/* 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.
 */

/*
 * undo.c: multi level undo facility
 *
 * The saved lines are stored in a list of lists (one for each buffer):
 *
 * b_u_oldhead------------------------------------------------+
 *							      |
 *							      V
 *		  +--------------+    +--------------+	  +--------------+
 * b_u_newhead--->| u_header	 |    | u_header     |	  | u_header	 |
 *		  |	uh_next------>|     uh_next------>|	uh_next---->NULL
 *	   NULL<--------uh_prev  |<---------uh_prev  |<---------uh_prev  |
 *		  |	uh_entry |    |     uh_entry |	  |	uh_entry |
 *		  +--------|-----+    +--------|-----+	  +--------|-----+
 *			   |		       |		   |
 *			   V		       V		   V
 *		  +--------------+    +--------------+	  +--------------+
 *		  | u_entry	 |    | u_entry      |	  | u_entry	 |
 *		  |	ue_next  |    |     ue_next  |	  |	ue_next  |
 *		  +--------|-----+    +--------|-----+	  +--------|-----+
 *			   |		       |		   |
 *			   V		       V		   V
 *		  +--------------+	      NULL		  NULL
 *		  | u_entry	 |
 *		  |	ue_next  |
 *		  +--------|-----+
 *			   |
 *			   V
 *			  etc.
 *
 * Each u_entry list contains the information for one undo or redo.
 * curbuf->b_u_curhead points to the header of the last undo (the next redo),
 * or is NULL if nothing has been undone (end of the branch).
 *
 * For keeping alternate undo/redo branches the uh_alt field is used.  Thus at
 * each point in the list a branch may appear for an alternate to redo.  The
 * uh_seq field is numbered sequentially to be able to find a newer or older
 * branch.
 *
 *		   +---------------+	+---------------+
 * b_u_oldhead --->| u_header	   |	| u_header	|
 *		   |   uh_alt_next ---->|   uh_alt_next ----> NULL
 *	   NULL <----- uh_alt_prev |<------ uh_alt_prev |
 *		   |   uh_prev	   |	|   uh_prev	|
 *		   +-----|---------+	+-----|---------+
 *			 |		      |
 *			 V		      V
 *		   +---------------+	+---------------+
 *		   | u_header	   |	| u_header	|
 *		   |   uh_alt_next |	|   uh_alt_next |
 * b_u_newhead --->|   uh_alt_prev |	|   uh_alt_prev |
 *		   |   uh_prev	   |	|   uh_prev	|
 *		   +-----|---------+	+-----|---------+
 *			 |		      |
 *			 V		      V
 *		       NULL		+---------------+    +---------------+
 *					| u_header	|    | u_header      |
 *					|   uh_alt_next ---->|	 uh_alt_next |
 *					|   uh_alt_prev |<------ uh_alt_prev |
 *					|   uh_prev	|    |	 uh_prev     |
 *					+-----|---------+    +-----|---------+
 *					      |			   |
 *					     etc.		  etc.
 *
 *
 * All data is allocated and will all be freed when the buffer is unloaded.
 */

/* Uncomment the next line for including the u_check() function.  This warns
 * for errors in the debug information. */
/* #define U_DEBUG 1 */
#define UH_MAGIC 0x18dade	/* value for uh_magic when in use */
#define UE_MAGIC 0xabc123	/* value for ue_magic when in use */

#if defined(MSDOS) || defined(WIN16) || defined(WIN32) || defined(_WIN64)
# include "vimio.h"	/* for vim_read(), must be before vim.h */
#endif

#include "vim.h"

static void u_unch_branch __ARGS((u_header_T *uhp));
static u_entry_T *u_get_headentry __ARGS((void));
static void u_getbot __ARGS((void));
static int u_savecommon __ARGS((linenr_T, linenr_T, linenr_T));
static void u_doit __ARGS((int count));
static void u_undoredo __ARGS((int undo));
static void u_undo_end __ARGS((int did_undo, int absolute));
static void u_add_time __ARGS((char_u *buf, size_t buflen, time_t tt));
static void u_freeheader __ARGS((buf_T *buf, u_header_T *uhp, u_header_T **uhpp));
static void u_freebranch __ARGS((buf_T *buf, u_header_T *uhp, u_header_T **uhpp));
static void u_freeentries __ARGS((buf_T *buf, u_header_T *uhp, u_header_T **uhpp));
static void u_freeentry __ARGS((u_entry_T *, long));
#ifdef FEAT_PERSISTENT_UNDO
static void corruption_error __ARGS((char *mesg, char_u *file_name));
static void u_free_uhp __ARGS((u_header_T *uhp));
static size_t fwrite_crypt __ARGS((buf_T *buf UNUSED, char_u *ptr, size_t len, FILE *fp));
static char_u *read_string_decrypt __ARGS((buf_T *buf UNUSED, FILE *fd, int len));
static int serialize_header __ARGS((FILE *fp, buf_T *buf, char_u *hash));
static int serialize_uhp __ARGS((FILE *fp, buf_T *buf, u_header_T *uhp));
static u_header_T *unserialize_uhp __ARGS((FILE *fp, char_u *file_name, int new_version));
static int serialize_uep __ARGS((FILE *fp, buf_T *buf, u_entry_T *uep));
static u_entry_T *unserialize_uep __ARGS((FILE *fp, int *error, char_u *file_name));
static void serialize_pos __ARGS((pos_T pos, FILE *fp));
static void unserialize_pos __ARGS((pos_T *pos, FILE *fp));
static void serialize_visualinfo __ARGS((visualinfo_T *info, FILE *fp));
static void unserialize_visualinfo __ARGS((visualinfo_T *info, FILE *fp));
static void put_header_ptr __ARGS((FILE	*fp, u_header_T *uhp));
#endif

#define U_ALLOC_LINE(size) lalloc((long_u)(size), FALSE)
static char_u *u_save_line __ARGS((linenr_T));

/* used in undo_end() to report number of added and deleted lines */
static long	u_newcount, u_oldcount;

/*
 * When 'u' flag included in 'cpoptions', we behave like vi.  Need to remember
 * the action that "u" should do.
 */
static int	undo_undoes = FALSE;

static int	lastmark = 0;

#if defined(U_DEBUG) || defined(PROTO)
/*
 * Check the undo structures for being valid.  Print a warning when something
 * looks wrong.
 */
static int seen_b_u_curhead;
static int seen_b_u_newhead;
static int header_count;

    static void
u_check_tree(u_header_T *uhp,
	u_header_T *exp_uh_next,
	u_header_T *exp_uh_alt_prev)
{
    u_entry_T *uep;

    if (uhp == NULL)
	return;
    ++header_count;
    if (uhp == curbuf->b_u_curhead && ++seen_b_u_curhead > 1)
    {
	EMSG("b_u_curhead found twice (looping?)");
	return;
    }
    if (uhp == curbuf->b_u_newhead && ++seen_b_u_newhead > 1)
    {
	EMSG("b_u_newhead found twice (looping?)");
	return;
    }

    if (uhp->uh_magic != UH_MAGIC)
	EMSG("uh_magic wrong (may be using freed memory)");
    else
    {
	/* Check pointers back are correct. */
	if (uhp->uh_next.ptr != exp_uh_next)
	{
	    EMSG("uh_next wrong");
	    smsg((char_u *)"expected: 0x%x, actual: 0x%x",
					       exp_uh_next, uhp->uh_next.ptr);
	}
	if (uhp->uh_alt_prev.ptr != exp_uh_alt_prev)
	{
	    EMSG("uh_alt_prev wrong");
	    smsg((char_u *)"expected: 0x%x, actual: 0x%x",
				       exp_uh_alt_prev, uhp->uh_alt_prev.ptr);
	}

	/* Check the undo tree at this header. */
	for (uep = uhp->uh_entry; uep != NULL; uep = uep->ue_next)
	{
	    if (uep->ue_magic != UE_MAGIC)
	    {
		EMSG("ue_magic wrong (may be using freed memory)");
		break;
	    }
	}

	/* Check the next alt tree. */
	u_check_tree(uhp->uh_alt_next.ptr, uhp->uh_next.ptr, uhp);

	/* Check the next header in this branch. */
	u_check_tree(uhp->uh_prev.ptr, uhp, NULL);
    }
}

    static void
u_check(int newhead_may_be_NULL)
{
    seen_b_u_newhead = 0;
    seen_b_u_curhead = 0;
    header_count = 0;

    u_check_tree(curbuf->b_u_oldhead, NULL, NULL);

    if (seen_b_u_newhead == 0 && curbuf->b_u_oldhead != NULL
	    && !(newhead_may_be_NULL && curbuf->b_u_newhead == NULL))
	EMSGN("b_u_newhead invalid: 0x%x", curbuf->b_u_newhead);
    if (curbuf->b_u_curhead != NULL && seen_b_u_curhead == 0)
	EMSGN("b_u_curhead invalid: 0x%x", curbuf->b_u_curhead);
    if (header_count != curbuf->b_u_numhead)
    {
	EMSG("b_u_numhead invalid");
	smsg((char_u *)"expected: %ld, actual: %ld",
			       (long)header_count, (long)curbuf->b_u_numhead);
    }
}
#endif

/*
 * Save the current line for both the "u" and "U" command.
 * Returns OK or FAIL.
 */
    int
u_save_cursor()
{
    return (u_save((linenr_T)(curwin->w_cursor.lnum - 1),
				      (linenr_T)(curwin->w_cursor.lnum + 1)));
}

/*
 * Save the lines between "top" and "bot" for both the "u" and "U" command.
 * "top" may be 0 and bot may be curbuf->b_ml.ml_line_count + 1.
 * Careful: may trigger autocommands that reload the buffer.
 * Returns FAIL when lines could not be saved, OK otherwise.
 */
    int
u_save(top, bot)
    linenr_T top, bot;
{
    if (undo_off)
	return OK;

    if (top > curbuf->b_ml.ml_line_count ||
			    top >= bot || bot > curbuf->b_ml.ml_line_count + 1)
	return FALSE;	/* rely on caller to do error messages */

    if (top + 2 == bot)
	u_saveline((linenr_T)(top + 1));

    return (u_savecommon(top, bot, (linenr_T)0));
}

/*
 * Save the line "lnum" (used by ":s" and "~" command).
 * The line is replaced, so the new bottom line is lnum + 1.
 * Careful: may trigger autocommands that reload the buffer.
 * Returns FAIL when lines could not be saved, OK otherwise.
 */
    int
u_savesub(lnum)
    linenr_T	lnum;
{
    if (undo_off)
	return OK;

    return (u_savecommon(lnum - 1, lnum + 1, lnum + 1));
}

/*
 * A new line is inserted before line "lnum" (used by :s command).
 * The line is inserted, so the new bottom line is lnum + 1.
 * Careful: may trigger autocommands that reload the buffer.
 * Returns FAIL when lines could not be saved, OK otherwise.
 */
    int
u_inssub(lnum)
    linenr_T	lnum;
{
    if (undo_off)
	return OK;

    return (u_savecommon(lnum - 1, lnum, lnum + 1));
}

/*
 * Save the lines "lnum" - "lnum" + nlines (used by delete command).
 * The lines are deleted, so the new bottom line is lnum, unless the buffer
 * becomes empty.
 * Careful: may trigger autocommands that reload the buffer.
 * Returns FAIL when lines could not be saved, OK otherwise.
 */
    int
u_savedel(lnum, nlines)
    linenr_T	lnum;
    long	nlines;
{
    if (undo_off)
	return OK;

    return (u_savecommon(lnum - 1, lnum + nlines,
			nlines == curbuf->b_ml.ml_line_count ? 2 : lnum));
}

/*
 * Return TRUE when undo is allowed.  Otherwise give an error message and
 * return FALSE.
 */
    int
undo_allowed()
{
    /* Don't allow changes when 'modifiable' is off.  */
    if (!curbuf->b_p_ma)
    {
	EMSG(_(e_modifiable));
	return FALSE;
    }

#ifdef HAVE_SANDBOX
    /* In the sandbox it's not allowed to change the text. */
    if (sandbox != 0)
    {
	EMSG(_(e_sandbox));
	return FALSE;
    }
#endif

    /* Don't allow changes in the buffer while editing the cmdline.  The
     * caller of getcmdline() may get confused. */
    if (textlock != 0)
    {
	EMSG(_(e_secure));
	return FALSE;
    }

    return TRUE;
}

/*
 * Common code for various ways to save text before a change.
 * "top" is the line above the first changed line.
 * "bot" is the line below the last changed line.
 * Careful: may trigger autocommands that reload the buffer.
 * Returns FAIL when lines could not be saved, OK otherwise.
 */
    static int
u_savecommon(top, bot, newbot)
    linenr_T	top, bot;
    linenr_T	newbot;
{
    linenr_T	lnum;
    long	i;
    u_header_T	*uhp;
    u_header_T	*old_curhead;
    u_entry_T	*uep;
    u_entry_T	*prev_uep;
    long	size;

    /* When making changes is not allowed return FAIL.  It's a crude way to
     * make all change commands fail. */
    if (!undo_allowed())
	return FAIL;

