neovim

undo.c (97307B)

---

// 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

#include <assert.h>
#include <fcntl.h>
#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <uv.h>

#include "auto/config.h"
#include "klib/kvec.h"
#include "nvim/ascii_defs.h"
#include "nvim/autocmd.h"
#include "nvim/buffer.h"
#include "nvim/buffer_defs.h"
#include "nvim/buffer_updates.h"
#include "nvim/change.h"
#include "nvim/cursor.h"
#include "nvim/drawscreen.h"
#include "nvim/edit.h"
#include "nvim/errors.h"
#include "nvim/eval/funcs.h"
#include "nvim/eval/typval.h"
#include "nvim/ex_cmds_defs.h"
#include "nvim/ex_docmd.h"
#include "nvim/ex_getln.h"
#include "nvim/extmark.h"
#include "nvim/extmark_defs.h"
#include "nvim/fileio.h"
#include "nvim/fold.h"
#include "nvim/garray.h"
#include "nvim/garray_defs.h"
#include "nvim/getchar.h"
#include "nvim/gettext_defs.h"
#include "nvim/globals.h"
#include "nvim/highlight_defs.h"
#include "nvim/macros_defs.h"
#include "nvim/mark.h"
#include "nvim/mark_defs.h"
#include "nvim/mbyte.h"
#include "nvim/memline.h"
#include "nvim/memline_defs.h"
#include "nvim/memory.h"
#include "nvim/message.h"
#include "nvim/option.h"
#include "nvim/option_vars.h"
#include "nvim/os/fs.h"
#include "nvim/os/fs_defs.h"
#include "nvim/os/input.h"
#include "nvim/os/os_defs.h"
#include "nvim/os/time.h"
#include "nvim/os/time_defs.h"
#include "nvim/path.h"
#include "nvim/pos_defs.h"
#include "nvim/sha256.h"
#include "nvim/spell.h"
#include "nvim/state.h"
#include "nvim/strings.h"
#include "nvim/types_defs.h"
#include "nvim/undo.h"
#include "nvim/undo_defs.h"
#include "nvim/vim_defs.h"

/// Structure passed around between undofile functions.
typedef struct {
 buf_T *bi_buf;
 FILE *bi_fp;
} bufinfo_T;

#include "undo.c.generated.h"

static const char e_undo_list_corrupt[]
 = N_("E439: Undo list corrupt");
static const char e_undo_line_missing[]
 = N_("E440: Undo line missing");
static const char e_write_error_in_undo_file_str[]
 = N_("E829: Write error in undo file: %s");

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

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

static int lastmark = 0;

#if defined(U_DEBUG)
// 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)
{
 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(0, "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(0, "expected: 0x%x, actual: 0x%x",
          exp_uh_alt_prev, uhp->uh_alt_prev.ptr);
   }

   // Check the undo tree at this header.
   for (u_entry_T *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)) {
   semsg("b_u_newhead invalid: 0x%x", curbuf->b_u_newhead);
 }
 if (curbuf->b_u_curhead != NULL && seen_b_u_curhead == 0) {
   semsg("b_u_curhead invalid: 0x%x", curbuf->b_u_curhead);
 }
 if (header_count != curbuf->b_u_numhead) {
   emsg("b_u_numhead invalid");
   smsg(0, "expected: %" PRId64 ", actual: %" PRId64,
        (int64_t)header_count, (int64_t)curbuf->b_u_numhead);
 }
}

#endif

/// Save the current line for both the "u" and "U" command.
/// Careful: may trigger autocommands that reload the buffer.
/// Returns OK or FAIL.
int u_save_cursor(void)
{
 linenr_T cur = curwin->w_cursor.lnum;
 linenr_T top = cur > 0 ? cur - 1 : 0;
 linenr_T bot = cur + 1;

 return u_save(top, bot);
}

/// 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(linenr_T top, linenr_T bot)
{
 return u_save_buf(curbuf, top, bot);
}

int u_save_buf(buf_T *buf, linenr_T top, linenr_T bot)
{
 if (top >= bot || bot > (buf->b_ml.ml_line_count + 1)) {
   return FAIL;        // rely on caller to do error messages
 }

 if (top + 2 == bot) {
   u_saveline(buf, top + 1);
 }

 return u_savecommon(buf, top, bot, 0, false);
}

/// 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(linenr_T lnum)
{
 return u_savecommon(curbuf, lnum - 1, lnum + 1, lnum + 1, false);
}

/// 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(linenr_T lnum)
{
 return u_savecommon(curbuf, lnum - 1, lnum, lnum + 1, false);
}

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

/// Return true when undo is allowed. Otherwise print an error message and
/// return false.
///
/// @return true if undo is allowed.
bool undo_allowed(buf_T *buf)
{
 // Don't allow changes when 'modifiable' is off.
 if (!MODIFIABLE(buf)) {
   emsg(_(e_modifiable));
   return false;
 }

