Function ValidateAutoDrag

src/rail_cmd.cpp:814–859

static CommandCost ValidateAutoDrag(Trackdir *trackdir, TileIndex start, TileIndex end)
{
	int x = TileX(start);
	int y = TileY(start);
	int ex = TileX(end);
	int ey = TileY(end);

	if (!ValParamTrackOrientation(TrackdirToTrack(*trackdir))) return CMD_ERROR;

	/* calculate delta x,y from start to end tile */
	int dx = ex - x;
	int dy = ey - y;

	/* calculate delta x,y for the first direction */
	int trdx = _trackdelta[*trackdir].x;
	int trdy = _trackdelta[*trackdir].y;

	if (!IsDiagonalTrackdir(*trackdir)) {
		trdx += _trackdelta[*trackdir ^ 1].x;
		trdy += _trackdelta[*trackdir ^ 1].y;
	}

	/* validate the direction */
	while ((trdx <= 0 && dx > 0) ||
			(trdx >= 0 && dx < 0) ||
			(trdy <= 0 && dy > 0) ||
			(trdy >= 0 && dy < 0)) {
		if (!HasBit(*trackdir, 3)) { // first direction is invalid, try the other
			SetBit(*trackdir, 3); // reverse the direction
			trdx = -trdx;
			trdy = -trdy;
		} else { // other direction is invalid too, invalid drag
			return CMD_ERROR;
		}
	}

	/* (for diagonal tracks, this is already made sure of by above test), but:
	 * for non-diagonal tracks, check if the start and end tile are on 1 line */
	if (!IsDiagonalTrackdir(*trackdir)) {
		trdx = _trackdelta[*trackdir].x;
		trdy = _trackdelta[*trackdir].y;
		if (abs(dx) != abs(dy) && abs(dx) + abs(trdy) != abs(dy) + abs(trdx)) return CMD_ERROR;
	}

	return CommandCost();
}

/**
 * Build or remove a stretch of railroad tracks.