Add more arithmetics operations to BigNumber

[0xclem]

I'm trying to get rid of BigNumber.js in my dApp to only use BigNumber from ethers but it looks like its API is quite limited.
Some arithmetic operations are missing such as sqr, which would be quite difficult to implement from scratch.

I was just wondering if it was possible to add more of these operations to BigNumber? I understand you are using BN.js which already has them all.

[PaulRBerg]

I had a similar problem, and I solved by writing convertor functions between ethers.BigNumber and mathjs.BigNumber and vice-versa.

You could do the same between ethers.BigNumber and BigNumber.js.

But I agree that it would be useful to have some advanced math functions (logs, exp, sqrt) shipped with the canonical BigNumber type.

[zeim839]

@clementbalestrat @PaulRBerg I've addressed this issue here. Hopefully it'll get merged. I've implemented the neg(), sqr(), divMod() and divRound() functions from bn.js, but do let me know If there's anything else you'd like me to add/propose.

[0xclem]

This is great, thanks guys!

@zeim839 would you be able to add min and max?

[0xclem]

Also a couple of nice to have, which aren't in BN.js so no pressure.

• A toFormat() function like in BigNumber.js https://mikemcl.github.io/bignumber.js/#toFor. Very useful for the dApps.
• multiplyDecimal() & divideDecimal(). Have a look at our implementation below:

    /*
     * Multiplies x and y interpreting them as fixed point decimal numbers.
     */
    const multiplyDecimal = (x, y, unit = UNIT) => {
        const xBN = BN.isBN(x) ? x : new BN(x);
        const yBN = BN.isBN(y) ? y : new BN(y);
        return xBN.mul(yBN).div(unit);
    };

    /*
     * Divides x and y interpreting them as fixed point decimal numbers.
     */
    const divideDecimal = (x, y, unit = UNIT) => {
        const xBN = BN.isBN(x) ? x : new BN(x);
        const yBN = BN.isBN(y) ? y : new BN(y);
        return xBN.mul(unit).div(yBN);
    };
    

EDIT: default UNIT would be 18.