merge changes from upstream

[MacVim/jjgod.git] / src / move.c

/* vi:set ts=8 sts=4 sw=4:
 *
 * VIM - Vi IMproved    by Bram Moolenaar
 *
 * Do ":help uganda"  in Vim to read copying and usage conditions.
 * Do ":help credits" in Vim to see a list of people who contributed.
 * See README.txt for an overview of the Vim source code.
 */
/*
 * move.c: Functions for moving the cursor and scrolling text.
 *
 * There are two ways to move the cursor:
 * 1. Move the cursor directly, the text is scrolled to keep the cursor in the
 *    window.
 * 2. Scroll the text, the cursor is moved into the text visible in the
 *    window.
 * The 'scrolloff' option makes this a bit complicated.
 */

#include "vim.h"

static void comp_botline __ARGS((win_T *wp));
static int scrolljump_value __ARGS((void));
static int check_top_offset __ARGS((void));
static void curs_rows __ARGS((win_T *wp, int do_botline));
static void validate_botline_win __ARGS((win_T *wp));
static void validate_cheight __ARGS((void));

typedef struct
{
    linenr_T        lnum;       /* line number */
#ifdef FEAT_DIFF
    int             fill;       /* filler lines */
#endif
    int             height;     /* height of added line */
} lineoff_T;

static void topline_back __ARGS((lineoff_T *lp));
static void botline_forw __ARGS((lineoff_T *lp));
#ifdef FEAT_DIFF
static void botline_topline __ARGS((lineoff_T *lp));
static void topline_botline __ARGS((lineoff_T *lp));
static void max_topfill __ARGS((void));
#endif

/*
 * Compute wp->w_botline for the current wp->w_topline.  Can be called after
 * wp->w_topline changed.
 */
    static void
comp_botline(wp)
    win_T       *wp;
{
    int         n;
    linenr_T    lnum;
    int         done;
#ifdef FEAT_FOLDING
    linenr_T    last;
    int         folded;
#endif

    /*
     * If w_cline_row is valid, start there.
     * Otherwise have to start at w_topline.
     */
    check_cursor_moved(wp);
    if (wp->w_valid & VALID_CROW)
    {
        lnum = wp->w_cursor.lnum;
        done = wp->w_cline_row;
    }
    else
    {
        lnum = wp->w_topline;
        done = 0;
    }

    for ( ; lnum <= wp->w_buffer->b_ml.ml_line_count; ++lnum)
    {
#ifdef FEAT_FOLDING
        last = lnum;
        folded = FALSE;
        if (hasFoldingWin(wp, lnum, NULL, &last, TRUE, NULL))
        {
            n = 1;
            folded = TRUE;
        }
        else
#endif
#ifdef FEAT_DIFF
            if (lnum == wp->w_topline)
                n = plines_win_nofill(wp, lnum, TRUE) + wp->w_topfill;
            else
#endif
                n = plines_win(wp, lnum, TRUE);
        if (
#ifdef FEAT_FOLDING
                lnum <= wp->w_cursor.lnum && last >= wp->w_cursor.lnum
#else
                lnum == wp->w_cursor.lnum
#endif
           )
        {
            wp->w_cline_row = done;
            wp->w_cline_height = n;
#ifdef FEAT_FOLDING
            wp->w_cline_folded = folded;
#endif
            wp->w_valid |= (VALID_CROW|VALID_CHEIGHT);
        }
        if (done + n > wp->w_height)
            break;
        done += n;
#ifdef FEAT_FOLDING
        lnum = last;
#endif
    }

    /* wp->w_botline is the line that is just below the window */
    wp->w_botline = lnum;
    wp->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;

    set_empty_rows(wp, done);
}

/*
 * Update curwin->w_topline and redraw if necessary.
 * Used to update the screen before printing a message.
 */
    void
update_topline_redraw()
{
    update_topline();
    if (must_redraw)
        update_screen(0);
}

/*
 * Update curwin->w_topline to move the cursor onto the screen.
 */
    void
update_topline()
{
    long        line_count;
    int         halfheight;
    int         n;
    linenr_T    old_topline;
#ifdef FEAT_DIFF
    int         old_topfill;
#endif
#ifdef FEAT_FOLDING
    linenr_T    lnum;
#endif
    int         check_topline = FALSE;
    int         check_botline = FALSE;
#ifdef FEAT_MOUSE
    int         save_so = p_so;
#endif

    if (!screen_valid(TRUE))
        return;

    check_cursor_moved(curwin);
    if (curwin->w_valid & VALID_TOPLINE)
        return;

#ifdef FEAT_MOUSE
    /* When dragging with the mouse, don't scroll that quickly */
    if (mouse_dragging > 0)
        p_so = mouse_dragging - 1;
#endif

    old_topline = curwin->w_topline;
#ifdef FEAT_DIFF
    old_topfill = curwin->w_topfill;
#endif

    /*
     * If the buffer is empty, always set topline to 1.
     */
    if (bufempty())             /* special case - file is empty */
    {
        if (curwin->w_topline != 1)
            redraw_later(NOT_VALID);
        curwin->w_topline = 1;
#ifdef FEAT_DIFF
        curwin->w_topfill = 0;
#endif
        curwin->w_botline = 2;
        curwin->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;
#ifdef FEAT_SCROLLBIND
        curwin->w_scbind_pos = 1;
#endif
    }

    /*
     * If the cursor is above or near the top of the window, scroll the window
     * to show the line the cursor is in, with 'scrolloff' context.
     */
    else
    {
        if (curwin->w_topline > 1)
        {
            /* If the cursor is above topline, scrolling is always needed.
             * If the cursor is far below topline and there is no folding,
             * scrolling down is never needed. */
            if (curwin->w_cursor.lnum < curwin->w_topline)
                check_topline = TRUE;
            else if (check_top_offset())
                check_topline = TRUE;
        }
#ifdef FEAT_DIFF
            /* Check if there are more filler lines than allowed. */
        if (!check_topline && curwin->w_topfill > diff_check_fill(curwin,
                                                           curwin->w_topline))
            check_topline = TRUE;
#endif

        if (check_topline)
        {
            halfheight = curwin->w_height / 2 - 1;
            if (halfheight < 2)
                halfheight = 2;

#ifdef FEAT_FOLDING
            if (hasAnyFolding(curwin))
            {
                /* Count the number of logical lines between the cursor and
                 * topline + p_so (approximation of how much will be
                 * scrolled). */
                n = 0;
                for (lnum = curwin->w_cursor.lnum;
                                      lnum < curwin->w_topline + p_so; ++lnum)
                {
                    ++n;
                    /* stop at end of file or when we know we are far off */
                    if (lnum >= curbuf->b_ml.ml_line_count || n >= halfheight)
                        break;
                    (void)hasFolding(lnum, NULL, &lnum);
                }
            }
            else
#endif
                n = curwin->w_topline + p_so - curwin->w_cursor.lnum;

            /* If we weren't very close to begin with, we scroll to put the
             * cursor in the middle of the window.  Otherwise put the cursor
             * near the top of the window. */
            if (n >= halfheight)
                scroll_cursor_halfway(FALSE);
            else
            {
                scroll_cursor_top(scrolljump_value(), FALSE);
                check_botline = TRUE;
            }
        }

        else
        {
#ifdef FEAT_FOLDING
            /* Make sure topline is the first line of a fold. */
            (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif
            check_botline = TRUE;
        }
    }

    /*
     * If the cursor is below the bottom of the window, scroll the window
     * to put the cursor on the window.
     * When w_botline is invalid, recompute it first, to avoid a redraw later.
     * If w_botline was approximated, we might need a redraw later in a few
     * cases, but we don't want to spend (a lot of) time recomputing w_botline
     * for every small change.
     */
    if (check_botline)
    {
        if (!(curwin->w_valid & VALID_BOTLINE_AP))
            validate_botline();

        if (curwin->w_botline <= curbuf->b_ml.ml_line_count)
        {
            if (curwin->w_cursor.lnum < curwin->w_botline
                    && ((long)curwin->w_cursor.lnum
                                             >= (long)curwin->w_botline - p_so
#ifdef FEAT_FOLDING
                        || hasAnyFolding(curwin)
#endif
                        ))
            {
                lineoff_T       loff;

                /* Cursor is above botline, check if there are 'scrolloff'
                 * window lines below the cursor.  If not, need to scroll. */
                n = curwin->w_empty_rows;
                loff.lnum = curwin->w_cursor.lnum;
#ifdef FEAT_FOLDING
                /* In a fold go to its last line. */
                (void)hasFolding(loff.lnum, NULL, &loff.lnum);
#endif
#ifdef FEAT_DIFF
                loff.fill = 0;
                n += curwin->w_filler_rows;
#endif
                loff.height = 0;
                while (loff.lnum < curwin->w_botline
#ifdef FEAT_DIFF
                        && (loff.lnum + 1 < curwin->w_botline || loff.fill == 0)
#endif
                        )
                {
                    n += loff.height;
                    if (n >= p_so)
                        break;
                    botline_forw(&loff);
                }
                if (n >= p_so)
                    /* sufficient context, no need to scroll */
                    check_botline = FALSE;
            }
            if (check_botline)
            {
#ifdef FEAT_FOLDING
                if (hasAnyFolding(curwin))
                {
                    /* Count the number of logical lines between the cursor and
                     * botline - p_so (approximation of how much will be
                     * scrolled). */
                    line_count = 0;
                    for (lnum = curwin->w_cursor.lnum;
                                     lnum >= curwin->w_botline - p_so; --lnum)
                    {
                        ++line_count;
                        /* stop at end of file or when we know we are far off */
                        if (lnum <= 0 || line_count > curwin->w_height + 1)
                            break;
                        (void)hasFolding(lnum, &lnum, NULL);
                    }
                }
                else
#endif
                    line_count = curwin->w_cursor.lnum - curwin->w_botline
                                                                   + 1 + p_so;
                if (line_count <= curwin->w_height + 1)
                    scroll_cursor_bot(scrolljump_value(), FALSE);
                else
                    scroll_cursor_halfway(FALSE);
            }
        }
    }
    curwin->w_valid |= VALID_TOPLINE;

