// SPDX-License-Identifier: BSD-3-Clause
// gz_pbuf.c - LD_PRELOAD shim that gives an SDL2 GL client a GLX pbuffer
// context instead of a window context. The pbuffer path is proven on the
// emulated vGPU (gl_frames); the window drawable path wedges the X server.
// Build: gcc -shared -fPIC -O2 -o gz_pbuf.so gz_pbuf.c -lGL -lX11 -ldl

#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <GL/glx.h>
#include <GL/gl.h>

typedef struct SDL_Window SDL_Window;

static GLXContext g_ctx;
static GLXPbuffer g_pb;
static Display *g_dpy;
static int g_w, g_h;
static void *g_win;

static void *(*real_GetWindowSize)(SDL_Window *, int *, int *);
static int (*real_MakeCurrent)(SDL_Window *, void *);
static void (*real_SwapWindow)(SDL_Window *);

static void resolve(void)
{
	if (!real_GetWindowSize)
		real_GetWindowSize = dlsym(RTLD_NEXT, "SDL_GetWindowSize");
	if (!real_MakeCurrent)
		real_MakeCurrent = dlsym(RTLD_NEXT, "SDL_GL_MakeCurrent");
	if (!real_SwapWindow)
		real_SwapWindow = dlsym(RTLD_NEXT, "SDL_GL_SwapWindow");
}

static int make_pb(int w, int h)
{
	static int attribs[] = {
		GLX_DRAWABLE_TYPE, GLX_PBUFFER_BIT,
		GLX_RENDER_TYPE, GLX_RGBA_BIT,
		GLX_RED_SIZE, 8, GLX_GREEN_SIZE, 8, GLX_BLUE_SIZE, 8,
		GLX_ALPHA_SIZE, 8,
		GLX_DOUBLEBUFFER, False,
		None
	};
	GLXFBConfig *fb;
	int n = 0, scr;

	if (!g_dpy)
		g_dpy = XOpenDisplay(NULL);
	if (!g_dpy)
		return 0;
	scr = DefaultScreen(g_dpy);
	fb = glXChooseFBConfig(g_dpy, scr, attribs, &n);
	if (!fb || n < 1)
		return 0;
	g_pb = glXCreatePbuffer(g_dpy, fb[0], (int[]){
		GLX_PBUFFER_WIDTH, w, GLX_PBUFFER_HEIGHT, h, None });
	if (!g_pb)
		return 0;
	g_ctx = glXCreateNewContext(g_dpy, fb[0], GLX_RGBA_TYPE, NULL, True);
	if (!g_ctx) {
		glXDestroyPbuffer(g_dpy, g_pb);
		g_pb = 0;
		return 0;
	}
	g_w = w;
	g_h = h;
	return 1;
}

void *SDL_GL_CreateContext(SDL_Window *win)
{
	int w = 640, h = 480;
	const char *ew, *eh;
	resolve();
	ew = getenv("GZPBUF_W"); eh = getenv("GZPBUF_H");
	if (ew && eh) { w = atoi(ew); h = atoi(eh); }
	else if (real_GetWindowSize) {
		real_GetWindowSize(win, &w, &h);
		if (w <= 0 || h <= 0) { w = 640; h = 480; }
	}
	if (!make_pb(w, h)) {
		fprintf(stderr, "[gzpbuf] pbuffer create failed\n");
		return NULL;
	}
	g_win = win;
	glXMakeCurrent(g_dpy, g_pb, g_ctx);
	fprintf(stderr, "[gzpbuf] pbuffer %dx%d ctx=%p\n", g_w, g_h, (void *)g_ctx);
	return (void *)g_ctx;
}

int SDL_GL_MakeCurrent(SDL_Window *win, void *ctx)
{
	resolve();
	if (ctx && g_pb) {
		glXMakeCurrent(g_dpy, g_pb, g_ctx);
		return 0;
	}
	if (real_MakeCurrent)
		return real_MakeCurrent(win, ctx);
	return 0;
}

void SDL_GL_SwapWindow(SDL_Window *win)
{
	(void)win;
	glFinish();
}

void SDL_GL_GetDrawableSize(SDL_Window *win, int *w, int *h)
{
	resolve();
	if (g_w > 0 && g_h > 0) {
		if (w) *w = g_w;
		if (h) *h = g_h;
		return;
	}
	if (real_GetWindowSize) {
		real_GetWindowSize(win, w, h);
		return;
	}
	if (w) *w = 640;
	if (h) *h = 480;
}

void SDL_GL_GetWindowSize(SDL_Window *win, int *w, int *h)
{
	SDL_GL_GetDrawableSize(win, w, h);
}

void *SDL_GL_GetCurrentContext(void)
{
	return (void *)g_ctx;
}

SDL_Window *SDL_GL_GetCurrentWindow(void)
{
	return (SDL_Window *)g_win;
}

void SDL_GL_DeleteContext(void *ctx)
{
	if (g_ctx && (!ctx || ctx == (void *)g_ctx)) {
		glXMakeCurrent(g_dpy, None, NULL);
		glXDestroyContext(g_dpy, g_ctx);
		if (g_pb)
			glXDestroyPbuffer(g_dpy, g_pb);
		g_ctx = NULL;
		g_pb = 0;
	}
}