int n=strlen(x) // where x is pointer to hex string
int sum=0;
int leftshift=0;
while(n>0)
{
  if((x[n-1]>='0') && (x[n-1]<='9'))
    sum+=(x[n-1]-'0')<<leftshift;
  if((x[n-1]>='A') && (x[n-1]<='F'))
    sum+=(x[n-1]-'A'+10)<<leftshift;
  if((x[n-1]>='a') && (x[n-1]<='f'))
    sum+=(x[n-1]-'a'+10)<<leftshift;
  n--;
  leftshift+=4;
}