    /*
     * Need to redraw when topline changed.
     */
    if (curwin->w_topline != old_topline
#ifdef FEAT_DIFF
            || curwin->w_topfill != old_topfill
#endif
            )
    {
        dollar_vcol = 0;
        if (curwin->w_skipcol != 0)
        {
            curwin->w_skipcol = 0;
            redraw_later(NOT_VALID);
        }
        else
            redraw_later(VALID);
        /* May need to set w_skipcol when cursor in w_topline. */
        if (curwin->w_cursor.lnum == curwin->w_topline)
            validate_cursor();
    }

#ifdef FEAT_MOUSE
    p_so = save_so;
#endif
}

/*
 * Return the scrolljump value to use for the current window.
 * When 'scrolljump' is positive use it as-is.
 * When 'scrolljump' is negative use it as a percentage of the window height.
 */
    static int
scrolljump_value()
{
    if (p_sj >= 0)
        return (int)p_sj;
    return (curwin->w_height * -p_sj) / 100;
}

/*
 * Return TRUE when there are not 'scrolloff' lines above the cursor for the
 * current window.
 */
    static int
check_top_offset()
{
    lineoff_T   loff;
    int         n;

    if (curwin->w_cursor.lnum < curwin->w_topline + p_so
#ifdef FEAT_FOLDING
                    || hasAnyFolding(curwin)
#endif
            )
    {
        loff.lnum = curwin->w_cursor.lnum;
#ifdef FEAT_DIFF
        loff.fill = 0;
        n = curwin->w_topfill;      /* always have this context */
#else
        n = 0;
#endif
        /* Count the visible screen lines above the cursor line. */
        while (n < p_so)
        {
            topline_back(&loff);
            /* Stop when included a line above the window. */
            if (loff.lnum < curwin->w_topline
#ifdef FEAT_DIFF
                    || (loff.lnum == curwin->w_topline && loff.fill > 0)
#endif
                    )
                break;
            n += loff.height;
        }
        if (n < p_so)
            return TRUE;
    }
    return FALSE;
}

    void
update_curswant()
{
    if (curwin->w_set_curswant)
    {
        validate_virtcol();
        curwin->w_curswant = curwin->w_virtcol;
        curwin->w_set_curswant = FALSE;
    }
}

/*
 * Check if the cursor has moved.  Set the w_valid flag accordingly.
 */
    void
check_cursor_moved(wp)
    win_T       *wp;
{
    if (wp->w_cursor.lnum != wp->w_valid_cursor.lnum)
    {
        wp->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL
                                     |VALID_CHEIGHT|VALID_CROW|VALID_TOPLINE);
        wp->w_valid_cursor = wp->w_cursor;
        wp->w_valid_leftcol = wp->w_leftcol;
    }
    else if (wp->w_cursor.col != wp->w_valid_cursor.col
             || wp->w_leftcol != wp->w_valid_leftcol
#ifdef FEAT_VIRTUALEDIT
             || wp->w_cursor.coladd != wp->w_valid_cursor.coladd
#endif
             )
    {
        wp->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL);
        wp->w_valid_cursor.col = wp->w_cursor.col;
        wp->w_valid_leftcol = wp->w_leftcol;
#ifdef FEAT_VIRTUALEDIT
        wp->w_valid_cursor.coladd = wp->w_cursor.coladd;
#endif
    }
}

/*
 * Call this function when some window settings have changed, which require
 * the cursor position, botline and topline to be recomputed and the window to
 * be redrawn.  E.g, when changing the 'wrap' option or folding.
 */
    void
changed_window_setting()
{
    changed_window_setting_win(curwin);
}

    void
changed_window_setting_win(wp)
    win_T       *wp;
{
    wp->w_lines_valid = 0;
    changed_line_abv_curs_win(wp);
    wp->w_valid &= ~(VALID_BOTLINE|VALID_BOTLINE_AP|VALID_TOPLINE);
    redraw_win_later(wp, NOT_VALID);
}

/*
 * Set wp->w_topline to a certain number.
 */
    void
set_topline(wp, lnum)
    win_T       *wp;
    linenr_T    lnum;
{
#ifdef FEAT_FOLDING
    /* go to first of folded lines */
    (void)hasFoldingWin(wp, lnum, &lnum, NULL, TRUE, NULL);
#endif
    /* Approximate the value of w_botline */
    wp->w_botline += lnum - wp->w_topline;
    wp->w_topline = lnum;
#ifdef FEAT_DIFF
    wp->w_topfill = 0;
#endif
    wp->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE|VALID_TOPLINE);
    /* Don't set VALID_TOPLINE here, 'scrolloff' needs to be checked. */
    redraw_later(VALID);
}

/*
 * Call this function when the length of the cursor line (in screen
 * characters) has changed, and the change is before the cursor.
 * Need to take care of w_botline separately!
 */
    void
changed_cline_bef_curs()
{
    curwin->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL
                                                |VALID_CHEIGHT|VALID_TOPLINE);
}

    void
changed_cline_bef_curs_win(wp)
    win_T       *wp;
{
    wp->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL
                                                |VALID_CHEIGHT|VALID_TOPLINE);
}

#if 0 /* not used */
/*
 * Call this function when the length of the cursor line (in screen
 * characters) has changed, and the position of the cursor doesn't change.
 * Need to take care of w_botline separately!
 */
    void
changed_cline_aft_curs()
{
    curwin->w_valid &= ~VALID_CHEIGHT;
}
#endif

/*
 * Call this function when the length of a line (in screen characters) above
 * the cursor have changed.
 * Need to take care of w_botline separately!
 */
    void
changed_line_abv_curs()
{
    curwin->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL|VALID_CROW
                                                |VALID_CHEIGHT|VALID_TOPLINE);
}

    void
changed_line_abv_curs_win(wp)
    win_T       *wp;
{
    wp->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL|VALID_CROW
                                                |VALID_CHEIGHT|VALID_TOPLINE);
}

/*
 * Make sure the value of curwin->w_botline is valid.
 */
    void
validate_botline()
{
    if (!(curwin->w_valid & VALID_BOTLINE))
        comp_botline(curwin);
}

/*
 * Make sure the value of wp->w_botline is valid.
 */
    static void
validate_botline_win(wp)
    win_T       *wp;
{
    if (!(wp->w_valid & VALID_BOTLINE))
        comp_botline(wp);
}

/*
 * Mark curwin->w_botline as invalid (because of some change in the buffer).
 */
    void
invalidate_botline()
{
    curwin->w_valid &= ~(VALID_BOTLINE|VALID_BOTLINE_AP);
}

    void
invalidate_botline_win(wp)
    win_T       *wp;
{
    wp->w_valid &= ~(VALID_BOTLINE|VALID_BOTLINE_AP);
}

#if 0 /* never used */
/*
 * Mark curwin->w_botline as approximated (because of some small change in the
 * buffer).
 */
    void
approximate_botline()
{
    curwin->w_valid &= ~VALID_BOTLINE;
}
#endif

    void
approximate_botline_win(wp)
    win_T       *wp;
{
    wp->w_valid &= ~VALID_BOTLINE;
}

#if 0 /* not used */
/*
 * Return TRUE if curwin->w_botline is valid.
 */
    int
botline_valid()
{
    return (curwin->w_valid & VALID_BOTLINE);
}
#endif

#if 0 /* not used */
/*
 * Return TRUE if curwin->w_botline is valid or approximated.
 */
    int
botline_approximated()
{
    return (curwin->w_valid & VALID_BOTLINE_AP);
}
#endif

/*
 * Return TRUE if curwin->w_wrow and curwin->w_wcol are valid.
 */
    int
cursor_valid()
{
    check_cursor_moved(curwin);
    return ((curwin->w_valid & (VALID_WROW|VALID_WCOL)) ==
                                                      (VALID_WROW|VALID_WCOL));
}

/*
 * Validate cursor position.  Makes sure w_wrow and w_wcol are valid.
 * w_topline must be valid, you may need to call update_topline() first!
 */
    void
validate_cursor()
{
    check_cursor_moved(curwin);
    if ((curwin->w_valid & (VALID_WCOL|VALID_WROW)) != (VALID_WCOL|VALID_WROW))
        curs_columns(TRUE);
}

#if defined(FEAT_GUI) || defined(PROTO)
/*
 * validate w_cline_row.
 */
    void
validate_cline_row()
{
    /*
     * First make sure that w_topline is valid (after moving the cursor).
     */
    update_topline();
    check_cursor_moved(curwin);
    if (!(curwin->w_valid & VALID_CROW))
        curs_rows(curwin, FALSE);
}
#endif

/*
 * Compute wp->w_cline_row and wp->w_cline_height, based on the current value
 * of wp->w_topine.
 *
 * Returns OK when cursor is in the window, FAIL when it isn't.
 */
    static void
curs_rows(wp, do_botline)
    win_T       *wp;
    int         do_botline;             /* also compute w_botline */
{
    linenr_T    lnum;
    int         i;
    int         all_invalid;
    int         valid;
#ifdef FEAT_FOLDING
    long        fold_count;
#endif

