// main.cpp
// Paradeigma gia arxeia tyxaias prospelasisis me fixed-width eggrafes
// Arxeio: students.txt (id 5, fName 20, sName 30, grade 4, + newline)

#include <iostream>
#include <fstream>
#include <sstream>
#include <iomanip>
#include <string>
#include <limits>

using namespace std;

struct Student {
    string id;     // 5
    string fName;  // 20
    string sName;  // 30
    double grade;  // 0..10 me 1 dekadiko
};

static const int DATA_LEN = 59;           // 5+20+30+4
static int g_recordSize = 60;             // DATA_LEN + newlineLen (1 i 2)
static string g_newline = "\n";           // "\n" i "\r\n"

// -----------------------------
// Helpers
// -----------------------------
static string rtrim(const string& s) {
    size_t end = s.find_last_not_of(' ');
    if (end == string::npos) return "";
    return s.substr(0, end + 1);
}

static string fitLeft(const string& s, size_t width) {
    if (s.size() >= width) return s.substr(0, width);
    return s + string(width - s.size(), ' ');
}

static string gradeToField(double g) {
    // width 4, 1 dekadiko (px 9.8, 10.0, 0.0)
    ostringstream oss;
    oss << fixed << setprecision(1) << g;
    string gs = oss.str(); // px "9.8" i "10.0"
    if (gs.size() > 4) gs = gs.substr(0, 4);
    // left-align se 4 chars, opote "8.5 " ant gia " 8.5"
    return fitLeft(gs, 4);
}

static bool detectRecordLayout(const string& path) {
    ifstream file(path, ios::binary);
    if (!file) {
        cerr << "Sfalma: Den mporei na anoiksei to arxeio: " << path << "\n";
        return false;
    }

    string line;
    if (!getline(file, line)) {
        cerr << "Sfalma: To arxeio einai adeio.\n";
        return false;
    }

    // Se Windows, to getline afairei to '\n' alla mporei na meinei '\r'
    bool hasCR = (!line.empty() && line.back() == '\r');
    if (hasCR) line.pop_back();

    if ((int)line.size() != DATA_LEN) {
        cerr << "Proeidopoiisi: I proti grammi den exei " << DATA_LEN
             << " xaraktires (exei " << line.size()
             << "). Ta offsets mporei na min einai akrivi.\n";
    }

    g_newline = hasCR ? "\r\n" : "\n";
    g_recordSize = DATA_LEN + (hasCR ? 2 : 1);
    return true;
}

static string formatRecord(const Student& s) {
    // Pedia fixed width
    string rec;
    rec.reserve(DATA_LEN + g_newline.size());
    rec += fitLeft(s.id, 5);
    rec += fitLeft(s.fName, 20);
    rec += fitLeft(s.sName, 30);
    rec += gradeToField(s.grade);
    rec += g_newline;
    return rec;
}

static bool fileHasRecord(ifstream& file, int recNo) {
    if (recNo <= 0) return false;
    file.seekg(0, ios::end);
    streamoff size = file.tellg();
    streamoff need = (streamoff)recNo * (streamoff)g_recordSize;
    return size >= need;
}

static Student inputStudentFromUser() {
    Student s;

    cout << "Dose ID (mexri 5 xaraktires): ";
    cin >> s.id;

    cin.ignore(numeric_limits<streamsize>::max(), '\n');

    cout << "Dose onoma (First Name, mexri 20): ";
    getline(cin, s.fName);

    cout << "Dose epitheto (Surname, mexri 30): ";
    getline(cin, s.sName);

    cout << "Dose vathmo (0.0 ews 10.0 me 1 dekadiko): ";
    cin >> s.grade;

    // Kanonopoiisi vathmou
    if (s.grade < 0.0) s.grade = 0.0;
    if (s.grade > 10.0) s.grade = 10.0;

    return s;
}

static void printStudent(const Student& s) {
    cout << "ID: " << rtrim(s.id) << "\n";
    cout << "First Name: " << rtrim(s.fName) << "\n";
    cout << "Surname: " << rtrim(s.sName) << "\n";
    cout << "Grade: " << fixed << setprecision(1) << s.grade << "\n";
}

// =========================================================
// synartiseis diaxeirishs
// =========================================================

// 1) readStudentAt(int recordNumber)
Student readStudentAt(int recordNumber) {
    Student s;

    ifstream file("..\\students.txt", ios::binary);
    if (!file) {
        cerr << "Sfalma: Den mporei na anoiksei to students.txt\n";
        return s;
    }

