Refactor win-check to O(1) counter-based approach, scalable to any NxN

- Win detection now uses per-player row/col/diagonal counters instead of
  scanning lines; O(1) per move and works for any board size (change N)
- Moved spielfeld into gameState as board[N][N]; removed global
- Replaced recursive getUserInput with a loop to avoid stack overflow
  on repeated bad input; also handles non-integer cin failures
- Initialized gameState::winner to ' ' to avoid undefined behaviour
- Fixed draw output: prints "Unentschieden!" instead of "Der Gewinner ist: -"
- Removed stale TODO comment that incorrectly claimed draw detection was broken

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.cpp

@@ -1,6 +1,5 @@
 #include <iostream>
 #include <iomanip>
-#include <vector>
 
 #ifdef _WIN32
 #include <windows.h>
@@ -8,7 +7,7 @@
 
 using namespace std;
 
-char spielfeld[3][3];
+const int N = 3;
 
 struct userInput {
     bool ok;
@@ -18,115 +17,120 @@ struct userInput {
 
 struct gameState {
     bool running = true;
-    char winner;
+    char winner = ' ';
+    char board[N][N];
     int filledCells = 0;
+    // Per-player (0=X, 1=O) counters for rows, cols, and both diagonals.
+    // Win-check is O(1): increment on each move, compare to N.
+    int rowCount[2][N] = {};
+    int colCount[2][N] = {};
+    int diagCount[2] = {};
+    int antiDiagCount[2] = {};
 };
 
-void render() {
-    #ifdef _WIN32
+void render(const gameState &state) {
+#ifdef _WIN32
     system("cls");
-    #endif
-    
-    #ifdef linux
+#endif
+#ifdef linux
     system("clear");
-    #endif
+#endif
 
-    cout << "  | 1 | 2 | 3 |" << endl;
-    cout << "---------------" << endl;
-    for (int i = 0; i < 3; i++) {
-        cout << i+1 << " | ";
-        for (int j = 0; j < 3; j++) {
-            cout << spielfeld[i][j] << " | ";
-        }
+    cout << "  | ";
+    for (int j = 0; j < N; j++) cout << j + 1 << " | ";
+    cout << endl;
+    for (int i = 0; i < N * 4 + 3; i++) cout << "-";
+    cout << endl;
+    for (int i = 0; i < N; i++) {
+        cout << i + 1 << " | ";
+        for (int j = 0; j < N; j++) cout << state.board[i][j] << " | ";
+        cout << endl;
+        for (int k = 0; k < N * 4 + 3; k++) cout << "-";
         cout << endl;
-        cout << "---------------" << endl;
     }
 }
 
-userInput getUserInput() {
-    int inputSpalte, inputZeile;
+userInput getUserInput(const gameState &state) {
+    while (true) {
+        int inputZeile, inputSpalte;
         cout << "Zeile: ";
         cin >> inputZeile;
         cout << "Spalte: ";
         cin >> inputSpalte;
         cout << endl;
-    if (spielfeld[inputZeile-1][inputSpalte-1] == ' ') {
-        return {true, inputZeile-1, inputSpalte-1};
-    } else {
-        cout << endl << "Input konnte nicht gelesen werden" << endl;
-        return getUserInput();
+
+        if (!cin) {
+            cin.clear();
+            cin.ignore(1000, '\n');
+            cout << "Ungueltige Eingabe, bitte nochmal." << endl;
+            continue;
+        }
+
+        if (inputZeile >= 1 && inputZeile <= N &&
+            inputSpalte >= 1 && inputSpalte <= N &&
+            state.board[inputZeile - 1][inputSpalte - 1] == ' ') {
+            return {true, inputZeile - 1, inputSpalte - 1};
+        }
+
+        cout << "Feld bereits belegt oder ausserhalb des Spielfelds." << endl;
     }
 }
 
-void checkforWin(gameState &state) {
-    // TODO: implement a proper efficent check that doesnt end when every tile is filled
-    // simplest way would be to check all 8 different solutions. 3 from top to bottom, 3 from left to right and 2 diagonals
-    for (int i = 0; i < 3; i++) {
-        // left to right
-        if (spielfeld[i][0] == spielfeld[i][1] && spielfeld[i][2] == spielfeld[i][1] && spielfeld[i][2] != ' ') {
+void checkforWin(gameState &state, int zeile, int spalte, char player) {
+    int p = (player == 'X') ? 0 : 1;
+
+    state.rowCount[p][zeile]++;
+    state.colCount[p][spalte]++;
+    if (zeile == spalte) state.diagCount[p]++;
+    if (zeile + spalte == N - 1) state.antiDiagCount[p]++;
+
+    if (state.rowCount[p][zeile] == N ||
+        state.colCount[p][spalte] == N ||
+        state.diagCount[p] == N ||
+        state.antiDiagCount[p] == N) {
         state.running = false;
-            state.winner = spielfeld[i][1];
-            return;
-        }
-        // top to bottom
-        if (spielfeld[0][i] == spielfeld[1][i] && spielfeld[1][i] == spielfeld[2][i] && spielfeld[1][i] != ' ') {
-            state.running = false;
-            state.winner = spielfeld[1][i];
-            return;
-        }
-    }
-    // top left to bottom right
-    if (spielfeld[0][0] == spielfeld[1][1] && spielfeld[1][1] == spielfeld[2][2] && spielfeld[2][2] != ' ') {
-        state.running = false;
-        state.winner = spielfeld[2][2];
+        state.winner = player;
         return;
     }
 
-    // top right to bottom left
-    if (spielfeld[0][2] == spielfeld[1][1] && spielfeld[1][1] == spielfeld[2][0] && spielfeld[2][0] != ' ') {
-        state.running = false;
-        state.winner = spielfeld[2][0];
-        return;
-    }
-
-    //draw
-    if (state.filledCells >= 9) {
+    if (state.filledCells >= N * N) {
         state.running = false;
         state.winner = '-';
-        return;
     }
 }
 
 int main() {
-    #ifdef _WIN32
+#ifdef _WIN32
     SetConsoleOutputCP(CP_UTF8);
-    #endif
-    cout << fixed << setprecision(2);
-
-    for (int i = 0; i < 3; i++) {
-        for (int j = 0; j < 3; j++) {
-            spielfeld[i][j] = ' ';
-        }
-    }
+#endif
 
     gameState state;
+    for (int i = 0; i < N; i++)
+        for (int j = 0; j < N; j++)
+            state.board[i][j] = ' ';
 
     int counter = 1;
     while (state.running) {
-        render();
+        render(state);
         char current_player = counter++ % 2 == 1 ? 'X' : 'O';
         cout << endl << "Am Zug ist Spieler: " << current_player << endl;
-        if (auto [ok, zeile, spalte] = getUserInput(); ok) {
-            spielfeld[zeile][spalte] = current_player;
-            state.filledCells += 1;
+        auto [ok, zeile, spalte] = getUserInput(state);
+        if (ok) {
+            state.board[zeile][spalte] = current_player;
+            state.filledCells++;
+            checkforWin(state, zeile, spalte, current_player);
         }
-        checkforWin(state);
     }
-    render();
-    cout << endl << "Der gewinner ist: " << state.winner << endl;
 
-    #ifdef _WIN32
+    render(state);
+    if (state.winner == '-') {
+        cout << endl << "Unentschieden!" << endl;
+    } else {
+        cout << endl << "Der Gewinner ist: " << state.winner << endl;
+    }
+
+#ifdef _WIN32
     system("pause");
-    #endif
+#endif
     return 0;
 }