/*
THIS CODE HAS BEEN TESTED AND IS FULLY OPERATIONAL.

Problem Statement: Department maintains a student information. The file contains roll number, name, division and address. Allow user to add, delete information of student. Display information of particular employee. If record of student does not exist an appropriate message is displayed. If it is, then the system displays the student details. Use sequential file to main the data.

Code from DataStructuresAndAlgorithms (SPPU - Second Year - Computer Engineering - Content) repository on KSKA Git: https://git.kska.io/sppu-se-comp-content/DataStructuresAndAlgorithms/
*/

// BEGINNING OF CODE
#include <string.h>
#include <fstream>
#include <iostream>
#include <vector>
using namespace std;

typedef struct employee {
    int empId;
    char name[50];
    char designation[50];
    int salary;
} employee;

typedef struct index_pair {
    int key;
    int position;
} index_pair;

class employeeDatabase {
    string data_file_name = "ind_employee_data.dat";
    string index_file_name = "ind_employee_index.dat";
   public:
    employeeDatabase();
    void addEmployee(int eid, char name[50], char dest[50], int sal);
    void searchEmployee(int);
    void deleteEmployee(int);
    bool isPresent(int);
    void display_all();
    employee readEmp(int pos);
};

employee employeeDatabase::readEmp(int pos) {
    fstream data_file(data_file_name, ios::binary | ios::in | ios::ate);
    employee emp;
    if (pos >= data_file.tellg() || pos == -1) {
        emp.empId = -1;
        return emp;
    }
    data_file.seekg(pos, ios::beg);
    data_file.read((char *)&emp, sizeof(emp));
    data_file.close();
    return emp;
}

employeeDatabase::employeeDatabase() {
    fstream data_file(data_file_name);
    fstream index_file(index_file_name);
    if (data_file.fail() | index_file.fail()) {
        cout << "Created a new file." << endl;
        data_file.open(data_file_name, ios::binary | ios::out | ios::app);
        index_file.open(index_file_name, ios::binary | ios::out | ios::app);
    } else {
        cout << "File already exists." << endl;
    }
    data_file.close();
    index_file.close();
}

void employeeDatabase::addEmployee(int eid, char name[50], char dest[50], int sal) {
    fstream data_file(data_file_name, ios::binary | ios::out | ios::app);
    fstream index_file(index_file_name, ios::binary | ios::in);
    index_pair current, temp;
    char *all_data;
    employee emp;
    int pos, ipos;
    bool update = false;

    while (index_file.read((char *)&current, sizeof(index_pair))) {
      
        if (current.key == eid) {
            if (current.position == -1) {
                ipos = (int)index_file.tellg() - sizeof(current);
                update = true;
                break;
            } else {
                cout << "Cannot add employee, already exists."
                     << endl;
                goto exit_addEmployee;
            }
        }
    }
    index_file.close();

    emp.empId = eid;
    strcpy(emp.name, name);
    strcpy(emp.designation, dest);
    emp.salary = sal;

    data_file.seekp(0, ios::end);
    pos = data_file.tellp();
    data_file.write((char *)&emp, sizeof(emp));
    current.key = eid;
    current.position = pos;
    // cout << pos << endl;
    if (update) {
        index_file.open(index_file_name, ios::binary | ios::out);
        index_file.seekp(ipos, ios::beg);
        index_file.write((char *)&current, sizeof(current));
    } else {
        bool written = false;
        // inserting record in sorted order
        vector<index_pair> index_pairs;
        index_file.open(index_file_name, ios::binary | ios::in);
        while (index_file.read((char *)&temp, sizeof(index_pair))) {
            if (!written and temp.key > eid) {
                written = true;
                index_pairs.push_back(current);
            }
            index_pairs.push_back(temp);
        }
        if (!written) {
            index_pairs.push_back(current);
        }

        index_file.clear();
        index_file.close();

