Minimal length of substring that forms cyclic string

Question

there is a knows problem stated as "Cyclic string":

A string S has been recorded many times, and then from the resulting line took a substring, and give to you.Your task is to determine the minimum possible length of a string S.

Input

In the first and only line of the input file contains a string that contains only letters, line length up to 50000 characters.

Output

The output file you want to display a single number - the answer to the problem.

It can be solved with substring hashes (source).

I composed a solution:

 bool substring_eq(const vector<uint64_t> &p_pow, const vector<uint64_t> &h,
                   size_t i1, size_t i2, size_t len) {
     uint64_t h1 = h[i1 + len - 1];
     if (i1) h1 -= h[i1 - 1];
     uint64_t h2 = h[i2 + len - 1];
     if (i2) h2 -= h[i2 - 1];

     return (i1 < i2 && (h1 * p_pow[i2 - i1]) == h2) ||
            (i1 > i2 && h1 == (h2 * p_pow[i1 - i2]));
 }

 void compute_hashes(const string &s, vector<uint64_t> &p_pow, vector<uint64_t> &h) {
     const int p = 191;
     p_pow[0] = 1;
     for (size_t i = 1; i < p_pow.size(); ++i) {
         p_pow[i] = p_pow[i - 1] * p;
     }

     for (size_t i = 0; i < s.length(); ++i) {
         h[i] = (s[i] - 'A' + 1) * p_pow[i];
         if (i) h[i] += h[i - 1];
     }
 }

 size_t solve(const string &s) {
     vector<uint64_t> p_pow(s.length()), hashes(s.length());

     compute_hashes(s, p_pow, hashes);

     size_t cycle_len = s.length();
     for (size_t k = 1; k < s.length(); ++k) {
         if (substring_eq(p_pow, hashes, 0, k, s.length() - k)) {
             cycle_len = k;
             break;
         }
     }
     return cycle_len;
 }

 int main() {
     //test();

     string input;
     cin >> input;
     cout << solve(input) << std::endl;
     return 0;
 }

I can't passed the last test in e-judging system (I don't know the contents of it).

I even composed a function that generates input strings, but it catches no errors:

 //////////////////////////
 /// Stress testing
 //////////////////////////
 #include <random>

 std::string random_string(std::string::size_type length) {
     static auto &chrs = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

     static std::mt19937 rg{std::random_device{}()};
     static std::uniform_int_distribution<std::string::size_type> pick(0, sizeof(chrs) - 2);

     std::string s;

     s.reserve(length);

     while (s.length() < length) {
         char ch = chrs[pick(rg)];
         if (s.find(ch) == string::npos) {
             s += ch;
         }
     }

     return s;
 }

 size_t solve(const string &s);
 void test() {
     srand(100);
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wmissing-noreturn"
     while (true) {
         string::size_type len = (unsigned long) (rand() % 50 + 1);
         string s = random_string(len);
         int count = rand() % 5000 + 2;
         string input;
         for (int i = 0; i < count; ++i) {
             input += s;
         }
         string input1 = input;
         input1 = input1.substr(rand() % (input1.length() - s.length()));
         input1 = input1.substr(0, rand() % (input1.length() - s.length()) + s.length());

         if (input1.empty()) continue;

         size_t k = solve(input1);
         if (k != s.length()) {
             cout << "boom:" << input1;
         }
     }
 #pragma clang diagnostic pop
 }
 //////////////////////////

What else did I miss in the solution?