Honda door unlock rolling code

string key = "AABBCCDDEEFF0011";
string rolling = "1A2B3C4D5E6F";
string rolling_code_s = rolling.substr(1, 2);
string rolling_code_s_dec = "";
char rolling_code_c = rolling_code_s[0];
int rolling_code_d = 0;
int rolling_code_c_dec = 0;
if (isdigit(rolling_code_c))
{
  rolling_code_d = rolling_code_c - '0';
  rolling_code_c_dec = rolling_code_d;
  rolling_code_s_dec.append(1, rolling_code_c);
}
else if (isalpha(rolling_code_c))
{
  rolling_code_d = rolling_code_c - 'A' + 10;
  rolling_code_c_dec = rolling_code_d;
  rolling_code_s_dec.append(1, rolling_code_c);
}
else
{
  rolling