// This is an open source non-commercial project. Dear PVS-Studio, please check
// it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com

/*
 * 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 <assert.h>
#include <inttypes.h>
#include <stdbool.h>

#include "nvim/vim.h"
#include "nvim/ascii.h"
#include "nvim/move.h"
#include "nvim/charset.h"
#include "nvim/cursor.h"
#include "nvim/diff.h"
#include "nvim/edit.h"
#include "nvim/fold.h"
#include "nvim/mbyte.h"
#include "nvim/memline.h"
#include "nvim/misc1.h"
#include "nvim/option.h"
#include "nvim/popupmnu.h"
#include "nvim/screen.h"
#include "nvim/strings.h"
#include "nvim/window.h"

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

#ifdef INCLUDE_GENERATED_DECLARATIONS
# include "move.c.generated.h"
#endif


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

  /*
   * 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) {
    int n;
    linenr_T last = lnum;
    bool folded = hasFoldingWin(wp, lnum, NULL, &last, true, NULL);
    if (folded) {
      n = 1;
    } else if (lnum == wp->w_topline) {
      n = plines_win_nofill(wp, lnum, true) + wp->w_topfill;
    } else {
      n = plines_win(wp, lnum, true);
    }
    if (lnum <= wp->w_cursor.lnum && last >= wp->w_cursor.lnum) {
      wp->w_cline_row = done;
      wp->w_cline_height = n;
      wp->w_cline_folded = folded;
      redraw_for_cursorline(wp);
      wp->w_valid |= (VALID_CROW|VALID_CHEIGHT);
    }
    if (done + n > wp->w_height)
      break;
    done += n;
    lnum = last;
  }

  /* 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);
}

/*
* Redraw when w_cline_row changes and 'relativenumber' or 'cursorline' is
* set.
*/
static void redraw_for_cursorline(win_T *wp)
{
  if ((wp->w_p_rnu || wp->w_p_cul)
      && (wp->w_valid & VALID_CROW) == 0
      && !pum_visible()) {
    redraw_win_later(wp, SOME_VALID);
  }
}

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

/*
 * Update curwin->w_topline to move the cursor onto the screen.
 */
void update_topline(void)
{
  linenr_T old_topline;
  int old_topfill;
  bool check_topline = false;
  bool check_botline = false;
  long save_so = p_so;

  if (!screen_valid(true))
    return;

  // If the window height is zero, just use the cursor line.
  if (curwin->w_height == 0) {
    curwin->w_topline = curwin->w_cursor.lnum;
    curwin->w_botline = curwin->w_topline;
    curwin->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;
    curwin->w_scbind_pos = 1;
    return;
  }

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

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

  old_topline = curwin->w_topline;
  old_topfill = curwin->w_topfill;

  /*
   * 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;
    curwin->w_botline = 2;
    curwin->w_valid |= VALID_BOTLINE|VALID_BOTLINE_AP;
    curwin->w_scbind_pos = 1;
  }
  /*
   * 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;
    }
    /* 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;

    if (check_topline) {
      int halfheight = curwin->w_height / 2 - 1;
      if (halfheight < 2)
        halfheight = 2;
      long n;
      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 (linenr_T 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
        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 {
      /* Make sure topline is the first line of a fold. */
      (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
      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) {
        if (((long)curwin->w_cursor.lnum
             >= (long)curwin->w_botline - p_so
             || hasAnyFolding(curwin)
             )) {
          lineoff_T loff;

          /* Cursor is (a few lines) above botline, check if there are
           * 'scrolloff' window lines below the cursor.  If not, need to
           * scroll. */
          int n = curwin->w_empty_rows;
          loff.lnum = curwin->w_cursor.lnum;
          /* In a fold go to its last line. */
          (void)hasFolding(loff.lnum, NULL, &loff.lnum);
          loff.fill = 0;
          n += curwin->w_filler_rows;
          loff.height = 0;
          while (loff.lnum < curwin->w_botline
                 && (loff.lnum + 1 < curwin->w_botline || loff.fill == 0)
                 ) {
            n += loff.height;
            if (n >= p_so)
              break;
            botline_forw(&loff);
          }
          if (n >= p_so)
            /* sufficient context, no need to scroll */
            check_botline = false;
        } else {
          /* sufficient context, no need to scroll */
          check_botline = false;
        }
      }
      if (check_botline) {
        long line_count = 0;
        if (hasAnyFolding(curwin)) {
          /* Count the number of logical lines between the cursor and
           * botline - p_so (approximation of how much will be
           * scrolled). */
          for (linenr_T 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
          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
      || curwin->w_topfill != old_topfill
      ) {
    dollar_vcol = -1;
    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();
  }