#ifdef U_DEBUG
    u_check(FALSE);
#endif
#ifdef FEAT_NETBEANS_INTG
    /*
     * Netbeans defines areas that cannot be modified.  Bail out here when
     * trying to change text in a guarded area.
     */
    if (netbeans_active())
    {
	if (netbeans_is_guarded(top, bot))
	{
	    EMSG(_(e_guarded));
	    return FAIL;
	}
	if (curbuf->b_p_ro)
	{
	    EMSG(_(e_nbreadonly));
	    return FAIL;
	}
    }
#endif

#ifdef FEAT_AUTOCMD
    /*
     * Saving text for undo means we are going to make a change.  Give a
     * warning for a read-only file before making the change, so that the
     * FileChangedRO event can replace the buffer with a read-write version
     * (e.g., obtained from a source control system).
     */
    change_warning(0);
    if (bot > curbuf->b_ml.ml_line_count + 1)
    {
	/* This happens when the FileChangedRO autocommand changes the file in
	 * a way it becomes shorter. */
	EMSG(_("E834: Line count changed unexpectedly"));
	return FAIL;
    }
#endif

    size = bot - top - 1;

    /*
     * If curbuf->b_u_synced == TRUE make a new header.
     */
    if (curbuf->b_u_synced)
    {
#ifdef FEAT_JUMPLIST
	/* Need to create new entry in b_changelist. */
	curbuf->b_new_change = TRUE;
#endif

	if (p_ul >= 0)
	{
	    /*
	     * Make a new header entry.  Do this first so that we don't mess
	     * up the undo info when out of memory.
	     */
	    uhp = (u_header_T *)U_ALLOC_LINE(sizeof(u_header_T));
	    if (uhp == NULL)
		goto nomem;
#ifdef U_DEBUG
	    uhp->uh_magic = UH_MAGIC;
#endif
	}
	else
	    uhp = NULL;

	/*
	 * If we undid more than we redid, move the entry lists before and
	 * including curbuf->b_u_curhead to an alternate branch.
	 */
	old_curhead = curbuf->b_u_curhead;
	if (old_curhead != NULL)
	{
	    curbuf->b_u_newhead = old_curhead->uh_next.ptr;
	    curbuf->b_u_curhead = NULL;
	}

	/*
	 * free headers to keep the size right
	 */
	while (curbuf->b_u_numhead > p_ul && curbuf->b_u_oldhead != NULL)
	{
	    u_header_T	    *uhfree = curbuf->b_u_oldhead;

	    if (uhfree == old_curhead)
		/* Can't reconnect the branch, delete all of it. */
		u_freebranch(curbuf, uhfree, &old_curhead);
	    else if (uhfree->uh_alt_next.ptr == NULL)
		/* There is no branch, only free one header. */
		u_freeheader(curbuf, uhfree, &old_curhead);
	    else
	    {
		/* Free the oldest alternate branch as a whole. */
		while (uhfree->uh_alt_next.ptr != NULL)
		    uhfree = uhfree->uh_alt_next.ptr;
		u_freebranch(curbuf, uhfree, &old_curhead);
	    }
#ifdef U_DEBUG
	    u_check(TRUE);
#endif
	}

	if (uhp == NULL)		/* no undo at all */
	{
	    if (old_curhead != NULL)
		u_freebranch(curbuf, old_curhead, NULL);
	    curbuf->b_u_synced = FALSE;
	    return OK;
	}

	uhp->uh_prev.ptr = NULL;
	uhp->uh_next.ptr = curbuf->b_u_newhead;
	uhp->uh_alt_next.ptr = old_curhead;
	if (old_curhead != NULL)
	{
	    uhp->uh_alt_prev.ptr = old_curhead->uh_alt_prev.ptr;
	    if (uhp->uh_alt_prev.ptr != NULL)
		uhp->uh_alt_prev.ptr->uh_alt_next.ptr = uhp;
	    old_curhead->uh_alt_prev.ptr = uhp;
	    if (curbuf->b_u_oldhead == old_curhead)
		curbuf->b_u_oldhead = uhp;
	}
	else
	    uhp->uh_alt_prev.ptr = NULL;
	if (curbuf->b_u_newhead != NULL)
	    curbuf->b_u_newhead->uh_prev.ptr = uhp;

	uhp->uh_seq = ++curbuf->b_u_seq_last;
	curbuf->b_u_seq_cur = uhp->uh_seq;
	uhp->uh_time = time(NULL);
	uhp->uh_save_nr = 0;
	curbuf->b_u_time_cur = uhp->uh_time + 1;

	uhp->uh_walk = 0;
	uhp->uh_entry = NULL;
	uhp->uh_getbot_entry = NULL;
	uhp->uh_cursor = curwin->w_cursor;	/* save cursor pos. for undo */
#ifdef FEAT_VIRTUALEDIT
	if (virtual_active() && curwin->w_cursor.coladd > 0)
	    uhp->uh_cursor_vcol = getviscol();
	else
	    uhp->uh_cursor_vcol = -1;
#endif

	/* save changed and buffer empty flag for undo */
	uhp->uh_flags = (curbuf->b_changed ? UH_CHANGED : 0) +
		       ((curbuf->b_ml.ml_flags & ML_EMPTY) ? UH_EMPTYBUF : 0);

	/* save named marks and Visual marks for undo */
	mch_memmove(uhp->uh_namedm, curbuf->b_namedm, sizeof(pos_T) * NMARKS);
#ifdef FEAT_VISUAL
	uhp->uh_visual = curbuf->b_visual;
#endif

	curbuf->b_u_newhead = uhp;
	if (curbuf->b_u_oldhead == NULL)
	    curbuf->b_u_oldhead = uhp;
	++curbuf->b_u_numhead;
    }
    else
    {
	if (p_ul < 0)		/* no undo at all */
	    return OK;

	/*
	 * When saving a single line, and it has been saved just before, it
	 * doesn't make sense saving it again.  Saves a lot of memory when
	 * making lots of changes inside the same line.
	 * This is only possible if the previous change didn't increase or
	 * decrease the number of lines.
	 * Check the ten last changes.  More doesn't make sense and takes too
	 * long.
	 */
	if (size == 1)
	{
	    uep = u_get_headentry();
	    prev_uep = NULL;
	    for (i = 0; i < 10; ++i)
	    {
		if (uep == NULL)
		    break;

		/* If lines have been inserted/deleted we give up.
		 * Also when the line was included in a multi-line save. */
		if ((curbuf->b_u_newhead->uh_getbot_entry != uep
			    ? (uep->ue_top + uep->ue_size + 1
				!= (uep->ue_bot == 0
				    ? curbuf->b_ml.ml_line_count + 1
				    : uep->ue_bot))
			    : uep->ue_lcount != curbuf->b_ml.ml_line_count)
			|| (uep->ue_size > 1
			    && top >= uep->ue_top
			    && top + 2 <= uep->ue_top + uep->ue_size + 1))
		    break;

		/* If it's the same line we can skip saving it again. */
		if (uep->ue_size == 1 && uep->ue_top == top)
		{
		    if (i > 0)
		    {
			/* It's not the last entry: get ue_bot for the last
			 * entry now.  Following deleted/inserted lines go to
			 * the re-used entry. */
			u_getbot();
			curbuf->b_u_synced = FALSE;

			/* Move the found entry to become the last entry.  The
			 * order of undo/redo doesn't matter for the entries
			 * we move it over, since they don't change the line
			 * count and don't include this line.  It does matter
			 * for the found entry if the line count is changed by
			 * the executed command. */
			prev_uep->ue_next = uep->ue_next;
			uep->ue_next = curbuf->b_u_newhead->uh_entry;
			curbuf->b_u_newhead->uh_entry = uep;
		    }

		    /* The executed command may change the line count. */
		    if (newbot != 0)
			uep->ue_bot = newbot;
		    else if (bot > curbuf->b_ml.ml_line_count)
			uep->ue_bot = 0;
		    else
		    {
			uep->ue_lcount = curbuf->b_ml.ml_line_count;
			curbuf->b_u_newhead->uh_getbot_entry = uep;
		    }
		    return OK;
		}
		prev_uep = uep;
		uep = uep->ue_next;
	    }
	}

	/* find line number for ue_bot for previous u_save() */
	u_getbot();
    }

#if !defined(UNIX) && !defined(DJGPP) && !defined(WIN32) && !defined(__EMX__)
	/*
	 * With Amiga and MSDOS 16 bit we can't handle big undo's, because
	 * then u_alloc_line would have to allocate a block larger than 32K
	 */
    if (size >= 8000)
	goto nomem;
#endif

    /*
     * add lines in front of entry list
     */
    uep = (u_entry_T *)U_ALLOC_LINE(sizeof(u_entry_T));
    if (uep == NULL)
	goto nomem;
    vim_memset(uep, 0, sizeof(u_entry_T));
#ifdef U_DEBUG
    uep->ue_magic = UE_MAGIC;
#endif

    uep->ue_size = size;
    uep->ue_top = top;
    if (newbot != 0)
	uep->ue_bot = newbot;
    /*
     * Use 0 for ue_bot if bot is below last line.
     * Otherwise we have to compute ue_bot later.
     */
    else if (bot > curbuf->b_ml.ml_line_count)
	uep->ue_bot = 0;
    else
    {
	uep->ue_lcount = curbuf->b_ml.ml_line_count;
	curbuf->b_u_newhead->uh_getbot_entry = uep;
    }

    if (size > 0)
    {
	if ((uep->ue_array = (char_u **)U_ALLOC_LINE(
					    sizeof(char_u *) * size)) == NULL)
	{
	    u_freeentry(uep, 0L);
	    goto nomem;
	}
	for (i = 0, lnum = top + 1; i < size; ++i)
	{
	    fast_breakcheck();
	    if (got_int)
	    {
		u_freeentry(uep, i);
		return FAIL;
	    }
	    if ((uep->ue_array[i] = u_save_line(lnum++)) == NULL)
	    {
		u_freeentry(uep, i);
		goto nomem;
	    }
	}
    }
    else
	uep->ue_array = NULL;
    uep->ue_next = curbuf->b_u_newhead->uh_entry;
    curbuf->b_u_newhead->uh_entry = uep;
    curbuf->b_u_synced = FALSE;
    undo_undoes = FALSE;

#ifdef U_DEBUG
    u_check(FALSE);
#endif
    return OK;

nomem:
    msg_silent = 0;	/* must display the prompt */
    if (ask_yesno((char_u *)_("No undo possible; continue anyway"), TRUE)
								       == 'y')
    {
	undo_off = TRUE;	    /* will be reset when character typed */
	return OK;
    }
    do_outofmem_msg((long_u)0);
    return FAIL;
}

#if defined(FEAT_PERSISTENT_UNDO) || defined(PROTO)

# define UF_START_MAGIC	    "Vim\237UnDo\345"  /* magic at start of undofile */
# define UF_START_MAGIC_LEN	9
# define UF_HEADER_MAGIC	0x5fd0	/* magic at start of header */
# define UF_HEADER_END_MAGIC	0xe7aa	/* magic after last header */
# define UF_ENTRY_MAGIC		0xf518	/* magic at start of entry */
# define UF_ENTRY_END_MAGIC	0x3581	/* magic after last entry */
# define UF_VERSION_PREV	1	/* 2-byte undofile version number */
# define UF_VERSION		2	/* 2-byte undofile version number */
# define UF_VERSION_CRYPT_PREV	0x8001	/* idem, encrypted */
# define UF_VERSION_CRYPT	0x8002	/* idem, encrypted */

/* extra fields for header */
# define UF_LAST_SAVE_NR	1

/* extra fields for uhp */
# define UHP_SAVE_NR		1

static char_u e_not_open[] = N_("E828: Cannot open undo file for writing: %s");

/*
 * Compute the hash for the current buffer text into hash[UNDO_HASH_SIZE].
 */
    void
u_compute_hash(hash)
    char_u *hash;
{
    context_sha256_T	ctx;
    linenr_T		lnum;
    char_u		*p;

    sha256_start(&ctx);
    for (lnum = 1; lnum < curbuf->b_ml.ml_line_count; ++lnum)
    {
	p = ml_get(lnum);
	sha256_update(&ctx, p, (UINT32_T)(STRLEN(p) + 1));
    }
    sha256_finish(&ctx, hash);
}

/*
 * Return an allocated string of the full path of the target undofile.
 * When "reading" is TRUE find the file to read, go over all directories in
 * 'undodir'.
 * When "reading" is FALSE use the first name where the directory exists.
 * Returns NULL when there is no place to write or no file to read.
 */
    char_u *
u_get_undo_file_name(buf_ffname, reading)
    char_u	*buf_ffname;
    int		reading;
{
    char_u	*dirp;
    char_u	dir_name[IOSIZE + 1];
    char_u	*munged_name = NULL;
    char_u	*undo_file_name = NULL;
    int		dir_len;
    char_u	*p;
    struct stat st;
    char_u	*ffname = buf_ffname;
#ifdef HAVE_READLINK
    char_u	fname_buf[MAXPATHL];
#endif

    if (ffname == NULL)
	return NULL;

#ifdef HAVE_READLINK
    /* Expand symlink in the file name, so that we put the undo file with the
     * actual file instead of with the symlink. */
    if (resolve_symlink(ffname, fname_buf) == OK)
	ffname = fname_buf;
#endif

