`multipleOf` doesn't account for rounding errors

[lpersin]

Hi,

I've encountered a problem with the v1 where the code checking for the multipleOf condition of the json-schema quickly gets thrown off by any rounding error.

Here's how the current code attempts to validate the rule:

  if (schema.multipleOf !== undefined && value % schema.multipleOf !== 0) {
    errors.push({ path, validation: 'multipleOf', schema, value })
  }

But this fails in very simple cases like:

> 100 % 0.01
0.009999999999997919

Other json-schema validation libraries seem to handle this by allowing to pass an arbitrary tolerance, something looking like this could solve this issue
Eg:

if (schema.multipleOf !== undefined) {
    const remainder = value % schema.multipleOf
    const precision = 1e-7
    if (!(Math.abs(remainder) < precision || Math.abs(remainder - schema.multipleOf) < precision)) {
      errors.push({ path, validation: 'multipleOf', schema, value })
    }
  }

I chose an arbitrary default precision here but you get the idea.

I wanted to open a PR with this suggestion but found that ultimately it should be up to the user to pass the desired tolerance when calling createHeadlessForm or handleValidation, yet these functions don't currently allow passing in additional validation options.

I'm not sure what the desired design direction is here, so I'm opening this issue to get some feedback on a potential fix.

Thanks

[lukad]

Hi @lpersin, thanks for raising this issue! As it turns out the official json-schema test suite does not specifically test for these floating point errors so this totally flew under our radar.
Instead of using a configurable precision value I would prefer to use a lightweight arbitrary precision math library like big.js to avoid floating point errors all together.

It could look something like this:

import Big from 'big.js';

if (schema.multipleOf !== undefined) {
    const valueBig = new Big(value);
    const multipleOfBig = new Big(schema.multipleOf);
    const remainder = valueBig.mod(multipleOfBig);
    
    if (!remainder.eq(0)) {
      errors.push({ path, validation: 'multipleOf', schema, value });
    }
}

Let me know if that's something you would like to help out with and submit a PR, that would be much appreciated :)