andlabs-ui/area_windows.go

// 24 march 2014

package ui

import (
	"fmt"
	"syscall"
	"unsafe"
	"image"
)

const (
	areastyle = _WS_HSCROLL | _WS_VSCROLL | controlstyle
	areaxstyle = 0 | controlxstyle
)

var (
	areaWndClass = toUTF16("gouiarea")
)

func getScrollPos(hwnd _HWND) (xpos int32, ypos int32) {
	var si _SCROLLINFO

	si.cbSize = uint32(unsafe.Sizeof(si))
	si.fMask = _SIF_POS | _SIF_TRACKPOS
	r1, _, err := _getScrollInfo.Call(
		uintptr(hwnd),
		uintptr(_SB_HORZ),
		uintptr(unsafe.Pointer(&si)))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error getting horizontal scroll position for Area: %v", err))
	}
	xpos = si.nPos
	si.cbSize = uint32(unsafe.Sizeof(si))			// MSDN example code reinitializes this each time, so we'll do it too just to be safe
	si.fMask = _SIF_POS | _SIF_TRACKPOS
	r1, _, err = _getScrollInfo.Call(
		uintptr(hwnd),
		uintptr(_SB_VERT),
		uintptr(unsafe.Pointer(&si)))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error getting vertical scroll position for Area: %v", err))
	}
	ypos = si.nPos
	return xpos, ypos
}

var (
	_alphaBlend = msimg32.NewProc("AlphaBlend")
	_beginPaint = user32.NewProc("BeginPaint")
	_bitBlt = gdi32.NewProc("BitBlt")
	_createCompatibleBitmap = gdi32.NewProc("CreateCompatibleBitmap")
	_createCompatibleDC = gdi32.NewProc("CreateCompatibleDC")
	_createDIBSection = gdi32.NewProc("CreateDIBSection")
	_deleteDC = gdi32.NewProc("DeleteDC")
	_deleteObject = gdi32.NewProc("DeleteObject")
	_endPaint = user32.NewProc("EndPaint")
	_fillRect = user32.NewProc("FillRect")
	_getUpdateRect = user32.NewProc("GetUpdateRect")
	// _selectObject in prefsize_windows.go
)

const (
	areaBackgroundBrush = _HBRUSH(_COLOR_BTNFACE + 1)
)

func paintArea(s *sysData) {
	var xrect _RECT
	var ps _PAINTSTRUCT

	r1, _, _ := _getUpdateRect.Call(
		uintptr(s.hwnd),
		uintptr(unsafe.Pointer(&xrect)),
		uintptr(_TRUE))		// erase the update rect with the background color
	if r1 == 0 {			// no update rect; do nothing
		return
	}

	hscroll, vscroll := getScrollPos(s.hwnd)

	// both Windows RECT and Go image.Rect are point..point, so the following is correct
	cliprect := image.Rect(int(xrect.left), int(xrect.top), int(xrect.right), int(xrect.bottom))
	cliprect = cliprect.Add(image.Pt(int(hscroll), int(vscroll)))			// adjust by scroll position
	// make sure the cliprect doesn't fall outside the size of the Area
	cliprect = cliprect.Intersect(image.Rect(0, 0, s.areawidth, s.areaheight))
	if cliprect.Empty() {		// still no update rect
		return
	}

	// TODO don't do the above, but always draw the background color?

	r1, _, err := _beginPaint.Call(
		uintptr(s.hwnd),
		uintptr(unsafe.Pointer(&ps)))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error beginning Area repaint: %v", err))
	}
	hdc := _HANDLE(r1)

	// very big thanks to Ninjifox for suggesting this technique and helping me go through it

