draw.lua

local ffi = require 'ffi'
ffi.cdef[[
typedef struct _XDisplay Display;
typedef struct Screen Screen;
extern Display *XOpenDisplay(
    const char*	/* display_name */
);
extern int XDefaultScreen(
    Display*		/* display */
);
typedef unsigned long XID;
typedef XID Window;
typedef unsigned long Atom;
typedef XID Drawable;
typedef XID Colormap;
extern Window XCreateSimpleWindow(
    Display*		/* display */,
    Window		/* parent */,
    int			/* x */,
    int			/* y */,
    unsigned int	/* width */,
    unsigned int	/* height */,
    unsigned int	/* border_width */,
    unsigned long	/* border */,
    unsigned long	/* background */
);
extern Window XRootWindow(
    Display*		/* display */,
    int			/* screen_number */
);
extern unsigned long XBlackPixel(
    Display*		/* display */,
    int			/* screen_number */
);
extern unsigned long XWhitePixel(
    Display*		/* display */,
    int			/* screen_number */
);
extern int XSelectInput(
    Display*		/* display */,
    Window		/* w */,
    long		/* event_mask */
);
extern int XMapWindow(
    Display*		/* display */,
    Window		/* w */
);

typedef int Bool;
typedef unsigned long Time;
/*
 * Definitions of specific events.
 */
typedef struct {
	int type;		/* of event */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;	        /* "event" window it is reported relative to */
	Window root;	        /* root window that the event occurred on */
	Window subwindow;	/* child window */
	Time time;		/* milliseconds */
	int x, y;		/* pointer x, y coordinates in event window */
	int x_root, y_root;	/* coordinates relative to root */
	unsigned int state;	/* key or button mask */
	unsigned int keycode;	/* detail */
	Bool same_screen;	/* same screen flag */
} XKeyEvent;
typedef XKeyEvent XKeyPressedEvent;
typedef XKeyEvent XKeyReleasedEvent;

typedef struct {
	int type;		/* of event */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;	        /* "event" window it is reported relative to */
	Window root;	        /* root window that the event occurred on */
	Window subwindow;	/* child window */
	Time time;		/* milliseconds */
	int x, y;		/* pointer x, y coordinates in event window */
	int x_root, y_root;	/* coordinates relative to root */
	unsigned int state;	/* key or button mask */
	unsigned int button;	/* detail */
	Bool same_screen;	/* same screen flag */
} XButtonEvent;
typedef XButtonEvent XButtonPressedEvent;
typedef XButtonEvent XButtonReleasedEvent;

typedef struct {
	int type;		/* of event */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;	        /* "event" window reported relative to */
	Window root;	        /* root window that the event occurred on */
	Window subwindow;	/* child window */
	Time time;		/* milliseconds */
	int x, y;		/* pointer x, y coordinates in event window */
	int x_root, y_root;	/* coordinates relative to root */
	unsigned int state;	/* key or button mask */
	char is_hint;		/* detail */
	Bool same_screen;	/* same screen flag */
} XMotionEvent;
typedef XMotionEvent XPointerMovedEvent;

typedef struct {
	int type;		/* of event */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;	        /* "event" window reported relative to */
	Window root;	        /* root window that the event occurred on */
	Window subwindow;	/* child window */
	Time time;		/* milliseconds */
	int x, y;		/* pointer x, y coordinates in event window */
	int x_root, y_root;	/* coordinates relative to root */
	int mode;		/* NotifyNormal, NotifyGrab, NotifyUngrab */
	int detail;
	/*
	 * NotifyAncestor, NotifyVirtual, NotifyInferior,
	 * NotifyNonlinear,NotifyNonlinearVirtual
	 */
	Bool same_screen;	/* same screen flag */
	Bool focus;		/* boolean focus */
	unsigned int state;	/* key or button mask */
} XCrossingEvent;
typedef XCrossingEvent XEnterWindowEvent;
typedef XCrossingEvent XLeaveWindowEvent;

typedef struct {
	int type;		/* FocusIn or FocusOut */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;		/* window of event */
	int mode;		/* NotifyNormal, NotifyWhileGrabbed,
				   NotifyGrab, NotifyUngrab */
	int detail;
	/*
	 * NotifyAncestor, NotifyVirtual, NotifyInferior,
	 * NotifyNonlinear,NotifyNonlinearVirtual, NotifyPointer,
	 * NotifyPointerRoot, NotifyDetailNone
	 */
} XFocusChangeEvent;
typedef XFocusChangeEvent XFocusInEvent;
typedef XFocusChangeEvent XFocusOutEvent;

