Gets the position of the first zero bit (Little-Endian) of a 255 bit number

findIndex.js

/**
 * Gets the position of the first zero bit (Little-Endian) of a 255 bit number.
 * 
 * 001010111111111
 *      ^
 * By doing an AND operation between the input and it's 2's complement, we get a
 * number whose single 1 bit is the first zero byte in the sequence. 
 */

const { constants, BigNumber } = require("ethers");

const input = BigNumber.from(72)

// NOT operator = XOR with a bitmask of all ones
const bitMask = constants.MaxUint256
const reverse = input.xor(bitMask)

// EthersJS two's complement is not working here so we are just doing
// it manually. NOT(input) + 1 (https://en.wikipedia.org/wiki/Two%27s_complement)
const twosComplement = reverse.add(BigNumber.from(1))

// Find position of resulting bit by doing a log2
const index = Math.log2(input.and(twosComplement).toNumber())

console.log(index)