    /* Check if wp->w_lines[].wl_size is invalid */
    all_invalid = (!redrawing()
                        || wp->w_lines_valid == 0
                        || wp->w_lines[0].wl_lnum > wp->w_topline);
    i = 0;
    wp->w_cline_row = 0;
    for (lnum = wp->w_topline; lnum < wp->w_cursor.lnum; ++i)
    {
        valid = FALSE;
        if (!all_invalid && i < wp->w_lines_valid)
        {
            if (wp->w_lines[i].wl_lnum < lnum || !wp->w_lines[i].wl_valid)
                continue;               /* skip changed or deleted lines */
            if (wp->w_lines[i].wl_lnum == lnum)
            {
#ifdef FEAT_FOLDING
                /* Check for newly inserted lines below this row, in which
                 * case we need to check for folded lines. */
                if (!wp->w_buffer->b_mod_set
                        || wp->w_lines[i].wl_lastlnum < wp->w_cursor.lnum
                        || wp->w_buffer->b_mod_top
                                             > wp->w_lines[i].wl_lastlnum + 1)
#endif
                valid = TRUE;
            }
            else if (wp->w_lines[i].wl_lnum > lnum)
                --i;                    /* hold at inserted lines */
        }
        if (valid
#ifdef FEAT_DIFF
                && (lnum != wp->w_topline || !wp->w_p_diff)
#endif
                )
        {
#ifdef FEAT_FOLDING
            lnum = wp->w_lines[i].wl_lastlnum + 1;
            /* Cursor inside folded lines, don't count this row */
            if (lnum > wp->w_cursor.lnum)
                break;
#else
            ++lnum;
#endif
            wp->w_cline_row += wp->w_lines[i].wl_size;
        }
        else
        {
#ifdef FEAT_FOLDING
            fold_count = foldedCount(wp, lnum, NULL);
            if (fold_count)
            {
                lnum += fold_count;
                if (lnum > wp->w_cursor.lnum)
                    break;
                ++wp->w_cline_row;
            }
            else
#endif
#ifdef FEAT_DIFF
                if (lnum == wp->w_topline)
                    wp->w_cline_row += plines_win_nofill(wp, lnum++, TRUE)
                                                              + wp->w_topfill;
                else
#endif
                    wp->w_cline_row += plines_win(wp, lnum++, TRUE);
        }
    }

    check_cursor_moved(wp);
    if (!(wp->w_valid & VALID_CHEIGHT))
    {
        if (all_invalid
                || i == wp->w_lines_valid
                || (i < wp->w_lines_valid
                    && (!wp->w_lines[i].wl_valid
                        || wp->w_lines[i].wl_lnum != wp->w_cursor.lnum)))
        {
#ifdef FEAT_DIFF
            if (wp->w_cursor.lnum == wp->w_topline)
                wp->w_cline_height = plines_win_nofill(wp, wp->w_cursor.lnum,
                                                        TRUE) + wp->w_topfill;
            else
#endif
                wp->w_cline_height = plines_win(wp, wp->w_cursor.lnum, TRUE);
#ifdef FEAT_FOLDING
            wp->w_cline_folded = hasFoldingWin(wp, wp->w_cursor.lnum,
                                                      NULL, NULL, TRUE, NULL);
#endif
        }
        else if (i > wp->w_lines_valid)
        {
            /* a line that is too long to fit on the last screen line */
            wp->w_cline_height = 0;
#ifdef FEAT_FOLDING
            wp->w_cline_folded = hasFoldingWin(wp, wp->w_cursor.lnum,
                                                      NULL, NULL, TRUE, NULL);
#endif
        }
        else
        {
            wp->w_cline_height = wp->w_lines[i].wl_size;
#ifdef FEAT_FOLDING
            wp->w_cline_folded = wp->w_lines[i].wl_folded;
#endif
        }
    }

    wp->w_valid |= VALID_CROW|VALID_CHEIGHT;

    /* validate botline too, if update_screen doesn't do it */
    if (do_botline && all_invalid)
        validate_botline_win(wp);
}

/*
 * Validate curwin->w_virtcol only.
 */
    void
validate_virtcol()
{
    validate_virtcol_win(curwin);
}

/*
 * Validate wp->w_virtcol only.
 */
    void
validate_virtcol_win(wp)
    win_T       *wp;
{
    check_cursor_moved(wp);
    if (!(wp->w_valid & VALID_VIRTCOL))
    {
        getvvcol(wp, &wp->w_cursor, NULL, &(wp->w_virtcol), NULL);
        wp->w_valid |= VALID_VIRTCOL;
#ifdef FEAT_SYN_HL
        if (wp->w_p_cuc
# ifdef FEAT_INS_EXPAND
                && !pum_visible()
# endif
                )
            redraw_win_later(wp, SOME_VALID);
#endif
    }
}

/*
 * Validate curwin->w_cline_height only.
 */
    static void
validate_cheight()
{
    check_cursor_moved(curwin);
    if (!(curwin->w_valid & VALID_CHEIGHT))
    {
#ifdef FEAT_DIFF
        if (curwin->w_cursor.lnum == curwin->w_topline)
            curwin->w_cline_height = plines_nofill(curwin->w_cursor.lnum)
                                                          + curwin->w_topfill;
        else
#endif
            curwin->w_cline_height = plines(curwin->w_cursor.lnum);
#ifdef FEAT_FOLDING
        curwin->w_cline_folded = hasFolding(curwin->w_cursor.lnum, NULL, NULL);
#endif
        curwin->w_valid |= VALID_CHEIGHT;
    }
}

/*
 * validate w_wcol and w_virtcol only.  Only correct when 'wrap' on!
 */
    void
validate_cursor_col()
{
    colnr_T off;
    colnr_T col;

    validate_virtcol();
    if (!(curwin->w_valid & VALID_WCOL))
    {
        col = curwin->w_virtcol;
        off = curwin_col_off();
        col += off;

        /* long line wrapping, adjust curwin->w_wrow */
        if (curwin->w_p_wrap && col >= (colnr_T)W_WIDTH(curwin)
                && W_WIDTH(curwin) - off + curwin_col_off2() > 0)
        {
            col -= W_WIDTH(curwin);
            col = col % (W_WIDTH(curwin) - off + curwin_col_off2());
        }
        curwin->w_wcol = col;
        curwin->w_valid |= VALID_WCOL;
    }
}

/*
 * Compute offset of a window, occupied by line number, fold column and sign
 * column (these don't move when scrolling horizontally).
 */
    int
win_col_off(wp)
    win_T       *wp;
{
    return ((wp->w_p_nu ? number_width(wp) + 1 : 0)
#ifdef FEAT_CMDWIN
            + (cmdwin_type == 0 || wp != curwin ? 0 : 1)
#endif
#ifdef FEAT_FOLDING
            + wp->w_p_fdc
#endif
#ifdef FEAT_SIGNS
            + (
# ifdef FEAT_NETBEANS_INTG
                /* always show glyph gutter in netbeans */
                usingNetbeans ||
# endif
                wp->w_buffer->b_signlist != NULL ? 2 : 0)
#endif
           );
}

    int
curwin_col_off()
{
    return win_col_off(curwin);
}

/*
 * Return the difference in column offset for the second screen line of a
 * wrapped line.  It's 8 if 'number' is on and 'n' is in 'cpoptions'.
 */
    int
win_col_off2(wp)
    win_T       *wp;
{
    if (wp->w_p_nu && vim_strchr(p_cpo, CPO_NUMCOL) != NULL)
        return number_width(wp) + 1;
    return 0;
}

    int
curwin_col_off2()
{
    return win_col_off2(curwin);
}

/*
 * compute curwin->w_wcol and curwin->w_virtcol.
 * Also updates curwin->w_wrow and curwin->w_cline_row.
 * Also updates curwin->w_leftcol.
 */
    void
curs_columns(scroll)
    int         scroll;         /* when TRUE, may scroll horizontally */
{
    int         diff;
    int         extra;          /* offset for first screen line */
    int         off_left, off_right;
    int         n;
    int         p_lines;
    int         width = 0;
    int         textwidth;
    int         new_leftcol;
    colnr_T     startcol;
    colnr_T     endcol;
    colnr_T     prev_skipcol;

    /*
     * First make sure that w_topline is valid (after moving the cursor).
     */
    update_topline();

    /*
     * Next make sure that w_cline_row is valid.
     */
    if (!(curwin->w_valid & VALID_CROW))
        curs_rows(curwin, FALSE);

    /*
     * Compute the number of virtual columns.
     */
#ifdef FEAT_FOLDING
    if (curwin->w_cline_folded)
        /* In a folded line the cursor is always in the first column */
        startcol = curwin->w_virtcol = endcol = curwin->w_leftcol;
    else
#endif
        getvvcol(curwin, &curwin->w_cursor,
                                &startcol, &(curwin->w_virtcol), &endcol);

    /* remove '$' from change command when cursor moves onto it */
    if (startcol > dollar_vcol)
        dollar_vcol = 0;

    extra = curwin_col_off();
    curwin->w_wcol = curwin->w_virtcol + extra;
    endcol += extra;

    /*
     * Now compute w_wrow, counting screen lines from w_cline_row.
     */
    curwin->w_wrow = curwin->w_cline_row;

    textwidth = W_WIDTH(curwin) - extra;
    if (textwidth <= 0)
    {
        /* No room for text, put cursor in last char of window. */
        curwin->w_wcol = W_WIDTH(curwin) - 1;
        curwin->w_wrow = curwin->w_height - 1;
    }
    else if (curwin->w_p_wrap
#ifdef FEAT_VERTSPLIT
            && curwin->w_width != 0
#endif
            )
    {
        width = textwidth + curwin_col_off2();

        /* long line wrapping, adjust curwin->w_wrow */
        if (curwin->w_wcol >= W_WIDTH(curwin))
        {
            n = (curwin->w_wcol - W_WIDTH(curwin)) / width + 1;
            curwin->w_wcol -= n * width;
            curwin->w_wrow += n;

#ifdef FEAT_LINEBREAK
            /* When cursor wraps to first char of next line in Insert
             * mode, the 'showbreak' string isn't shown, backup to first
             * column */
            if (*p_sbr && *ml_get_cursor() == NUL
                    && curwin->w_wcol == (int)vim_strsize(p_sbr))
                curwin->w_wcol = 0;
#endif
        }
    }

    /* No line wrapping: compute curwin->w_leftcol if scrolling is on and line
     * is not folded.
     * If scrolling is off, curwin->w_leftcol is assumed to be 0 */
    else if (scroll
#ifdef FEAT_FOLDING
            && !curwin->w_cline_folded
#endif
            )
    {
        /*
         * If Cursor is left of the screen, scroll rightwards.
         * If Cursor is right of the screen, scroll leftwards
         * If we get closer to the edge than 'sidescrolloff', scroll a little
         * extra
         */
        off_left = (int)startcol - (int)curwin->w_leftcol - p_siso;
        off_right = (int)endcol - (int)(curwin->w_leftcol + W_WIDTH(curwin)
                                                                - p_siso) + 1;
        if (off_left < 0 || off_right > 0)
        {
            if (off_left < 0)
                diff = -off_left;
            else
                diff = off_right;