  p_so = save_so;
}

/*
 * Update win->w_topline to move the cursor onto the screen.
 */
void update_topline_win(win_T* win)
{
  win_T *save_curwin;
  switch_win(&save_curwin, NULL, win, NULL, true);
  update_topline();
  restore_win(save_curwin, NULL, true);
}

/*
 * 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(void)
{
  long result = p_sj >= 0 ? p_sj : (curwin->w_height * -p_sj) / 100;
  assert(result <= INT_MAX);
  return (int)result;
}

/*
 * Return true when there are not 'scrolloff' lines above the cursor for the
 * current window.
 */
static bool check_top_offset(void)
{
  if (curwin->w_cursor.lnum < curwin->w_topline + p_so
      || hasAnyFolding(curwin)
      ) {
    lineoff_T loff;
    loff.lnum = curwin->w_cursor.lnum;
    loff.fill = 0;
    int n = curwin->w_topfill;          /* always have this context */
    /* 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
          || (loff.lnum == curwin->w_topline && loff.fill > 0)
          )
        break;
      n += loff.height;
    }
    if (n < p_so)
      return true;
  }
  return false;
}

void update_curswant(void)
{
  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(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
             || wp->w_cursor.coladd != wp->w_valid_cursor.coladd
             ) {
    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;
    wp->w_valid_cursor.coladd = wp->w_cursor.coladd;
  }
}

/*
 * 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(void)
{
  changed_window_setting_win(curwin);
}

void changed_window_setting_win(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(win_T *wp, linenr_T lnum)
{
  /* go to first of folded lines */
  (void)hasFoldingWin(wp, lnum, &lnum, NULL, true, NULL);
  /* Approximate the value of w_botline */
  wp->w_botline += lnum - wp->w_topline;
  wp->w_topline = lnum;
  wp->w_topline_was_set = true;
  wp->w_topfill = 0;
  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(void)
{
  curwin->w_valid &= ~(VALID_WROW|VALID_WCOL|VALID_VIRTCOL
                       |VALID_CHEIGHT|VALID_TOPLINE);
}

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

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

void changed_line_abv_curs_win(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(void)
{
  if (!(curwin->w_valid & VALID_BOTLINE))
    comp_botline(curwin);
}

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

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

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

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

/*
 * Compute wp->w_cline_row and wp->w_cline_height, based on the current value
 * of wp->w_topline.
 */
static void curs_rows(win_T *wp)
{
  /* Check if wp->w_lines[].wl_size is invalid */
  int all_invalid = (!redrawing()
                     || wp->w_lines_valid == 0
                     || wp->w_lines[0].wl_lnum > wp->w_topline);
  int i = 0;
  wp->w_cline_row = 0;
  for (linenr_T lnum = wp->w_topline; lnum < wp->w_cursor.lnum; ++i) {
    bool 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) {
        /* 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)
          valid = true;
      } else if (wp->w_lines[i].wl_lnum > lnum) {
        --i;                            /* hold at inserted lines */
      }
    }
    if (valid
        && (lnum != wp->w_topline || !wp->w_p_diff)
        ) {
      lnum = wp->w_lines[i].wl_lastlnum + 1;
      /* Cursor inside folded lines, don't count this row */
      if (lnum > wp->w_cursor.lnum)
        break;
      wp->w_cline_row += wp->w_lines[i].wl_size;
    } else {
      long fold_count = foldedCount(wp, lnum, NULL);
      if (fold_count) {
        lnum += fold_count;
        if (lnum > wp->w_cursor.lnum)
          break;
        ++wp->w_cline_row;
      } else if (lnum == wp->w_topline) {
        wp->w_cline_row += plines_win_nofill(wp, lnum++, true)
                           + wp->w_topfill;
      } else {
        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))) {
      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
        wp->w_cline_height = plines_win(wp, wp->w_cursor.lnum, true);
      wp->w_cline_folded = hasFoldingWin(wp, wp->w_cursor.lnum,
          NULL, NULL, true, NULL);
    } 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;
      wp->w_cline_folded = hasFoldingWin(wp, wp->w_cursor.lnum,
          NULL, NULL, true, NULL);
    } else {
      wp->w_cline_height = wp->w_lines[i].wl_size;
      wp->w_cline_folded = wp->w_lines[i].wl_folded;
    }
  }