    /* Loop over 'undodir'.  When reading find the first file that exists.
     * When not reading use the first directory that exists or ".". */
    dirp = p_udir;
    while (*dirp != NUL)
    {
	dir_len = copy_option_part(&dirp, dir_name, IOSIZE, ",");
	if (dir_len == 1 && dir_name[0] == '.')
	{
	    /* Use same directory as the ffname,
	     * "dir/name" -> "dir/.name.un~" */
	    undo_file_name = vim_strnsave(ffname, (int)(STRLEN(ffname) + 5));
	    if (undo_file_name == NULL)
		break;
	    p = gettail(undo_file_name);
	    mch_memmove(p + 1, p, STRLEN(p) + 1);
	    *p = '.';
	    STRCAT(p, ".un~");
	}
	else
	{
	    dir_name[dir_len] = NUL;
	    if (mch_isdir(dir_name))
	    {
		if (munged_name == NULL)
		{
		    munged_name = vim_strsave(ffname);
		    if (munged_name == NULL)
			return NULL;
		    for (p = munged_name; *p != NUL; mb_ptr_adv(p))
			if (vim_ispathsep(*p))
			    *p = '%';
		}
		undo_file_name = concat_fnames(dir_name, munged_name, TRUE);
	    }
	}

	/* When reading check if the file exists. */
	if (undo_file_name != NULL && (!reading
			       || mch_stat((char *)undo_file_name, &st) >= 0))
	    break;
	vim_free(undo_file_name);
	undo_file_name = NULL;
    }

    vim_free(munged_name);
    return undo_file_name;
}

    static void
corruption_error(mesg, file_name)
    char *mesg;
    char_u *file_name;
{
    EMSG3(_("E825: Corrupted undo file (%s): %s"), mesg, file_name);
}

    static void
u_free_uhp(uhp)
    u_header_T	*uhp;
{
    u_entry_T	*nuep;
    u_entry_T	*uep;

    uep = uhp->uh_entry;
    while (uep != NULL)
    {
	nuep = uep->ue_next;
	u_freeentry(uep, uep->ue_size);
	uep = nuep;
    }
    vim_free(uhp);
}

/*
 * Like fwrite() but crypt the bytes when 'key' is set.
 * Returns 1 if successful.
 */
    static size_t
fwrite_crypt(buf, ptr, len, fp)
    buf_T	*buf UNUSED;
    char_u	*ptr;
    size_t	len;
    FILE	*fp;
{
#ifdef FEAT_CRYPT
    char_u  *copy;
    char_u  small_buf[100];
    size_t  i;

    if (*buf->b_p_key == NUL)
	return fwrite(ptr, len, (size_t)1, fp);
    if (len < 100)
	copy = small_buf;  /* no malloc()/free() for short strings */
    else
    {
	copy = lalloc(len, FALSE);
	if (copy == NULL)
	    return 0;
    }
    crypt_encode(ptr, len, copy);
    i = fwrite(copy, len, (size_t)1, fp);
    if (copy != small_buf)
	vim_free(copy);
    return i;
#else
    return fwrite(ptr, len, (size_t)1, fp);
#endif
}

/*
 * Read a string of length "len" from "fd".
 * When 'key' is set decrypt the bytes.
 */
    static char_u *
read_string_decrypt(buf, fd, len)
    buf_T   *buf UNUSED;
    FILE    *fd;
    int	    len;
{
    char_u  *ptr;

    ptr = read_string(fd, len);
#ifdef FEAT_CRYPT
    if (ptr != NULL || *buf->b_p_key != NUL)
	crypt_decode(ptr, len);
#endif
    return ptr;
}

    static int
serialize_header(fp, buf, hash)
    FILE	*fp;
    buf_T	*buf;
    char_u	*hash;
{
    int len;

    /* Start writing, first the magic marker and undo info version. */
    if (fwrite(UF_START_MAGIC, (size_t)UF_START_MAGIC_LEN, (size_t)1, fp) != 1)
	return FAIL;

    /* If the buffer is encrypted then all text bytes following will be
     * encrypted.  Numbers and other info is not crypted. */
#ifdef FEAT_CRYPT
    if (*buf->b_p_key)
    {
	char_u *header;
	int    header_len;

	put_bytes(fp, (long_u)UF_VERSION_CRYPT, 2);
	header = prepare_crypt_write(buf, &header_len);
	if (header == NULL)
	    return FAIL;
	len = (int)fwrite(header, (size_t)header_len, (size_t)1, fp);
	vim_free(header);
	if (len != 1)
	{
	    crypt_pop_state();
	    return FAIL;
	}
    }
    else
#endif
	put_bytes(fp, (long_u)UF_VERSION, 2);


    /* Write a hash of the buffer text, so that we can verify it is still the
     * same when reading the buffer text. */
    if (fwrite(hash, (size_t)UNDO_HASH_SIZE, (size_t)1, fp) != 1)
	return FAIL;

    /* buffer-specific data */
    put_bytes(fp, (long_u)buf->b_ml.ml_line_count, 4);
    len = buf->b_u_line_ptr != NULL ? (int)STRLEN(buf->b_u_line_ptr) : 0;
    put_bytes(fp, (long_u)len, 4);
    if (len > 0 && fwrite_crypt(buf, buf->b_u_line_ptr, (size_t)len, fp) != 1)
	return FAIL;
    put_bytes(fp, (long_u)buf->b_u_line_lnum, 4);
    put_bytes(fp, (long_u)buf->b_u_line_colnr, 4);

    /* Undo structures header data */
    put_header_ptr(fp, buf->b_u_oldhead);
    put_header_ptr(fp, buf->b_u_newhead);
    put_header_ptr(fp, buf->b_u_curhead);

    put_bytes(fp, (long_u)buf->b_u_numhead, 4);
    put_bytes(fp, (long_u)buf->b_u_seq_last, 4);
    put_bytes(fp, (long_u)buf->b_u_seq_cur, 4);
    put_time(fp, buf->b_u_time_cur);

    /* Optional fields. */
    putc(4, fp);
    putc(UF_LAST_SAVE_NR, fp);
    put_bytes(fp, (long_u)buf->b_u_save_nr_last, 4);

    putc(0, fp);  /* end marker */

    return OK;
}

    static int
serialize_uhp(fp, buf, uhp)
    FILE	*fp;
    buf_T	*buf;
    u_header_T	*uhp;
{
    int		i;
    u_entry_T	*uep;

    if (put_bytes(fp, (long_u)UF_HEADER_MAGIC, 2) == FAIL)
	return FAIL;

    put_header_ptr(fp, uhp->uh_next.ptr);
    put_header_ptr(fp, uhp->uh_prev.ptr);
    put_header_ptr(fp, uhp->uh_alt_next.ptr);
    put_header_ptr(fp, uhp->uh_alt_prev.ptr);
    put_bytes(fp, uhp->uh_seq, 4);
    serialize_pos(uhp->uh_cursor, fp);
#ifdef FEAT_VIRTUALEDIT
    put_bytes(fp, (long_u)uhp->uh_cursor_vcol, 4);
#else
    put_bytes(fp, (long_u)0, 4);
#endif
    put_bytes(fp, (long_u)uhp->uh_flags, 2);
    /* Assume NMARKS will stay the same. */
    for (i = 0; i < NMARKS; ++i)
	serialize_pos(uhp->uh_namedm[i], fp);
#ifdef FEAT_VISUAL
    serialize_visualinfo(&uhp->uh_visual, fp);
#else
    {
	visualinfo_T info;

	memset(&info, 0, sizeof(visualinfo_T));
	serialize_visualinfo(&info, fp);
    }
#endif
    put_time(fp, uhp->uh_time);

    /* Optional fields. */
    putc(4, fp);
    putc(UHP_SAVE_NR, fp);
    put_bytes(fp, (long_u)uhp->uh_save_nr, 4);

    putc(0, fp);  /* end marker */

    /* Write all the entries. */
    for (uep = uhp->uh_entry; uep != NULL; uep = uep->ue_next)
    {
	put_bytes(fp, (long_u)UF_ENTRY_MAGIC, 2);
	if (serialize_uep(fp, buf, uep) == FAIL)
	    return FAIL;
    }
    put_bytes(fp, (long_u)UF_ENTRY_END_MAGIC, 2);
    return OK;
}

    static u_header_T *
unserialize_uhp(fp, file_name, new_version)
    FILE	*fp;
    char_u	*file_name;
    int		new_version;
{
    u_header_T	*uhp;
    int		i;
    u_entry_T	*uep, *last_uep;
    int		c;
    int		error;

    uhp = (u_header_T *)U_ALLOC_LINE(sizeof(u_header_T));
    if (uhp == NULL)
	return NULL;
    vim_memset(uhp, 0, sizeof(u_header_T));
#ifdef U_DEBUG
    uhp->uh_magic = UH_MAGIC;
#endif
    uhp->uh_next.seq = get4c(fp);
    uhp->uh_prev.seq = get4c(fp);
    uhp->uh_alt_next.seq = get4c(fp);
    uhp->uh_alt_prev.seq = get4c(fp);
    uhp->uh_seq = get4c(fp);
    if (uhp->uh_seq <= 0)
    {
	corruption_error("uh_seq", file_name);
	vim_free(uhp);
	return NULL;
    }
    unserialize_pos(&uhp->uh_cursor, fp);
#ifdef FEAT_VIRTUALEDIT
    uhp->uh_cursor_vcol = get4c(fp);
#else
    (void)get4c(fp);
#endif
    uhp->uh_flags = get2c(fp);
    for (i = 0; i < NMARKS; ++i)
	unserialize_pos(&uhp->uh_namedm[i], fp);
#ifdef FEAT_VISUAL
    unserialize_visualinfo(&uhp->uh_visual, fp);
#else
    {
	visualinfo_T info;
	unserialize_visualinfo(&info, fp);
    }
#endif
    uhp->uh_time = get8ctime(fp);

    /* Optional fields. */
    if (new_version)
	for (;;)
	{
	    int len = getc(fp);
	    int what;

	    if (len == 0)
		break;
	    what = getc(fp);
	    switch (what)
	    {
		case UHP_SAVE_NR:
		    uhp->uh_save_nr = get4c(fp);
		    break;
		default:
		    /* field not supported, skip */
		    while (--len >= 0)
			(void)getc(fp);
	    }
	}

    /* Unserialize the uep list. */
    last_uep = NULL;
    while ((c = get2c(fp)) == UF_ENTRY_MAGIC)
    {
	error = FALSE;
	uep = unserialize_uep(fp, &error, file_name);
	if (last_uep == NULL)
	    uhp->uh_entry = uep;
	else
	    last_uep->ue_next = uep;
	last_uep = uep;
	if (uep == NULL || error)
	{
	    u_free_uhp(uhp);
	    return NULL;
	}
    }
    if (c != UF_ENTRY_END_MAGIC)
    {
	corruption_error("entry end", file_name);
	u_free_uhp(uhp);
	return NULL;
    }

    return uhp;
}

/*
 * Serialize "uep" to "fp".
 */
    static int
serialize_uep(fp, buf, uep)
    FILE	*fp;
    buf_T	*buf;
    u_entry_T	*uep;
{
    int		i;
    size_t	len;

    put_bytes(fp, (long_u)uep->ue_top, 4);
    put_bytes(fp, (long_u)uep->ue_bot, 4);
    put_bytes(fp, (long_u)uep->ue_lcount, 4);
    put_bytes(fp, (long_u)uep->ue_size, 4);
    for (i = 0; i < uep->ue_size; ++i)
    {
	len = STRLEN(uep->ue_array[i]);
	if (put_bytes(fp, (long_u)len, 4) == FAIL)
	    return FAIL;
	if (len > 0 && fwrite_crypt(buf, uep->ue_array[i], len, fp) != 1)
	    return FAIL;
    }
    return OK;
}

    static u_entry_T *
unserialize_uep(fp, error, file_name)
    FILE	*fp;
    int		*error;
    char_u	*file_name;
{
    int		i;
    u_entry_T	*uep;
    char_u	**array;
    char_u	*line;
    int		line_len;

    uep = (u_entry_T *)U_ALLOC_LINE(sizeof(u_entry_T));
    if (uep == NULL)
	return NULL;
    vim_memset(uep, 0, sizeof(u_entry_T));
#ifdef U_DEBUG
    uep->ue_magic = UE_MAGIC;
#endif
    uep->ue_top = get4c(fp);
    uep->ue_bot = get4c(fp);
    uep->ue_lcount = get4c(fp);
    uep->ue_size = get4c(fp);
    if (uep->ue_size > 0)
    {
	array = (char_u **)U_ALLOC_LINE(sizeof(char_u *) * uep->ue_size);
	if (array == NULL)
	{
	    *error = TRUE;
	    return uep;
	}
	vim_memset(array, 0, sizeof(char_u *) * uep->ue_size);
    }
    else
	array = NULL;
    uep->ue_array = array;

    for (i = 0; i < uep->ue_size; ++i)
    {
	line_len = get4c(fp);
	if (line_len >= 0)
	    line = read_string_decrypt(curbuf, fp, line_len);
	else
	{
	    line = NULL;
	    corruption_error("line length", file_name);
	}
	if (line == NULL)
	{
	    *error = TRUE;
	    return uep;
	}
	array[i] = line;
    }
    return uep;
}