	// first let's create the destination image, which we fill with the windows background color
	// this is how we fake drawing the background; see also http://msdn.microsoft.com/en-us/library/ms969905.aspx
	r1, _, err = _createCompatibleDC.Call(uintptr(hdc))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error creating off-screen rendering DC: %v", err))
	}
	rdc := _HANDLE(r1)
	// the bitmap has to be compatible with the window
	// if we create a bitmap compatible with the DC we just created, it'll be monochrome
	// thanks to David Heffernan in http://stackoverflow.com/questions/23033636/winapi-gdi-fillrectcolor-btnface-fills-with-strange-grid-like-brush-on-window
	r1, _, err = _createCompatibleBitmap.Call(
		uintptr(hdc),
		uintptr(xrect.right - xrect.left),
		uintptr(xrect.bottom - xrect.top))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error creating off-screen rendering bitmap: %v", err))
	}
	rbitmap := _HANDLE(r1)
	r1, _, err = _selectObject.Call(
		uintptr(rdc),
		uintptr(rbitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error connecting off-screen rendering bitmap to off-screen rendering DC: %v", err))
	}
	prevrbitmap := _HANDLE(r1)
	rrect := _RECT{
		left:		0,
		right:	xrect.right - xrect.left,
		top:		0,
		bottom:	xrect.bottom - xrect.top,
	}
	r1, _, err = _fillRect.Call(
		uintptr(rdc),
		uintptr(unsafe.Pointer(&rrect)),
		uintptr(areaBackgroundBrush))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error filling off-screen rendering bitmap with the system background color: %v", err))
	}

	i := s.handler.Paint(cliprect)
	// don't convert to BRGA just yet; see below

	// now we need to shove realbits into a bitmap
	// technically bitmaps don't know about alpha; they just ignore the alpha byte
	// AlphaBlend(), however, sees it - see http://msdn.microsoft.com/en-us/library/windows/desktop/dd183352%28v=vs.85%29.aspx
	bi := _BITMAPINFO{}
	bi.bmiHeader.biSize = uint32(unsafe.Sizeof(bi.bmiHeader))
	bi.bmiHeader.biWidth = int32(i.Rect.Dx())
	bi.bmiHeader.biHeight = -int32(i.Rect.Dy())		// negative height to force top-down drawing
	bi.bmiHeader.biPlanes = 1
	bi.bmiHeader.biBitCount = 32
	bi.bmiHeader.biCompression = _BI_RGB
	bi.bmiHeader.biSizeImage = uint32(i.Rect.Dx() * i.Rect.Dy() * 4)
	// this is all we need, but because this confused me at first, I will say the two pixels-per-meter fields are unused (see http://blogs.msdn.com/b/oldnewthing/archive/2013/05/15/10418646.aspx and page 581 of Charles Petzold's Programming Windows, Fifth Edition)
	ppvBits := uintptr(0)		// now for the trouble: CreateDIBSection() allocates the memory for us...
	r1, _, err = _createDIBSection.Call(
		uintptr(_NULL),		// Ninjifox does this, so do some wine tests (http://source.winehq.org/source/dlls/gdi32/tests/bitmap.c#L725, thanks vpovirk in irc.freenode.net/#winehackers) and even Raymond Chen (http://blogs.msdn.com/b/oldnewthing/archive/2006/11/16/1086835.aspx), so.
		uintptr(unsafe.Pointer(&bi)),
		uintptr(_DIB_RGB_COLORS),
		uintptr(unsafe.Pointer(&ppvBits)),
		uintptr(0),		// we're not dealing with hSection or dwOffset
		uintptr(0))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error creating HBITMAP for image returned by AreaHandler.Paint(): %v", err))
	}
	ibitmap := _HANDLE(r1)

	// now we have to do TWO MORE things before we can finally do alpha blending
	// first, we need to load the bitmap memory, because Windows makes it for us
	// the pixels are arranged in RGBA order, but GDI requires BGRA
	// this turns out to be just ARGB in little endian; let's convert into this memory
	// the bitmap Windows gives us has a stride == width
	toARGB(i, ppvBits, i.Rect.Dx() * 4)

