ be _scaledM_ mod 1.
-      1. Let _e_ be the exponent of _d_.
-      1. Let _eStr_ be the unique decimal string representation of _e_ without leading zeroes.
-      1. If _numCoefficientDigits_ ≤ _numCoefficientDigits_, then
-        1. Let _adjustedMantissa_ be _m_ - (_fractionalPart_ × 10<sup>-_numDigits</sup>).
-        1. Assert: _adjustedMantissa_ is an integer.
-        1. Let _adjustedMantissaStr_ be the unique decimal string representation of _adjustedMantissa_ without leading zeroes and without trailing zeroes.
-        1. If _e_ ≥ 0, set _eStr_ to the concation of *"+"* and _eStr_.
-        1. Return the concation of _signPrefix_, _adjustedMantissaStr_, *"e"*, and _eStr_.
+          1. Let _additionalZeroes_ be the string *"0"* repeated _precision_ - _numDigits_ times.
+          1. Return the string concatenationof _signPrefix, _s_ and _additionalZeroes_.
+      1. Else if _numDigits_ = _precision_, then
+        1. Return _coefficient_.
       1. Otherwise:
-        1. Let _extraZeroes_ be the string *"0"* repeated _numCoefficientDigits_ - 1 - _numCoefficientDigits_ times.
-        1. Let _s_ be the string concatenation of _firstDigit_, *"."*, _remainingDigits_, _extraZeroes_, *"e"*, and _eStr_.
-        1. Return _s_.
+        1. Let _exp_ be _q_ + _precision_.
+        1. Let _scaled_ be _d_ × 10<sup>-_exp_</sup>.
+        1. Let _scaledAndRounded_ be ApplyRoundingModeToPositive(_scaled_, _roundingMode_).
+        1. Assert: _scaledAndRounded_ is not an integer.
+        1. Let _s_ be Decimal128ToDecimalString(_scaledAndRounded_).
+        1. Let _renderedQ_ be the unique decimal string representation of _q_.
+        1. Return the string concatenation of _signPrefix_, _s_, *"e"*, and _renderedQ_.
     </emu-alg>
     <emu-note>
       <p>This operation follows the specification of the conversion of IEEE 754-2019 Decimal128 values to strings (external character sequences) discussed in Section 5.12 of <emu-xref href="#sec-bibliography">IEEE 754-2019</emu-xref>.</p>