/*
 * Serialize "pos" to "fp".
 */
    static void
serialize_pos(pos, fp)
    pos_T pos;
    FILE  *fp;
{
    put_bytes(fp, (long_u)pos.lnum, 4);
    put_bytes(fp, (long_u)pos.col, 4);
#ifdef FEAT_VIRTUALEDIT
    put_bytes(fp, (long_u)pos.coladd, 4);
#else
    put_bytes(fp, (long_u)0, 4);
#endif
}

/*
 * Unserialize the pos_T at the current position in fp.
 */
    static void
unserialize_pos(pos, fp)
    pos_T *pos;
    FILE  *fp;
{
    pos->lnum = get4c(fp);
    if (pos->lnum < 0)
	pos->lnum = 0;
    pos->col = get4c(fp);
    if (pos->col < 0)
	pos->col = 0;
#ifdef FEAT_VIRTUALEDIT
    pos->coladd = get4c(fp);
    if (pos->coladd < 0)
	pos->coladd = 0;
#else
    (void)get4c(fp);
#endif
}

/*
 * Serialize "info" to "fp".
 */
    static void
serialize_visualinfo(info, fp)
    visualinfo_T    *info;
    FILE	    *fp;
{
    serialize_pos(info->vi_start, fp);
    serialize_pos(info->vi_end, fp);
    put_bytes(fp, (long_u)info->vi_mode, 4);
    put_bytes(fp, (long_u)info->vi_curswant, 4);
}

/*
 * Unserialize the visualinfo_T at the current position in fp.
 */
    static void
unserialize_visualinfo(info, fp)
    visualinfo_T    *info;
    FILE	    *fp;
{
    unserialize_pos(&info->vi_start, fp);
    unserialize_pos(&info->vi_end, fp);
    info->vi_mode = get4c(fp);
    info->vi_curswant = get4c(fp);
}

/*
 * Write the pointer to an undo header.  Instead of writing the pointer itself
 * we use the sequence number of the header.  This is converted back to
 * pointers when reading. */
    static void
put_header_ptr(fp, uhp)
    FILE	*fp;
    u_header_T	*uhp;
{
    put_bytes(fp, (long_u)(uhp != NULL ? uhp->uh_seq : 0), 4);
}

/*
 * Write the undo tree in an undo file.
 * When "name" is not NULL, use it as the name of the undo file.
 * Otherwise use buf->b_ffname to generate the undo file name.
 * "buf" must never be null, buf->b_ffname is used to obtain the original file
 * permissions.
 * "forceit" is TRUE for ":wundo!", FALSE otherwise.
 * "hash[UNDO_HASH_SIZE]" must be the hash value of the buffer text.
 */
    void
u_write_undo(name, forceit, buf, hash)
    char_u	*name;
    int		forceit;
    buf_T	*buf;
    char_u	*hash;
{
    u_header_T	*uhp;
    char_u	*file_name;
    int		mark;
#ifdef U_DEBUG
    int		headers_written = 0;
#endif
    int		fd;
    FILE	*fp = NULL;
    int		perm;
    int		write_ok = FALSE;
#ifdef UNIX
    int		st_old_valid = FALSE;
    struct stat	st_old;
    struct stat	st_new;
#endif
#ifdef FEAT_CRYPT
    int		do_crypt = FALSE;
#endif

    if (name == NULL)
    {
	file_name = u_get_undo_file_name(buf->b_ffname, FALSE);
	if (file_name == NULL)
	{
	    if (p_verbose > 0)
	    {
		verbose_enter();
		smsg((char_u *)
		   _("Cannot write undo file in any directory in 'undodir'"));
		verbose_leave();
	    }
	    return;
	}
    }
    else
	file_name = name;

    /*
     * Decide about the permission to use for the undo file.  If the buffer
     * has a name use the permission of the original file.  Otherwise only
     * allow the user to access the undo file.
     */
    perm = 0600;
    if (buf->b_ffname != NULL)
    {
#ifdef UNIX
	if (mch_stat((char *)buf->b_ffname, &st_old) >= 0)
	{
	    perm = st_old.st_mode;
	    st_old_valid = TRUE;
	}
#else
	perm = mch_getperm(buf->b_ffname);
	if (perm < 0)
	    perm = 0600;
#endif
    }

    /* strip any s-bit */
    perm = perm & 0777;

    /* If the undo file already exists, verify that it actually is an undo
     * file, and delete it. */
    if (mch_getperm(file_name) >= 0)
    {
	if (name == NULL || !forceit)
	{
	    /* Check we can read it and it's an undo file. */
	    fd = mch_open((char *)file_name, O_RDONLY|O_EXTRA, 0);
	    if (fd < 0)
	    {
		if (name != NULL || p_verbose > 0)
		{
		    if (name == NULL)
			verbose_enter();
		    smsg((char_u *)
		      _("Will not overwrite with undo file, cannot read: %s"),
								   file_name);
		    if (name == NULL)
			verbose_leave();
		}
		goto theend;
	    }
	    else
	    {
		char_u	mbuf[UF_START_MAGIC_LEN];
		int	len;

		len = vim_read(fd, mbuf, UF_START_MAGIC_LEN);
		close(fd);
		if (len < UF_START_MAGIC_LEN
		      || memcmp(mbuf, UF_START_MAGIC, UF_START_MAGIC_LEN) != 0)
		{
		    if (name != NULL || p_verbose > 0)
		    {
			if (name == NULL)
			    verbose_enter();
			smsg((char_u *)
			_("Will not overwrite, this is not an undo file: %s"),
								   file_name);
			if (name == NULL)
			    verbose_leave();
		    }
		    goto theend;
		}
	    }
	}
	mch_remove(file_name);
    }

    /* If there is no undo information at all, quit here after deleting any
     * existing undo file. */
    if (buf->b_u_numhead == 0 && buf->b_u_line_ptr == NULL)
    {
	if (p_verbose > 0)
	    verb_msg((char_u *)_("Skipping undo file write, nothing to undo"));
	goto theend;
    }

    fd = mch_open((char *)file_name,
			    O_CREAT|O_EXTRA|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
    if (fd < 0)
    {
	EMSG2(_(e_not_open), file_name);
	goto theend;
    }
    (void)mch_setperm(file_name, perm);
    if (p_verbose > 0)
    {
	verbose_enter();
	smsg((char_u *)_("Writing undo file: %s"), file_name);
	verbose_leave();
    }

#ifdef U_DEBUG
    /* Check there is no problem in undo info before writing. */
    u_check(FALSE);
#endif

#ifdef UNIX
    /*
     * Try to set the group of the undo file same as the original file. If
     * this fails, set the protection bits for the group same as the
     * protection bits for others.
     */
    if (st_old_valid && (mch_stat((char *)file_name, &st_new) >= 0
		&& st_new.st_gid != st_old.st_gid
# ifdef HAVE_FCHOWN  /* sequent-ptx lacks fchown() */
		&& fchown(fd, (uid_t)-1, st_old.st_gid) != 0)
# endif
       )
	mch_setperm(file_name, (perm & 0707) | ((perm & 07) << 3));
# ifdef HAVE_SELINUX
    if (buf->b_ffname != NULL)
	mch_copy_sec(buf->b_ffname, file_name);
# endif
#endif

    fp = fdopen(fd, "w");
    if (fp == NULL)
    {
	EMSG2(_(e_not_open), file_name);
	close(fd);
	mch_remove(file_name);
	goto theend;
    }

    /* Undo must be synced. */
    u_sync(TRUE);

    /*
     * Write the header.
     */
    if (serialize_header(fp, buf, hash) == FAIL)
	goto write_error;
#ifdef FEAT_CRYPT
    if (*buf->b_p_key)
	do_crypt = TRUE;
#endif

    /*
     * Iteratively serialize UHPs and their UEPs from the top down.
     */
    mark = ++lastmark;
    uhp = buf->b_u_oldhead;
    while (uhp != NULL)
    {
	/* Serialize current UHP if we haven't seen it */
	if (uhp->uh_walk != mark)
	{
	    uhp->uh_walk = mark;
#ifdef U_DEBUG
	    ++headers_written;
#endif
	    if (serialize_uhp(fp, buf, uhp) == FAIL)
		goto write_error;
	}

	/* Now walk through the tree - algorithm from undo_time(). */
	if (uhp->uh_prev.ptr != NULL && uhp->uh_prev.ptr->uh_walk != mark)
	    uhp = uhp->uh_prev.ptr;
	else if (uhp->uh_alt_next.ptr != NULL
				     && uhp->uh_alt_next.ptr->uh_walk != mark)
	    uhp = uhp->uh_alt_next.ptr;
	else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
					 && uhp->uh_next.ptr->uh_walk != mark)
	    uhp = uhp->uh_next.ptr;
	else if (uhp->uh_alt_prev.ptr != NULL)
	    uhp = uhp->uh_alt_prev.ptr;
	else
	    uhp = uhp->uh_next.ptr;
    }

    if (put_bytes(fp, (long_u)UF_HEADER_END_MAGIC, 2) == OK)
	write_ok = TRUE;
#ifdef U_DEBUG
    if (headers_written != buf->b_u_numhead)
	EMSG3("Written %ld headers, but numhead is %ld",
					   headers_written, buf->b_u_numhead);
#endif

write_error:
    fclose(fp);
    if (!write_ok)
	EMSG2(_("E829: write error in undo file: %s"), file_name);

#if defined(MACOS_CLASSIC) || defined(WIN3264)
    /* Copy file attributes; for systems where this can only be done after
     * closing the file. */
    if (buf->b_ffname != NULL)
	(void)mch_copy_file_attribute(buf->b_ffname, file_name);
#endif
#ifdef HAVE_ACL
    if (buf->b_ffname != NULL)
    {
	vim_acl_T	    acl;

	/* For systems that support ACL: get the ACL from the original file. */
	acl = mch_get_acl(buf->b_ffname);
	mch_set_acl(file_name, acl);
    }
#endif

theend:
#ifdef FEAT_CRYPT
    if (do_crypt)
	crypt_pop_state();
#endif
    if (file_name != name)
	vim_free(file_name);
}

/*
 * Load the undo tree from an undo file.
 * If "name" is not NULL use it as the undo file name.  This also means being
 * a bit more verbose.
 * Otherwise use curbuf->b_ffname to generate the undo file name.
 * "hash[UNDO_HASH_SIZE]" must be the hash value of the buffer text.
 */
    void
u_read_undo(name, hash, orig_name)
    char_u *name;
    char_u *hash;
    char_u *orig_name;
{
    char_u	*file_name;
    FILE	*fp;
    long	version, str_len;
    int		new_version;
    char_u	*line_ptr = NULL;
    linenr_T	line_lnum;
    colnr_T	line_colnr;
    linenr_T	line_count;
    int		num_head = 0;
    long	old_header_seq, new_header_seq, cur_header_seq;
    long	seq_last, seq_cur;
    long	last_save_nr = 0;
    short	old_idx = -1, new_idx = -1, cur_idx = -1;
    long	num_read_uhps = 0;
    time_t	seq_time;
    int		i, j;
    int		c;
    u_header_T	*uhp;
    u_header_T	**uhp_table = NULL;
    char_u	read_hash[UNDO_HASH_SIZE];
    char_u	magic_buf[UF_START_MAGIC_LEN];
#ifdef U_DEBUG
    int		*uhp_table_used;
#endif
#ifdef UNIX
    struct stat	st_orig;
    struct stat	st_undo;
#endif
#ifdef FEAT_CRYPT
    int		do_decrypt = FALSE;
#endif

    if (name == NULL)
    {
	file_name = u_get_undo_file_name(curbuf->b_ffname, TRUE);
	if (file_name == NULL)
	    return;

#ifdef UNIX
	/* For safety we only read an undo file if the owner is equal to the
	 * owner of the text file. */
	if (mch_stat((char *)orig_name, &st_orig) >= 0
		&& mch_stat((char *)file_name, &st_undo) >= 0
		&& st_orig.st_uid != st_undo.st_uid)
	{
	    if (p_verbose > 0)
	    {
		verbose_enter();
		smsg((char_u *)_("Not reading undo file, owner differs: %s"),
								   file_name);
		verbose_leave();
	    }
	    return;
	}
#endif
    }
    else
	file_name = name;

    if (p_verbose > 0)
    {
	verbose_enter();
	smsg((char_u *)_("Reading undo file: %s"), file_name);
	verbose_leave();
    }

    fp = mch_fopen((char *)file_name, "r");
    if (fp == NULL)
    {
	if (name != NULL || p_verbose > 0)
	    EMSG2(_("E822: Cannot open undo file for reading: %s"), file_name);
	goto error;
    }