/* generated on EnterWindow and FocusIn  when KeyMapState selected */
typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	char key_vector[32];
} XKeymapEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	int x, y;
	int width, height;
	int count;		/* if non-zero, at least this many more */
} XExposeEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Drawable drawable;
	int x, y;
	int width, height;
	int count;		/* if non-zero, at least this many more */
	int major_code;		/* core is CopyArea or CopyPlane */
	int minor_code;		/* not defined in the core */
} XGraphicsExposeEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Drawable drawable;
	int major_code;		/* core is CopyArea or CopyPlane */
	int minor_code;		/* not defined in the core */
} XNoExposeEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	int state;		/* Visibility state */
} XVisibilityEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window parent;		/* parent of the window */
	Window window;		/* window id of window created */
	int x, y;		/* window location */
	int width, height;	/* size of window */
	int border_width;	/* border width */
	Bool override_redirect;	/* creation should be overridden */
} XCreateWindowEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
} XDestroyWindowEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	Bool from_configure;
} XUnmapEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	Bool override_redirect;	/* boolean, is override set... */
} XMapEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window parent;
	Window window;
} XMapRequestEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	Window parent;
	int x, y;
	Bool override_redirect;
} XReparentEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	int x, y;
	int width, height;
	int border_width;
	Window above;
	Bool override_redirect;
} XConfigureEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	int x, y;
} XGravityEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	int width, height;
} XResizeRequestEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window parent;
	Window window;
	int x, y;
	int width, height;
	int border_width;
	Window above;
	int detail;		/* Above, Below, TopIf, BottomIf, Opposite */
	unsigned long value_mask;
} XConfigureRequestEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window event;
	Window window;
	int place;		/* PlaceOnTop, PlaceOnBottom */
} XCirculateEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window parent;
	Window window;
	int place;		/* PlaceOnTop, PlaceOnBottom */
} XCirculateRequestEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	Atom atom;
	Time time;
	int state;		/* NewValue, Deleted */
} XPropertyEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	Atom selection;
	Time time;
} XSelectionClearEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window owner;
	Window requestor;
	Atom selection;
	Atom target;
	Atom property;
	Time time;
} XSelectionRequestEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window requestor;
	Atom selection;
	Atom target;
	Atom property;		/* ATOM or None */
	Time time;
} XSelectionEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	Colormap colormap;	/* COLORMAP or None */
	Bool new;
	int state;		/* ColormapInstalled, ColormapUninstalled */
} XColormapEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	Atom message_type;
	int format;
	union {
		char b[20];
		short s[10];
		long l[5];
		} data;
} XClientMessageEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;		/* unused */
	int request;		/* one of MappingModifier, MappingKeyboard,
				   MappingPointer */
	int first_keycode;	/* first keycode */
	int count;		/* defines range of change w. first_keycode*/
} XMappingEvent;

typedef struct {
	int type;
	Display *display;	/* Display the event was read from */
	XID resourceid;		/* resource id */
	unsigned long serial;	/* serial number of failed request */
	unsigned char error_code;	/* error code of failed request */
	unsigned char request_code;	/* Major op-code of failed request */
	unsigned char minor_code;	/* Minor op-code of failed request */
} XErrorEvent;

typedef struct {
	int type;
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;/* Display the event was read from */
	Window window;	/* window on which event was requested in event mask */
} XAnyEvent;

typedef struct
    {
    int            type;         /* of event. Always GenericEvent */
    unsigned long  serial;       /* # of last request processed */
    Bool           send_event;   /* true if from SendEvent request */
    Display        *display;     /* Display the event was read from */
    int            extension;    /* major opcode of extension that caused the event */
    int            evtype;       /* actual event type. */
    } XGenericEvent;

typedef struct {
    int            type;         /* of event. Always GenericEvent */
    unsigned long  serial;       /* # of last request processed */
    Bool           send_event;   /* true if from SendEvent request */
    Display        *display;     /* Display the event was read from */
    int            extension;    /* major opcode of extension that caused the event */
    int            evtype;       /* actual event type. */
    unsigned int   cookie;
    void           *data;
} XGenericEventCookie;

