raycast

static const s32 w = SCREEN_WIDTH;
static const s32 h = SCREEN_HEIGHT;

static f32 pos_x;
static f32 pos_y;

static f32 dir_x;
static f32 dir_y;

static f32 plane_x;
static f32 plane_y;

void
raycast_draw_walls(void)
{
	const u8 max_steps = 32;

	for (s32 x = 0; x < w; x++) {
		u8 map_x = pos_x;
		u8 map_y = pos_y;

		// x-coordinate in camera space
		f32 cam_x = (f32)x / w * 2 - 1;

		f32 ray_dir_x = dir_x + plane_x * cam_x;
		f32 ray_dir_y = dir_y + plane_y * cam_x;

		f32 delta_x = ABS(1 / ray_dir_x);
		f32 delta_y = ABS(1 / ray_dir_y);

		s8 step_x = SIGN(ray_dir_x);
		s8 step_y = SIGN(ray_dir_y);

		// distance from starting pos to first grid intersection
		f32 dist_x = step_x < 0 ?
			delta_x * (pos_x - map_x) :
			delta_x * (map_x - pos_x + 1);
		f32 dist_y = step_y < 0 ?
			delta_y * (pos_y - map_y) :
			delta_y * (map_y - pos_y + 1);

		BOOL side;
		u8 tile;
		u8 steps = 0;

		while (1) {
			if (++steps > max_steps)
				goto next_ray;

			if (dist_x < dist_y) {
				dist_x += delta_x;
				map_x += step_x;
				side = 0;
			} else {
				dist_y += delta_y;
				map_y += step_y;
				side = 1;
			}

			tile = map_at(map_x, map_y, MAP_WALL);
			if (tile)
				break;
		}

		f32 dist = side ?
			(dist_y - delta_y) :
			(dist_x - delta_x);

		s32 line_h = h / dist;

		s32 y0 = (h - line_h) / 2;
		s32 y1 = (h + line_h) / 2;

		f32 wall_x = side ?
			pos_x + dist * ray_dir_x :
			pos_y + dist * ray_dir_y;
		wall_x -= (s32)wall_x;

		s32 tex_x = wall_x * 64;
		if (side == 0 && ray_dir_x > 0) tex_x = 64 - tex_x - 1;
		if (side == 1 && ray_dir_y < 0) tex_x = 64 - tex_x - 1;

		tex_x += (tile - 1) * 64;

		f32 tex_step = 64.0 / line_h;
		f32 tex_pos = 0;

		s32 tex_y = 0;

		for (s32 y = y0; y < y1; y++) {
			tex_pos += tex_step;

			if (y < 0 || y >= h)
				continue;

			u32 col = PIXEL(g_bitmaps[BITMAP_TILES], tex_x, tex_y);

			tex_y = roundf(tex_pos);

			if (side)
				col = (col >> 1) & 0x7F7F7F;

			col = video_blend_color(col, 0, (f32)steps / max_steps);

			PIXEL(g_screen, x, y) = col;
		}

		next_ray:
	}
}