    /*
     * Read the undo file header.
     */
    if (fread(magic_buf, UF_START_MAGIC_LEN, 1, fp) != 1
		|| memcmp(magic_buf, UF_START_MAGIC, UF_START_MAGIC_LEN) != 0)
    {
	EMSG2(_("E823: Not an undo file: %s"), file_name);
	goto error;
    }
    version = get2c(fp);
    if (version == UF_VERSION_CRYPT || version == UF_VERSION_CRYPT_PREV)
    {
#ifdef FEAT_CRYPT
	if (*curbuf->b_p_key == NUL)
	{
	    EMSG2(_("E832: Non-encrypted file has encrypted undo file: %s"),
								   file_name);
	    goto error;
	}
	if (prepare_crypt_read(fp) == FAIL)
	{
	    EMSG2(_("E826: Undo file decryption failed: %s"), file_name);
	    goto error;
	}
	do_decrypt = TRUE;
#else
	EMSG2(_("E827: Undo file is encrypted: %s"), file_name);
	goto error;
#endif
    }
    else if (version != UF_VERSION && version != UF_VERSION_PREV)
    {
	EMSG2(_("E824: Incompatible undo file: %s"), file_name);
	goto error;
    }
    new_version = (version == UF_VERSION || version == UF_VERSION_CRYPT);

    if (fread(read_hash, UNDO_HASH_SIZE, 1, fp) != 1)
    {
	corruption_error("hash", file_name);
	goto error;
    }
    line_count = (linenr_T)get4c(fp);
    if (memcmp(hash, read_hash, UNDO_HASH_SIZE) != 0
				  || line_count != curbuf->b_ml.ml_line_count)
    {
	if (p_verbose > 0 || name != NULL)
	{
	    if (name == NULL)
		verbose_enter();
	    give_warning((char_u *)
		      _("File contents changed, cannot use undo info"), TRUE);
	    if (name == NULL)
		verbose_leave();
	}
	goto error;
    }

    /* Read undo data for "U" command. */
    str_len = get4c(fp);
    if (str_len < 0)
	goto error;
    if (str_len > 0)
	line_ptr = read_string_decrypt(curbuf, fp, str_len);
    line_lnum = (linenr_T)get4c(fp);
    line_colnr = (colnr_T)get4c(fp);
    if (line_lnum < 0 || line_colnr < 0)
    {
	corruption_error("line lnum/col", file_name);
	goto error;
    }

    /* Begin general undo data */
    old_header_seq = get4c(fp);
    new_header_seq = get4c(fp);
    cur_header_seq = get4c(fp);
    num_head = get4c(fp);
    seq_last = get4c(fp);
    seq_cur = get4c(fp);
    seq_time = get8ctime(fp);

    /* Optional header fields, not in previous version. */
    if (new_version)
	for (;;)
	{
	    int len = getc(fp);
	    int what;

	    if (len == 0 || len == EOF)
		break;
	    what = getc(fp);
	    switch (what)
	    {
		case UF_LAST_SAVE_NR:
		    last_save_nr = get4c(fp);
		    break;
		default:
		    /* field not supported, skip */
		    while (--len >= 0)
			(void)getc(fp);
	    }
	}

    /* uhp_table will store the freshly created undo headers we allocate
     * until we insert them into curbuf. The table remains sorted by the
     * sequence numbers of the headers.
     * When there are no headers uhp_table is NULL. */
    if (num_head > 0)
    {
	uhp_table = (u_header_T **)U_ALLOC_LINE(
					     num_head * sizeof(u_header_T *));
	if (uhp_table == NULL)
	    goto error;
    }

    while ((c = get2c(fp)) == UF_HEADER_MAGIC)
    {
	if (num_read_uhps >= num_head)
	{
	    corruption_error("num_head too small", file_name);
	    goto error;
	}

	uhp = unserialize_uhp(fp, file_name, new_version);
	if (uhp == NULL)
	    goto error;
	uhp_table[num_read_uhps++] = uhp;
    }

    if (num_read_uhps != num_head)
    {
	corruption_error("num_head", file_name);
	goto error;
    }
    if (c != UF_HEADER_END_MAGIC)
    {
	corruption_error("end marker", file_name);
	goto error;
    }

#ifdef U_DEBUG
    uhp_table_used = (int *)alloc_clear(
				     (unsigned)(sizeof(int) * num_head + 1));
# define SET_FLAG(j) ++uhp_table_used[j]
#else
# define SET_FLAG(j)
#endif

    /* We have put all of the headers into a table. Now we iterate through the
     * table and swizzle each sequence number we have stored in uh_*_seq into
     * a pointer corresponding to the header with that sequence number. */
    for (i = 0; i < num_head; i++)
    {
	uhp = uhp_table[i];
	if (uhp == NULL)
	    continue;
	for (j = 0; j < num_head; j++)
	    if (uhp_table[j] != NULL && i != j
			      && uhp_table[i]->uh_seq == uhp_table[j]->uh_seq)
	    {
		corruption_error("duplicate uh_seq", file_name);
		goto error;
	    }
	for (j = 0; j < num_head; j++)
	    if (uhp_table[j] != NULL
				  && uhp_table[j]->uh_seq == uhp->uh_next.seq)
	    {
		uhp->uh_next.ptr = uhp_table[j];
		SET_FLAG(j);
		break;
	    }
	for (j = 0; j < num_head; j++)
	    if (uhp_table[j] != NULL
				  && uhp_table[j]->uh_seq == uhp->uh_prev.seq)
	    {
		uhp->uh_prev.ptr = uhp_table[j];
		SET_FLAG(j);
		break;
	    }
	for (j = 0; j < num_head; j++)
	    if (uhp_table[j] != NULL
			      && uhp_table[j]->uh_seq == uhp->uh_alt_next.seq)
	    {
		uhp->uh_alt_next.ptr = uhp_table[j];
		SET_FLAG(j);
		break;
	    }
	for (j = 0; j < num_head; j++)
	    if (uhp_table[j] != NULL
			      && uhp_table[j]->uh_seq == uhp->uh_alt_prev.seq)
	    {
		uhp->uh_alt_prev.ptr = uhp_table[j];
		SET_FLAG(j);
		break;
	    }
	if (old_header_seq > 0 && old_idx < 0 && uhp->uh_seq == old_header_seq)
	{
	    old_idx = i;
	    SET_FLAG(i);
	}
	if (new_header_seq > 0 && new_idx < 0 && uhp->uh_seq == new_header_seq)
	{
	    new_idx = i;
	    SET_FLAG(i);
	}
	if (cur_header_seq > 0 && cur_idx < 0 && uhp->uh_seq == cur_header_seq)
	{
	    cur_idx = i;
	    SET_FLAG(i);
	}
    }

    /* Now that we have read the undo info successfully, free the current undo
     * info and use the info from the file. */
    u_blockfree(curbuf);
    curbuf->b_u_oldhead = old_idx < 0 ? NULL : uhp_table[old_idx];
    curbuf->b_u_newhead = new_idx < 0 ? NULL : uhp_table[new_idx];
    curbuf->b_u_curhead = cur_idx < 0 ? NULL : uhp_table[cur_idx];
    curbuf->b_u_line_ptr = line_ptr;
    curbuf->b_u_line_lnum = line_lnum;
    curbuf->b_u_line_colnr = line_colnr;
    curbuf->b_u_numhead = num_head;
    curbuf->b_u_seq_last = seq_last;
    curbuf->b_u_seq_cur = seq_cur;
    curbuf->b_u_time_cur = seq_time;
    curbuf->b_u_save_nr_last = last_save_nr;

    curbuf->b_u_synced = TRUE;
    vim_free(uhp_table);

#ifdef U_DEBUG
    for (i = 0; i < num_head; ++i)
	if (uhp_table_used[i] == 0)
	    EMSGN("uhp_table entry %ld not used, leaking memory", i);
    vim_free(uhp_table_used);
    u_check(TRUE);
#endif

    if (name != NULL)
	smsg((char_u *)_("Finished reading undo file %s"), file_name);
    goto theend;

error:
    vim_free(line_ptr);
    if (uhp_table != NULL)
    {
	for (i = 0; i < num_read_uhps; i++)
	    if (uhp_table[i] != NULL)
		u_free_uhp(uhp_table[i]);
	vim_free(uhp_table);
    }

theend:
#ifdef FEAT_CRYPT
    if (do_decrypt)
	crypt_pop_state();
#endif
    if (fp != NULL)
	fclose(fp);
    if (file_name != name)
	vim_free(file_name);
    return;
}

#endif /* FEAT_PERSISTENT_UNDO */


/*
 * If 'cpoptions' contains 'u': Undo the previous undo or redo (vi compatible).
 * If 'cpoptions' does not contain 'u': Always undo.
 */
    void
u_undo(count)
    int count;
{
    /*
     * If we get an undo command while executing a macro, we behave like the
     * original vi. If this happens twice in one macro the result will not
     * be compatible.
     */
    if (curbuf->b_u_synced == FALSE)
    {
	u_sync(TRUE);
	count = 1;
    }

    if (vim_strchr(p_cpo, CPO_UNDO) == NULL)
	undo_undoes = TRUE;
    else
	undo_undoes = !undo_undoes;
    u_doit(count);
}

/*
 * If 'cpoptions' contains 'u': Repeat the previous undo or redo.
 * If 'cpoptions' does not contain 'u': Always redo.
 */
    void
u_redo(count)
    int count;
{
    if (vim_strchr(p_cpo, CPO_UNDO) == NULL)
	undo_undoes = FALSE;
    u_doit(count);
}

/*
 * Undo or redo, depending on 'undo_undoes', 'count' times.
 */
    static void
u_doit(startcount)
    int startcount;
{
    int count = startcount;

    if (!undo_allowed())
	return;

    u_newcount = 0;
    u_oldcount = 0;
    if (curbuf->b_ml.ml_flags & ML_EMPTY)
	u_oldcount = -1;
    while (count--)
    {
	/* Do the change warning now, so that it triggers FileChangedRO when
	 * needed.  This may cause the file to be reloaded, that must happen
	 * before we do anything, because it may change curbuf->b_u_curhead
	 * and more. */
	change_warning(0);

	if (undo_undoes)
	{
	    if (curbuf->b_u_curhead == NULL)		/* first undo */
		curbuf->b_u_curhead = curbuf->b_u_newhead;
	    else if (p_ul > 0)				/* multi level undo */
		/* get next undo */
		curbuf->b_u_curhead = curbuf->b_u_curhead->uh_next.ptr;
	    /* nothing to undo */
	    if (curbuf->b_u_numhead == 0 || curbuf->b_u_curhead == NULL)
	    {
		/* stick curbuf->b_u_curhead at end */
		curbuf->b_u_curhead = curbuf->b_u_oldhead;
		beep_flush();
		if (count == startcount - 1)
		{
		    MSG(_("Already at oldest change"));
		    return;
		}
		break;
	    }

	    u_undoredo(TRUE);
	}
	else
	{
	    if (curbuf->b_u_curhead == NULL || p_ul <= 0)
	    {
		beep_flush();	/* nothing to redo */
		if (count == startcount - 1)
		{
		    MSG(_("Already at newest change"));
		    return;
		}
		break;
	    }

	    u_undoredo(FALSE);

	    /* Advance for next redo.  Set "newhead" when at the end of the
	     * redoable changes. */
	    if (curbuf->b_u_curhead->uh_prev.ptr == NULL)
		curbuf->b_u_newhead = curbuf->b_u_curhead;
	    curbuf->b_u_curhead = curbuf->b_u_curhead->uh_prev.ptr;
	}
    }
    u_undo_end(undo_undoes, FALSE);
}

/*
 * Undo or redo over the timeline.
 * When "step" is negative go back in time, otherwise goes forward in time.
 * When "sec" is FALSE make "step" steps, when "sec" is TRUE use "step" as
 * seconds.
 * When "file" is TRUE use "step" as a number of file writes.
 * When "absolute" is TRUE use "step" as the sequence number to jump to.
 * "sec" must be FALSE then.
 */
    void
undo_time(step, sec, file, absolute)
    long	step;
    int		sec;
    int		file;
    int		absolute;
{
    long	    target;
    long	    closest;
    long	    closest_start;
    long	    closest_seq = 0;
    long	    val;
    u_header_T	    *uhp;
    u_header_T	    *last;
    int		    mark;
    int		    nomark;
    int		    round;
    int		    dosec = sec;
    int		    dofile = file;
    int		    above = FALSE;
    int		    did_undo = TRUE;

    /* First make sure the current undoable change is synced. */
    if (curbuf->b_u_synced == FALSE)
	u_sync(TRUE);

    u_newcount = 0;
    u_oldcount = 0;
    if (curbuf->b_ml.ml_flags & ML_EMPTY)
	u_oldcount = -1;

