// Bitcoin Base58 encoder/decoder algorithm
const btcTable = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
console.assert(btcTable.length === 58);

// Base58 decoder/encoder for BigInt
function b58ToBi(chars, table = btcTable) {
  const carry = BigInt(table.length);
  let total = 0n, base = 1n;
  for (let i = chars.length - 1; i >= 0; i--) {
    const n = table.indexOf(chars[i]);
    if (n < 0) throw TypeError(`invalid letter contained: '${chars[i]}'`);
    total += base * BigInt(n);
    base *= carry;
  }
  return total;
}
function biToB58(num, table = btcTable) {
  const carry = BigInt(table.length);
  let r = [];
  while (num > 0n) {
    r.unshift(table[num % carry]);
    num /= carry;
  }
  return r;
}

// Base58 decoder/encoder for bytes
function b58decode(str, table = btcTable) {
  const chars = [...str];
  const trails = chars.findIndex(c => c !== table[0]);
  const head0s = Array(trails).fill(0);
  if (trails === chars.length) return Uint8Array.from(head0s);
  const beBytes = [];
  let num = b58ToBi(chars.slice(trails), table);
  while (num > 0n) {
    beBytes.unshift(Number(num % 256n));
    num /= 256n;
  }
  return Uint8Array.from(head0s.concat(beBytes));
}

function b58encode(beBytes, table = btcTable) {
  if (!(beBytes instanceof Uint8Array)) throw TypeError(`must be Uint8Array`);
  const trails = beBytes.findIndex(n => n !== 0);
  const head0s = table[0].repeat(trails);
  if (trails === beBytes.length) return head0s;
  const num = beBytes.slice(trails).reduce((r, n) => r * 256n + BigInt(n), 0n);
  return head0s + biToB58(num, table).join("");
}

// example
{
  //BASE58: heading byte value 0s are converted to "1"s (letter of table[0])
  console.log(b58encode(new Uint8Array([0, 1, 2]))); // => "15T"
  console.log(b58decode("15T")); // => Uint8Array([0, 1, 2])
}