#define _POSIX_C_SOURCE 200809L

#include <assert.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/select.h>
#include <unistd.h>
#include <xf86drm.h>
#include <xf86drmMode.h>

typedef struct {
	uint32_t *buf;
	uint32_t handle;
	uint32_t id;
	uint32_t pitch;
	uint32_t size;
} fb_info_t;

fb_info_t fb_init(int fd, drmModeModeInfo mode)
{
	int err;

	struct drm_mode_create_dumb create = {
		.width = mode.hdisplay,
		.height = mode.vdisplay,
		.bpp = 32,
	};
	err = ioctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create);
	assert(err != -1);

	uint32_t id;
	err = drmModeAddFB(
	    fd, create.width, create.height, 24, 32, create.pitch, create.handle,
	    &id);
	assert(err == 0);

	struct drm_mode_map_dumb map = { .handle = create.handle };
	err = ioctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &map);
	assert(err != -1);

	uint32_t *buf = mmap(
	    0, create.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, map.offset);
	assert(buf != MAP_FAILED);

	return (fb_info_t) {
		.buf = buf,
		.handle = create.handle,
		.id = id,
		.pitch = create.pitch,
		.size = create.size,
	};
}

void fb_cleanup(int fd, fb_info_t fb)
{
	munmap(fb.buf, fb.size);
	drmModeRmFB(fd, fb.id);
	struct drm_mode_destroy_dumb delete = { .handle = fb.handle };
	ioctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &delete);
}

static void page_flip_handler(int, unsigned, unsigned, unsigned, void *)
{
}

int main()
{
	int err;

	const int fd = open("/dev/dri/card1", O_RDWR | O_CLOEXEC);
	assert(fd != -1);

	drmModeRes *res = drmModeGetResources(fd);
	assert(res != nullptr);
	drmModeConnector *conn = drmModeGetConnector(fd, res->connectors[0]);
	assert(conn != nullptr);
	drmModeModeInfo mode = conn->modes[0];
	uint32_t conn_id = conn->connector_id;

	fb_info_t fbs[2] = {
		fb_init(fd, mode),
		fb_init(fd, mode),
	};
	int front = 0, back = 1;

	err = drmModeSetCrtc(
	    fd, res->crtcs[0], fbs[front].id, 0, 0, &conn_id, 1, &mode);
	assert(err == 0);
	drmModeCrtc *crtc = drmModeGetCrtc(fd, res->crtcs[0]);
	assert(crtc != nullptr);

	fd_set set;
	FD_ZERO(&set);
	FD_SET(fd, &set);

	drmEventContext ev = {
		.version = DRM_EVENT_CONTEXT_VERSION,
		.page_flip_handler = page_flip_handler,
	};

	for (uint64_t frame = 0; frame < (10 * 60); ++frame) {
		for (int y = 0; y < mode.vdisplay; ++y) {
			for (int x = 0; x < mode.hdisplay; ++x) {
				const uint8_t r = (x + frame) % 256;
				const uint8_t g = (y + frame) & 256;
				const uint8_t b = (x + y + frame) % 256;
				fbs[back].buf[y * (fbs[back].pitch / 4) + x]
				    = (r << 16) | (g << 8) | b;
			}
		}

		drmModePageFlip(
		    fd, crtc->crtc_id, fbs[back].id, DRM_MODE_PAGE_FLIP_EVENT,
		    nullptr);
		select(fd + 1, &set, nullptr, nullptr, nullptr);
		drmHandleEvent(fd, &ev);

		front = (front + 1) & 1;
		back = (back + 1) & 1;
	}

	drmModeSetCrtc(
	    fd, crtc->crtc_id, crtc->buffer_id, crtc->x, crtc->y, &conn_id, 1,
	    &crtc->mode);

	fb_cleanup(fd, fbs[0]);
	fb_cleanup(fd, fbs[1]);
	drmModeFreeCrtc(crtc);
	drmModeFreeConnector(conn);
	drmModeFreeResources(res);
	close(fd);

	return 0;
}