Support number format digit options for PluralRules

[hsivonen]

Currently, the options from https://tc39.es/ecma402/#sec-setnfdigitoptions are unsupported. These are:

• minimumIntegerDigits
• minimumFractionDigits
• maximumFractionDigits
• minimumSignificantDigits
• maximumSignificantDigits
• roundingPriority
• roundingIncrement
• roundingMode
• trailingZeroDisplay

These need to be supported for ECMA-402 compat.

[sffc]

We support these already:

• minimumIntegerDigits
• minimumFractionDigits
• maximumFractionDigits
• roundingMode

The following can be implemented fairly easily in wrapper code, but it may still be useful to move them into the library:

• minimumSignificantDigits
• maximumSignificantDigits
• roundingPriority
• trailingZeroDisplay

The following is less trivial to implement in user code so should definitely land in ICU4X:

• roundingIncrement

[sffc]

I made an issue #3942 in order to make an API that more directly handles the rounding-related ECMA-402 options listed above (which I think is all but trailingZeroDisplay).

[sffc]

@sffc to write a design doc for this and for #3942

[jedel1043]

FWIW, most of the options were very straightforward to apply:

https://github.com/boa-dev/boa/blob/caac9049c40f7361ede282cf358d3c5981e5d6db/boa_engine/src/builtins/intl/number_format/utils.rs#L274-L301

However, I couldn't find a way to apply the roundingIncrement option without support for division from FixedDecimal, which would let me do:

let increment: FixedDecimal = FixedDecimal::from(_rounding_increment).multiplied_pow10(-max_fraction);
number /= increment;
round(number, 0, rounding_mode);
number *= increment;

and that is essentially what ICU4C does:

https://github.com/unicode-org/icu/blob/c4689841c001a0b98e5b59adaab34656bd2a998f/icu4c/source/i18n/number_decimalquantity.cpp#L202-L208

[sffc]

Yeah, roundingIncrement in ECMA-402 is limited to common integer values that evenly divide the next highest power of 10 (like 20, 25, and 50). This is in part to enable a more efficient implementation in libraries like ICU4X, avoiding the need for division.

It would be an excellent first issue for someone to write the following API:

pub enum RoundingIncrement {
    I1, I2, I5, I10, I20, I25, I50, I100, I200, I250, I500, I1000, I2000, I2500, I5000
}

impl FixedDecimal {
    pub fn round_half_even_with_increment(&mut self, position: i16, increment: RoundingIncrement) { ... }
}

Come to think of it, we should really only need I2, I5, and I25. The expressiveness of the FixedDecimal API in ICU4X allows all of the others to be derived.

[sffc]

Discussion from team meeting on 2023-10-26:

• @sffc - other option to have multiple methods: trunc_to_multiple_of_5
• @sffc - RoundingIncrement is used in Ecma and ICU4C
• @sffc - trunc_to_multiple, with enum Multiple {...}
• @robertbastian - I don't like being constrained by Ecma or ICU4C naming on our API design
• @sffc - I think niche concepts should have consistent naming because it helps make concepts more transferrable. For higher-level things, if we can shape an API significantly better, then we can diverge.
• @robertbastian - niche APIs don't affect as many users, so in a way less problematic to use different names/concepts
• @sffc - Multiple functions could be trunc_to_increment_of_5, or trunc_to_incr_5
• @robertbastian - should be trunc_to_increment_of_multiples_of_5 then
• @sffc - trunc_to_increment with enum RoundingIncrement?
• @robertbastian - I'm not opposed to that suggestion and not particularly attached to any other suggestion that's been made.

Conclusion: trunc_to_increment(-2, RoundingIncrement::MultiplesOf5)

LGTM: @sffc (@robertbastian) (@Manishearth)

Enum values:

enum RoundingIncrement {
    #[default]
    MultiplesOf1,
    MultiplesOf2,
    MultiplesOf5,
    MultiplesOf25,
}

LGTM: @sffc @robertbastian (@Manishearth)