hexadecimal ,a=10, b=11, c=12, d=13, e=14, f=15

int hex2dec(char c){
  switch(c){
    case 'a': return 10;
    case 'b': return 11;
    case 'c': return 12;
    case 'd': return 13;
    case 'e': return 14;
    case 'f': return 15;
    default:  return c - '0';
  }
}

char dec2hex(int n){
  switch(n){
    case 10: return 'a';
    case 11: return 'b';
    case 12: return 'c';
    case 13: return 'd';
    case 14: return 'e';
    case 15: return 'f';
    default: return n + '0';
  }
}