            /* When far off or not enough room on either side, put cursor in
             * middle of window. */
            if (p_ss == 0 || diff >= textwidth / 2 || off_right >= off_left)
                new_leftcol = curwin->w_wcol - extra - textwidth / 2;
            else
            {
                if (diff < p_ss)
                    diff = p_ss;
                if (off_left < 0)
                    new_leftcol = curwin->w_leftcol - diff;
                else
                    new_leftcol = curwin->w_leftcol + diff;
            }
            if (new_leftcol < 0)
                new_leftcol = 0;
            if (new_leftcol != (int)curwin->w_leftcol)
            {
                curwin->w_leftcol = new_leftcol;
                /* screen has to be redrawn with new curwin->w_leftcol */
                redraw_later(NOT_VALID);
            }
        }
        curwin->w_wcol -= curwin->w_leftcol;
    }
    else if (curwin->w_wcol > (int)curwin->w_leftcol)
        curwin->w_wcol -= curwin->w_leftcol;
    else
        curwin->w_wcol = 0;

#ifdef FEAT_DIFF
    /* Skip over filler lines.  At the top use w_topfill, there
     * may be some filler lines above the window. */
    if (curwin->w_cursor.lnum == curwin->w_topline)
        curwin->w_wrow += curwin->w_topfill;
    else
        curwin->w_wrow += diff_check_fill(curwin, curwin->w_cursor.lnum);
#endif

    prev_skipcol = curwin->w_skipcol;

    p_lines = 0;
    if ((curwin->w_wrow >= curwin->w_height
                || ((prev_skipcol > 0
                        || curwin->w_wrow + p_so >= curwin->w_height)
                    && (p_lines =
#ifdef FEAT_DIFF
                        plines_win_nofill
#else
                        plines_win
#endif
                        (curwin, curwin->w_cursor.lnum, FALSE))
                                                    - 1 >= curwin->w_height))
            && curwin->w_height != 0
            && curwin->w_cursor.lnum == curwin->w_topline
            && width > 0
#ifdef FEAT_VERTSPLIT
            && curwin->w_width != 0
#endif
            )
    {
        /* Cursor past end of screen.  Happens with a single line that does
         * not fit on screen.  Find a skipcol to show the text around the
         * cursor.  Avoid scrolling all the time. compute value of "extra":
         * 1: Less than "p_so" lines above
         * 2: Less than "p_so" lines below
         * 3: both of them */
        extra = 0;
        if (curwin->w_skipcol + p_so * width > curwin->w_virtcol)
            extra = 1;
        /* Compute last display line of the buffer line that we want at the
         * bottom of the window. */
        if (p_lines == 0)
            p_lines = plines_win(curwin, curwin->w_cursor.lnum, FALSE);
        --p_lines;
        if (p_lines > curwin->w_wrow + p_so)
            n = curwin->w_wrow + p_so;
        else
            n = p_lines;
        if ((colnr_T)n >= curwin->w_height + curwin->w_skipcol / width)
            extra += 2;

        if (extra == 3 || p_lines < p_so * 2)
        {
            /* not enough room for 'scrolloff', put cursor in the middle */
            n = curwin->w_virtcol / width;
            if (n > curwin->w_height / 2)
                n -= curwin->w_height / 2;
            else
                n = 0;
            /* don't skip more than necessary */
            if (n > p_lines - curwin->w_height + 1)
                n = p_lines - curwin->w_height + 1;
            curwin->w_skipcol = n * width;
        }
        else if (extra == 1)
        {
            /* less then 'scrolloff' lines above, decrease skipcol */
            extra = (curwin->w_skipcol + p_so * width - curwin->w_virtcol
                                     + width - 1) / width;
            if (extra > 0)
            {
                if ((colnr_T)(extra * width) > curwin->w_skipcol)
                    extra = curwin->w_skipcol / width;
                curwin->w_skipcol -= extra * width;
            }
        }
        else if (extra == 2)
        {
            /* less then 'scrolloff' lines below, increase skipcol */
            endcol = (n - curwin->w_height + 1) * width;
            while (endcol > curwin->w_virtcol)
                endcol -= width;
            if (endcol > curwin->w_skipcol)
                curwin->w_skipcol = endcol;
        }

        curwin->w_wrow -= curwin->w_skipcol / width;
        if (curwin->w_wrow >= curwin->w_height)
        {
            /* small window, make sure cursor is in it */
            extra = curwin->w_wrow - curwin->w_height + 1;
            curwin->w_skipcol += extra * width;
            curwin->w_wrow -= extra;
        }

        extra = ((int)prev_skipcol - (int)curwin->w_skipcol) / width;
        if (extra > 0)
            win_ins_lines(curwin, 0, extra, FALSE, FALSE);
        else if (extra < 0)
            win_del_lines(curwin, 0, -extra, FALSE, FALSE);
    }
    else
        curwin->w_skipcol = 0;
    if (prev_skipcol != curwin->w_skipcol)
        redraw_later(NOT_VALID);

#ifdef FEAT_SYN_HL
    /* Redraw when w_virtcol changes and 'cursorcolumn' is set, or when w_row
     * changes and 'cursorline' is set. */
    if (((curwin->w_p_cuc && (curwin->w_valid & VALID_VIRTCOL) == 0)
                || (curwin->w_p_cul && (curwin->w_valid & VALID_WROW) == 0))
# ifdef FEAT_INS_EXPAND
            && !pum_visible()
# endif
            )
        redraw_later(SOME_VALID);
#endif

    curwin->w_valid |= VALID_WCOL|VALID_WROW|VALID_VIRTCOL;
}

/*
 * Scroll the current window down by "line_count" logical lines.  "CTRL-Y"
 */
/*ARGSUSED*/
    void
scrolldown(line_count, byfold)
    long        line_count;
    int         byfold;         /* TRUE: count a closed fold as one line */
{
    long        done = 0;       /* total # of physical lines done */
    int         wrow;
    int         moved = FALSE;

#ifdef FEAT_FOLDING
    linenr_T    first;

    /* Make sure w_topline is at the first of a sequence of folded lines. */
    (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif
    validate_cursor();          /* w_wrow needs to be valid */
    while (line_count-- > 0)
    {
#ifdef FEAT_DIFF
        if (curwin->w_topfill < diff_check(curwin, curwin->w_topline))
        {
            ++curwin->w_topfill;
            ++done;
        }
        else
#endif
        {
            if (curwin->w_topline == 1)
                break;
            --curwin->w_topline;
#ifdef FEAT_DIFF
            curwin->w_topfill = 0;
#endif
#ifdef FEAT_FOLDING
            /* A sequence of folded lines only counts for one logical line */
            if (hasFolding(curwin->w_topline, &first, NULL))
            {
                ++done;
                if (!byfold)
                    line_count -= curwin->w_topline - first - 1;
                curwin->w_botline -= curwin->w_topline - first;
                curwin->w_topline = first;
            }
            else
#endif
#ifdef FEAT_DIFF
                done += plines_nofill(curwin->w_topline);
#else
                done += plines(curwin->w_topline);
#endif
        }
        --curwin->w_botline;            /* approximate w_botline */
        invalidate_botline();
    }
    curwin->w_wrow += done;             /* keep w_wrow updated */
    curwin->w_cline_row += done;        /* keep w_cline_row updated */

#ifdef FEAT_DIFF
    if (curwin->w_cursor.lnum == curwin->w_topline)
        curwin->w_cline_row = 0;
    check_topfill(curwin, TRUE);
#endif

    /*
     * Compute the row number of the last row of the cursor line
     * and move the cursor onto the displayed part of the window.
     */
    wrow = curwin->w_wrow;
    if (curwin->w_p_wrap
#ifdef FEAT_VERTSPLIT
                && curwin->w_width != 0
#endif
            )
    {
        validate_virtcol();
        validate_cheight();
        wrow += curwin->w_cline_height - 1 -
            curwin->w_virtcol / W_WIDTH(curwin);
    }
    while (wrow >= curwin->w_height && curwin->w_cursor.lnum > 1)
    {
#ifdef FEAT_FOLDING
        if (hasFolding(curwin->w_cursor.lnum, &first, NULL))
        {
            --wrow;
            if (first == 1)
                curwin->w_cursor.lnum = 1;
            else
                curwin->w_cursor.lnum = first - 1;
        }
        else
#endif
            wrow -= plines(curwin->w_cursor.lnum--);
        curwin->w_valid &=
              ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW|VALID_VIRTCOL);
        moved = TRUE;
    }
    if (moved)
    {
#ifdef FEAT_FOLDING
        /* Move cursor to first line of closed fold. */
        foldAdjustCursor();
#endif
        coladvance(curwin->w_curswant);
    }
}

/*
 * Scroll the current window up by "line_count" logical lines.  "CTRL-E"
 */
/*ARGSUSED*/
    void
scrollup(line_count, byfold)
    long        line_count;
    int         byfold;         /* TRUE: count a closed fold as one line */
{
#if defined(FEAT_FOLDING) || defined(FEAT_DIFF)
    linenr_T    lnum;

    if (
# ifdef FEAT_FOLDING
            (byfold && hasAnyFolding(curwin))
#  ifdef FEAT_DIFF
            ||
#  endif
# endif
# ifdef FEAT_DIFF
            curwin->w_p_diff
# endif
            )
    {
        /* count each sequence of folded lines as one logical line */
        lnum = curwin->w_topline;
        while (line_count--)
        {
# ifdef FEAT_DIFF
            if (curwin->w_topfill > 0)
                --curwin->w_topfill;
            else
# endif
            {
# ifdef FEAT_FOLDING
                if (byfold)
                    (void)hasFolding(lnum, NULL, &lnum);
# endif
                if (lnum >= curbuf->b_ml.ml_line_count)
                    break;
                ++lnum;
# ifdef FEAT_DIFF
                curwin->w_topfill = diff_check_fill(curwin, lnum);
# endif
            }
        }
        /* approximate w_botline */
        curwin->w_botline += lnum - curwin->w_topline;
        curwin->w_topline = lnum;
    }
    else
#endif
    {
        curwin->w_topline += line_count;
        curwin->w_botline += line_count;        /* approximate w_botline */
    }

    if (curwin->w_topline > curbuf->b_ml.ml_line_count)
        curwin->w_topline = curbuf->b_ml.ml_line_count;
    if (curwin->w_botline > curbuf->b_ml.ml_line_count + 1)
        curwin->w_botline = curbuf->b_ml.ml_line_count + 1;

#ifdef FEAT_DIFF
    check_topfill(curwin, FALSE);
#endif

#ifdef FEAT_FOLDING
    if (hasAnyFolding(curwin))
        /* Make sure w_topline is at the first of a sequence of folded lines. */
        (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif

    curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
    if (curwin->w_cursor.lnum < curwin->w_topline)
    {
        curwin->w_cursor.lnum = curwin->w_topline;
        curwin->w_valid &=
              ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW|VALID_VIRTCOL);
        coladvance(curwin->w_curswant);
    }
}

