checksum_calculator.cpp

#include <string>
#include <iostream>

using namespace std;

int main(){
    string s;
    cin>>s;
    string s1;
    s1=s;
    for(int i=20;i<24;i++){
        s1[i]='0';
    }
    unsigned int sum=0;
    for(int i=0;i<40;i+=4){
        int a=0,b;
        b=(int)s1[i]-48;
        if(b<=9){
            a+=b;
            a=a<<4;
        }
        else{
            b=(int)s1[i]-87;
            a+=b;
            a=a<<4;
        }

        b=(int)s1[i+1]-48;
        if(b<=9){
            a+=b;
            a=a<<4;
        }
        else{
            b=(int)s1[i+1]-87;
            a+=b;
            a=a<<4;
        }
        b=(int)s1[i+2]-48;
        if(b<=9){
            a+=b;
            a=a<<4;
        }
        else{
            b=(int)s1[i+2]-87;
            a+=b;
            a=a<<4;
        }

        b=(int)s1[i+3]-48;
        if(b<=9){
            a+=b;
        }
        else{
            b=(int)s1[i+3]-87;
            a+=b;
        }
        sum+=a;
        if(sum>=65536){
            sum-=65536;
            sum+=1;
        }
    }
    sum=~sum;
    string ans="";
    for(int i=0;i<4;i++){
        int a=sum%16;
        if(a<10){
            char c=(char)(a+48);
            ans=c+ans;
            sum/=16;
        }
        else{
            char c=(char)(a+87);
            ans=c+ans;
            sum/=16;
        }
    }
    cout<<ans<<endl;
}