    // Elegxos yparxis eggrafis
    if (!fileHasRecord(file, recordNumber)) {
        cerr << "Sfalma: Den yparxei eggrafi me arithmo " << recordNumber << "\n";
        return s;
    }

    streamoff offset = (streamoff)(recordNumber - 1) * (streamoff)g_recordSize;
    file.seekg(offset, ios::beg);

    string buf(DATA_LEN, '\0');
    file.read(&buf[0], DATA_LEN);

    if (!file) {
        cerr << "Sfalma: Apotyxia anagnosis eggrafis.\n";
        return s;
    }

    s.id    = buf.substr(0, 5);
    s.fName = buf.substr(5, 20);
    s.sName = buf.substr(25, 30);

    // grade field (4 chars), stod agnoei leading/trailing spaces
    string gfield = buf.substr(55, 4);
    try {
        s.grade = stod(gfield);
    } catch (...) {
        s.grade = 0.0;
    }

    return s;
}

// 2) add(const Student&)
bool add(const Student& st) {
    ofstream file("..\\students.txt", ios::binary | ios::app);
    if (!file) {
        cerr << "Sfalma: Den mporei na anoiksei to students.txt gia prosthiki.\n";
        return false;
    }

    string rec = formatRecord(st);
    file.write(rec.c_str(), (streamsize)rec.size());

    if (!file) {
        cerr << "Sfalma: Apotyxia eggrafis sto arxeio.\n";
        return false;
    }

    return true;
}

// 3) replace(int recNo, const Student& replacement)
void replace(int recNo, const Student& replacement) {
    fstream file("..\\students.txt", ios::binary | ios::in | ios::out);
    if (!file) {
        cerr << "Sfalma: Den mporei na anoiksei to students.txt gia tropopoiisi.\n";
        return;
    }

    // Elegxos yparxis eggrafis
    file.seekg(0, ios::end);
    streamoff size = file.tellg();
    streamoff need = (streamoff)recNo * (streamoff)g_recordSize;
    if (recNo <= 0 || size < need) {
        cerr << "Sfalma: Den yparxei eggrafi me arithmo " << recNo << "\n";
        return;
    }

    streamoff offset = (streamoff)(recNo - 1) * (streamoff)g_recordSize;
    file.seekp(offset, ios::beg);

    string rec = formatRecord(replacement);

    // Grafoyme mono ta DATA_LEN bytes sto idio offset,
    // kai to newline (1 i 2 bytes) meta, wste na meinei idio recordSize.
    file.write(rec.c_str(), (streamsize)rec.size());

    if (!file) {
        cerr << "Sfalma: Apotyxia antikatastasis eggrafis.\n";
        return;
    }

    cout << "Epityxia: I eggrafi " << recNo << " antikatastathike.\n";
}

// -----------------------------
// Main (epideiksi me menu)
// -----------------------------
int main() {
    // Prospathoume na "katalavoume" an to arxeio exei \n i \r\n,
    // gia na exoume swsta offsets.
    if (!detectRecordLayout("..\\students.txt")) {
        return 1;
    }

    int choice = -1;

    while (choice != 0) {
        cout << "\n--- MENU ---\n";
        cout << "1) Diavase tin n-osti eggrafi (readStudentAt)\n";
        cout << "2) Prosthesi neas eggrafis sto telos (add)\n";
        cout << "3) Antikatastasi eggrafis (replace)\n";
        cout << "0) Eksodos\n";
        cout << "Epilekse: ";
        cin >> choice;

        if (cin.fail()) {
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
            cout << "Lathos eisodos. Prospathise ksana.\n";
            continue;
        }

        if (choice == 1) {
            int recNo;
            cout << "Dose arithmo eggrafis: ";
            cin >> recNo;

            Student s = readStudentAt(recNo);
            cout << "\n--- STOIXEIA FOITITI ---\n";
            printStudent(s);

        } else if (choice == 2) {
            cout << "\nDose stoixeia gia nea eggrafi:\n";
            Student s = inputStudentFromUser();

            if (add(s)) {
                cout << "Epityxia: I eggrafi prostethike sto telos.\n";
            } else {
                cout << "Apotyxia: Den prostethike i eggrafi.\n";
            }

        } else if (choice == 3) {
            int recNo;
            cout << "Dose arithmo eggrafis gia antikatastasi: ";
            cin >> recNo;

            cout << "\nDose stoixeia replacement:\n";
            Student repl = inputStudentFromUser();

            replace(recNo, repl);

        } else if (choice == 0) {
            cout << "Eksodos.\n";
        } else {
            cout << "Mi egkyri epilogi.\n";
        }
    }

    return 0;
}