Method IsLinkVisible

src/linkgraph/linkgraph_gui.cpp:143–206

* Determine if a certain link crosses through the area given by the dpi with some lee way. * @param pta First end of the link. * @param ptb Second end of the link. * @param dpi Visible area. * @param padding Width or thickness of the link. * @return If the link or any of its "thickness" is visible. This may return false positives. */

 * @return If the link or any of its "thickness" is visible. This may return false positives.
 */
inline bool LinkGraphOverlay::IsLinkVisible(Point pta, Point ptb, const DrawPixelInfo *dpi, int padding) const
{
	const int left = dpi->left - padding;
	const int right = dpi->left + dpi->width + padding;
	const int top = dpi->top - padding;
	const int bottom = dpi->top + dpi->height + padding;

	/*
	 * This method is an implementation of the Cohen-Sutherland line-clipping algorithm.
	 * See: https://en.wikipedia.org/wiki/Cohen%E2%80%93Sutherland_algorithm
	 */

	const uint8_t INSIDE = 0; // 0000
	const uint8_t LEFT   = 1; // 0001
	const uint8_t RIGHT  = 2; // 0010
	const uint8_t BOTTOM = 4; // 0100
	const uint8_t TOP    = 8; // 1000

	int x0 = pta.x;
	int y0 = pta.y;
	int x1 = ptb.x;
	int y1 = ptb.y;

	auto out_code = [&](int x, int y) -> uint8_t {
		uint8_t out = INSIDE;
		if (x < left) {
			out |= LEFT;
		} else if (x > right) {
			out |= RIGHT;
		}
		if (y < top) {
			out |= TOP;
		} else if (y > bottom) {
			out |= BOTTOM;
		}
		return out;
	};

	uint8_t c0 = out_code(x0, y0);
	uint8_t c1 = out_code(x1, y1);

	while (true) {
		if (c0 == 0 || c1 == 0) return true;
		if ((c0 & c1) != 0) return false;

		if (c0 & TOP) {           // point 0 is above the clip window
			x0 = x0 + (int)(((int64_t) (x1 - x0)) * ((int64_t) (top - y0)) / ((int64_t) (y1 - y0)));
			y0 = top;
		} else if (c0 & BOTTOM) { // point 0 is below the clip window
			x0 = x0 + (int)(((int64_t) (x1 - x0)) * ((int64_t) (bottom - y0)) / ((int64_t) (y1 - y0)));
			y0 = bottom;
		} else if (c0 & RIGHT) {  // point 0 is to the right of clip window
			y0 = y0 + (int)(((int64_t) (y1 - y0)) * ((int64_t) (right - x0)) / ((int64_t) (x1 - x0)));
			x0 = right;
		} else if (c0 & LEFT) {   // point 0 is to the left of clip window
			y0 = y0 + (int)(((int64_t) (y1 - y0)) * ((int64_t) (left - x0)) / ((int64_t) (x1 - x0)));
			x0 = left;
		}