Use significant digits only on the way in and out

Author: nicolo-ribaudo

spec.emu

@@ -262,32 +262,6 @@ location: https://github.com/tc39/proposal-amount/
         </emu-alg>
       </emu-clause>
 
-      <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 mathematical value
-        </h1>
-        <dl class="header">
-          <dt>description</dt>
-          <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 _roundingMode_ is *undefined*, set _roundingMode_ to *"halfEven"*.
-          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-rendermvwithfractiondigits" type="abstract operation">
         <h1>RenderAmountValueWithFractionDigits (
           _v_: an Intl mathematical value,
@@ -340,6 +314,46 @@ location: https://github.com/tc39/proposal-amount/
         </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. Let _integerDigitsMinusOne_ be 0.
+          1. Else
+            1. Let _e_ be the smallest integer such that _e_ > the base 10 logarithm of abs(_value_).
+            1. Let _integerDigits_ be 1 + _e_.
+          1. Assert: _integerDigits_ + _fractionDigits_ > 0.
+          1. Return _integerDigits_ + _fractionDigits_.
+        </emu-alg>
+      </emu-clause>
+
+      <emu-clause id="sec-amount-SignificantToFractionDigits" type="abstract operation">
+        <h1>SignificantToFractionDigits (
+          _value_: an Intl mathematical value,
+          _significantDigits_: an integer
+        ): an 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. Let _integerDigits_ be 1.
+          1. Else
+            1. Let _e_ be the smallest integer such that _e_ > the base 10 logarithm of abs(_value_).
+            1. Let _integerDigits_ be 1 + _e_.
+          1. Assert: _significantDigits_ > 0.
+          1. Return _significantDigits_ - _integerDigits_.
+        </emu-alg>
+      </emu-clause>
+
       <emu-clause id="sec-amount-getamountoptions" type="abstract operation">
         <h1>GetAmountOptions (
           _opts_: an Object
@@ -389,47 +403,26 @@ location: https://github.com/tc39/proposal-amount/
         1. Let _parsed_ be ParseText(_toParse_, |StringNumericLiteral|).
         1. If _parsed_ is a List of errors, then
           1. Let _amountValue_ be ~not-a-number~.
-          1. Let _numDigits_ be 0.
         1. Else,
           1. Let _intlMV_ be the StringIntlMV of _parsed_.
           1. Let _amountValue_ be _intlMV_[0].
-          1. Let _numDigits_ be _intlMV_[1].
         1. Let _validatedOpts_ be ? GetAmountOptions(_opts_).
-        1. Let _fractionDigits_ be _validatedOpts_.[[FractionDigits]].
         1. Let _roundingMode_ be _validatedOpts_.[[RoundingMode]].
+        1. Let _fractionDigits_ be _validatedOpts_.[[FractionDigits]].
         1. Let _significantDigits_ be _validatedOpts_.[[SignificantDigits]].
         1. Let _unit_ be _validatedOpts_.[[Unit]].
-        1. Let _O_ be OrdinaryObjectCreate(%Amount.prototype%, « [[FractionDigits]], [[SignificantDigits]], [[Unit]], [[Value]] »).
-        1. If _amountValue_ is a mathematical value, then
-          1. Let _roundedValue_ be _amountValue_.
-          1. If both _significantDigits_ and _fractionDigits_ are *undefined*, then
-            1. Set _significantDigits_ to _numDigits_.
+        1. Let _O_ be OrdinaryObjectCreate(%Amount.prototype%, « [[FractionDigits]], [[Unit]], [[Value]] »).
+        1. If _fractionDigits_ is *undefined*, then
+          1. If _significantDigits_ is not *undefined*, then
+            1. Set _fractionDigits_ to SignificantToFractionDigits(_amountValue_, _significantDigits_).
