How do I mimic Solidity/EVM signed modulo arithmetic with BigNumber?

[PaulRBerg]

Issue Description

Take the following negative big number:

-57896044618658097711785492504343953926634992332820282019728792003956564819968

Which happens to be the minimum value permitted by int156.

If one takes the modulo 1e18 of this number, one gets a different result in Ethers.js compared to what Solidity produces.

Solidity

int256 public constant foo = type(int256).min % 1e18;

Which yields:

-792003956564819968

Ethers.Js

const bn = ethers.BigNumber.from("-57896044618658097711785492504343953926634992332820282019728792003956564819968");
const foo = bn.mod(BigNumber.from(10).pow(18));

Which yields:

207996043435180032

[ricmoo]

That is correct. :)

The ethers BigNumber always returns a positive number in the range [0, N). Solidity (and EVM's SMOD) uses the C fmod-style method, which returns a value with the same sign as value.

If you want to mimic Solidity's behaviour, you can use a method similar to:

function solidityMod(k, N) {
    let result = k.mod(N);
    if (k.isNeg()) { result = result.sub(N); }
    return result;
}

Does that make sense?

[PaulRBerg]

It does. Thanks for the code snippet, it was helpful.

[PaulRBerg]

Just noticed that there is a bug here. The if block should be rewritten like this:

if (!result.isZero() && k.isNeg())

To account for the case when k is negative but is a multiple of N.

[ricmoo]

Oh! This is probably correct! Thanks. :)

[ricmoo]

(moving to discussions)

[PaulRBerg]

Idea: what if the solidityMod function written above was shipped with ethers by default?