  redraw_for_cursorline(curwin);
  wp->w_valid |= VALID_CROW|VALID_CHEIGHT;
}

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

/*
 * Validate wp->w_virtcol only.
 */
void validate_virtcol_win(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;
    if (wp->w_p_cuc
        && !pum_visible()
        )
      redraw_win_later(wp, SOME_VALID);
  }
}

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

/*
 * Validate w_wcol and w_virtcol only.
 */
void validate_cursor_col(void)
{
  validate_virtcol();
  if (!(curwin->w_valid & VALID_WCOL)) {
    colnr_T col = curwin->w_virtcol;
    colnr_T off = curwin_col_off();
    col += off;
    int width = curwin->w_width - off + curwin_col_off2();

    /* long line wrapping, adjust curwin->w_wrow */
    if (curwin->w_p_wrap
        && col >= (colnr_T)curwin->w_width
        && width > 0)
      /* use same formula as what is used in curs_columns() */
      col -= ((col - curwin->w_width) / width + 1) * width;
    if (col > (int)curwin->w_leftcol)
      col -= curwin->w_leftcol;
    else
      col = 0;
    curwin->w_wcol = col;

    curwin->w_valid |= VALID_WCOL;
  }
}

/*
 * Compute offset of a window, occupied by absolute or relative line number,
 * fold column and sign column (these don't move when scrolling horizontally).
 */
int win_col_off(win_T *wp)
{
  return ((wp->w_p_nu || wp->w_p_rnu) ? number_width(wp) + 1 : 0)
         + (cmdwin_type == 0 || wp != curwin ? 0 : 1)
         + (int)wp->w_p_fdc
         + (signcolumn_on(wp) ? 2 : 0);
}

int curwin_col_off(void)
{
  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' or 'relativenumber' is on and 'n' is in
 * 'cpoptions'.
 */
int win_col_off2(win_T *wp)
{
  if ((wp->w_p_nu || wp->w_p_rnu) && vim_strchr(p_cpo, CPO_NUMCOL) != NULL)
    return number_width(wp) + 1;
  return 0;
}

int curwin_col_off2(void)
{
  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(
    int may_scroll                 /* when true, may scroll horizontally */
)
{
  int n;
  int width = 0;
  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);
  }

  /*
   * Compute the number of virtual columns.
   */
  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
    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 = -1;

  int 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;

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

    /* long line wrapping, adjust curwin->w_wrow */
    if (curwin->w_wcol >= curwin->w_width) {
      /* this same formula is used in validate_cursor_col() */
      n = (curwin->w_wcol - curwin->w_width) / width + 1;
      curwin->w_wcol -= n * width;
      curwin->w_wrow += n;

      /* 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 && *get_cursor_pos_ptr() == NUL
          && curwin->w_wcol == (int)vim_strsize(p_sbr))
        curwin->w_wcol = 0;
    }
  }
  /* 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 (may_scroll
           && !curwin->w_cline_folded
           ) {
    /*
     * 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
     */
    assert(p_siso <= INT_MAX);
    int off_left = startcol - curwin->w_leftcol - (int)p_siso;
    int off_right =
        endcol - curwin->w_leftcol - curwin->w_width + (int)p_siso + 1;
    if (off_left < 0 || off_right > 0) {
      int diff = (off_left < 0) ? -off_left: off_right;