	// the second thing is... make a device context for the bitmap :|
	// Ninjifox just makes another compatible DC; we'll do the same
	r1, _, err = _createCompatibleDC.Call(uintptr(hdc))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error creating HDC for image returned by AreaHandler.Paint(): %v", err))
	}
	idc := _HANDLE(r1)
	r1, _, err = _selectObject.Call(
		uintptr(idc),
		uintptr(ibitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error connecting HBITMAP for image returned by AreaHandler.Paint() to its HDC: %v", err))
	}
	previbitmap := _HANDLE(r1)

	// AND FINALLY WE CAN DO THE ALPHA BLENDING!!!!!!111
	blendfunc := _BLENDFUNCTION{
		BlendOp:				_AC_SRC_OVER,
		BlendFlags:			0,
		SourceConstantAlpha:	255,					// only use per-pixel alphas
		AlphaFormat:			_AC_SRC_ALPHA,		// premultiplied
	}
	r1, _, err = _alphaBlend.Call(
		uintptr(rdc),	// destination
		uintptr(0),		// origin and size
		uintptr(0),
		uintptr(i.Rect.Dx()),
		uintptr(i.Rect.Dy()),
		uintptr(idc),	// source image
		uintptr(0),
		uintptr(0),
		uintptr(i.Rect.Dx()),
		uintptr(i.Rect.Dy()),
		blendfunc.arg())
	if r1 == _FALSE {		// failure
		panic(fmt.Errorf("error alpha-blending image returned by AreaHandler.Paint() onto background: %v", err))
	}

	// and finally we can just blit that into the window
	r1, _, err = _bitBlt.Call(
		uintptr(hdc),
		uintptr(xrect.left),
		uintptr(xrect.top),
		uintptr(xrect.right - xrect.left),
		uintptr(xrect.bottom - xrect.top),
		uintptr(rdc),
		uintptr(0),			// from the rdc's origin
		uintptr(0),
		uintptr(_SRCCOPY))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error blitting Area image to Area: %v", err))
	}

	// now to clean up
	r1, _, err = _selectObject.Call(
		uintptr(idc),
		uintptr(previbitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error reverting HDC for image returned by AreaHandler.Paint() to original HBITMAP: %v", err))
	}
	r1, _, err = _selectObject.Call(
		uintptr(rdc),
		uintptr(prevrbitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error reverting HDC for off-screen rendering to original HBITMAP: %v", err))
	}
	r1, _, err = _deleteObject.Call(uintptr(ibitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error deleting HBITMAP for image returned by AreaHandler.Paint(): %v", err))
	}
	r1, _, err = _deleteObject.Call(uintptr(rbitmap))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error deleting HBITMAP for off-screen rendering: %v", err))
	}
	r1, _, err = _deleteDC.Call(uintptr(idc))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error deleting HDC for image returned by AreaHandler.Paint(): %v", err))
	}
	r1, _, err = _deleteDC.Call(uintptr(rdc))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error deleting HDC for off-screen rendering: %v", err))
	}

	// return value always nonzero according to MSDN
	_endPaint.Call(
		uintptr(s.hwnd),
		uintptr(unsafe.Pointer(&ps)))
}

func getAreaControlSize(hwnd _HWND) (width int, height int) {
	var rect _RECT

	r1, _, err := _getClientRect.Call(
		uintptr(hwnd),
		uintptr(unsafe.Pointer(&rect)))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error getting size of actual Area control: %v", err))
	}
	return int(rect.right - rect.left),
		int(rect.bottom - rect.top)
}

func scrollArea(s *sysData, wparam _WPARAM, which uintptr) {
	var si _SCROLLINFO

	cwid, cht := getAreaControlSize(s.hwnd)
	pagesize := int32(cwid)
	maxsize := int32(s.areawidth)
	if which == uintptr(_SB_VERT) {
		pagesize = int32(cht)
		maxsize = int32(s.areaheight)
	}

	si.cbSize = uint32(unsafe.Sizeof(si))
	si.fMask = _SIF_POS | _SIF_TRACKPOS
	r1, _, err := _getScrollInfo.Call(
		uintptr(s.hwnd),
		which,
		uintptr(unsafe.Pointer(&si)))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error getting current scroll position for scrolling: %v", err))
	}

