block_prep(
    oparg_T		*oap,
    struct block_def	*bdp,
    linenr_T		lnum,
    int			is_del)
{
    int		incr = 0;
    char_u	*pend;
    char_u	*pstart;
    char_u	*line;
    char_u	*prev_pstart;
    char_u	*prev_pend;
    chartabsize_T cts;
#ifdef FEAT_LINEBREAK
    int		lbr_saved = curwin->w_p_lbr;

    // Avoid a problem with unwanted linebreaks in block mode.
    curwin->w_p_lbr = FALSE;
#endif
    bdp->startspaces = 0;
    bdp->endspaces = 0;
    bdp->textlen = 0;
    bdp->start_vcol = 0;
    bdp->end_vcol = 0;
    bdp->is_short = FALSE;
    bdp->is_oneChar = FALSE;
    bdp->pre_whitesp = 0;
    bdp->pre_whitesp_c = 0;
    bdp->end_char_vcols = 0;
    bdp->start_char_vcols = 0;

    line = ml_get(lnum);
    prev_pstart = line;
    init_chartabsize_arg(&cts, curwin, lnum, bdp->start_vcol, line, line);
    while (cts.cts_vcol < oap->start_vcol && *cts.cts_ptr != NUL)
    {
	// Count a tab for what it's worth (if list mode not on)
	incr = lbr_chartabsize(&cts);
	cts.cts_vcol += incr;
	if (VIM_ISWHITE(*cts.cts_ptr))
	{
	    bdp->pre_whitesp += incr;
	    bdp->pre_whitesp_c++;
	}
	else
	{
	    bdp->pre_whitesp = 0;
	    bdp->pre_whitesp_c = 0;
	}
	prev_pstart = cts.cts_ptr;
	MB_PTR_ADV(cts.cts_ptr);
    }
    bdp->start_vcol = cts.cts_vcol;
    pstart = cts.cts_ptr;
    clear_chartabsize_arg(&cts);

    bdp->start_char_vcols = incr;
    if (bdp->start_vcol < oap->start_vcol)	// line too short
    {
	bdp->end_vcol = bdp->start_vcol;
	bdp->is_short = TRUE;
	if (!is_del || oap->op_type == OP_APPEND)
	    bdp->endspaces = oap->end_vcol - oap->start_vcol + 1;
    }
    else
    {
	// notice: this converts partly selected Multibyte characters to
	// spaces, too.
	bdp->startspaces = bdp->start_vcol - oap->start_vcol;
	if (is_del && bdp->startspaces)
	    bdp->startspaces = bdp->start_char_vcols - bdp->startspaces;
	pend = pstart;
	bdp->end_vcol = bdp->start_vcol;
	if (bdp->end_vcol > oap->end_vcol)	// it's all in one character
	{
	    bdp->is_oneChar = TRUE;
	    if (oap->op_type == OP_INSERT)
		bdp->endspaces = bdp->start_char_vcols - bdp->startspaces;
	    else if (oap->op_type == OP_APPEND)
	    {
		bdp->startspaces += oap->end_vcol - oap->start_vcol + 1;
		bdp->endspaces = bdp->start_char_vcols - bdp->startspaces;
	    }
	    else
	    {
		bdp->startspaces = oap->end_vcol - oap->start_vcol + 1;
		if (is_del && oap->op_type != OP_LSHIFT)
		{
		    // just putting the sum of those two into
		    // bdp->startspaces doesn't work for Visual replace,
		    // so we have to split the tab in two
		    bdp->startspaces = bdp->start_char_vcols
					- (bdp->start_vcol - oap->start_vcol);
		    bdp->endspaces = bdp->end_vcol - oap->end_vcol - 1;
		}
	    }
	}
	else
	{
	    init_chartabsize_arg(&cts, curwin, lnum, bdp->end_vcol,
								  line, pend);
	    prev_pend = pend;
	    while (cts.cts_vcol <= oap->end_vcol && *cts.cts_ptr != NUL)
	    {
		// count a tab for what it's worth (if list mode not on)
		prev_pend = cts.cts_ptr;
		incr = lbr_chartabsize_adv(&cts);
		cts.cts_vcol += incr;
	    }
	    bdp->end_vcol = cts.cts_vcol;
	    pend = cts.cts_ptr;
	    clear_chartabsize_arg(&cts);

	    if (bdp->end_vcol <= oap->end_vcol
		    && (!is_del
			|| oap->op_type == OP_APPEND
			|| oap->op_type == OP_REPLACE)) // line too short
	    {
		bdp->is_short = TRUE;
		// Alternative: include spaces to fill up the block.
		// Disadvantage: can lead to trailing spaces when the line is
		// short where the text is put
		// if (!is_del || oap->op_type == OP_APPEND)
		if (oap->op_type == OP_APPEND || virtual_op)
		    bdp->endspaces = oap->end_vcol - bdp->end_vcol
							     + oap->inclusive;
		else
		    bdp->endspaces = 0; // replace doesn't add characters
	    }
	    else if (bdp->end_vcol > oap->end_vcol)
	    {
		bdp->endspaces = bdp->end_vcol - oap->end_vcol - 1;
		if (!is_del && bdp->endspaces)
		{
		    bdp->endspaces = incr - bdp->endspaces;
		    if (pend != pstart)
			pend = prev_pend;
		}
	    }
	}
	bdp->end_char_vcols = incr;
	if (is_del && bdp->startspaces)
	    pstart = prev_pstart;
	bdp->textlen = (int)(pend - pstart);
    }
    bdp->textcol = (colnr_T) (pstart - line);
    bdp->textstart = pstart;
#ifdef FEAT_LINEBREAK
    curwin->w_p_lbr = lbr_saved;
#endif
}