Rejection sampling random number

getPublicRandom.js

// https://drand.love/blog/2023/02/22/how-to-use-drand/
// https://research.kudelskisecurity.com/2020/07/28/the-definitive-guide-to-modulo-bias-and-how-to-avoid-it
// https://go.dev/src/crypto/rand/util.go

// https://go.dev/src/crypto/rand/util.go
//follows rejection sampling
function getPublicRandom(max, randomHexString){
    if(max<=0)
    throw "max cannot be <=0"

    let bitCount = Math.floor(Math.log2(max)+1)

    if (bitCount === 0) 
		return 0
	

    let byteCount = Math.ceil(bitCount/8)
    // number of bits in the most significant byte of max
    let sigBit = bitCount % 8
    if(sigBit==0)
    sigBit=8
    let firstByteNum =  Number("0x"+randomHexString.slice(0, 2))
    // Clear bits in the first byte to increase the probability that the candidate is <= max
    firstByteNum &= (1<<sigBit) - 1 


    const randomNumber = Number("0x" + firstByteNum.toString(16) + randomHexString.slice(2, (byteCount-1)*2)) 
    if(randomNumber<=max)
    return randomNumber
    
 }
// returns randomHexString
async function fetchPublicRandomness(round=0){
    let randJSON = await fetch(`https://drand.cloudflare.com/8990e7a9aaed2ffed73dbd7092123d6f289930540d7651336225dc172e51b2ce/public/${round}`).then(res=>res.json())
    return randJSON.randomness
}