    /* "target" is the node below which we want to be.
     * Init "closest" to a value we can't reach. */
    if (absolute)
    {
	target = step;
	closest = -1;
    }
    else
    {
	/* When doing computations with time_t subtract starttime, because
	 * time_t converted to a long may result in a wrong number. */
	if (dosec)
	    target = (long)(curbuf->b_u_time_cur - starttime) + step;
	else if (dofile)
	{
	    if (step < 0)
	    {
		/* Going back to a previous write. If there were changes after
		 * the last write, count that as moving one file-write, so
		 * that ":earlier 1f" undoes all changes since the last save. */
		uhp = curbuf->b_u_curhead;
		if (uhp != NULL)
		    uhp = uhp->uh_next.ptr;
		else
		    uhp = curbuf->b_u_newhead;
		if (uhp != NULL && uhp->uh_save_nr != 0)
		    /* "uh_save_nr" was set in the last block, that means
		     * there were no changes since the last write */
		    target = curbuf->b_u_save_nr_cur + step;
		else
		    /* count the changes since the last write as one step */
		    target = curbuf->b_u_save_nr_cur + step + 1;
		if (target <= 0)
		    /* Go to before first write: before the oldest change. Use
		     * the sequence number for that. */
		    dofile = FALSE;
	    }
	    else
	    {
		/* Moving forward to a newer write. */
		target = curbuf->b_u_save_nr_cur + step;
		if (target > curbuf->b_u_save_nr_last)
		{
		    /* Go to after last write: after the latest change. Use
		     * the sequence number for that. */
		    target = curbuf->b_u_seq_last + 1;
		    dofile = FALSE;
		}
	    }
	}
	else
	    target = curbuf->b_u_seq_cur + step;
	if (step < 0)
	{
	    if (target < 0)
		target = 0;
	    closest = -1;
	}
	else
	{
	    if (dosec)
		closest = (long)(time(NULL) - starttime + 1);
	    else if (dofile)
		closest = curbuf->b_u_save_nr_last + 2;
	    else
		closest = curbuf->b_u_seq_last + 2;
	    if (target >= closest)
		target = closest - 1;
	}
    }
    closest_start = closest;
    closest_seq = curbuf->b_u_seq_cur;

    /*
     * May do this twice:
     * 1. Search for "target", update "closest" to the best match found.
     * 2. If "target" not found search for "closest".
     *
     * When using the closest time we use the sequence number in the second
     * round, because there may be several entries with the same time.
     */
    for (round = 1; round <= 2; ++round)
    {
	/* Find the path from the current state to where we want to go.  The
	 * desired state can be anywhere in the undo tree, need to go all over
	 * it.  We put "nomark" in uh_walk where we have been without success,
	 * "mark" where it could possibly be. */
	mark = ++lastmark;
	nomark = ++lastmark;

	if (curbuf->b_u_curhead == NULL)	/* at leaf of the tree */
	    uhp = curbuf->b_u_newhead;
	else
	    uhp = curbuf->b_u_curhead;

	while (uhp != NULL)
	{
	    uhp->uh_walk = mark;
	    if (dosec)
		val = (long)(uhp->uh_time - starttime);
	    else if (dofile)
		val = uhp->uh_save_nr;
	    else
		val = uhp->uh_seq;

	    if (round == 1 && !(dofile && val == 0))
	    {
		/* Remember the header that is closest to the target.
		 * It must be at least in the right direction (checked with
		 * "b_u_seq_cur").  When the timestamp is equal find the
		 * highest/lowest sequence number. */
		if ((step < 0 ? uhp->uh_seq <= curbuf->b_u_seq_cur
			      : uhp->uh_seq > curbuf->b_u_seq_cur)
			&& ((dosec && val == closest)
			    ? (step < 0
				? uhp->uh_seq < closest_seq
				: uhp->uh_seq > closest_seq)
			    : closest == closest_start
				|| (val > target
				    ? (closest > target
					? val - target <= closest - target
					: val - target <= target - closest)
				    : (closest > target
					? target - val <= closest - target
					: target - val <= target - closest))))
		{
		    closest = val;
		    closest_seq = uhp->uh_seq;
		}
	    }

	    /* Quit searching when we found a match.  But when searching for a
	     * time we need to continue looking for the best uh_seq. */
	    if (target == val && !dosec)
	    {
		target = uhp->uh_seq;
		break;
	    }

	    /* go down in the tree if we haven't been there */
	    if (uhp->uh_prev.ptr != NULL && uhp->uh_prev.ptr->uh_walk != nomark
					 && uhp->uh_prev.ptr->uh_walk != mark)
		uhp = uhp->uh_prev.ptr;

	    /* go to alternate branch if we haven't been there */
	    else if (uhp->uh_alt_next.ptr != NULL
		    && uhp->uh_alt_next.ptr->uh_walk != nomark
		    && uhp->uh_alt_next.ptr->uh_walk != mark)
		uhp = uhp->uh_alt_next.ptr;

	    /* go up in the tree if we haven't been there and we are at the
	     * start of alternate branches */
	    else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
		    && uhp->uh_next.ptr->uh_walk != nomark
		    && uhp->uh_next.ptr->uh_walk != mark)
	    {
		/* If still at the start we don't go through this change. */
		if (uhp == curbuf->b_u_curhead)
		    uhp->uh_walk = nomark;
		uhp = uhp->uh_next.ptr;
	    }

	    else
	    {
		/* need to backtrack; mark this node as useless */
		uhp->uh_walk = nomark;
		if (uhp->uh_alt_prev.ptr != NULL)
		    uhp = uhp->uh_alt_prev.ptr;
		else
		    uhp = uhp->uh_next.ptr;
	    }
	}

	if (uhp != NULL)    /* found it */
	    break;

	if (absolute)
	{
	    EMSGN(_("E830: Undo number %ld not found"), step);
	    return;
	}

	if (closest == closest_start)
	{
	    if (step < 0)
		MSG(_("Already at oldest change"));
	    else
		MSG(_("Already at newest change"));
	    return;
	}

	target = closest_seq;
	dosec = FALSE;
	dofile = FALSE;
	if (step < 0)
	    above = TRUE;	/* stop above the header */
    }

    /* If we found it: Follow the path to go to where we want to be. */
    if (uhp != NULL)
    {
	/*
	 * First go up the tree as much as needed.
	 */
	while (!got_int)
	{
	    /* Do the change warning now, for the same reason as above. */
	    change_warning(0);

	    uhp = curbuf->b_u_curhead;
	    if (uhp == NULL)
		uhp = curbuf->b_u_newhead;
	    else
		uhp = uhp->uh_next.ptr;
	    if (uhp == NULL || uhp->uh_walk != mark
					 || (uhp->uh_seq == target && !above))
		break;
	    curbuf->b_u_curhead = uhp;
	    u_undoredo(TRUE);
	    uhp->uh_walk = nomark;	/* don't go back down here */
	}

	/*
	 * And now go down the tree (redo), branching off where needed.
	 */
	while (!got_int)
	{
	    /* Do the change warning now, for the same reason as above. */
	    change_warning(0);

	    uhp = curbuf->b_u_curhead;
	    if (uhp == NULL)
		break;

	    /* Go back to the first branch with a mark. */
	    while (uhp->uh_alt_prev.ptr != NULL
				     && uhp->uh_alt_prev.ptr->uh_walk == mark)
		uhp = uhp->uh_alt_prev.ptr;

	    /* Find the last branch with a mark, that's the one. */
	    last = uhp;
	    while (last->uh_alt_next.ptr != NULL
				    && last->uh_alt_next.ptr->uh_walk == mark)
		last = last->uh_alt_next.ptr;
	    if (last != uhp)
	    {
		/* Make the used branch the first entry in the list of
		 * alternatives to make "u" and CTRL-R take this branch. */
		while (uhp->uh_alt_prev.ptr != NULL)
		    uhp = uhp->uh_alt_prev.ptr;
		if (last->uh_alt_next.ptr != NULL)
		    last->uh_alt_next.ptr->uh_alt_prev.ptr =
							last->uh_alt_prev.ptr;
		last->uh_alt_prev.ptr->uh_alt_next.ptr = last->uh_alt_next.ptr;
		last->uh_alt_prev.ptr = NULL;
		last->uh_alt_next.ptr = uhp;
		uhp->uh_alt_prev.ptr = last;

		if (curbuf->b_u_oldhead == uhp)
		    curbuf->b_u_oldhead = last;
		uhp = last;
		if (uhp->uh_next.ptr != NULL)
		    uhp->uh_next.ptr->uh_prev.ptr = uhp;
	    }
	    curbuf->b_u_curhead = uhp;

	    if (uhp->uh_walk != mark)
		break;	    /* must have reached the target */

	    /* Stop when going backwards in time and didn't find the exact
	     * header we were looking for. */
	    if (uhp->uh_seq == target && above)
	    {
		curbuf->b_u_seq_cur = target - 1;
		break;
	    }

	    u_undoredo(FALSE);

	    /* Advance "curhead" to below the header we last used.  If it
	     * becomes NULL then we need to set "newhead" to this leaf. */
	    if (uhp->uh_prev.ptr == NULL)
		curbuf->b_u_newhead = uhp;
	    curbuf->b_u_curhead = uhp->uh_prev.ptr;
	    did_undo = FALSE;

	    if (uhp->uh_seq == target)	/* found it! */
		break;

	    uhp = uhp->uh_prev.ptr;
	    if (uhp == NULL || uhp->uh_walk != mark)
	    {
		/* Need to redo more but can't find it... */
		EMSG2(_(e_intern2), "undo_time()");
		break;
	    }
	}
    }
    u_undo_end(did_undo, absolute);
}

/*
 * u_undoredo: common code for undo and redo
 *
 * The lines in the file are replaced by the lines in the entry list at
 * curbuf->b_u_curhead. The replaced lines in the file are saved in the entry
 * list for the next undo/redo.
 *
 * When "undo" is TRUE we go up in the tree, when FALSE we go down.
 */
    static void
u_undoredo(undo)
    int		undo;
{
    char_u	**newarray = NULL;
    linenr_T	oldsize;
    linenr_T	newsize;
    linenr_T	top, bot;
    linenr_T	lnum;
    linenr_T	newlnum = MAXLNUM;
    long	i;
    u_entry_T	*uep, *nuep;
    u_entry_T	*newlist = NULL;
    int		old_flags;
    int		new_flags;
    pos_T	namedm[NMARKS];
#ifdef FEAT_VISUAL
    visualinfo_T visualinfo;
#endif
    int		empty_buffer;		    /* buffer became empty */
    u_header_T	*curhead = curbuf->b_u_curhead;

#ifdef FEAT_AUTOCMD
    /* Don't want autocommands using the undo structures here, they are
     * invalid till the end. */
    block_autocmds();
#endif

#ifdef U_DEBUG
    u_check(FALSE);
#endif
    old_flags = curhead->uh_flags;
    new_flags = (curbuf->b_changed ? UH_CHANGED : 0) +
	       ((curbuf->b_ml.ml_flags & ML_EMPTY) ? UH_EMPTYBUF : 0);
    setpcmark();

    /*
     * save marks before undo/redo
     */
    mch_memmove(namedm, curbuf->b_namedm, sizeof(pos_T) * NMARKS);
#ifdef FEAT_VISUAL
    visualinfo = curbuf->b_visual;
#endif
    curbuf->b_op_start.lnum = curbuf->b_ml.ml_line_count;
    curbuf->b_op_start.col = 0;
    curbuf->b_op_end.lnum = 0;
    curbuf->b_op_end.col = 0;

    for (uep = curhead->uh_entry; uep != NULL; uep = nuep)
    {
	top = uep->ue_top;
	bot = uep->ue_bot;
	if (bot == 0)
	    bot = curbuf->b_ml.ml_line_count + 1;
	if (top > curbuf->b_ml.ml_line_count || top >= bot
				      || bot > curbuf->b_ml.ml_line_count + 1)
	{
#ifdef FEAT_AUTOCMD
	    unblock_autocmds();
#endif
	    EMSG(_("E438: u_undo: line numbers wrong"));
	    changed();		/* don't want UNCHANGED now */
	    return;
	}

	oldsize = bot - top - 1;    /* number of lines before undo */
	newsize = uep->ue_size;	    /* number of lines after undo */

	if (top < newlnum)
	{
	    /* If the saved cursor is somewhere in this undo block, move it to
	     * the remembered position.  Makes "gwap" put the cursor back
	     * where it was. */
	    lnum = curhead->uh_cursor.lnum;
	    if (lnum >= top && lnum <= top + newsize + 1)
	    {
		curwin->w_cursor = curhead->uh_cursor;
		newlnum = curwin->w_cursor.lnum - 1;
	    }
	    else
	    {
		/* Use the first line that actually changed.  Avoids that
		 * undoing auto-formatting puts the cursor in the previous
		 * line. */
		for (i = 0; i < newsize && i < oldsize; ++i)
		    if (STRCMP(uep->ue_array[i], ml_get(top + 1 + i)) != 0)
			break;
		if (i == newsize && newlnum == MAXLNUM && uep->ue_next == NULL)
		{
		    newlnum = top;
		    curwin->w_cursor.lnum = newlnum + 1;
		}
		else if (i < newsize)
		{
		    newlnum = top + i;
		    curwin->w_cursor.lnum = newlnum + 1;
		}
	    }
	}

	empty_buffer = FALSE;