	newpos := si.nPos
	switch wparam & 0xFFFF {
	case _SB_LEFT:			// also _SB_TOP but Go won't let me
		newpos = 0
	case _SB_RIGHT:		// also _SB_BOTTOM
		// see comment in adjustAreaScrollbars() below
		newpos = maxsize - pagesize
	case _SB_LINELEFT:		// also _SB_LINEUP
		newpos--
	case _SB_LINERIGHT:	// also _SB_LINEDOWN
		newpos++
	case _SB_PAGELEFT:		// also _SB_PAGEUP
		newpos -= pagesize
	case _SB_PAGERIGHT:	// also _SB_PAGEDOWN
		newpos += pagesize
	case _SB_THUMBPOSITION:
		// raymond chen says to just set the newpos to the SCROLLINFO nPos for this message; see http://blogs.msdn.com/b/oldnewthing/archive/2003/07/31/54601.aspx and http://blogs.msdn.com/b/oldnewthing/archive/2003/08/05/54602.aspx
		// do nothing here; newpos already has nPos
	case _SB_THUMBTRACK:
		newpos = si.nTrackPos
	}		// otherwise just keep the current position (that's what MSDN example code says, anyway)

	// make sure we're not out of range
	if newpos < 0 {
		newpos = 0
	}
	if newpos > (maxsize - pagesize) {
		newpos = maxsize - pagesize
	}

	// this would be where we would put a check to not scroll if the scroll position changed, but see the note about SB_THUMBPOSITION above: Raymond Chen's code always does the scrolling anyway in this case

	delta := -(newpos - si.nPos)	// negative because ScrollWindowEx() scrolls in the opposite direction
	dx := delta
	dy := int32(0)
	if which == uintptr(_SB_VERT) {
		dx = int32(0)
		dy = delta
	}
	r1, _, err = _scrollWindowEx.Call(
		uintptr(s.hwnd),
		uintptr(dx),
		uintptr(dy),
		uintptr(0),			// these four change what is scrolled and record info about the scroll; we're scrolling the whole client area and don't care about the returned information here
		uintptr(0),
		uintptr(0),
		uintptr(0),
		uintptr(_SW_INVALIDATE | _SW_ERASE))					// mark the remaining rect as needing redraw and erase...
	if r1 == _ERROR {		// failure
		panic(fmt.Errorf("error scrolling Area: %v", err))
	}
	// ...but don't redraw the window yet; we need to apply our scroll changes

	// we actually have to commit the change back to the scrollbar; otherwise the scroll position will merely reset itself
	si.cbSize = uint32(unsafe.Sizeof(si))
	si.fMask = _SIF_POS
	si.nPos = newpos
	_setScrollInfo.Call(
		uintptr(s.hwnd),
		which,
		uintptr(unsafe.Pointer(&si)))

	// NOW redraw it
	r1, _, err = _updateWindow.Call(uintptr(s.hwnd))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error updating Area after scrolling: %v", err))
	}
}

func adjustAreaScrollbars(s *sysData) {
	var si _SCROLLINFO

	cwid, cht := getAreaControlSize(s.hwnd)

	// the trick is we want a page to be the width/height of the visible area
	// so the scroll range would go from [0..image_dimension - control_dimension]
	// but judging from the sample code on MSDN, we don't need to do this; the scrollbar will do it for us
	// we DO need to handle it when scrolling, though, since the thumb can only go up to this upper limit

	// have to do horizontal and vertical separately
	si.cbSize = uint32(unsafe.Sizeof(si))
	si.fMask = _SIF_RANGE | _SIF_PAGE
	si.nMin = 0
	si.nMax = int32(s.areawidth - 1)		// the max point is inclusive, so we have to pass in the last valid value, not the first invalid one (see http://blogs.msdn.com/b/oldnewthing/archive/2003/07/31/54601.aspx); if we don't, we get weird things like the scrollbar sometimes showing one extra scroll position at the end that you can never scroll to
	si.nPage = uint32(cwid)
	_setScrollInfo.Call(
		uintptr(s.hwnd),
		uintptr(_SB_HORZ),
		uintptr(unsafe.Pointer(&si)),
		uintptr(_TRUE))			// redraw the scroll bar