#ifdef FEAT_DIFF
/*
 * Don't end up with too many filler lines in the window.
 */
    void
check_topfill(wp, down)
    win_T       *wp;
    int         down;   /* when TRUE scroll down when not enough space */
{
    int         n;

    if (wp->w_topfill > 0)
    {
        n = plines_win_nofill(wp, wp->w_topline, TRUE);
        if (wp->w_topfill + n > wp->w_height)
        {
            if (down && wp->w_topline > 1)
            {
                --wp->w_topline;
                wp->w_topfill = 0;
            }
            else
            {
                wp->w_topfill = wp->w_height - n;
                if (wp->w_topfill < 0)
                    wp->w_topfill = 0;
            }
        }
    }
}

/*
 * Use as many filler lines as possible for w_topline.  Make sure w_topline
 * is still visible.
 */
    static void
max_topfill()
{
    int         n;

    n = plines_nofill(curwin->w_topline);
    if (n >= curwin->w_height)
        curwin->w_topfill = 0;
    else
    {
        curwin->w_topfill = diff_check_fill(curwin, curwin->w_topline);
        if (curwin->w_topfill + n > curwin->w_height)
            curwin->w_topfill = curwin->w_height - n;
    }
}
#endif

#if defined(FEAT_INS_EXPAND) || defined(PROTO)
/*
 * Scroll the screen one line down, but don't do it if it would move the
 * cursor off the screen.
 */
    void
scrolldown_clamp()
{
    int         end_row;
#ifdef FEAT_DIFF
    int         can_fill = (curwin->w_topfill
                                < diff_check_fill(curwin, curwin->w_topline));
#endif

    if (curwin->w_topline <= 1
#ifdef FEAT_DIFF
            && !can_fill
#endif
            )
        return;

    validate_cursor();      /* w_wrow needs to be valid */

    /*
     * Compute the row number of the last row of the cursor line
     * and make sure it doesn't go off the screen. Make sure the cursor
     * doesn't go past 'scrolloff' lines from the screen end.
     */
    end_row = curwin->w_wrow;
#ifdef FEAT_DIFF
    if (can_fill)
        ++end_row;
    else
        end_row += plines_nofill(curwin->w_topline - 1);
#else
    end_row += plines(curwin->w_topline - 1);
#endif
    if (curwin->w_p_wrap
#ifdef FEAT_VERTSPLIT
                && curwin->w_width != 0
#endif
            )
    {
        validate_cheight();
        validate_virtcol();
        end_row += curwin->w_cline_height - 1 -
            curwin->w_virtcol / W_WIDTH(curwin);
    }
    if (end_row < curwin->w_height - p_so)
    {
#ifdef FEAT_DIFF
        if (can_fill)
        {
            ++curwin->w_topfill;
            check_topfill(curwin, TRUE);
        }
        else
        {
            --curwin->w_topline;
            curwin->w_topfill = 0;
        }
#else
        --curwin->w_topline;
#endif
#ifdef FEAT_FOLDING
        hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif
        --curwin->w_botline;        /* approximate w_botline */
        curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
    }
}

/*
 * Scroll the screen one line up, but don't do it if it would move the cursor
 * off the screen.
 */
    void
scrollup_clamp()
{
    int     start_row;

    if (curwin->w_topline == curbuf->b_ml.ml_line_count
#ifdef FEAT_DIFF
            && curwin->w_topfill == 0
#endif
            )
        return;

    validate_cursor();      /* w_wrow needs to be valid */

    /*
     * Compute the row number of the first row of the cursor line
     * and make sure it doesn't go off the screen. Make sure the cursor
     * doesn't go before 'scrolloff' lines from the screen start.
     */
#ifdef FEAT_DIFF
    start_row = curwin->w_wrow - plines_nofill(curwin->w_topline)
                                                          - curwin->w_topfill;
#else
    start_row = curwin->w_wrow - plines(curwin->w_topline);
#endif
    if (curwin->w_p_wrap
#ifdef FEAT_VERTSPLIT
                && curwin->w_width != 0
#endif
            )
    {
        validate_virtcol();
        start_row -= curwin->w_virtcol / W_WIDTH(curwin);
    }
    if (start_row >= p_so)
    {
#ifdef FEAT_DIFF
        if (curwin->w_topfill > 0)
            --curwin->w_topfill;
        else
#endif
        {
#ifdef FEAT_FOLDING
            (void)hasFolding(curwin->w_topline, NULL, &curwin->w_topline);
#endif
            ++curwin->w_topline;
        }
        ++curwin->w_botline;            /* approximate w_botline */
        curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
    }
}
#endif /* FEAT_INS_EXPAND */

/*
 * Add one line above "lp->lnum".  This can be a filler line, a closed fold or
 * a (wrapped) text line.  Uses and sets "lp->fill".
 * Returns the height of the added line in "lp->height".
 * Lines above the first one are incredibly high.
 */
    static void
topline_back(lp)
    lineoff_T   *lp;
{
#ifdef FEAT_DIFF
    if (lp->fill < diff_check_fill(curwin, lp->lnum))
    {
        /* Add a filler line. */
        ++lp->fill;
        lp->height = 1;
    }
    else
#endif
    {
        --lp->lnum;
#ifdef FEAT_DIFF
        lp->fill = 0;
#endif
        if (lp->lnum < 1)
            lp->height = MAXCOL;
        else
#ifdef FEAT_FOLDING
            if (hasFolding(lp->lnum, &lp->lnum, NULL))
            /* Add a closed fold */
            lp->height = 1;
        else
#endif
        {
#ifdef FEAT_DIFF
            lp->height = plines_nofill(lp->lnum);
#else
            lp->height = plines(lp->lnum);
#endif
        }
    }
}

/*
 * Add one line below "lp->lnum".  This can be a filler line, a closed fold or
 * a (wrapped) text line.  Uses and sets "lp->fill".
 * Returns the height of the added line in "lp->height".
 * Lines below the last one are incredibly high.
 */
    static void
botline_forw(lp)
    lineoff_T   *lp;
{
#ifdef FEAT_DIFF
    if (lp->fill < diff_check_fill(curwin, lp->lnum + 1))
    {
        /* Add a filler line. */
        ++lp->fill;
        lp->height = 1;
    }
    else
#endif
    {
        ++lp->lnum;
#ifdef FEAT_DIFF
        lp->fill = 0;
#endif
        if (lp->lnum > curbuf->b_ml.ml_line_count)
            lp->height = MAXCOL;
        else
#ifdef FEAT_FOLDING
            if (hasFolding(lp->lnum, NULL, &lp->lnum))
            /* Add a closed fold */
            lp->height = 1;
        else
#endif
        {
#ifdef FEAT_DIFF
            lp->height = plines_nofill(lp->lnum);
#else
            lp->height = plines(lp->lnum);
#endif
        }
    }
}

#ifdef FEAT_DIFF
/*
 * Switch from including filler lines below lp->lnum to including filler
 * lines above loff.lnum + 1.  This keeps pointing to the same line.
 * When there are no filler lines nothing changes.
 */
    static void
botline_topline(lp)
    lineoff_T   *lp;
{
    if (lp->fill > 0)
    {
        ++lp->lnum;
        lp->fill = diff_check_fill(curwin, lp->lnum) - lp->fill + 1;
    }
}

/*
 * Switch from including filler lines above lp->lnum to including filler
 * lines below loff.lnum - 1.  This keeps pointing to the same line.
 * When there are no filler lines nothing changes.
 */
    static void
topline_botline(lp)
    lineoff_T   *lp;
{
    if (lp->fill > 0)
    {
        lp->fill = diff_check_fill(curwin, lp->lnum) - lp->fill + 1;
        --lp->lnum;
    }
}
#endif

/*
 * Recompute topline to put the cursor at the top of the window.
 * Scroll at least "min_scroll" lines.
 * If "always" is TRUE, always set topline (for "zt").
 */
    void
scroll_cursor_top(min_scroll, always)
    int         min_scroll;
    int         always;
{
    int         scrolled = 0;
    int         extra = 0;
    int         used;
    int         i;
    linenr_T    top;            /* just above displayed lines */
    linenr_T    bot;            /* just below displayed lines */
    linenr_T    old_topline = curwin->w_topline;
#ifdef FEAT_DIFF
    linenr_T    old_topfill = curwin->w_topfill;
#endif
    linenr_T    new_topline;
    int         off = p_so;

#ifdef FEAT_MOUSE
    if (mouse_dragging > 0)
        off = mouse_dragging - 1;
#endif

    /*
     * Decrease topline until:
     * - it has become 1
     * - (part of) the cursor line is moved off the screen or
     * - moved at least 'scrolljump' lines and
     * - at least 'scrolloff' lines above and below the cursor
     */
    validate_cheight();
    used = curwin->w_cline_height;
    if (curwin->w_cursor.lnum < curwin->w_topline)
        scrolled = used;

#ifdef FEAT_FOLDING
    if (hasFolding(curwin->w_cursor.lnum, &top, &bot))
    {
        --top;
        ++bot;
    }
    else
#endif
    {
        top = curwin->w_cursor.lnum - 1;
        bot = curwin->w_cursor.lnum + 1;
    }
    new_topline = top + 1;

#ifdef FEAT_DIFF
    /* count filler lines of the cursor window as context */
    i = diff_check_fill(curwin, curwin->w_cursor.lnum);
    used += i;
    extra += i;
#endif

    /*
     * Check if the lines from "top" to "bot" fit in the window.  If they do,
     * set new_topline and advance "top" and "bot" to include more lines.
     */
    while (top > 0)
    {
#ifdef FEAT_FOLDING
        if (hasFolding(top, &top, NULL))
            /* count one logical line for a sequence of folded lines */
            i = 1;
        else
#endif
            i = plines(top);
        used += i;
        if (extra + i <= off && bot < curbuf->b_ml.ml_line_count)
        {
#ifdef FEAT_FOLDING
            if (hasFolding(bot, NULL, &bot))
                /* count one logical line for a sequence of folded lines */
                ++used;
            else
#endif
                used += plines(bot);
        }
        if (used > curwin->w_height)
            break;
        if (top < curwin->w_topline)
            scrolled += i;