      /* When far off or not enough room on either side, put cursor in
       * middle of window. */
      int new_leftcol;
      if (p_ss == 0 || diff >= textwidth / 2 || off_right >= off_left)
        new_leftcol = curwin->w_wcol - extra - textwidth / 2;
      else {
        if (diff < p_ss) {
          assert(p_ss <= INT_MAX);
          diff = (int)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;

  /* 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);

  prev_skipcol = curwin->w_skipcol;

  int p_lines = 0;
  if ((curwin->w_wrow >= curwin->w_height
       || ((prev_skipcol > 0
            || curwin->w_wrow + p_so >= curwin->w_height)
           && (p_lines =
                 plines_win_nofill
                   (curwin, curwin->w_cursor.lnum, false))
           - 1 >= curwin->w_height))
      && curwin->w_height != 0
      && curwin->w_cursor.lnum == curwin->w_topline
      && width > 0
      && curwin->w_width != 0
      ) {
    /* 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) {
      assert(p_so <= INT_MAX);
      n = curwin->w_wrow + (int)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 */
      assert(p_so <= INT_MAX);
      extra = (curwin->w_skipcol + (int)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);

  /* Redraw when w_virtcol changes and 'cursorcolumn' is set */
  if (curwin->w_p_cuc && (curwin->w_valid & VALID_VIRTCOL) == 0
      && !pum_visible()) {
    redraw_later(SOME_VALID);
  }

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

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

  /* Make sure w_topline is at the first of a sequence of folded lines. */
  (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
  validate_cursor();            /* w_wrow needs to be valid */
  while (line_count-- > 0) {
    if (curwin->w_topfill < diff_check(curwin, curwin->w_topline)
        && curwin->w_topfill < curwin->w_height - 1) {
      ++curwin->w_topfill;
      ++done;
    } else {
      if (curwin->w_topline == 1)
        break;
      --curwin->w_topline;
      curwin->w_topfill = 0;
      /* A sequence of folded lines only counts for one logical line */
      linenr_T first;
      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
        done += plines_nofill(curwin->w_topline);
    }
    --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 */

  if (curwin->w_cursor.lnum == curwin->w_topline)
    curwin->w_cline_row = 0;
  check_topfill(curwin, true);

  /*
   * Compute the row number of the last row of the cursor line
   * and move the cursor onto the displayed part of the window.
   */
  int wrow = curwin->w_wrow;
  if (curwin->w_p_wrap
      && curwin->w_width != 0
      ) {
    validate_virtcol();
    validate_cheight();
    wrow += curwin->w_cline_height - 1 -
            curwin->w_virtcol / curwin->w_width;
  }
  bool moved = false;
  while (wrow >= curwin->w_height && curwin->w_cursor.lnum > 1) {
    linenr_T first;
    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
      wrow -= plines(curwin->w_cursor.lnum--);
    curwin->w_valid &=
      ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW|VALID_VIRTCOL);
    moved = true;
  }
  if (moved) {
    /* Move cursor to first line of closed fold. */
    foldAdjustCursor();
    coladvance(curwin->w_curswant);
  }
}

/*
 * Scroll the current window up by "line_count" logical lines.  "CTRL-E"
 */
void 
scrollup (
    long line_count,
    int byfold              /* true: count a closed fold as one line */
)
{
  if ((byfold && hasAnyFolding(curwin))
      || curwin->w_p_diff) {
    // count each sequence of folded lines as one logical line
    linenr_T lnum = curwin->w_topline;
    while (line_count--) {
      if (curwin->w_topfill > 0)
        --curwin->w_topfill;
      else {
        if (byfold)
          (void)hasFolding(lnum, NULL, &lnum);
        if (lnum >= curbuf->b_ml.ml_line_count)
          break;
        ++lnum;
        curwin->w_topfill = diff_check_fill(curwin, lnum);
      }
    }
    /* approximate w_botline */
    curwin->w_botline += lnum - curwin->w_topline;
    curwin->w_topline = lnum;
  } else {
    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;

  check_topfill(curwin, false);

  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);

  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);
  }
}

/*
 * Don't end up with too many filler lines in the window.
 */
void 
check_topfill (
    win_T *wp,
    bool down              /* when true scroll down when not enough space */
)
{
  if (wp->w_topfill > 0) {
    int 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(void)
{
  int 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;
  }
}