+          1. Else if _amountValue_ is a mathematical value or ~negative-zero~, then
             1. Set _fractionDigits_ to the CountFractionDigits of _toParse_.
-          1. Else if _significantDigits_ is *undefined*, then
-            1. Set _roundedValue_ be RoundAmountValueToFractionDigits(_amountValue_, _fractionDigits_, _roundingMode_).
-            1. Let _e_ be the smallest non-negative integer such that _roundedValue_ × 10<sup>-_e_</sup> is an integer.
-            1. Let _scaledRoundedValue_ be _roundedValue_ × 10<sup>-_e_</sup>.
-            1. If _scaledRoundedValue_ = 0, then
-              1. Set _significantDigits_ to 1.
-            1. Else,
-              1. Let _l_ be the log-10 of abs(_scaledRoundedValue_).
-              1. Set _significantDigits_ to floor(_l_) + 1.
-          1. Otherwise:
-            1. Set _roundedValue_ be RoundToSignificantDigits(_amountValue_, _significantDigits_, _roundingMode_).
-            1. Let _digitStr_ be the unique decimal string representation of _roundedValue_ without duplicate leading zeroes.
-            1. Set _fractionDigits_ to the CountFractionDigits of _digitStr_.
-          1. Set _O_.[[Value]] to _roundedValue_.
-          1. Set _O_.[[SignificantDigits]] to _significantDigits_.
-          1. Set _O_.[[FractionDigits]] to _fractionDigits_.
-        1. Else if _amountValue_ is ~minus-zero~, then
-          1. Set _O_.[[Value]] to ~minus-zero~.
-          1. Set _O_.[[SignificantDigits]] to _numDigits_.
-          1. Assert: _numDigits_ ≥ 1.
-          1. Set _O_.[[FractionDigits]] to _numDigits_ - 1.
-        1. Otherwise:
+          1. Else, set _fractionDigits_ to 0.
+        1. Set _O_.[[FractionDigits]] to _fractionDigits_.
+        1. If _amountValue_ is a mathematical value, then
+          1. Set _O_.[[Value]] to RoundAmountValueToFractionDigits(_amountValue_, _fractionDigits_, _roundingMode_).
+        1. Else
           1. Set _O_.[[Value]] to _amountValue_.
-          1. Set _O_.[[SignificantDigits]] to _numDigits_..
-          1. Set _O_.[[FractionDigits]] to 0.
         1. If _unit_ is not *undefined* and _amountValue_ is not ~not-a-number~, set _O_.[[Unit]] to _unit_.
         1. Return _O_.
       </emu-alg>
@@ -486,15 +479,14 @@ location: https://github.com/tc39/proposal-amount/
       1. Let _fractionDigits_ be _processedOptions_.[[FractionDigits]].
       1. Let _significantDigits_ be _processedOptions_.[[SignificantDigits]].
       1. Let _value_ be _O_.[[Value]].
+      1. If _significantDigits_ is not *undefined*, set _fractionDigits_ to SignificantToFractionDigits(_value_, _significantDigits_).
       1. If _fractionDigits_ is not *undefined*, set _value_ to RoundAmountValueToFractionDigits(_value_, _fractionDigits_).
-      1. Else if _significantDigits_ is not *undefined*, set _value_ to RoundToSignificantDigits(_value_, _significantDigits_).
-      1. Let _N_ be OrdinaryObjectCreate(*"%Amount.prototype%"*, « [[FractionDigits]], [[SignificantDigits]], [[Unit]], [[Value]] »).
+      1. Let _N_ be OrdinaryObjectCreate(*"%Amount.prototype%"*, « [[FractionDigits]], [[Unit]], [[Value]] »).
       1. If _unit_ is not *undefined* and _O_.[[Unit]] is not *undefined*, then
         1. If SameValueNonNumber(_unit_, _O_.[[Unit]]) is *false*, throw a *TypeError* exception.
       1. Set _N_.[[Value]] to _value_.
       1. Set _N_.[[Unit]] to _unit_.
       1. Set _N_.[[FractionDigits]] to ℝ(_fractionDigits_).
-      1. Set _N_.[[SignificantDigits]] to ℝ(_significantDigits_).
       1. Return _N_.
     </emu-alg>
   </emu-clause>