        /*
         * If scrolling is needed, scroll at least 'sj' lines.
         */
        if ((new_topline >= curwin->w_topline || scrolled > min_scroll)
                && extra >= off)
            break;

        extra += i;
        new_topline = top;
        --top;
        ++bot;
    }

    /*
     * If we don't have enough space, put cursor in the middle.
     * This makes sure we get the same position when using "k" and "j"
     * in a small window.
     */
    if (used > curwin->w_height)
        scroll_cursor_halfway(FALSE);
    else
    {
        /*
         * If "always" is FALSE, only adjust topline to a lower value, higher
         * value may happen with wrapping lines
         */
        if (new_topline < curwin->w_topline || always)
            curwin->w_topline = new_topline;
        if (curwin->w_topline > curwin->w_cursor.lnum)
            curwin->w_topline = curwin->w_cursor.lnum;
#ifdef FEAT_DIFF
        curwin->w_topfill = diff_check_fill(curwin, curwin->w_topline);
        if (curwin->w_topfill > 0 && extra > off)
        {
            curwin->w_topfill -= extra - off;
            if (curwin->w_topfill < 0)
                curwin->w_topfill = 0;
        }
        check_topfill(curwin, FALSE);
#endif
        if (curwin->w_topline != old_topline
#ifdef FEAT_DIFF
                || curwin->w_topfill != old_topfill
#endif
                )
            curwin->w_valid &=
                      ~(VALID_WROW|VALID_CROW|VALID_BOTLINE|VALID_BOTLINE_AP);
        curwin->w_valid |= VALID_TOPLINE;
    }
}

/*
 * Set w_empty_rows and w_filler_rows for window "wp", having used up "used"
 * screen lines for text lines.
 */
    void
set_empty_rows(wp, used)
    win_T       *wp;
    int         used;
{
#ifdef FEAT_DIFF
    wp->w_filler_rows = 0;
#endif
    if (used == 0)
        wp->w_empty_rows = 0;   /* single line that doesn't fit */
    else
    {
        wp->w_empty_rows = wp->w_height - used;
#ifdef FEAT_DIFF
        if (wp->w_botline <= wp->w_buffer->b_ml.ml_line_count)
        {
            wp->w_filler_rows = diff_check_fill(wp, wp->w_botline);
            if (wp->w_empty_rows > wp->w_filler_rows)
                wp->w_empty_rows -= wp->w_filler_rows;
            else
            {
                wp->w_filler_rows = wp->w_empty_rows;
                wp->w_empty_rows = 0;
            }
        }
#endif
    }
}

/*
 * Recompute topline to put the cursor at the bottom of the window.
 * Scroll at least "min_scroll" lines.
 * If "set_topbot" is TRUE, set topline and botline first (for "zb").
 * This is messy stuff!!!
 */
    void
scroll_cursor_bot(min_scroll, set_topbot)
    int         min_scroll;
    int         set_topbot;
{
    int         used;
    int         scrolled = 0;
    int         extra = 0;
    int         i;
    linenr_T    line_count;
    linenr_T    old_topline = curwin->w_topline;
    lineoff_T   loff;
    lineoff_T   boff;
#ifdef FEAT_DIFF
    int         old_topfill = curwin->w_topfill;
    int         fill_below_window;
#endif
    linenr_T    old_botline = curwin->w_botline;
    linenr_T    old_valid = curwin->w_valid;
    int         old_empty_rows = curwin->w_empty_rows;
    linenr_T    cln;                /* Cursor Line Number */

    cln = curwin->w_cursor.lnum;
    if (set_topbot)
    {
        used = 0;
        curwin->w_botline = cln + 1;
#ifdef FEAT_DIFF
        loff.fill = 0;
#endif
        for (curwin->w_topline = curwin->w_botline;
                curwin->w_topline > 1;
                curwin->w_topline = loff.lnum)
        {
            loff.lnum = curwin->w_topline;
            topline_back(&loff);
            if (used + loff.height > curwin->w_height)
                break;
            used += loff.height;
#ifdef FEAT_DIFF
            curwin->w_topfill = loff.fill;
#endif
        }
        set_empty_rows(curwin, used);
        curwin->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;
        if (curwin->w_topline != old_topline
#ifdef FEAT_DIFF
                || curwin->w_topfill != old_topfill
#endif
                )
            curwin->w_valid &= ~(VALID_WROW|VALID_CROW);
    }
    else
        validate_botline();

    /* The lines of the cursor line itself are always used. */
#ifdef FEAT_DIFF
    used = plines_nofill(cln);
#else
    validate_cheight();
    used = curwin->w_cline_height;
#endif

    /* If the cursor is below botline, we will at least scroll by the height
     * of the cursor line.  Correct for empty lines, which are really part of
     * botline. */
    if (cln >= curwin->w_botline)
    {
        scrolled = used;
        if (cln == curwin->w_botline)
            scrolled -= curwin->w_empty_rows;
    }

    /*
     * Stop counting lines to scroll when
     * - hitting start of the file
     * - scrolled nothing or at least 'sj' lines
     * - at least 'so' lines below the cursor
     * - lines between botline and cursor have been counted
     */
#ifdef FEAT_FOLDING
    if (!hasFolding(curwin->w_cursor.lnum, &loff.lnum, &boff.lnum))
#endif
    {
        loff.lnum = cln;
        boff.lnum = cln;
    }
#ifdef FEAT_DIFF
    loff.fill = 0;
    boff.fill = 0;
    fill_below_window = diff_check_fill(curwin, curwin->w_botline)
                                                      - curwin->w_filler_rows;
#endif

    while (loff.lnum > 1)
    {
        /* Stop when scrolled nothing or at least "min_scroll", found "extra"
         * context for 'scrolloff' and counted all lines below the window. */
        if ((((scrolled <= 0 || scrolled >= min_scroll)
                        && extra >= (
#ifdef FEAT_MOUSE
                            mouse_dragging > 0 ? mouse_dragging - 1 :
#endif
                            p_so))
                    || boff.lnum + 1 > curbuf->b_ml.ml_line_count)
                && loff.lnum <= curwin->w_botline
#ifdef FEAT_DIFF
                && (loff.lnum < curwin->w_botline
                    || loff.fill >= fill_below_window)
#endif
                )
            break;

        /* Add one line above */
        topline_back(&loff);
        used += loff.height;
        if (used > curwin->w_height)
            break;
        if (loff.lnum >= curwin->w_botline
#ifdef FEAT_DIFF
                && (loff.lnum > curwin->w_botline
                    || loff.fill <= fill_below_window)
#endif
                )
        {
            /* Count screen lines that are below the window. */
            scrolled += loff.height;
            if (loff.lnum == curwin->w_botline
#ifdef FEAT_DIFF
                            && boff.fill == 0
#endif
                    )
                scrolled -= curwin->w_empty_rows;
        }

        if (boff.lnum < curbuf->b_ml.ml_line_count)
        {
            /* Add one line below */
            botline_forw(&boff);
            used += boff.height;
            if (used > curwin->w_height)
                break;
            if (extra < (
#ifdef FEAT_MOUSE
                        mouse_dragging > 0 ? mouse_dragging - 1 :
#endif
                        p_so) || scrolled < min_scroll)
            {
                extra += boff.height;
                if (boff.lnum >= curwin->w_botline
#ifdef FEAT_DIFF
                        || (boff.lnum + 1 == curwin->w_botline
                            && boff.fill > curwin->w_filler_rows)
#endif
                   )
                {
                    /* Count screen lines that are below the window. */
                    scrolled += boff.height;
                    if (boff.lnum == curwin->w_botline
#ifdef FEAT_DIFF
                            && boff.fill == 0
#endif
                            )
                        scrolled -= curwin->w_empty_rows;
                }
            }
        }
    }

    /* curwin->w_empty_rows is larger, no need to scroll */
    if (scrolled <= 0)
        line_count = 0;
    /* more than a screenfull, don't scroll but redraw */
    else if (used > curwin->w_height)
        line_count = used;
    /* scroll minimal number of lines */
    else
    {
        line_count = 0;
#ifdef FEAT_DIFF
        boff.fill = curwin->w_topfill;
#endif
        boff.lnum = curwin->w_topline - 1;
        for (i = 0; i < scrolled && boff.lnum < curwin->w_botline; )
        {
            botline_forw(&boff);
            i += boff.height;
            ++line_count;
        }
        if (i < scrolled)       /* below curwin->w_botline, don't scroll */
            line_count = 9999;
    }

    /*
     * Scroll up if the cursor is off the bottom of the screen a bit.
     * Otherwise put it at 1/2 of the screen.
     */
    if (line_count >= curwin->w_height && line_count > min_scroll)
        scroll_cursor_halfway(FALSE);
    else
        scrollup(line_count, TRUE);

    /*
     * If topline didn't change we need to restore w_botline and w_empty_rows
     * (we changed them).
     * If topline did change, update_screen() will set botline.
     */
    if (curwin->w_topline == old_topline && set_topbot)
    {
        curwin->w_botline = old_botline;
        curwin->w_empty_rows = old_empty_rows;
        curwin->w_valid = old_valid;
    }
    curwin->w_valid |= VALID_TOPLINE;
}

/*
 * Recompute topline to put the cursor halfway the window
 * If "atend" is TRUE, also put it halfway at the end of the file.
 */
    void
scroll_cursor_halfway(atend)
    int         atend;
{
    int         above = 0;
    linenr_T    topline;
#ifdef FEAT_DIFF
    int         topfill = 0;
#endif
    int         below = 0;
    int         used;
    lineoff_T   loff;
    lineoff_T   boff;

    loff.lnum = boff.lnum = curwin->w_cursor.lnum;
#ifdef FEAT_FOLDING
    (void)hasFolding(loff.lnum, &loff.lnum, &boff.lnum);
#endif
#ifdef FEAT_DIFF
    used = plines_nofill(loff.lnum);
    loff.fill = 0;
    boff.fill = 0;
#else
    used = plines(loff.lnum);
#endif
    topline = loff.lnum;
    while (topline > 1)
    {
        if (below <= above)         /* add a line below the cursor first */
        {
            if (boff.lnum < curbuf->b_ml.ml_line_count)
            {
                botline_forw(&boff);
                used += boff.height;
                if (used > curwin->w_height)
                    break;
                below += boff.height;
            }
            else
            {
                ++below;            /* count a "~" line */
                if (atend)
                    ++used;
            }
        }

        if (below > above)          /* add a line above the cursor */
        {
            topline_back(&loff);
            used += loff.height;
            if (used > curwin->w_height)
                break;
            above += loff.height;
            topline = loff.lnum;
#ifdef FEAT_DIFF
            topfill = loff.fill;
#endif
        }
    }
#ifdef FEAT_FOLDING
    if (!hasFolding(topline, &curwin->w_topline, NULL))
#endif
        curwin->w_topline = topline;
#ifdef FEAT_DIFF
    curwin->w_topfill = topfill;
    check_topfill(curwin, FALSE);
#endif
    curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE|VALID_BOTLINE_AP);
    curwin->w_valid |= VALID_TOPLINE;
}