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

  if (curwin->w_topline <= 1
      && !can_fill
      )
    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.
   */
  int end_row = curwin->w_wrow;
  if (can_fill)
    ++end_row;
  else
    end_row += plines_nofill(curwin->w_topline - 1);
  if (curwin->w_p_wrap
      && curwin->w_width != 0
      ) {
    validate_cheight();
    validate_virtcol();
    end_row += curwin->w_cline_height - 1 -
               curwin->w_virtcol / curwin->w_width;
  }
  if (end_row < curwin->w_height - p_so) {
    if (can_fill) {
      ++curwin->w_topfill;
      check_topfill(curwin, true);
    } else {
      --curwin->w_topline;
      curwin->w_topfill = 0;
    }
    (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
    --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(void)
{
  if (curwin->w_topline == curbuf->b_ml.ml_line_count
      && curwin->w_topfill == 0
      )
    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.
   */
  int start_row = curwin->w_wrow - plines_nofill(curwin->w_topline)
              - curwin->w_topfill;
  if (curwin->w_p_wrap
      && curwin->w_width != 0
      ) {
    validate_virtcol();
    start_row -= curwin->w_virtcol / curwin->w_width;
  }
  if (start_row >= p_so) {
    if (curwin->w_topfill > 0)
      --curwin->w_topfill;
    else {
      (void)hasFolding(curwin->w_topline, NULL, &curwin->w_topline);
      ++curwin->w_topline;
    }
    ++curwin->w_botline;                /* approximate w_botline */
    curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
  }
}

/*
 * 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: MAXCOL.
 */
static void topline_back(lineoff_T *lp)
{
  if (lp->fill < diff_check_fill(curwin, lp->lnum)) {
    /* Add a filler line. */
    ++lp->fill;
    lp->height = 1;
  } else {
    --lp->lnum;
    lp->fill = 0;
    if (lp->lnum < 1)
      lp->height = MAXCOL;
    else if (hasFolding(lp->lnum, &lp->lnum, NULL))
      /* Add a closed fold */
      lp->height = 1;
    else {
      lp->height = plines_nofill(lp->lnum);
    }
  }
}

/*
 * 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(lineoff_T *lp)
{
  if (lp->fill < diff_check_fill(curwin, lp->lnum + 1)) {
    /* Add a filler line. */
    ++lp->fill;
    lp->height = 1;
  } else {
    ++lp->lnum;
    lp->fill = 0;
    if (lp->lnum > curbuf->b_ml.ml_line_count)
      lp->height = MAXCOL;
    else if (hasFolding(lp->lnum, NULL, &lp->lnum))
      /* Add a closed fold */
      lp->height = 1;
    else {
      lp->height = plines_nofill(lp->lnum);
    }
  }
}

/*
 * 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(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(lineoff_T *lp)
{
  if (lp->fill > 0) {
    lp->fill = diff_check_fill(curwin, lp->lnum) - lp->fill + 1;
    --lp->lnum;
  }
}

/*
 * 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(int min_scroll, int always)
{
  int scrolled = 0;
  linenr_T top;                 /* just above displayed lines */
  linenr_T bot;                 /* just below displayed lines */
  linenr_T old_topline = curwin->w_topline;
  linenr_T old_topfill = curwin->w_topfill;
  linenr_T new_topline;
  assert(p_so <= INT_MAX);
  int off = (int)p_so;

  if (mouse_dragging > 0)
    off = mouse_dragging - 1;

  /*
   * 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();
  int used = curwin->w_cline_height;  // includes filler lines above
  if (curwin->w_cursor.lnum < curwin->w_topline) {
    scrolled = used;
  }

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

  // "used" already contains the number of filler lines above, don't add it
  // again.
  // Hide filler lines above cursor line by adding them to "extra".
  int extra = diff_check_fill(curwin, curwin->w_cursor.lnum);