	si.cbSize = uint32(unsafe.Sizeof(si))			// MSDN sample code does this a second time; let's do it too to be safe
	si.fMask = _SIF_RANGE | _SIF_PAGE
	si.nMin = 0
	si.nMax = int32(s.areaheight - 1)
	si.nPage = uint32(cht)
	_setScrollInfo.Call(
		uintptr(s.hwnd),
		uintptr(_SB_VERT),
		uintptr(unsafe.Pointer(&si)),
		uintptr(_TRUE))			// redraw the scroll bar
}

var (
	_invalidateRect = user32.NewProc("InvalidateRect")
)

func repaintArea(s *sysData) {
	r1, _, err := _invalidateRect.Call(
		uintptr(s.hwnd),
		uintptr(0),			// the whole area
		uintptr(_TRUE))		// have Windows erase if possible
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error flagging Area as needing repainting after event (last error: %v)", err))
	}
	r1, _, err = _updateWindow.Call(uintptr(s.hwnd))
	if r1 == 0 {		// failure
		panic(fmt.Errorf("error repainting Area after event: %v", err))
	}
}

var (
	_getKeyState = user32.NewProc("GetKeyState")
)

func getModifiers() (m Modifiers) {
	down := func(x uintptr) bool {
		// GetKeyState() gets the key state at the time of the message, so this is what we want
		r1, _, _ := _getKeyState.Call(x)
		return (r1 & 0x80) != 0
	}

	if down(_VK_CONTROL) {
		m |= Ctrl
	}
	if down(_VK_MENU) {
		m |= Alt
	}
	if down(_VK_SHIFT) {
		m |= Shift
	}
	if down(_VK_LWIN) || down(_VK_RWIN) {
		m |= Super
	}
	return m
}

var (
	_getMessageTime = user32.NewProc("GetMessageTime")
	_getDoubleClickTime = user32.NewProc("GetDoubleClickTime")
	_getSystemMetrics = user32.NewProc("GetSystemMetrics")
)

func areaMouseEvent(s *sysData, button uint, up bool, wparam _WPARAM, lparam _LPARAM) {
	var me MouseEvent

	xpos, ypos := getScrollPos(s.hwnd)		// mouse coordinates are relative to control; make them relative to Area
	xpos += lparam.X()
	ypos += lparam.Y()
	me.Pos = image.Pt(int(xpos), int(ypos))
	if !me.Pos.In(image.Rect(0, 0, s.areawidth, s.areaheight)) {		// outside the actual Area; no event
		return
	}
	if up {
		me.Up = button
	} else if button != 0 {			// don't run the click counter if the mouse was only moved
		me.Down = button
		// this returns a LONG, which is int32, but we don't need to worry about the signedness because for the same bit widths and two's complement arithmetic, s1-s2 == u1-u2 if bits(s1)==bits(s2) and bits(u1)==bits(u2) (and Windows requires two's complement: http://blogs.msdn.com/b/oldnewthing/archive/2005/05/27/422551.aspx)
		// signedness isn't much of an issue for these calls anyway because http://stackoverflow.com/questions/24022225/what-are-the-sign-extension-rules-for-calling-windows-api-functions-stdcall-t and that we're only using unsigned values (think back to how you (didn't) handle signedness in assembly language) AND because of the above AND because the statistics below (time interval and width/height) really don't make sense if negative
		time, _, _ := _getMessageTime.Call()
		maxTime, _, _ := _getDoubleClickTime.Call()
		// ignore zero returns and errors; MSDN says zero will be returned on error but that GetLastError() is meaningless
		xdist, _, _ := _getSystemMetrics.Call(_SM_CXDOUBLECLK)
		ydist, _, _ := _getSystemMetrics.Call(_SM_CYDOUBLECLK)
		me.Count = s.clickCounter.click(button, me.Pos.X, me.Pos.Y,
			time, maxTime, int(xdist / 2), int(ydist / 2))
	}
	// though wparam will contain control and shift state, let's use just one function to get modifiers for both keyboard and mouse events; it'll work the same anyway since we have to do this for alt and windows key (super)
	me.Modifiers = getModifiers()
	if button != 1 && (wparam & _MK_LBUTTON) != 0 {
		me.Held = append(me.Held, 1)
	}
	if button != 2 && (wparam & _MK_MBUTTON) != 0 {
		me.Held = append(me.Held, 2)
	}
	if button != 3 && (wparam & _MK_RBUTTON) != 0 {
		me.Held = append(me.Held, 3)
	}
	if button != 4 && (wparam & _MK_XBUTTON1) != 0 {
		me.Held = append(me.Held, 4)
	}
	if button != 5 && (wparam & _MK_XBUTTON2) != 0 {
		me.Held = append(me.Held, 5)
	}
	repaint := s.handler.Mouse(me)
	if repaint {
		repaintArea(s)
	}
}

