Solving String Factoring

Question

I'm trying to solve a question from kattis as shown here regarding string factorisation. I've tried adjusting my code for quite abit but it still seems theoretically correct. Not sure why it still fails for some of the test cases.

#include <stdio.h>
#include <iostream>
#include <string>
#include <map>
#include <algorithm>

using namespace std;

string shortener (string input) {
    map<string, int> freq;
    int flag = 0;
    for (int i = (input.length())/2; i >= 1 && !flag; i--) {
        for (int d = 0; d + i + i <= input.length(); d++) {
            if (input.substr(d, i) == input.substr(d + i, i)) {

                freq[input.substr(d,i)]++;
                flag = 1; // stop at this size
            }
        }
    }

    int largest = 0;
    if (freq.empty()) return input;

    //string largest;
    auto x = max_element(freq.begin(), freq.end(),
                         [](const pair<string, int>& p1, const pair<string, int>& p2) {
                             return p1.second < p2.second; });
    if (x->first == input) return input;

    int a = input.find(x->first);

    for (int i = 0; i < x->second ; i++) {
        input.replace(a, x->first.length(), "");
        a = input.find(x->first);
    }
    if (a != -1) {
        if (!input.substr(0, a).empty())
            input.replace(0, a, shortener(input.substr(0, a)));
        if (!input.substr(a + x->first.length()-1, input.length()-1).empty())
            input.replace(a + x->first.length()-1, input.length()-1, shortener(input.substr(a + x->first.length()-1, input.length()-1)));
        //cout << input.substr(a + x->first.length()-1, input.length()-1) << endl;
        input.replace(a, x->first.length(), shortener(x->first));
    }

    return input;
}

int main () {
    string input;
    cin >> input;

    cout << shortener(input).length() << endl;
}

I know my code may not be the most efficient, but I'm hoping to find out what kind of test case might potentially break my code.