  /*
   * 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) {
    int i = hasFolding(top, &top, NULL)
            ? 1  // count one logical line for a sequence of folded lines
            : plines_nofill(top);
    used += i;
    if (extra + i <= off && bot < curbuf->b_ml.ml_line_count) {
      if (hasFolding(bot, NULL, &bot))
        /* count one logical line for a sequence of folded lines */
        ++used;
      else
        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;
    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);
    if (curwin->w_topline != old_topline
        || curwin->w_topfill != old_topfill
        )
      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(win_T *wp, int used)
{
  wp->w_filler_rows = 0;
  if (used == 0)
    wp->w_empty_rows = 0;       /* single line that doesn't fit */
  else {
    wp->w_empty_rows = wp->w_height - used;
    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;
      }
    }
  }
}

/*
 * 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(int min_scroll, int set_topbot)
{
  int used;
  int scrolled = 0;
  int extra = 0;
  lineoff_T loff;
  lineoff_T boff;
  int fill_below_window;
  linenr_T old_topline    = curwin->w_topline;
  int      old_topfill    = curwin->w_topfill;
  linenr_T old_botline    = curwin->w_botline;
  int      old_valid      = curwin->w_valid;
  int      old_empty_rows = curwin->w_empty_rows;
  linenr_T cln            = curwin->w_cursor.lnum; /* Cursor Line Number */

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

  /* The lines of the cursor line itself are always used. */
  used = plines_nofill(cln);

  /* 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
   */
  if (!hasFolding(curwin->w_cursor.lnum, &loff.lnum, &boff.lnum)) {
    loff.lnum = cln;
    boff.lnum = cln;
  }
  loff.fill = 0;
  boff.fill = 0;
  fill_below_window = diff_check_fill(curwin, curwin->w_botline)
                      - curwin->w_filler_rows;

  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 >= (
            mouse_dragging > 0 ? mouse_dragging - 1 :
            p_so))
         || boff.lnum + 1 > curbuf->b_ml.ml_line_count)
        && loff.lnum <= curwin->w_botline
        && (loff.lnum < curwin->w_botline
            || loff.fill >= fill_below_window)
        )
      break;

    /* Add one line above */
    topline_back(&loff);
    if (loff.height == MAXCOL)
      used = MAXCOL;
    else
      used += loff.height;
    if (used > curwin->w_height)
      break;
    if (loff.lnum >= curwin->w_botline
        && (loff.lnum > curwin->w_botline
            || loff.fill <= fill_below_window)
        ) {
      /* Count screen lines that are below the window. */
      scrolled += loff.height;
      if (loff.lnum == curwin->w_botline
          && boff.fill == 0
          )
        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 < (
            mouse_dragging > 0 ? mouse_dragging - 1 :
            p_so) || scrolled < min_scroll) {
        extra += boff.height;
        if (boff.lnum >= curwin->w_botline
            || (boff.lnum + 1 == curwin->w_botline
                && boff.fill > curwin->w_filler_rows)
            ) {
          /* Count screen lines that are below the window. */
          scrolled += boff.height;
          if (boff.lnum == curwin->w_botline
              && boff.fill == 0
              )
            scrolled -= curwin->w_empty_rows;
        }
      }
    }
  }

  linenr_T line_count;
  /* 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;
    boff.fill = curwin->w_topfill;
    boff.lnum = curwin->w_topline - 1;
    int i;
    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 across the window
///
/// @param atend if true, also put the cursor halfway to the end of the file.
///
void scroll_cursor_halfway(int atend)
{
  int above = 0;
  int topfill = 0;
  int below = 0;
  lineoff_T loff;
  lineoff_T boff;
  linenr_T old_topline = curwin->w_topline;