func areaKeyEvent(s *sysData, up bool, wparam _WPARAM, lparam _LPARAM) {
	var ke KeyEvent

	// the numeric keypad keys when Num Lock is off are considered left-hand keys as the separate navigation buttons were added later
	// the numeric keypad enter, however, is a right-hand key because it has the same virtual-key code as the typewriter enter
	righthand := (lparam & 0x01000000) != 0

	scancode := byte((lparam >> 16) & 0xFF)
	ke.Modifiers = getModifiers()
	if extkey, ok := numpadextkeys[wparam]; ok && !righthand {
		// the above is special handling for numpad keys to ignore the state of Num Lock and Shift; see http://blogs.msdn.com/b/oldnewthing/archive/2004/09/06/226045.aspx and https://github.com/glfw/glfw/blob/master/src/win32_window.c#L152
		ke.ExtKey = extkey
	} else if wparam == _VK_RETURN && righthand {
		ke.ExtKey = NEnter
	} else if extkey, ok := extkeys[wparam]; ok {
		ke.ExtKey = extkey
	} else if mod, ok := modonlykeys[wparam]; ok {
		ke.Modifier = mod
		// don't include the modifier in ke.Modifiers
		ke.Modifiers &^= mod
	} else if xke, ok := fromScancode(uintptr(scancode)); ok {
		// one of these will be nonzero
		ke.Key = xke.Key
		ke.ExtKey = xke.ExtKey
	} else if ke.Modifiers == 0 {
		// no key, extkey, or modifiers; do nothing
		return
	}
	ke.Up = up
	repaint := s.handler.Key(ke)
	if repaint {
		repaintArea(s)
	}
}

// all mappings come from GLFW - https://github.com/glfw/glfw/blob/master/src/win32_window.c#L152
var numpadextkeys = map[_WPARAM]ExtKey{
	_VK_HOME:	N7,
	_VK_UP:		N8,
	_VK_PRIOR:	N9,
	_VK_LEFT:	N4,
	_VK_CLEAR:	N5,
	_VK_RIGHT:	N6,
	_VK_END:		N1,
	_VK_DOWN:	N2,
	_VK_NEXT:	N3,
	_VK_INSERT:	N0,
	_VK_DELETE:	NDot,
}

var extkeys = map[_WPARAM]ExtKey{
	_VK_ESCAPE:		Escape,
	_VK_INSERT:		Insert,
	_VK_DELETE:		Delete,
	_VK_HOME:		Home,
	_VK_END:			End,
	_VK_PRIOR:		PageUp,
	_VK_NEXT:		PageDown,
	_VK_UP:			Up,
	_VK_DOWN:		Down,
	_VK_LEFT:		Left,
	_VK_RIGHT:		Right,
	_VK_F1:			F1,
	_VK_F2:			F2,
	_VK_F3:			F3,
	_VK_F4:			F4,
	_VK_F5:			F5,
	_VK_F6:			F6,
	_VK_F7:			F7,
	_VK_F8:			F8,
	_VK_F9:			F9,
	_VK_F10:			F10,
	_VK_F11:			F11,
	_VK_F12:			F12,
	// numpad numeric keys and . are handled in events_notdarwin.go
	// numpad enter is handled in code above
	_VK_ADD:		NAdd,
	_VK_SUBTRACT:	NSubtract,
	_VK_MULTIPLY:	NMultiply,
	_VK_DIVIDE:		NDivide,
}

