function con(b) {
  if ((b & 0xc0) === 0x80) {
    return b & 0x3f;
  } else {
    throw new Error("invalid UTF-8 encoding");
  }
}

function code(min, n) {
  if (n < min || (0xd800 <= n && n < 0xe000) || n >= 0x10000) {
    throw new Error("invalid UTF-8 encoding");
  } else {
    return n;
  }
}

export function decode(bytes) {
  return _decode(bytes)
    .map((x) => String.fromCharCode(x))
    .join("");
}

function _decode(bytes) {
  const result = [];
  while (bytes.length > 0) {
    const b1 = bytes[0];
    if (b1 < 0x80) {
      result.push(code(0x0, b1));
      bytes = bytes.slice(1);
      continue;
    }

    if (b1 < 0xc0) {
      throw new Error("invalid UTF-8 encoding");
    }

    const b2 = bytes[1];
    if (b1 < 0xe0) {
      result.push(code(0x80, ((b1 & 0x1f) << 6) + con(b2)));
      bytes = bytes.slice(2);
      continue;
    }

    const b3 = bytes[2];
    if (b1 < 0xf0) {
      result.push(code(0x800, ((b1 & 0x0f) << 12) + (con(b2) << 6) + con(b3)));
      bytes = bytes.slice(3);
      continue;
    }

    const b4 = bytes[3];
    if (b1 < 0xf8) {
      result.push(
        code(
          0x10000,
          ((((b1 & 0x07) << 18) + con(b2)) << 12) + (con(b3) << 6) + con(b4)
        )
      );
      bytes = bytes.slice(4);
      continue;
    }

    throw new Error("invalid UTF-8 encoding");
  }

  return result;
}