  loff.lnum = boff.lnum = curwin->w_cursor.lnum;
  (void)hasFolding(loff.lnum, &loff.lnum, &boff.lnum);
  int used = plines_nofill(loff.lnum);
  loff.fill = 0;
  boff.fill = 0;
  linenr_T 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);
      if (loff.height == MAXCOL)
        used = MAXCOL;
      else
        used += loff.height;
      if (used > curwin->w_height)
        break;
      above += loff.height;
      topline = loff.lnum;
      topfill = loff.fill;
    }
  }
  if (!hasFolding(topline, &curwin->w_topline, NULL))
    curwin->w_topline = topline;
  curwin->w_topfill = topfill;
  if (old_topline > curwin->w_topline + curwin->w_height)
    curwin->w_botfill = false;
  check_topfill(curwin, false);
  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(void)
{
  /*
   * 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.
   */
  assert(p_so <= INT_MAX);
  int above_wanted = (int)p_so;
  int below_wanted = (int)p_so;
  if (mouse_dragging > 0) {
    above_wanted = mouse_dragging - 1;
    below_wanted = mouse_dragging - 1;
  }
  if (curwin->w_topline == 1) {
    above_wanted = 0;
    int 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
      && mouse_dragging == 0
      ) {
    below_wanted = 0;
    int 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.
   */
  linenr_T cln = curwin->w_cursor.lnum; /* Cursor Line Number */
  if (cln >= curwin->w_topline + above_wanted
      && cln < curwin->w_botline - below_wanted
      && !hasAnyFolding(curwin)
      )
    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
   */
  linenr_T topline = curwin->w_topline;
  linenr_T botline = curwin->w_botline - 1;
  /* count filler lines as context */
  int above = curwin->w_topfill; /* screen lines above topline */
  int below = curwin->w_filler_rows; /* screen lines below botline */
  while ((above < above_wanted || below < below_wanted) && topline < botline) {
    if (below < below_wanted && (below <= above || above >= above_wanted)) {
      if (hasFolding(botline, &botline, NULL))
        ++below;
      else
        below += plines(botline);
      --botline;
    }
    if (above < above_wanted && (above < below || below >= below_wanted)) {
      if (hasFolding(topline, NULL, &topline))
        ++above;
      else
        above += plines_nofill(topline);

      /* Count filler lines below this line as context. */
      if (topline < botline)
        above += diff_check_fill(curwin, topline + 1);
      ++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;
}


/*
 * move screen 'count' pages up or down and update screen
 *
 * return FAIL for failure, OK otherwise
 */
int onepage(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
           && curwin->w_topfill ==
           diff_check_fill(curwin, curwin->w_topline)
           )) {
      beep_flush();
      retval = FAIL;
      break;
    }

    loff.fill = 0;
    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;
        curwin->w_topfill = 0;
        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;
        loff.fill = diff_check_fill(curwin, loff.lnum)
                    - curwin->w_filler_rows;
        get_scroll_overlap(&loff, -1);
        curwin->w_topline = loff.lnum;
        curwin->w_topfill = loff.fill;
        check_topfill(curwin, false);
        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 */
      if (curwin->w_topline == 1) {
        /* Include max number of filler lines */
        max_topfill();
        continue;
      }
      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;
      loff.fill = diff_check_fill(curwin, loff.lnum + 1)
                  - curwin->w_topfill;
      get_scroll_overlap(&loff, 1);

      if (loff.lnum >= curbuf->b_ml.ml_line_count) {
        loff.lnum = curbuf->b_ml.ml_line_count;
        loff.fill = 0;
      } else {
        botline_topline(&loff);
      }
      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);
        if (loff.height == MAXCOL)
          n = MAXCOL;
        else
          n += loff.height;
      }
      if (loff.lnum < 1) {                      /* at begin of file */
        curwin->w_topline = 1;
        max_topfill();
        curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
      } else {
        /* Go two lines forward again. */
        topline_botline(&loff);
        botline_forw(&loff);
        botline_forw(&loff);
        botline_topline(&loff);
        /* We're at the wrong end of a fold now. */
        (void)hasFolding(loff.lnum, &loff.lnum, NULL);

        /* Always scroll at least one line.  Avoid getting stuck on
         * very long lines. */
        if (loff.lnum >= curwin->w_topline
            && (loff.lnum > curwin->w_topline
                || loff.fill >= curwin->w_topfill)
            ) {
          /* 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) {
            --curwin->w_topline;
            curwin->w_topfill = 0;
          }
          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;
          curwin->w_topfill = loff.fill;
          check_topfill(curwin, false);
          curwin->w_valid &= ~(VALID_WROW|VALID_CROW|VALID_BOTLINE);
        }
      }
    }
  }
  foldAdjustCursor();
  cursor_correct();
  check_cursor_col();
  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;
      (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
    }
  }