// sanity check
func init() {
	included := make([]bool, _nextkeys)
	for _, v := range extkeys {
		included[v] = true
	}
	for i := 1; i < int(_nextkeys); i++ {
		if i >= int(N0) && i <= int(N9) {		// skip numpad numbers, ., and enter
			continue
		}
		if i == int(NDot) || i == int(NEnter) {
			continue
		}
		if !included[i] {
			panic(fmt.Errorf("error: not all ExtKeys defined on Windows (missing %d)", i))
		}
	}
}

var modonlykeys = map[_WPARAM]Modifiers{
	// even if the separate left/right aren't necessary, have them here anyway, just to be safe
	_VK_CONTROL:	Ctrl,
	_VK_LCONTROL:	Ctrl,
	_VK_RCONTROL:	Ctrl,
	_VK_MENU:		Alt,
	_VK_LMENU:		Alt,
	_VK_RMENU:		Alt,
	_VK_SHIFT:		Shift,
	_VK_LSHIFT:		Shift,
	_VK_RSHIFT:		Shift,
	// there's no combined Windows key virtual-key code as there is with the others
	_VK_LWIN:		Super,
	_VK_RWIN:		Super,
}

var (
	_setFocus = user32.NewProc("SetFocus")
)

func areaWndProc(hwnd _HWND, uMsg uint32, wParam _WPARAM, lParam _LPARAM) _LRESULT {
	s := getSysData(hwnd)
	if s == nil {		// not yet saved
		return storeSysData(hwnd, uMsg, wParam, lParam)
	}
	switch uMsg {
	case _WM_PAINT:
		paintArea(s)
		return 0
	case _WM_ERASEBKGND:
		// don't draw a background; we'll do so when painting
		// this is to make things flicker-free; see http://msdn.microsoft.com/en-us/library/ms969905.aspx
		return 1
	case _WM_HSCROLL:
		scrollArea(s, wParam, _SB_HORZ)
		return 0
	case _WM_VSCROLL:
		scrollArea(s, wParam, _SB_VERT)
		return 0
	case _WM_SIZE:
		adjustAreaScrollbars(s)
		return 0
	case _WM_ACTIVATE:
		// don't keep the double-click timer running if the user switched programs in between clicks
		s.clickCounter.reset()
		return 0
	case _WM_MOUSEACTIVATE:
		// this happens on every mouse click (apparently), so DON'T reset the click counter, otherwise it will always be reset (not an issue, as MSDN says WM_ACTIVATE is sent alongside WM_MOUSEACTIVATE when necessary)
		// transfer keyboard focus to our Area on an activating click
		// (see http://www.catch22.net/tuts/custom-controls)
		// don't bother checking SetFocus()'s error; see http://stackoverflow.com/questions/24073695/winapi-can-setfocus-return-null-without-an-error-because-thats-what-im-see/24074912#24074912
		_setFocus.Call(uintptr(s.hwnd))
		// and don't eat the click, as we want to handle clicks that switch into Windows with Areas from other windows
		return _MA_ACTIVATE
	case _WM_MOUSEMOVE:
		areaMouseEvent(s, 0, false, wParam, lParam)
		return 0
	case _WM_LBUTTONDOWN:
		areaMouseEvent(s, 1, false, wParam, lParam)
		return 0
	case _WM_LBUTTONUP:
		areaMouseEvent(s, 1, true, wParam, lParam)
		return 0
	case _WM_MBUTTONDOWN:
		areaMouseEvent(s, 2, false, wParam, lParam)
		return 0
	case _WM_MBUTTONUP:
		areaMouseEvent(s, 2, true, wParam, lParam)
		return 0
	case _WM_RBUTTONDOWN:
		areaMouseEvent(s, 3, false, wParam, lParam)
		return 0
	case _WM_RBUTTONUP:
		areaMouseEvent(s, 3, true, wParam, lParam)
		return 0
	case _WM_XBUTTONDOWN:
		which := uint((wParam >> 16) & 0xFFFF) + 3		// values start at 1; we want them to start at 4
		areaMouseEvent(s, which, false, wParam, lParam)
		return _LRESULT(_TRUE)		// XBUTTON messages are different!
	case _WM_XBUTTONUP:
		which := uint((wParam >> 16) & 0xFFFF) + 3
		areaMouseEvent(s, which, true, wParam, lParam)
		return _LRESULT(_TRUE)
	case _WM_KEYDOWN:
		areaKeyEvent(s, false, wParam, lParam)
		return 0
	case _WM_KEYUP:
		areaKeyEvent(s, true, wParam, lParam)
		return 0
	// Alt+[anything] and F10 send these instead and require us to return to DefWindowProc() so global keystrokes such as Alt+Tab can be processed
	case _WM_SYSKEYDOWN:
		areaKeyEvent(s, false, wParam, lParam)
		return defWindowProc(hwnd, uMsg, wParam, lParam)
	case _WM_SYSKEYUP:
		areaKeyEvent(s, true, wParam, lParam)
		return defWindowProc(hwnd, uMsg, wParam, lParam)
	case msgSetAreaSize:
		s.areawidth = int(wParam)		// see setAreaSize() in sysdata_windows.go
		s.areaheight = int(lParam)
		adjustAreaScrollbars(s)
		repaintArea(s)					// this calls for an update
		return 0
	default:
		return defWindowProc(hwnd, uMsg, wParam, lParam)
	}
	panic(fmt.Sprintf("areaWndProc message %d did not return: internal bug in ui library", uMsg))
}