/*
 * Correct the cursor position so that it is in a part of the screen at least
 * 'so' lines from the top and bottom, if possible.
 * If not possible, put it at the same position as scroll_cursor_halfway().
 * When called topline must be valid!
 */
    void
cursor_correct()
{
    int         above = 0;          /* screen lines above topline */
    linenr_T    topline;
    int         below = 0;          /* screen lines below botline */
    linenr_T    botline;
    int         above_wanted, below_wanted;
    linenr_T    cln;                /* Cursor Line Number */
    int         max_off;

    /*
     * How many lines we would like to have above/below the cursor depends on
     * whether the first/last line of the file is on screen.
     */
    above_wanted = p_so;
    below_wanted = p_so;
#ifdef FEAT_MOUSE
    if (mouse_dragging > 0)
    {
        above_wanted = mouse_dragging - 1;
        below_wanted = mouse_dragging - 1;
    }
#endif
    if (curwin->w_topline == 1)
    {
        above_wanted = 0;
        max_off = curwin->w_height / 2;
        if (below_wanted > max_off)
            below_wanted = max_off;
    }
    validate_botline();
    if (curwin->w_botline == curbuf->b_ml.ml_line_count + 1
#ifdef FEAT_MOUSE
            && mouse_dragging == 0
#endif
            )
    {
        below_wanted = 0;
        max_off = (curwin->w_height - 1) / 2;
        if (above_wanted > max_off)
            above_wanted = max_off;
    }

    /*
     * If there are sufficient file-lines above and below the cursor, we can
     * return now.
     */
    cln = curwin->w_cursor.lnum;
    if (cln >= curwin->w_topline + above_wanted
            && cln < curwin->w_botline - below_wanted
#ifdef FEAT_FOLDING
            && !hasAnyFolding(curwin)
#endif
            )
        return;

    /*
     * Narrow down the area where the cursor can be put by taking lines from
     * the top and the bottom until:
     * - the desired context lines are found
     * - the lines from the top is past the lines from the bottom
     */
    topline = curwin->w_topline;
    botline = curwin->w_botline - 1;
#ifdef FEAT_DIFF
    /* count filler lines as context */
    above = curwin->w_topfill;
    below = curwin->w_filler_rows;
#endif
    while ((above < above_wanted || below < below_wanted) && topline < botline)
    {
        if (below < below_wanted && (below <= above || above >= above_wanted))
        {
#ifdef FEAT_FOLDING
            if (hasFolding(botline, &botline, NULL))
                ++below;
            else
#endif
                below += plines(botline);
            --botline;
        }
        if (above < above_wanted && (above < below || below >= below_wanted))
        {
#ifdef FEAT_FOLDING
            if (hasFolding(topline, NULL, &topline))
                ++above;
            else
#endif
#ifndef FEAT_DIFF
                above += plines(topline);
#else
                above += plines_nofill(topline);

            /* Count filler lines below this line as context. */
            if (topline < botline)
                above += diff_check_fill(curwin, topline + 1);
#endif
            ++topline;
        }
    }
    if (topline == botline || botline == 0)
        curwin->w_cursor.lnum = topline;
    else if (topline > botline)
        curwin->w_cursor.lnum = botline;
    else
    {
        if (cln < topline && curwin->w_topline > 1)
        {
            curwin->w_cursor.lnum = topline;
            curwin->w_valid &=
                            ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW);
        }
        if (cln > botline && curwin->w_botline <= curbuf->b_ml.ml_line_count)
        {
            curwin->w_cursor.lnum = botline;
            curwin->w_valid &=
                            ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW);
        }
    }
    curwin->w_valid |= VALID_TOPLINE;
}

static void get_scroll_overlap __ARGS((lineoff_T *lp, int dir));

/*
 * move screen 'count' pages up or down and update screen
 *
 * return FAIL for failure, OK otherwise
 */
    int
onepage(dir, count)
    int         dir;
    long        count;
{
    long        n;
    int         retval = OK;
    lineoff_T   loff;
    linenr_T    old_topline = curwin->w_topline;

    if (curbuf->b_ml.ml_line_count == 1)    /* nothing to do */
    {
        beep_flush();
        return FAIL;
    }

    for ( ; count > 0; --count)
    {
        validate_botline();
        /*
         * It's an error to move a page up when the first line is already on
         * the screen.  It's an error to move a page down when the last line
         * is on the screen and the topline is 'scrolloff' lines from the
         * last line.
         */
        if (dir == FORWARD
                ? ((curwin->w_topline >= curbuf->b_ml.ml_line_count - p_so)
                    && curwin->w_botline > curbuf->b_ml.ml_line_count)
                : (curwin->w_topline == 1
#ifdef FEAT_DIFF
                    && curwin->w_topfill ==
                                    diff_check_fill(curwin, curwin->w_topline)
#endif
                    ))
        {
            beep_flush();
            retval = FAIL;
            break;
        }

#ifdef FEAT_DIFF
        loff.fill = 0;
#endif
        if (dir == FORWARD)
        {
            if (firstwin == lastwin && p_window > 0 && p_window < Rows - 1)
            {
                /* Vi compatible scrolling */
                if (p_window <= 2)
                    ++curwin->w_topline;
                else
                    curwin->w_topline += p_window - 2;
                if (curwin->w_topline > curbuf->b_ml.ml_line_count)
                    curwin->w_topline = curbuf->b_ml.ml_line_count;
                curwin->w_cursor.lnum = curwin->w_topline;
            }
            else if (curwin->w_botline > curbuf->b_ml.ml_line_count)
            {
                /* at end of file */
                curwin->w_topline = curbuf->b_ml.ml_line_count;
#ifdef FEAT_DIFF
                curwin->w_topfill = 0;
#endif
                curwin->w_valid &= ~(VALID_WROW|VALID_CROW);
            }
            else
            {
                /* For the overlap, start with the line just below the window
                 * and go upwards. */
                loff.lnum = curwin->w_botline;
#ifdef FEAT_DIFF
                loff.fill = diff_check_fill(curwin, loff.lnum)
                                                      - curwin->w_filler_rows;
#endif
                get_scroll_overlap(&loff, -1);
                curwin->w_topline = loff.lnum;
#ifdef FEAT_DIFF
                curwin->w_topfill = loff.fill;
                check_topfill(curwin, FALSE);
#endif
                curwin->w_cursor.lnum = curwin->w_topline;
                curwin->w_valid &= ~(VALID_WCOL|VALID_CHEIGHT|VALID_WROW|
                                   VALID_CROW|VALID_BOTLINE|VALID_BOTLINE_AP);
            }
        }
        else    /* dir == BACKWARDS */
        {
#ifdef FEAT_DIFF
            if (curwin->w_topline == 1)
            {
                /* Include max number of filler lines */
                max_topfill();
                continue;
            }
#endif
            if (firstwin == lastwin && p_window > 0 && p_window < Rows - 1)
            {
                /* Vi compatible scrolling (sort of) */
                if (p_window <= 2)
                    --curwin->w_topline;
                else
                    curwin->w_topline -= p_window - 2;
                if (curwin->w_topline < 1)
                    curwin->w_topline = 1;
                curwin->w_cursor.lnum = curwin->w_topline + p_window - 1;
                if (curwin->w_cursor.lnum > curbuf->b_ml.ml_line_count)
                    curwin->w_cursor.lnum = curbuf->b_ml.ml_line_count;
                continue;
            }

            /* Find the line at the top of the window that is going to be the
             * line at the bottom of the window.  Make sure this results in
             * the same line as before doing CTRL-F. */
            loff.lnum = curwin->w_topline - 1;
#ifdef FEAT_DIFF
            loff.fill = diff_check_fill(curwin, loff.lnum + 1)
                                                          - curwin->w_topfill;
#endif
            get_scroll_overlap(&loff, 1);

            if (loff.lnum >= curbuf->b_ml.ml_line_count)
            {
                loff.lnum = curbuf->b_ml.ml_line_count;
#ifdef FEAT_DIFF
                loff.fill = 0;
            }
            else
            {
                botline_topline(&loff);
#endif
            }
            curwin->w_cursor.lnum = loff.lnum;

            /* Find the line just above the new topline to get the right line
             * at the bottom of the window. */
            n = 0;
            while (n <= curwin->w_height && loff.lnum >= 1)
            {
                topline_back(&loff);
                n += loff.height;
            }
            if (n <= curwin->w_height)              /* at begin of file */
            {
                curwin->w_topline = 1;
#ifdef FEAT_DIFF
                max_topfill();
#endif
                curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
            }
            else
            {
                /* Go two lines forward again. */
#ifdef FEAT_DIFF
                topline_botline(&loff);
#endif
                botline_forw(&loff);
                botline_forw(&loff);
#ifdef FEAT_DIFF
                botline_topline(&loff);
#endif
#ifdef FEAT_FOLDING
                /* We're at the wrong end of a fold now. */
                (void)hasFolding(loff.lnum, &loff.lnum, NULL);
#endif

