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Use significant digits only on the way in and out #66

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Merged
jessealama merged 6 commits into from
Sep 24, 2025
Merged

Use significant digits only on the way in and out #66

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2407c34
Use significant digits only on the way in and out
nicolo-ribaudo Sep 22, 2025
8ff04f9
Fix off-by-on error
nicolo-ribaudo Sep 23, 2025
843ff7a
Ensure 0 has at least 1 significant digit
nicolo-ribaudo Sep 23, 2025
b2fecbc
Update SignificantToFractionDigits type
nicolo-ribaudo Sep 23, 2025
4d177e1
Fix signature of RoundAmountValueToFractionDigits
nicolo-ribaudo Sep 23, 2025
b444517
Allow negative fractionDigits in GetAmountOptions
nicolo-ribaudo Sep 23, 2025
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118 changes: 55 additions & 63 deletions spec.emu
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Original file line number Diff line number Diff line change
Expand Up @@ -237,7 +237,7 @@ location: https://github.com/tc39/proposal-amount/
<emu-clause id="sec-amount-roundtofractiondigits" type="abstract operation">
<h1>RoundAmountValueToFractionDigits(
_v_: a mathematical value,
_n_: a non-negative integer,
_n_: an integer,
optional _roundingMode_: a rounding mode
): a mathematical value
</h1>
Expand All @@ -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,
Expand Down Expand Up @@ -340,6 +314,45 @@ 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. 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.
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@waldemarhorwat waldemarhorwat Sep 22, 2025

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This assert can fail for zeroes. That's one of the reasons why I don't think we should compute significantDigits from fractionDigits.

FractionToSignificantDigits is not used anywhere. Just delete it.

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_: a positive 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 _integerDigits_ be the smallest integer such that 10<sup>_integerDigits_</sup> > abs(_value_).
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
Expand All @@ -357,7 +370,7 @@ location: https://github.com/tc39/proposal-amount/
1. Let _unit_ be ? GetOption(_opts_, *"unit"*, ~string~, ~empty~, *undefined*).
1. If _fractionDigits_ is not *undefined*, then
1. If _significantDigits_ is not *undefined*, throw a *RangeError* exception.
1. If _fractionDigits_ is not a non-negative integral number, throw a *RangeError* exception.
1. If _fractionDigits_ is not an integral number, throw a *RangeError* exception.
1. Else if _significantDigits_ is not *undefined*, then
1. If _significantDigits_ is not an integral number, throw a *RangeError* exception.
1. If ℝ(_significantDigits_) < 1, throw a *RangeError* exception.
Expand Down Expand Up @@ -389,47 +402,27 @@ 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,
1. 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>
Expand Down Expand Up @@ -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>
Expand Down