[PIU] Click Box

#include <windows.h>
#define DIVISIONS 5

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Click Check");
    HWND hwnd;
    MSG msg;
    WNDCLASS wndclass;

    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;

    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("Nie udało się zarejestrować klasy okna!"), szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName, TEXT("Click Box"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hInstance, NULL);

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    static BOOL fState[DIVISIONS][DIVISIONS];
    static int cxBlock, cyBlock;
    HDC hdc;
    int x, y;
    PAINTSTRUCT ps;
    RECT rect;

    switch (msg)
    {
    case WM_SIZE:
        cxBlock = LOWORD(lParam) / DIVISIONS;
        cyBlock = HIWORD(lParam) / DIVISIONS;
        return 0;

    case WM_LBUTTONDOWN:
        x = LOWORD(lParam) / cxBlock;
        y = HIWORD(lParam) / cyBlock;

        if (x < DIVISIONS && y < DIVISIONS)
        {
            fState[x][y] ^= 1;

            rect.left = x * cxBlock;
            rect.top = y * cyBlock;
            rect.right = (x + 1) * cxBlock;
            rect.bottom = (y + 1) * cyBlock;

            InvalidateRect(hwnd, &rect, FALSE);
        }
        else
            MessageBeep(0);
        return 0;

    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);

        for (x = 0; x < DIVISIONS; x++)
            for (y = 0; y < DIVISIONS; y++)
            {
                Rectangle(hdc, x * cxBlock, y * cyBlock, (x + 1) * cxBlock, (y + 1) * cyBlock);

                if (fState[x][y])
                {
                    MoveToEx(hdc, x * cxBlock, y * cyBlock, NULL);
                    LineTo(hdc, (x + 1) * cxBlock, (y + 1) * cyBlock);
                    MoveToEx(hdc, x * cxBlock, (y + 1) * cyBlock, NULL);
                    LineTo(hdc, (x + 1) * cxBlock, y * cyBlock);
                }
            }
        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}