                /* Always scroll at least one line.  Avoid getting stuck on
                 * very long lines. */
                if (loff.lnum >= curwin->w_topline
#ifdef FEAT_DIFF
                        && (loff.lnum > curwin->w_topline
                            || loff.fill >= curwin->w_topfill)
#endif
                        )
                {
#ifdef FEAT_DIFF
                    /* First try using the maximum number of filler lines.  If
                     * that's not enough, backup one line. */
                    loff.fill = curwin->w_topfill;
                    if (curwin->w_topfill < diff_check_fill(curwin,
                                                           curwin->w_topline))
                        max_topfill();
                    if (curwin->w_topfill == loff.fill)
#endif
                    {
                        --curwin->w_topline;
#ifdef FEAT_DIFF
                        curwin->w_topfill = 0;
#endif
                    }
                    comp_botline(curwin);
                    curwin->w_cursor.lnum = curwin->w_botline - 1;
                    curwin->w_valid &= ~(VALID_WCOL|VALID_CHEIGHT|
                            VALID_WROW|VALID_CROW);
                }
                else
                {
                    curwin->w_topline = loff.lnum;
#ifdef FEAT_DIFF
                    curwin->w_topfill = loff.fill;
                    check_topfill(curwin, FALSE);
#endif
                    curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
                }
            }
        }
    }
#ifdef FEAT_FOLDING
    foldAdjustCursor();
#endif
    cursor_correct();
    if (retval == OK)
        beginline(BL_SOL | BL_FIX);
    curwin->w_valid &= ~(VALID_WCOL|VALID_WROW|VALID_VIRTCOL);

    /*
     * Avoid the screen jumping up and down when 'scrolloff' is non-zero.
     * But make sure we scroll at least one line (happens with mix of long
     * wrapping lines and non-wrapping line).
     */
    if (retval == OK && dir == FORWARD && check_top_offset())
    {
        scroll_cursor_top(1, FALSE);
        if (curwin->w_topline <= old_topline
                                  && old_topline < curbuf->b_ml.ml_line_count)
        {
            curwin->w_topline = old_topline + 1;
#ifdef FEAT_FOLDING
            (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif
        }
    }

    redraw_later(VALID);
    return retval;
}

/*
 * Decide how much overlap to use for page-up or page-down scrolling.
 * This is symmetric, so that doing both keeps the same lines displayed.
 * Three lines are examined:
 *
 *  before CTRL-F           after CTRL-F / before CTRL-B
 *     etc.                     l1
 *  l1 last but one line        ------------
 *  l2 last text line           l2 top text line
 *  -------------               l3 second text line
 *  l3                             etc.
 */
    static void
get_scroll_overlap(lp, dir)
    lineoff_T   *lp;
    int         dir;
{
    int         h1, h2, h3, h4;
    int         min_height = curwin->w_height - 2;
    lineoff_T   loff0, loff1, loff2;

#ifdef FEAT_DIFF
    if (lp->fill > 0)
        lp->height = 1;
    else
        lp->height = plines_nofill(lp->lnum);
#else
    lp->height = plines(lp->lnum);
#endif
    h1 = lp->height;
    if (h1 > min_height)
        return;         /* no overlap */

    loff0 = *lp;
    if (dir > 0)
        botline_forw(lp);
    else
        topline_back(lp);
    h2 = lp->height;
    if (h2 + h1 > min_height)
    {
        *lp = loff0;    /* no overlap */
        return;
    }

    loff1 = *lp;
    if (dir > 0)
        botline_forw(lp);
    else
        topline_back(lp);
    h3 = lp->height;
    if (h3 + h2 > min_height)
    {
        *lp = loff0;    /* no overlap */
        return;
    }

    loff2 = *lp;
    if (dir > 0)
        botline_forw(lp);
    else
        topline_back(lp);
    h4 = lp->height;
    if (h4 + h3 + h2 > min_height || h3 + h2 + h1 > min_height)
        *lp = loff1;    /* 1 line overlap */
    else
        *lp = loff2;    /* 2 lines overlap */
    return;
}

/* #define KEEP_SCREEN_LINE */
/*
 * Scroll 'scroll' lines up or down.
 */
    void
halfpage(flag, Prenum)
    int         flag;
    linenr_T    Prenum;
{
    long        scrolled = 0;
    int         i;
    int         n;
    int         room;

    if (Prenum)
        curwin->w_p_scr = (Prenum > curwin->w_height) ?
                                                curwin->w_height : Prenum;
    n = (curwin->w_p_scr <= curwin->w_height) ?
                                    curwin->w_p_scr : curwin->w_height;

    validate_botline();
    room = curwin->w_empty_rows;
#ifdef FEAT_DIFF
    room += curwin->w_filler_rows;
#endif
    if (flag)
    {
        /*
         * scroll the text up
         */
        while (n > 0 && curwin->w_botline <= curbuf->b_ml.ml_line_count)
        {
#ifdef FEAT_DIFF
            if (curwin->w_topfill > 0)
            {
                i = 1;
                if (--n < 0 && scrolled > 0)
                    break;
                --curwin->w_topfill;
            }
            else
#endif
            {
#ifdef FEAT_DIFF
                i = plines_nofill(curwin->w_topline);
#else
                i = plines(curwin->w_topline);
#endif
                n -= i;
                if (n < 0 && scrolled > 0)
                    break;
#ifdef FEAT_FOLDING
                (void)hasFolding(curwin->w_topline, NULL, &curwin->w_topline);
#endif
                ++curwin->w_topline;
#ifdef FEAT_DIFF
                curwin->w_topfill = diff_check_fill(curwin, curwin->w_topline);
#endif

#ifndef KEEP_SCREEN_LINE
                if (curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count)
                {
                    ++curwin->w_cursor.lnum;
                    curwin->w_valid &=
                                    ~(VALID_VIRTCOL|VALID_CHEIGHT|VALID_WCOL);
                }
#endif
            }
            curwin->w_valid &= ~(VALID_CROW|VALID_WROW);
            scrolled += i;

            /*
             * Correct w_botline for changed w_topline.
             * Won't work when there are filler lines.
             */
#ifdef FEAT_DIFF
            if (curwin->w_p_diff)
                curwin->w_valid &= ~(VALID_BOTLINE|VALID_BOTLINE_AP);
            else
#endif
            {
                room += i;
                do
                {
                    i = plines(curwin->w_botline);
                    if (i > room)
                        break;
#ifdef FEAT_FOLDING
                    (void)hasFolding(curwin->w_botline, NULL,
                                                          &curwin->w_botline);
#endif
                    ++curwin->w_botline;
                    room -= i;
                } while (curwin->w_botline <= curbuf->b_ml.ml_line_count);
            }
        }

#ifndef KEEP_SCREEN_LINE
        /*
         * When hit bottom of the file: move cursor down.
         */
        if (n > 0)
        {
# ifdef FEAT_FOLDING
            if (hasAnyFolding(curwin))
            {
                while (--n >= 0
                        && curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count)
                {
                    (void)hasFolding(curwin->w_cursor.lnum, NULL,
                                                      &curwin->w_cursor.lnum);
                    ++curwin->w_cursor.lnum;
                }
            }
            else
# endif
                curwin->w_cursor.lnum += n;
            check_cursor_lnum();
        }
#else
        /* try to put the cursor in the same screen line */
        while ((curwin->w_cursor.lnum < curwin->w_topline || scrolled > 0)
                             && curwin->w_cursor.lnum < curwin->w_botline - 1)
        {
            scrolled -= plines(curwin->w_cursor.lnum);
            if (scrolled < 0 && curwin->w_cursor.lnum >= curwin->w_topline)
                break;
# ifdef FEAT_FOLDING
            (void)hasFolding(curwin->w_cursor.lnum, NULL,
                                                      &curwin->w_cursor.lnum);
# endif
            ++curwin->w_cursor.lnum;
        }
#endif
    }
    else
    {
        /*
         * scroll the text down
         */
        while (n > 0 && curwin->w_topline > 1)
        {
#ifdef FEAT_DIFF
            if (curwin->w_topfill < diff_check_fill(curwin, curwin->w_topline))
            {
                i = 1;
                if (--n < 0 && scrolled > 0)
                    break;
                ++curwin->w_topfill;
            }
            else
#endif
            {
#ifdef FEAT_DIFF
                i = plines_nofill(curwin->w_topline - 1);
#else
                i = plines(curwin->w_topline - 1);
#endif
                n -= i;
                if (n < 0 && scrolled > 0)
                    break;
                --curwin->w_topline;
#ifdef FEAT_FOLDING
                (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
#endif
#ifdef FEAT_DIFF
                curwin->w_topfill = 0;
#endif
            }
            curwin->w_valid &= ~(VALID_CROW|VALID_WROW|
                                              VALID_BOTLINE|VALID_BOTLINE_AP);
            scrolled += i;
#ifndef KEEP_SCREEN_LINE
            if (curwin->w_cursor.lnum > 1)
            {
                --curwin->w_cursor.lnum;
                curwin->w_valid &= ~(VALID_VIRTCOL|VALID_CHEIGHT|VALID_WCOL);
            }
#endif
        }
#ifndef KEEP_SCREEN_LINE
        /*
         * When hit top of the file: move cursor up.
         */
        if (n > 0)
        {
            if (curwin->w_cursor.lnum <= (linenr_T)n)
                curwin->w_cursor.lnum = 1;
            else
# ifdef FEAT_FOLDING
            if (hasAnyFolding(curwin))
            {
                while (--n >= 0 && curwin->w_cursor.lnum > 1)
                {
                    --curwin->w_cursor.lnum;
                    (void)hasFolding(curwin->w_cursor.lnum,
                                                &curwin->w_cursor.lnum, NULL);
                }
            }
            else
# endif
                curwin->w_cursor.lnum -= n;
        }
#else
        /* try to put the cursor in the same screen line */
        scrolled += n;      /* move cursor when topline is 1 */
        while (curwin->w_cursor.lnum > curwin->w_topline
              && (scrolled > 0 || curwin->w_cursor.lnum >= curwin->w_botline))
        {
            scrolled -= plines(curwin->w_cursor.lnum - 1);
            if (scrolled < 0 && curwin->w_cursor.lnum < curwin->w_botline)
                break;
            --curwin->w_cursor.lnum;
# ifdef FEAT_FOLDING
            foldAdjustCursor();
# endif
        }
#endif
    }
# ifdef FEAT_FOLDING
    /* Move cursor to first line of closed fold. */
    foldAdjustCursor();
# endif
#ifdef FEAT_DIFF
    check_topfill(curwin, !flag);
#endif
    cursor_correct();
    beginline(BL_SOL | BL_FIX);
    redraw_later(VALID);
}