typedef union _XEvent {
        int type;		/* must not be changed; first element */
	XAnyEvent xany;
	XKeyEvent xkey;
	XButtonEvent xbutton;
	XMotionEvent xmotion;
	XCrossingEvent xcrossing;
	XFocusChangeEvent xfocus;
	XExposeEvent xexpose;
	XGraphicsExposeEvent xgraphicsexpose;
	XNoExposeEvent xnoexpose;
	XVisibilityEvent xvisibility;
	XCreateWindowEvent xcreatewindow;
	XDestroyWindowEvent xdestroywindow;
	XUnmapEvent xunmap;
	XMapEvent xmap;
	XMapRequestEvent xmaprequest;
	XReparentEvent xreparent;
	XConfigureEvent xconfigure;
	XGravityEvent xgravity;
	XResizeRequestEvent xresizerequest;
	XConfigureRequestEvent xconfigurerequest;
	XCirculateEvent xcirculate;
	XCirculateRequestEvent xcirculaterequest;
	XPropertyEvent xproperty;
	XSelectionClearEvent xselectionclear;
	XSelectionRequestEvent xselectionrequest;
	XSelectionEvent xselection;
	XColormapEvent xcolormap;
	XClientMessageEvent xclient;
	XMappingEvent xmapping;
	XErrorEvent xerror;
	XKeymapEvent xkeymap;
	XGenericEvent xgeneric;
	XGenericEventCookie xcookie;
	long pad[24];
} XEvent;
extern int XNextEvent(
    Display*		/* display */,
    XEvent*		/* event_return */
);

typedef struct _XGC *GC;
extern int XDrawLine(
    Display*		/* display */,
    Drawable		/* d */,
    GC			/* gc */,
    int			/* x1 */,
    int			/* y1 */,
    int			/* x2 */,
    int			/* y2 */
);
extern GC XDefaultGC(
    Display*		/* display */,
    int			/* screen_number */
);

extern int XCloseDisplay(
    Display*		/* display */
);
]]
local X = ffi.load('X11')
local ExposureMask = bit.lshift(1, 15)
local KeyPressMask = bit.lshift(1, 0)  
local PointerMotionMask = bit.lshift(1, 6)
local KeyPress = 2
local MotionNotify = 6
local Expose = 12
local bit = require 'bit'

local function new_point(x, y)
   return {x=x, y=y}
end

local function load_points()
   local f = loadfile('./save.txt')
   if f == nil then
      return {new_point(-1, -1)}
   end

   return f()
end

local function save_points(ll)
   local fptr = io.open('./save.txt', 'w')

   fptr:write('return {\n')
   for _, head in ipairs(ll) do
      fptr:write(string.format('{x=%i, y=%i},\n', head.x, head.y))
   end
   fptr:write('}')
   fptr:close()
end

local ll = load_points()

local doRefresh

local display = X.XOpenDisplay(nil)
if display == nil then
   error("Cannot open display")
end

local s = X.XDefaultScreen(display)

local window = X.XCreateSimpleWindow(display, X.XRootWindow(display, s), 10, 10, 200, 200, 1, X.XBlackPixel(display, s), X.XWhitePixel(display, s))

X.XSelectInput(display, window, bit.bor(ExposureMask, KeyPressMask, PointerMotionMask))

X.XMapWindow(display, window)

local event = ffi.new('XEvent[1]') -- https://stackoverflow.com/a/24113572
while true do
   X.XNextEvent(display, event)

   if event[0].type == MotionNotify then
      local points_tip_new = new_point(ffi.cast('XMotionEvent*', event).x, ffi.cast('XMotionEvent*', event).y)
      table.insert(ll, points_tip_new)

      doRefresh = true
   end

   if event[0].type == Expose then
      local points_tip_new = new_point(-1, -1)
      table.insert(ll, points_tip_new)

      doRefresh = true
   end

   if event[0].type == KeyPress and (event[0].xkey.keycode == 0x09 or event[0].xkey.keycode == 61) then -- diff keycodes on diff OSes?
      break
   end

   if doRefresh then
      local lastX = -1
      local lastY = -1
      for _, p in ipairs(ll) do
         if p.x ~= -1 and lastX ~= -1 then
            X.XDrawLine(display, window, X.XDefaultGC(display, s), p.x, p.y, lastX, lastY)
         end
         lastX = p.x
         lastY = p.y
      end

      doRefresh = false
   end
end
save_points(ll)
X.XCloseDisplay(display)