func registerAreaWndClass() (err error) {
	wc := &_WNDCLASS{
		style:			_CS_HREDRAW | _CS_VREDRAW,		// no CS_DBLCLKS because do that manually
		lpszClassName:	utf16ToArg(areaWndClass),
		lpfnWndProc:		syscall.NewCallback(areaWndProc),
		hInstance:		hInstance,
		hIcon:			icon,
		hCursor:			cursor,
		hbrBackground:	_HBRUSH(_NULL),		// no brush; we handle WM_ERASEBKGND
	}
	r1, _, err := _registerClass.Call(uintptr(unsafe.Pointer(wc)))
	if r1 == 0 {		// failure
		return err
	}
	return nil
}

type _BITMAPINFO struct {
	bmiHeader	_BITMAPINFOHEADER
	bmiColors	[32]uintptr	// we don't use it; make it an arbitrary number that wouldn't cause issues
}

type _BITMAPINFOHEADER struct {
	biSize			uint32
	biWidth			int32
	biHeight			int32
	biPlanes			uint16
	biBitCount		uint16
	biCompression		uint32
	biSizeImage		uint32
	biXPelsPerMeter	int32
	biYPelsPerMeter	int32
	biClrUsed			uint32
	biClrImportant		uint32
}

type _BLENDFUNCTION struct {
	BlendOp				byte
	BlendFlags			byte
	SourceConstantAlpha	byte
	AlphaFormat			byte
}

// AlphaBlend() takes a BLENDFUNCTION value
func (b _BLENDFUNCTION) arg() (x uintptr) {
	// little endian
	x = uintptr(b.AlphaFormat) << 24
	x |= uintptr(b.SourceConstantAlpha) << 16
	x |= uintptr(b.BlendFlags) << 8
	x |= uintptr(b.BlendOp)
	return x
}

type _PAINTSTRUCT struct {
	hdc			_HANDLE
	fErase		int32		// originally BOOL
	rcPaint		_RECT
	fRestore		int32		// originally BOOL
	fIncUpdate	int32		// originally BOOL
	rgbReserved	[32]byte
}