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Add/comment/multipleof #1219

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Add/comment/multipleof #1219

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96729e9
added instruction for multiple of
akshat09867 Dec 26, 2024
961a51b
#1113 added validation of multipleof
akshat09867 Dec 27, 2024
c9305a4
#1113 added validation
akshat09867 Dec 27, 2024
949c3fd
#1113 added validation
akshat09867 Dec 27, 2024
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4 changes: 4 additions & 0 deletions public/draft/2020-12/draft-bhutton-json-schema-validation-01.html
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Expand Up @@ -1744,6 +1744,10 @@ <h4 id="name-multipleof">
<p id="section-6.2.1-2">
A numeric instance is valid only if division by this keyword's value results in
an integer.<a href="#section-6.2.1-2" class="pilcrow">¶</a></p>
<p id="section-6.2.1-2">
To ensure consistent validation:
- Use scaled integers (e.g., represent $4.02 as 402 with 'multipleOf: 1')
- If floating-point values are necessary, use binary-friendly numbers (e.g., 0.5, 0.25, 0.125)</p><a href="#section-6.2.1-2" class="pilcrow">¶</a></p>
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Is this really the recommendation we want to make?

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@mwadams mwadams Dec 27, 2024 •
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I'd say "no".

Use of scaled integers is ok but is a choice not a recommendation. And we don't have direct support for a scaled integer - e.g. defining the scale. You push a whole new problem onto consumers.

When it comes to non-integers it isn't really meaningful; why would we be recommending that people only use "particular values". And then there is the problem of representation on all platforms (e.g. 1.3, 2.4, 5.6 cannot be represented precisely as a 32 bit float).

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@gregsdennis gregsdennis Dec 27, 2024 •
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A use case could be defining currency, for example dollars & cents. You'd want to ensure that your data only has a penny ($0.01) resolution.

But the point is that we don't want to force users into a scenario where they should be trying to figure out the nuances of binary float arithmetic while writing their schemas.

I think that the very most, we should be leaving a note that some implementations may use IEEE754 math.

I do think the way that the spec is worded is pretty clever, though.

6.2.1. multipleOf

The value of "multipleOf" MUST be a number, strictly greater than 0.
A numeric instance is valid only if division by this keyword's value results in an integer.

In C#,

double a = .0075;  // IEEE754
double b = .0001;
decimal c = .0075m;
decimal d = .0001m;

Console.WriteLine("double");
Console.WriteLine(a / b);
Console.WriteLine(a % b);
Console.WriteLine();
Console.WriteLine("decimal");
Console.WriteLine(c / d);
Console.WriteLine(c % d);

outputs

double
75
9.999999999999937E-05

decimal
75
0.0000

So following the method prescribed by the spec (using division instead of modulation) actually produces the correct result, even with IEEE754 math.

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Storing monetary values as cents is a good idea, to keep all math as integers, but a better way of specifying that in the schema is with type: integer not multipleOf: 1.

I don't think we want to add anything like scale into the spec, though -- this should be defined and controlled by the individual application.

We may want to add a note that tooling might implement the multipleOf keyword using modulo calculations, as a lead-in to the warning about imprecision with floating point storage.

</section>
<section id="section-6.2.2">
<h4 id="name-maximum">
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