  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(lineoff_T *lp, int dir)
{
  int min_height = curwin->w_height - 2;

  if (lp->fill > 0)
    lp->height = 1;
  else
    lp->height = plines_nofill(lp->lnum);
  int h1 = lp->height;
  if (h1 > min_height)
    return;             /* no overlap */

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

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

  lineoff_T loff2 = *lp;
  if (dir > 0)
    botline_forw(lp);
  else
    topline_back(lp);
  int h4 = lp->height;
  if (h4 == MAXCOL || 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(bool flag, linenr_T Prenum)
{
  long scrolled = 0;
  int i;

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

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

#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.
       */
      if (curwin->w_p_diff)
        curwin->w_valid &= ~(VALID_BOTLINE|VALID_BOTLINE_AP);
      else {
        room += i;
        do {
          i = plines(curwin->w_botline);
          if (i > room)
            break;
          (void)hasFolding(curwin->w_botline, NULL,
              &curwin->w_botline);
          ++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) {
      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
        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;
      (void)hasFolding(curwin->w_cursor.lnum, NULL,
          &curwin->w_cursor.lnum);
      ++curwin->w_cursor.lnum;
    }
#endif
  } else {
    /*
     * scroll the text down
     */
    while (n > 0 && curwin->w_topline > 1) {
      if (curwin->w_topfill < diff_check_fill(curwin, curwin->w_topline)) {
        i = 1;
        if (--n < 0 && scrolled > 0)
          break;
        ++curwin->w_topfill;
      } else {
        i = plines_nofill(curwin->w_topline - 1);
        n -= i;
        if (n < 0 && scrolled > 0)
          break;
        --curwin->w_topline;
        (void)hasFolding(curwin->w_topline, &curwin->w_topline, NULL);
        curwin->w_topfill = 0;
      }
      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 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
        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;
      foldAdjustCursor();
    }
#endif
  }
  /* Move cursor to first line of closed fold. */
  foldAdjustCursor();
  check_topfill(curwin, !flag);
  cursor_correct();
  beginline(BL_SOL | BL_FIX);
  redraw_later(VALID);
}

void do_check_cursorbind(void)
{
  linenr_T line    = curwin->w_cursor.lnum;
  colnr_T col      = curwin->w_cursor.col;
  colnr_T coladd   = curwin->w_cursor.coladd;
  colnr_T curswant = curwin->w_curswant;
  int set_curswant = curwin->w_set_curswant;
  win_T *old_curwin = curwin;
  buf_T *old_curbuf = curbuf;
  int old_VIsual_select = VIsual_select;
  int old_VIsual_active = VIsual_active;

  /*
   * loop through the cursorbound windows
   */
  VIsual_select = VIsual_active = 0;
  FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
    curwin = wp;
    curbuf = curwin->w_buffer;
    /* skip original window  and windows with 'noscrollbind' */
    if (curwin != old_curwin && curwin->w_p_crb) {
      if (curwin->w_p_diff)
        curwin->w_cursor.lnum
          = diff_get_corresponding_line(old_curbuf,
            line,
            curbuf,
            curwin->w_cursor.lnum);
      else
        curwin->w_cursor.lnum = line;
      curwin->w_cursor.col = col;
      curwin->w_cursor.coladd = coladd;
      curwin->w_curswant = curswant;
      curwin->w_set_curswant = set_curswant;

      /* Make sure the cursor is in a valid position.  Temporarily set
       * "restart_edit" to allow the cursor to be beyond the EOL. */
      {
        int restart_edit_save = restart_edit;
        restart_edit = true;
        check_cursor();
        restart_edit = restart_edit_save;
      }
      /* Correct cursor for multi-byte character. */
      if (has_mbyte)
        mb_adjust_cursor();
      redraw_later(VALID);

      /* Only scroll when 'scrollbind' hasn't done this. */
      if (!curwin->w_p_scb)
        update_topline();
      curwin->w_redr_status = true;
    }
  }

  /*
   * reset current-window
   */
  VIsual_select = old_VIsual_select;
  VIsual_active = old_VIsual_active;
  curwin = old_curwin;
  curbuf = old_curbuf;
}

