win condition check

lib/main.dart

@@ -91,6 +91,7 @@ class _MyHomePageState extends State<MyHomePage> {
 
 enum TTCState { empty, x, o }
 
+/// Board of a single game of tic tac toe
 class TTCGame extends StatefulWidget {
   const TTCGame({
     super.key,
@@ -126,11 +127,63 @@ class _TTCGameState extends State<TTCGame> {
           };
         });
         widget.onClick?.call();
+        notifyWin();
         break;
       default:
         ScaffoldMessenger.of(context)
           ..clearSnackBars()
-          ..showSnackBar(const SnackBar(content: Text("Invalid Choice")));
+          ..showSnackBar(const SnackBar(
+              content: Text("Invalid Choice: cell already has value")));
+    }
+  }
+
+  Iterable<TTCState> getRow(int index) => data.getRange(index, index + 3);
+  Iterable<TTCState> getCol(int index) => [
+        data[index],
+        data[index + 3],
+        data[index + 3 * 2],
+      ];
+  Iterable<TTCState> getCross(int index) {
+    if (index == 0) {
+      return [data[0], data[4], data[8]];
+    }
+    return [data[3], data[4], data[6]];
+  }
+
+  TTCState _checkRow(Iterable<TTCState> row) => row.reduce((value, element) =>
+      (element == TTCState.empty || value == TTCState.empty || element != value)
+          ? TTCState.empty
+          : value);
+
+  TTCState _checkBoard(Iterable<TTCState> rowValues) =>
+      rowValues.reduce((value, element) {
+        if (value != TTCState.empty) {
+          return value;
+        }
+        if (element != TTCState.empty) {
+          return element;
+        }
+
+        return TTCState.empty;
+      });
+
+  TTCState _checkWin() {
+    Iterable<TTCState> rows = Iterable.generate(3, (i) => _checkRow(getRow(i)));
+    Iterable<TTCState> cols = Iterable.generate(3, (i) => _checkRow(getCol(i)));
+    Iterable<TTCState> crosses =
+        Iterable.generate(2, (i) => _checkRow(getCross(i)));
+
+    return _checkBoard([...rows, ...cols, ...crosses]);
+  }
+
+  void notifyWin() {
+    TTCState state = _checkWin();
+
+    if (state != TTCState.empty) {
+      ScaffoldMessenger.of(context)
+        ..clearSnackBars()
+        ..showSnackBar(
+            SnackBar(content: Text("${state.name.toUpperCase()} wins")));
     }
   }