BIGNUM C++

int sot(string s){
    int ans = 0;
    Fora(v, s){
        ans *= 10;
        ans += (v - '0');
    }
    return ans;
}

string tos(int x){
    if (x == 0) return "0";
    string s = "";
    while (x){
        s += (char)(x % 10 + '0');
        x /= 10;
    }
    reverse(bend(s));
    return s;
}

bool cmp(string a, string b){
    if (a.length() != b.length()) return a.length() < b.length();
    return strcmp(a.c_str(), b.c_str()) < 0;
}

string add(string a, string b){
    string ans = "";
    while (a.length() < b.length()) a = "0" + a;
    while (b.length() < a.length()) b = "0" + b;
    int carry = 0, rem;
    for (int i = a.length() - 1; i >= 0; --i) {
        rem = (a[i] - '0' + b[i] - '0') + carry;
        ans = (char(rem % 10 + '0')) + ans;
        carry = rem / 10;
    }
    if (carry > 0) ans = "1" + ans;
    return ans;
}

string sub(string a, string b){
    string ans = "";
    bool neg = 0;
    while (a.length() < b.length()) a = "0" + a;
    while (b.length() < a.length()) b = "0" + b;
    if (a < b) swap(a, b), neg = 1;
    int carry = 0, rem;
    for (int i = a.length() - 1; i >= 0; --i) {
        rem = a[i] - b[i] - carry;
        carry = 0;
        while (rem < 0) {
            ++ carry;
            rem += 10;
        }
        ans = (char(rem % 10 + '0')) + ans;
    }
    while (ans.length() > 1 && ans[0] == '0') ans.erase(0, 1);
    if (neg) ans = "-" + ans;
    return ans;
}

string mul(string a, string b){
    long long sav[2050];
    for (int i = 0; i < 2050; i++)
        sav[i] = 0;
    string ans = "";
    int carry = 0, rem;
    a = "0" + a;
    b = "0" + b;
    for (int i = a.length() - 1; i >= 0; --i)
        for (int j = b.length() - 1; j >= 0; --j)
            sav[i + j] += ((a[i] - '0') * (b[j] - '0'));
    for (int i = a.length() + b.length() - 2; i >= 0; --i){
        rem = sav[i] + carry;
        carry = rem / 10;
        ans = char(rem % 10 + '0') + ans;
    }
    while (ans.length() > 1 && ans[0] == '0') ans.erase(0, 1);
    return ans;
}

string divnum(string a, int b){
    string ans;
    int idx = 0;
    int tmp = a[idx] - '0';
    while (tmp < b){
       tmp = tmp * 10 + (a[++idx] - '0');
   }
    while (a.size() > idx){
        ans += (tmp / b) + '0';
        tmp = (tmp % b) * 10 + a[++idx] - '0';
    }
    if (ans.length() == 0){
        return "0";
    }
    return ans;
}

int modnum(string a, int b){
    string c = divnum(a, b), d = mul(c, tos(b));
    return sot(sub(a, d));
}

int strgcd(string a, int b){
    return __gcd(modnum(a, b), b);
}

string lcm(string x, int y){
    return mul(divnum(x, strgcd(x, y)), tos(y));
}