#include <iostream>
#include <fstream>
#include <cstring>
#include <cctype>

using namespace std;

int main()
{

    char filename[256];
    cout << "Enter file name: ";
    cin.getline(filename, sizeof(filename));

    bool isValid = true;

    const char* forbiddenChars = "*\"<>?|";
    if (strpbrk(filename, forbiddenChars) != 0)
        isValid = false;

    if (strchr(filename, ':') != nullptr &&
        (filename[1] != '\\' || !isalpha(filename[0]) ||
        strchr(filename, '\\') != strrchr(filename, '\\')))
        isValid = false;

    if (strchr(filename, '.') == NULL) {
        strcat(filename, ".txt");
    }

    if (isValid)
    {
        ifstream file(filename, ios::binary | ios::ate);
        if (file.is_open())
        {
            streampos fileSize = file.tellg();
            char* buffer = new char[fileSize];
            file.seekg(0, ios::beg);
            file.read(buffer, fileSize);
            file.close();

            char searchString[256];
            cout << "Enter a search string: ";
            cin.getline(searchString, sizeof(searchString));

            int count = 0;
            char* p = buffer;
            while ((p = strstr(p, searchString)) != nullptr)
            {
                count++;
                p += strlen(searchString);
            }

            cout << "Number of occurrences of a string \"" << searchString << "\" in a text file: " << count << endl;

            delete[] buffer;
        }
        else
            cout << "Error opening file." << endl;
    }
    else
        cout << "Invalid file name." << endl;

    system("pause");
    return 0;
}