	/* delete the lines between top and bot and save them in newarray */
	if (oldsize > 0)
	{
	    if ((newarray = (char_u **)U_ALLOC_LINE(
					 sizeof(char_u *) * oldsize)) == NULL)
	    {
		do_outofmem_msg((long_u)(sizeof(char_u *) * oldsize));
		/*
		 * We have messed up the entry list, repair is impossible.
		 * we have to free the rest of the list.
		 */
		while (uep != NULL)
		{
		    nuep = uep->ue_next;
		    u_freeentry(uep, uep->ue_size);
		    uep = nuep;
		}
		break;
	    }
	    /* delete backwards, it goes faster in most cases */
	    for (lnum = bot - 1, i = oldsize; --i >= 0; --lnum)
	    {
		/* what can we do when we run out of memory? */
		if ((newarray[i] = u_save_line(lnum)) == NULL)
		    do_outofmem_msg((long_u)0);
		/* remember we deleted the last line in the buffer, and a
		 * dummy empty line will be inserted */
		if (curbuf->b_ml.ml_line_count == 1)
		    empty_buffer = TRUE;
		ml_delete(lnum, FALSE);
	    }
	}
	else
	    newarray = NULL;

	/* insert the lines in u_array between top and bot */
	if (newsize)
	{
	    for (lnum = top, i = 0; i < newsize; ++i, ++lnum)
	    {
		/*
		 * If the file is empty, there is an empty line 1 that we
		 * should get rid of, by replacing it with the new line
		 */
		if (empty_buffer && lnum == 0)
		    ml_replace((linenr_T)1, uep->ue_array[i], TRUE);
		else
		    ml_append(lnum, uep->ue_array[i], (colnr_T)0, FALSE);
		vim_free(uep->ue_array[i]);
	    }
	    vim_free((char_u *)uep->ue_array);
	}

	/* adjust marks */
	if (oldsize != newsize)
	{
	    mark_adjust(top + 1, top + oldsize, (long)MAXLNUM,
					       (long)newsize - (long)oldsize);
	    if (curbuf->b_op_start.lnum > top + oldsize)
		curbuf->b_op_start.lnum += newsize - oldsize;
	    if (curbuf->b_op_end.lnum > top + oldsize)
		curbuf->b_op_end.lnum += newsize - oldsize;
	}

	changed_lines(top + 1, 0, bot, newsize - oldsize);

	/* set '[ and '] mark */
	if (top + 1 < curbuf->b_op_start.lnum)
	    curbuf->b_op_start.lnum = top + 1;
	if (newsize == 0 && top + 1 > curbuf->b_op_end.lnum)
	    curbuf->b_op_end.lnum = top + 1;
	else if (top + newsize > curbuf->b_op_end.lnum)
	    curbuf->b_op_end.lnum = top + newsize;

	u_newcount += newsize;
	u_oldcount += oldsize;
	uep->ue_size = oldsize;
	uep->ue_array = newarray;
	uep->ue_bot = top + newsize + 1;

	/*
	 * insert this entry in front of the new entry list
	 */
	nuep = uep->ue_next;
	uep->ue_next = newlist;
	newlist = uep;
    }

    curhead->uh_entry = newlist;
    curhead->uh_flags = new_flags;
    if ((old_flags & UH_EMPTYBUF) && bufempty())
	curbuf->b_ml.ml_flags |= ML_EMPTY;
    if (old_flags & UH_CHANGED)
	changed();
    else
#ifdef FEAT_NETBEANS_INTG
	/* per netbeans undo rules, keep it as modified */
	if (!isNetbeansModified(curbuf))
#endif
	unchanged(curbuf, FALSE);

    /*
     * restore marks from before undo/redo
     */
    for (i = 0; i < NMARKS; ++i)
	if (curhead->uh_namedm[i].lnum != 0)
	{
	    curbuf->b_namedm[i] = curhead->uh_namedm[i];
	    curhead->uh_namedm[i] = namedm[i];
	}
#ifdef FEAT_VISUAL
    if (curhead->uh_visual.vi_start.lnum != 0)
    {
	curbuf->b_visual = curhead->uh_visual;
	curhead->uh_visual = visualinfo;
    }
#endif

    /*
     * If the cursor is only off by one line, put it at the same position as
     * before starting the change (for the "o" command).
     * Otherwise the cursor should go to the first undone line.
     */
    if (curhead->uh_cursor.lnum + 1 == curwin->w_cursor.lnum
						 && curwin->w_cursor.lnum > 1)
	--curwin->w_cursor.lnum;
    if (curhead->uh_cursor.lnum == curwin->w_cursor.lnum)
    {
	curwin->w_cursor.col = curhead->uh_cursor.col;
#ifdef FEAT_VIRTUALEDIT
	if (virtual_active() && curhead->uh_cursor_vcol >= 0)
	    coladvance((colnr_T)curhead->uh_cursor_vcol);
	else
	    curwin->w_cursor.coladd = 0;
#endif
    }
    else if (curwin->w_cursor.lnum <= curbuf->b_ml.ml_line_count)
	beginline(BL_SOL | BL_FIX);
    else
    {
	/* We get here with the current cursor line being past the end (eg
	 * after adding lines at the end of the file, and then undoing it).
	 * check_cursor() will move the cursor to the last line.  Move it to
	 * the first column here. */
	curwin->w_cursor.col = 0;
#ifdef FEAT_VIRTUALEDIT
	curwin->w_cursor.coladd = 0;
#endif
    }

    /* Make sure the cursor is on an existing line and column. */
    check_cursor();

    /* Remember where we are for "g-" and ":earlier 10s". */
    curbuf->b_u_seq_cur = curhead->uh_seq;
    if (undo)
	/* We are below the previous undo.  However, to make ":earlier 1s"
	 * work we compute this as being just above the just undone change. */
	--curbuf->b_u_seq_cur;

    /* Remember where we are for ":earlier 1f" and ":later 1f". */
    if (curhead->uh_save_nr != 0)
    {
	if (undo)
	    curbuf->b_u_save_nr_cur = curhead->uh_save_nr - 1;
	else
	    curbuf->b_u_save_nr_cur = curhead->uh_save_nr;
    }

    /* The timestamp can be the same for multiple changes, just use the one of
     * the undone/redone change. */
    curbuf->b_u_time_cur = curhead->uh_time;

#ifdef FEAT_AUTOCMD
    unblock_autocmds();
#endif
#ifdef U_DEBUG
    u_check(FALSE);
#endif
}

/*
 * If we deleted or added lines, report the number of less/more lines.
 * Otherwise, report the number of changes (this may be incorrect
 * in some cases, but it's better than nothing).
 */
    static void
u_undo_end(did_undo, absolute)
    int		did_undo;	/* just did an undo */
    int		absolute;	/* used ":undo N" */
{
    char	*msgstr;
    u_header_T	*uhp;
    char_u	msgbuf[80];

#ifdef FEAT_FOLDING
    if ((fdo_flags & FDO_UNDO) && KeyTyped)
	foldOpenCursor();
#endif

    if (global_busy	    /* no messages now, wait until global is finished */
	    || !messaging())  /* 'lazyredraw' set, don't do messages now */
	return;

    if (curbuf->b_ml.ml_flags & ML_EMPTY)
	--u_newcount;

    u_oldcount -= u_newcount;
    if (u_oldcount == -1)
	msgstr = N_("more line");
    else if (u_oldcount < 0)
	msgstr = N_("more lines");
    else if (u_oldcount == 1)
	msgstr = N_("line less");
    else if (u_oldcount > 1)
	msgstr = N_("fewer lines");
    else
    {
	u_oldcount = u_newcount;
	if (u_newcount == 1)
	    msgstr = N_("change");
	else
	    msgstr = N_("changes");
    }

    if (curbuf->b_u_curhead != NULL)
    {
	/* For ":undo N" we prefer a "after #N" message. */
	if (absolute && curbuf->b_u_curhead->uh_next.ptr != NULL)
	{
	    uhp = curbuf->b_u_curhead->uh_next.ptr;
	    did_undo = FALSE;
	}
	else if (did_undo)
	    uhp = curbuf->b_u_curhead;
	else
	    uhp = curbuf->b_u_curhead->uh_next.ptr;
    }
    else
	uhp = curbuf->b_u_newhead;

    if (uhp == NULL)
	*msgbuf = NUL;
    else
	u_add_time(msgbuf, sizeof(msgbuf), uhp->uh_time);

#ifdef FEAT_CONCEAL
    {
	win_T	*wp;

	FOR_ALL_WINDOWS(wp)
	{
	    if (wp->w_buffer == curbuf && wp->w_p_conceal)
		redraw_win_later(wp, NOT_VALID);
	}
    }
#endif

    smsg((char_u *)_("%ld %s; %s #%ld  %s"),
	    u_oldcount < 0 ? -u_oldcount : u_oldcount,
	    _(msgstr),
	    did_undo ? _("before") : _("after"),
	    uhp == NULL ? 0L : uhp->uh_seq,
	    msgbuf);
}

/*
 * u_sync: stop adding to the current entry list
 */
    void
u_sync(force)
    int	    force;	/* Also sync when no_u_sync is set. */
{
    /* Skip it when already synced or syncing is disabled. */
    if (curbuf->b_u_synced || (!force && no_u_sync > 0))
	return;
#if defined(FEAT_XIM) && defined(FEAT_GUI_GTK)
    if (im_is_preediting())
	return;		    /* XIM is busy, don't break an undo sequence */
#endif
    if (p_ul < 0)
	curbuf->b_u_synced = TRUE;  /* no entries, nothing to do */
    else
    {
	u_getbot();		    /* compute ue_bot of previous u_save */
	curbuf->b_u_curhead = NULL;
    }
}

/*
 * ":undolist": List the leafs of the undo tree
 */
    void
ex_undolist(eap)
    exarg_T *eap UNUSED;
{
    garray_T	ga;
    u_header_T	*uhp;
    int		mark;
    int		nomark;
    int		changes = 1;
    int		i;

    /*
     * 1: walk the tree to find all leafs, put the info in "ga".
     * 2: sort the lines
     * 3: display the list
     */
    mark = ++lastmark;
    nomark = ++lastmark;
    ga_init2(&ga, (int)sizeof(char *), 20);

    uhp = curbuf->b_u_oldhead;
    while (uhp != NULL)
    {
	if (uhp->uh_prev.ptr == NULL && uhp->uh_walk != nomark
						      && uhp->uh_walk != mark)
	{
	    if (ga_grow(&ga, 1) == FAIL)
		break;
	    vim_snprintf((char *)IObuff, IOSIZE, "%6ld %7ld  ",
							uhp->uh_seq, changes);
	    u_add_time(IObuff + STRLEN(IObuff), IOSIZE - STRLEN(IObuff),
								uhp->uh_time);
	    if (uhp->uh_save_nr > 0)
	    {
		while (STRLEN(IObuff) < 32)
		    STRCAT(IObuff, " ");
		vim_snprintf_add((char *)IObuff, IOSIZE,
						   "  %3ld", uhp->uh_save_nr);
	    }
	    ((char_u **)(ga.ga_data))[ga.ga_len++] = vim_strsave(IObuff);
	}

	uhp->uh_walk = mark;

	/* go down in the tree if we haven't been there */
	if (uhp->uh_prev.ptr != NULL && uhp->uh_prev.ptr->uh_walk != nomark
					 && uhp->uh_prev.ptr->uh_walk != mark)
	{
	    uhp = uhp->uh_prev.ptr;
	    ++changes;
	}

	/* go to alternate branch if we haven't been there */
	else if (uhp->uh_alt_next.ptr != NULL
		&& uhp->uh_alt_next.ptr->uh_walk != nomark
		&& uhp->uh_alt_next.ptr->uh_walk != mark)
	    uhp = uhp->uh_alt_next.ptr;

	/* go up in the tree if we haven't been there and we are at the
	 * start of alternate branches */
	else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
		&& uhp->uh_next.ptr->uh_walk != nomark
		&& uhp->uh_next.ptr->uh_walk != mark)
	{
	    uhp = uhp->uh_next.ptr;
	    --changes;
	}

	else
	{
	    /* need to backtrack; mark this node as done */
	    uhp->uh_walk = nomark;
	    if (uhp->uh_alt_prev.ptr != NULL)
		uhp = uhp->uh_alt_prev.ptr;
	    else
	    {
		uhp = uhp->uh_next.ptr;
		--changes;
	    }
	}
    }

    if (ga.ga_len == 0)
	MSG(_("Nothing to undo"));
    else
    {
	sort_strings((char_u **)ga.ga_data, ga.ga_len);

	msg_start();
	msg_puts_attr((char_u *)_("number changes  time            saved"),
							      hl_attr(HLF_T));
	for (i = 0; i < ga.ga_len && !got_int; ++i)
	{
	    msg_putchar('\n');
	    if (got_int)
		break;
	    msg_puts(((char_u **)ga.ga_data)[i]);
	}
	msg_end();

	ga_clear_strings(&ga);
    }
}

/*
 * Put the timestamp of an undo header in "buf[buflen]" in a nice format.
 */
    static void
u_add_time(buf, buflen, tt)
    char_u	*buf;
    size_t	buflen;
    time_t	tt;
{
#ifdef HAVE_STRFTIME
    struct tm	*curtime;

    if (time(NULL) - tt >= 100)
    {
	curtime = localtime(&tt);
	(void)strftime((char *)buf, buflen, "%H:%M:%S", curtime);
    }
    else
#endif
	vim_snprintf((char *)buf, buflen, _("%ld seconds ago"),
						     (long)(time(NULL) - tt));
}