        // write records
        index_file.open(index_file_name, ios::binary | ios::out);

        for (int i = 0; i < index_pairs.size(); i++) {
            current = index_pairs[i];
            if (current.position != -1) {
                index_file.write((char *)&current, sizeof(index_pair));
            }
        }
    }
    cout << "Employee added successfully." << endl;
// close and exit
exit_addEmployee:
    data_file.close();
    index_file.close();
}

void display_emp(employee emp) {
    cout << "ID:\t" << emp.empId << endl;
    cout << "Name:\t" << emp.name << endl;
    cout << "Designation:\t" << emp.designation << endl;
    cout << "Salary:\tRs. " << emp.salary << endl;
}

void employeeDatabase::searchEmployee(int eid) {
    fstream index_file(index_file_name, ios::binary | ios::in);
    index_pair current;
    int pos = -1;

    while (index_file.read((char *)&current, sizeof(index_pair))) {
        if (current.key == eid) {
            pos = current.position;
            break;
        }
    }

    employee emp = readEmp(pos);
    if (emp.empId == -1) {
        cout << "Employee does not exist." << endl;
    } else {
        cout << "--- EMPLOYEE FOUND ---" << endl;
        display_emp(emp);
        cout << "--- END OF RECORD ---" << endl;
    }

    index_file.close();
}

bool employeeDatabase::isPresent(int eid) {
    fstream index_file(index_file_name, ios::binary | ios::in);
    index_pair current;

    while (index_file.read((char *)&current, sizeof(index_pair))) {
        if (current.key == eid) {
            if (current.position == -1) {
                index_file.close();
                return false;
            } else {
                index_file.close();
                return true;
            }
        }
    }
    index_file.close();
    return false;
}

void employeeDatabase::deleteEmployee(int eid) {
    fstream index_file(index_file_name, ios::binary | ios::in);
    index_pair current; 
    bool deleted = false;
    vector<index_pair> pairs;

    while (index_file.read((char *)&current, sizeof(index_pair))) {
        if (current.key == eid) {
            deleted = true;
        } else {
            pairs.push_back(current);
        }
    }
    index_file.close();
    if (deleted) {
        index_file.open(index_file_name, ios::binary | ios::out);
        index_file.clear();
        index_file.seekp(0, ios::beg);

        for (int i = 0; i < pairs.size(); i++) {
            current = pairs[i];
            index_file.write((char *)&current, sizeof(index_pair));
        }
        index_file.close();
        cout << "Employee removed from record." << endl;
    } else {
        cout << "Employee does not exist in the record." << endl;
    }
}

void employeeDatabase::display_all() {
    fstream index_file(index_file_name, ios::binary | ios::in);
    fstream data_file(data_file_name, ios::binary | ios::in);
    index_pair current;
    employee emp;
    while (index_file.read((char *)&current, sizeof(index_pair))) {
        if (current.position != -1) {
            data_file.seekg(current.position, ios::beg);
            data_file.read((char *)&emp, sizeof(emp));
            cout << emp.empId << " | " << emp.name << " | " << emp.designation
                 << " | " << emp.salary << endl;
        }
    }
    index_file.close();
    data_file.close();
}

int main() {
    employeeDatabase db;
    int eid, sal;
    char name[50], dest[50];
    int ch;
    do {
        cout << endl << "--- MAIN MENU ---" << endl;
        cout << "1 -> Add employee" << endl;
        cout << "2 -> Search employee" << endl;
        cout << "3 -> Delete employee" << endl;
        cout << "4 -> Display all" << endl;
        cout << "5 -> Exit" << endl;
        cout << "Choose an option (1-5):\t";
        cin >> ch;
        switch (ch) {
            case 1:
            reenter_eid:
                cout << "Employee ID:\t";
                cin >> eid;
                if (db.isPresent(eid)) {
                    cout << "Employee already exists in record. Please try again."
                         << endl;
                    goto reenter_eid;
                }
                cout << "Name:\t";
                cin >> name;
                cout << "Designation:\t";
                cin >> dest;
                cout << "Salary:\tRs. ";
                cin >> sal;

                db.addEmployee(eid, name, dest, sal);
                break;
            case 2:
                cout << "Employee ID:\t";
                cin >> eid;
                db.searchEmployee(eid);
                break;
            case 3:
                cout << "Employee ID:\t";
                cin >> eid;
                db.deleteEmployee(eid);
                break;
            case 4:
                db.display_all();
                break;
            case 5:
                cout << "\n\n// END OF CODE\n\n" << endl;
                break;
            default:
                cout << "Please choose a valid option (1-5)." << endl;
                break;
        }
    } while (ch != 5);

    return 0;
}
// END OF CODE