#include <Windows.h>

HWND child;
HDC bbuf = 0;

LRESULT CALLBACK WndProc(HWND window, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    switch (uMsg) {
    case WM_CREATE:
        return 0;

    case WM_MOUSEMOVE:
        InvalidateRect(window, NULL, true);

        return 0;

    case WM_PAINT: {
        PAINTSTRUCT ps;
        POINT mpos;
        GetCursorPos(&mpos);
        ScreenToClient(window, &mpos);

        BeginPaint(window, &ps);

        bbuf = bbuf ? bbuf : CreateCompatibleDC(ps.hdc);

        static auto& backbuffer = bbuf;

        // hardcoded size is the same in the CreateWindowEx call
        static auto backbmp = CreateCompatibleBitmap(ps.hdc, 400, 400);

        static auto unused = SelectObject(backbuffer, backbmp);

        POINT points[2] = {
            { 0, 0 },
            { mpos.x, mpos.y }
        };

        // give ourselves a white background so we can see the wblack line
        SelectObject(backbuffer, (HBRUSH)GetStockObject(WHITE_BRUSH));
        Rectangle(backbuffer, 0, 0, 400, 400);
        SelectObject(backbuffer, (HBRUSH)GetStockObject(BLACK_BRUSH));
        Polyline(backbuffer, points, 2);

        SendMessage(child, WM_PAINT, 0, 0);

        BitBlt(ps.hdc, 0, 0, 400, 400, backbuffer, 0, 0, SRCCOPY);

        EndPaint(window, &ps);

        return 0;
    }

    case WM_ERASEBKGND:
        return 0;

    case WM_NCDESTROY:
        PostQuitMessage(0);
        break;
    }

    return DefWindowProc(window, uMsg, wParam, lParam);
}

LRESULT CALLBACK ChildWndProc(HWND window, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    switch (uMsg) {
    case WM_CREATE:
        return 0;

    case WM_PAINT: {
        PAINTSTRUCT ps;

        BeginPaint(window, &ps);

        static auto backbuffer = CreateCompatibleDC(ps.hdc);
        static auto backbmp = CreateCompatibleBitmap(ps.hdc, 100, 50);

        static auto unused = SelectObject(backbuffer, backbmp);

        BitBlt(backbuffer, 0, 0, 100, 50, bbuf, 50, 150, SRCCOPY);

        RECT r = { 0, 0, 50, 100 };

        SetBkMode(backbuffer, TRANSPARENT);
        SetTextColor(backbuffer, RGB(255, 0, 0));
        DrawText(backbuffer, "hello", 5, &r, DT_NOCLIP | DT_TABSTOP | DT_EXPANDTABS | DT_NOPREFIX);

        BitBlt(bbuf, 50, 150, 100, 50, backbuffer, 0, 0, SRCCOPY);

        EndPaint(window, &ps);

        return 0;
    }

    case WM_ERASEBKGND:
        return 0;
    }

    return DefWindowProc(window, uMsg, wParam, lParam);
}

int PASCAL WinMain(HINSTANCE hinst, HINSTANCE, LPSTR, int nShowCmd) {
    WNDCLASS wc;
    wc.style         = 0;
    wc.lpfnWndProc   = WndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hinst;
    wc.hIcon         = NULL;
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = "test_window";

    RegisterClass(&wc);

    wc.style         = 0;
    wc.lpfnWndProc   = ChildWndProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hinst;
    wc.hIcon         = NULL;
    wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = "transparent_window";

    RegisterClass(&wc);

    HWND wnd = CreateWindowEx(NULL, "test_window", "test", WS_OVERLAPPEDWINDOW | WS_VISIBLE, 50, 50, 400, 400, NULL, NULL, hinst, NULL);

    child = CreateWindowEx(NULL, "transparent_window", "", WS_VISIBLE | WS_CLIPSIBLINGS | WS_CLIPCHILDREN | WS_CHILD, 50, 150, 100, 50, wnd, NULL, hinst, NULL);

    MSG m;

    while (GetMessage(&m, NULL, 0, 0)) {
        TranslateMessage(&m);
        DispatchMessage(&m);
    }

    return 0;
}