/*
 * ":undojoin": continue adding to the last entry list
 */
    void
ex_undojoin(eap)
    exarg_T *eap UNUSED;
{
    if (curbuf->b_u_newhead == NULL)
	return;		    /* nothing changed before */
    if (curbuf->b_u_curhead != NULL)
    {
	EMSG(_("E790: undojoin is not allowed after undo"));
	return;
    }
    if (!curbuf->b_u_synced)
	return;		    /* already unsynced */
    if (p_ul < 0)
	return;		    /* no entries, nothing to do */
    else
    {
	/* Go back to the last entry */
	curbuf->b_u_curhead = curbuf->b_u_newhead;
	curbuf->b_u_synced = FALSE;  /* no entries, nothing to do */
    }
}

/*
 * Called after writing the file and setting b_changed to FALSE.
 * Now an undo means that the buffer is modified.
 */
    void
u_unchanged(buf)
    buf_T	*buf;
{
    u_unch_branch(buf->b_u_oldhead);
    buf->b_did_warn = FALSE;
}

/*
 * Increase the write count, store it in the last undo header, what would be
 * used for "u".
 */
    void
u_update_save_nr(buf)
    buf_T *buf;
{
    u_header_T	*uhp;

    ++buf->b_u_save_nr_last;
    buf->b_u_save_nr_cur = buf->b_u_save_nr_last;
    uhp = buf->b_u_curhead;
    if (uhp != NULL)
	uhp = uhp->uh_next.ptr;
    else
	uhp = buf->b_u_newhead;
    if (uhp != NULL)
	uhp->uh_save_nr = buf->b_u_save_nr_last;
}

    static void
u_unch_branch(uhp)
    u_header_T	*uhp;
{
    u_header_T	*uh;

    for (uh = uhp; uh != NULL; uh = uh->uh_prev.ptr)
    {
	uh->uh_flags |= UH_CHANGED;
	if (uh->uh_alt_next.ptr != NULL)
	    u_unch_branch(uh->uh_alt_next.ptr);	    /* recursive */
    }
}

/*
 * Get pointer to last added entry.
 * If it's not valid, give an error message and return NULL.
 */
    static u_entry_T *
u_get_headentry()
{
    if (curbuf->b_u_newhead == NULL || curbuf->b_u_newhead->uh_entry == NULL)
    {
	EMSG(_("E439: undo list corrupt"));
	return NULL;
    }
    return curbuf->b_u_newhead->uh_entry;
}

/*
 * u_getbot(): compute the line number of the previous u_save
 *		It is called only when b_u_synced is FALSE.
 */
    static void
u_getbot()
{
    u_entry_T	*uep;
    linenr_T	extra;

    uep = u_get_headentry();	/* check for corrupt undo list */
    if (uep == NULL)
	return;

    uep = curbuf->b_u_newhead->uh_getbot_entry;
    if (uep != NULL)
    {
	/*
	 * the new ue_bot is computed from the number of lines that has been
	 * inserted (0 - deleted) since calling u_save. This is equal to the
	 * old line count subtracted from the current line count.
	 */
	extra = curbuf->b_ml.ml_line_count - uep->ue_lcount;
	uep->ue_bot = uep->ue_top + uep->ue_size + 1 + extra;
	if (uep->ue_bot < 1 || uep->ue_bot > curbuf->b_ml.ml_line_count)
	{
	    EMSG(_("E440: undo line missing"));
	    uep->ue_bot = uep->ue_top + 1;  /* assume all lines deleted, will
					     * get all the old lines back
					     * without deleting the current
					     * ones */
	}

	curbuf->b_u_newhead->uh_getbot_entry = NULL;
    }

    curbuf->b_u_synced = TRUE;
}

/*
 * Free one header "uhp" and its entry list and adjust the pointers.
 */
    static void
u_freeheader(buf, uhp, uhpp)
    buf_T	    *buf;
    u_header_T	    *uhp;
    u_header_T	    **uhpp;	/* if not NULL reset when freeing this header */
{
    u_header_T	    *uhap;

    /* When there is an alternate redo list free that branch completely,
     * because we can never go there. */
    if (uhp->uh_alt_next.ptr != NULL)
	u_freebranch(buf, uhp->uh_alt_next.ptr, uhpp);

    if (uhp->uh_alt_prev.ptr != NULL)
	uhp->uh_alt_prev.ptr->uh_alt_next.ptr = NULL;

    /* Update the links in the list to remove the header. */
    if (uhp->uh_next.ptr == NULL)
	buf->b_u_oldhead = uhp->uh_prev.ptr;
    else
	uhp->uh_next.ptr->uh_prev.ptr = uhp->uh_prev.ptr;

    if (uhp->uh_prev.ptr == NULL)
	buf->b_u_newhead = uhp->uh_next.ptr;
    else
	for (uhap = uhp->uh_prev.ptr; uhap != NULL;
						 uhap = uhap->uh_alt_next.ptr)
	    uhap->uh_next.ptr = uhp->uh_next.ptr;

    u_freeentries(buf, uhp, uhpp);
}

/*
 * Free an alternate branch and any following alternate branches.
 */
    static void
u_freebranch(buf, uhp, uhpp)
    buf_T	    *buf;
    u_header_T	    *uhp;
    u_header_T	    **uhpp;	/* if not NULL reset when freeing this header */
{
    u_header_T	    *tofree, *next;

    /* If this is the top branch we may need to use u_freeheader() to update
     * all the pointers. */
    if (uhp == buf->b_u_oldhead)
    {
	u_freeheader(buf, uhp, uhpp);
	return;
    }

    if (uhp->uh_alt_prev.ptr != NULL)
	uhp->uh_alt_prev.ptr->uh_alt_next.ptr = NULL;

    next = uhp;
    while (next != NULL)
    {
	tofree = next;
	if (tofree->uh_alt_next.ptr != NULL)
	    u_freebranch(buf, tofree->uh_alt_next.ptr, uhpp);   /* recursive */
	next = tofree->uh_prev.ptr;
	u_freeentries(buf, tofree, uhpp);
    }
}

/*
 * Free all the undo entries for one header and the header itself.
 * This means that "uhp" is invalid when returning.
 */
    static void
u_freeentries(buf, uhp, uhpp)
    buf_T	    *buf;
    u_header_T	    *uhp;
    u_header_T	    **uhpp;	/* if not NULL reset when freeing this header */
{
    u_entry_T	    *uep, *nuep;

    /* Check for pointers to the header that become invalid now. */
    if (buf->b_u_curhead == uhp)
	buf->b_u_curhead = NULL;
    if (buf->b_u_newhead == uhp)
	buf->b_u_newhead = NULL;  /* freeing the newest entry */
    if (uhpp != NULL && uhp == *uhpp)
	*uhpp = NULL;

    for (uep = uhp->uh_entry; uep != NULL; uep = nuep)
    {
	nuep = uep->ue_next;
	u_freeentry(uep, uep->ue_size);
    }

#ifdef U_DEBUG
    uhp->uh_magic = 0;
#endif
    vim_free((char_u *)uhp);
    --buf->b_u_numhead;
}

/*
 * free entry 'uep' and 'n' lines in uep->ue_array[]
 */
    static void
u_freeentry(uep, n)
    u_entry_T	*uep;
    long	    n;
{
    while (n > 0)
	vim_free(uep->ue_array[--n]);
    vim_free((char_u *)uep->ue_array);
#ifdef U_DEBUG
    uep->ue_magic = 0;
#endif
    vim_free((char_u *)uep);
}

/*
 * invalidate the undo buffer; called when storage has already been released
 */
    void
u_clearall(buf)
    buf_T	*buf;
{
    buf->b_u_newhead = buf->b_u_oldhead = buf->b_u_curhead = NULL;
    buf->b_u_synced = TRUE;
    buf->b_u_numhead = 0;
    buf->b_u_line_ptr = NULL;
    buf->b_u_line_lnum = 0;
}

/*
 * save the line "lnum" for the "U" command
 */
    void
u_saveline(lnum)
    linenr_T lnum;
{
    if (lnum == curbuf->b_u_line_lnum)	    /* line is already saved */
	return;
    if (lnum < 1 || lnum > curbuf->b_ml.ml_line_count) /* should never happen */
	return;
    u_clearline();
    curbuf->b_u_line_lnum = lnum;
    if (curwin->w_cursor.lnum == lnum)
	curbuf->b_u_line_colnr = curwin->w_cursor.col;
    else
	curbuf->b_u_line_colnr = 0;
    if ((curbuf->b_u_line_ptr = u_save_line(lnum)) == NULL)
	do_outofmem_msg((long_u)0);
}

/*
 * clear the line saved for the "U" command
 * (this is used externally for crossing a line while in insert mode)
 */
    void
u_clearline()
{
    if (curbuf->b_u_line_ptr != NULL)
    {
	vim_free(curbuf->b_u_line_ptr);
	curbuf->b_u_line_ptr = NULL;
	curbuf->b_u_line_lnum = 0;
    }
}

/*
 * Implementation of the "U" command.
 * Differentiation from vi: "U" can be undone with the next "U".
 * We also allow the cursor to be in another line.
 * Careful: may trigger autocommands that reload the buffer.
 */
    void
u_undoline()
{
    colnr_T t;
    char_u  *oldp;

    if (undo_off)
	return;

    if (curbuf->b_u_line_ptr == NULL
			|| curbuf->b_u_line_lnum > curbuf->b_ml.ml_line_count)
    {
	beep_flush();
	return;
    }

    /* first save the line for the 'u' command */
    if (u_savecommon(curbuf->b_u_line_lnum - 1,
				curbuf->b_u_line_lnum + 1, (linenr_T)0) == FAIL)
	return;
    oldp = u_save_line(curbuf->b_u_line_lnum);
    if (oldp == NULL)
    {
	do_outofmem_msg((long_u)0);
	return;
    }
    ml_replace(curbuf->b_u_line_lnum, curbuf->b_u_line_ptr, TRUE);
    changed_bytes(curbuf->b_u_line_lnum, 0);
    vim_free(curbuf->b_u_line_ptr);
    curbuf->b_u_line_ptr = oldp;

    t = curbuf->b_u_line_colnr;
    if (curwin->w_cursor.lnum == curbuf->b_u_line_lnum)
	curbuf->b_u_line_colnr = curwin->w_cursor.col;
    curwin->w_cursor.col = t;
    curwin->w_cursor.lnum = curbuf->b_u_line_lnum;
    check_cursor_col();
}

/*
 * Free all allocated memory blocks for the buffer 'buf'.
 */
    void
u_blockfree(buf)
    buf_T	*buf;
{
    while (buf->b_u_oldhead != NULL)
	u_freeheader(buf, buf->b_u_oldhead, NULL);
    vim_free(buf->b_u_line_ptr);
}

/*
 * u_save_line(): allocate memory and copy line 'lnum' into it.
 * Returns NULL when out of memory.
 */
    static char_u *
u_save_line(lnum)
    linenr_T	lnum;
{
    return vim_strsave(ml_get(lnum));
}

/*
 * Check if the 'modified' flag is set, or 'ff' has changed (only need to
 * check the first character, because it can only be "dos", "unix" or "mac").
 * "nofile" and "scratch" type buffers are considered to always be unchanged.
 */
    int
bufIsChanged(buf)
    buf_T	*buf;
{
    return
#ifdef FEAT_QUICKFIX
	    !bt_dontwrite(buf) &&
#endif
	    (buf->b_changed || file_ff_differs(buf));
}

    int
curbufIsChanged()
{
    return
#ifdef FEAT_QUICKFIX
	!bt_dontwrite(curbuf) &&
#endif
	(curbuf->b_changed || file_ff_differs(curbuf));
}

#if defined(FEAT_EVAL) || defined(PROTO)
/*
 * For undotree(): Append the list of undo blocks at "first_uhp" to "list".
 * Recursive.
 */
    void
u_eval_tree(first_uhp, list)
    u_header_T  *first_uhp;
    list_T	*list;
{
    u_header_T  *uhp = first_uhp;
    dict_T	*dict;

    while (uhp != NULL)
    {
	dict = dict_alloc();
	if (dict == NULL)
	    return;
	dict_add_nr_str(dict, "seq", uhp->uh_seq, NULL);
	dict_add_nr_str(dict, "time", (long)uhp->uh_time, NULL);
	if (uhp == curbuf->b_u_newhead)
	    dict_add_nr_str(dict, "newhead", 1, NULL);
	if (uhp == curbuf->b_u_curhead)
	    dict_add_nr_str(dict, "curhead", 1, NULL);
	if (uhp->uh_save_nr > 0)
	    dict_add_nr_str(dict, "save", uhp->uh_save_nr, NULL);

	if (uhp->uh_alt_next.ptr != NULL)
	{
	    list_T	*alt_list = list_alloc();

	    if (alt_list != NULL)
	    {
		/* Recursive call to add alternate undo tree. */
		u_eval_tree(uhp->uh_alt_next.ptr, alt_list);
		dict_add_list(dict, "alt", alt_list);
	    }
	}

	list_append_dict(list, dict);
	uhp = uhp->uh_prev.ptr;
    }
}
#endif