const hexCharset = "0123456789abcdef".split("");
const suffixCharset = [
    "0123456789", 
    "abcdefghijklmnopqrstuvwxyz",
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ", 
    "+-",
].flatMap(set => set.split(""));
const suffixCharsetBits = 6;

const sha256 = "SHA-256";

Uint8Array.prototype.toHex = function() {
    return [...this].map(c => hexCharset[c >> 4] + hexCharset[c & 15]).join("");
};

Uint8Array.prototype.hasPrefix = function(array, bits) {
    for(let i = 0; i < array.length && i * 8 < bits; i++) {
        const bitsForByte = Math.min(8, bits - i * 8);
        const bitMask = ((1 << bitsForByte) - 1) << (8 - bitsForByte);
        
        if ((this[i] & bitMask) != (array[i] & bitMask)) {
            return false;
        }
    }
    return true;
};

async function digest(algo, input) {
    return new Uint8Array(await crypto.subtle.digest(algo, new TextEncoder().encode(input)));
}

function makeSuffix(index) {
    let result = "";
    
    while (index > 0) {
        result += suffixCharset[index & ((1 << suffixCharsetBits) - 1)];
        index >>= suffixCharsetBits;
    }
    
    return result;
}

async function findHashWithPrefix(hashPrefixBits, inputPrefix) {
    const hashPrefix = new Uint8Array(Array(Math.ceil(hashPrefixBits / 8)).map(c => 0));
    let iteration = 0;
    
    do {
        var message = inputPrefix + makeSuffix(iteration++)
        var hash = await digest(sha256, message);
    } while (!hash.hasPrefix(hashPrefix, hashPrefixBits));
    
    return message;
}