 // In the sandbox it's not allowed to change the text.
 if (sandbox != 0) {
   emsg(_(e_sandbox));
   return false;
 }

 // Don't allow changes in the buffer while editing the cmdline.  The
 // caller of getcmdline() may get confused.
 if (textlock != 0 || expr_map_locked()) {
   emsg(_(e_textlock));
   return false;
 }

 return true;
}

/// Get the 'undolevels' value for the current buffer.
static OptInt get_undolevel(buf_T *buf)
{
 if (buf->b_p_ul == NO_LOCAL_UNDOLEVEL) {
   return p_ul;
 }
 return buf->b_p_ul;
}

static inline void zero_fmark_additional_data(fmark_T *fmarks)
{
 for (size_t i = 0; i < NMARKS; i++) {
   XFREE_CLEAR(fmarks[i].additional_data);
 }
}

/// 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.
/// "newbot" is the new bottom line.  Use zero when not known.
/// "reload" is true when saving for a buffer reload.
/// Careful: may trigger autocommands that reload the buffer.
/// Returns FAIL when lines could not be saved, OK otherwise.
int u_savecommon(buf_T *buf, linenr_T top, linenr_T bot, linenr_T newbot, bool reload)
{
 if (!reload) {
   // When making changes is not allowed return FAIL.  It's a crude way
   // to make all change commands fail.
   if (!undo_allowed(buf)) {
     return FAIL;
   }

   // 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).
   if (buf == curbuf) {
     change_warning(buf, 0);
   }

   if (bot > buf->b_ml.ml_line_count + 1) {
     //  This happens when the FileChangedRO autocommand changes the
     //  file in a way it becomes shorter.
     emsg(_("E881: Line count changed unexpectedly"));
     return FAIL;
   }
 }

#ifdef U_DEBUG
 u_check(false);
#endif

 u_entry_T *uep;
 u_entry_T *prev_uep;
 linenr_T size = bot - top - 1;

 // If buf->b_u_synced == true make a new header.
 if (buf->b_u_synced) {
   // Need to create new entry in b_changelist.
   buf->b_new_change = true;

   u_header_T *uhp;
   if (get_undolevel(buf) >= 0) {
     // Make a new header entry.  Do this first so that we don't mess
     // up the undo info when out of memory.
     uhp = xmalloc(sizeof(u_header_T));
     kv_init(uhp->uh_extmark);
#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 buf->b_u_curhead to an alternate branch.
   u_header_T *old_curhead = buf->b_u_curhead;
   if (old_curhead != NULL) {
     buf->b_u_newhead = old_curhead->uh_next.ptr;
     buf->b_u_curhead = NULL;
   }

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

     if (uhfree == old_curhead) {
       // Can't reconnect the branch, delete all of it.
       u_freebranch(buf, uhfree, &old_curhead);
     } else if (uhfree->uh_alt_next.ptr == NULL) {
       // There is no branch, only free one header.
       u_freeheader(buf, 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(buf, uhfree, &old_curhead);
     }
#ifdef U_DEBUG
     u_check(true);
#endif
   }

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

   uhp->uh_prev.ptr = NULL;
   uhp->uh_next.ptr = buf->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 (buf->b_u_oldhead == old_curhead) {
       buf->b_u_oldhead = uhp;
     }
   } else {
     uhp->uh_alt_prev.ptr = NULL;
   }

   if (buf->b_u_newhead != NULL) {
     buf->b_u_newhead->uh_prev.ptr = uhp;
   }

   uhp->uh_seq = ++buf->b_u_seq_last;
   buf->b_u_seq_cur = uhp->uh_seq;
   uhp->uh_time = time(NULL);
   uhp->uh_save_nr = 0;
   buf->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
   if (virtual_active(curwin) && curwin->w_cursor.coladd > 0) {
     uhp->uh_cursor_vcol = getviscol();
   } else {
     uhp->uh_cursor_vcol = -1;
   }

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

   // save named marks and Visual marks for undo
   zero_fmark_additional_data(buf->b_namedm);
   memmove(uhp->uh_namedm, buf->b_namedm,
           sizeof(buf->b_namedm[0]) * NMARKS);
   uhp->uh_visual = buf->b_visual;

   buf->b_u_newhead = uhp;

   if (buf->b_u_oldhead == NULL) {
     buf->b_u_oldhead = uhp;
   }
   buf->b_u_numhead++;
 } else {
   if (get_undolevel(buf) < 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(buf);
     prev_uep = NULL;
     for (int 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 ((buf->b_u_newhead->uh_getbot_entry != uep
            ? (uep->ue_top + uep->ue_size + 1
               != (uep->ue_bot == 0
                   ? buf->b_ml.ml_line_count + 1
                   : uep->ue_bot))
            : uep->ue_lcount != buf->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(buf);
           buf->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 = buf->b_u_newhead->uh_entry;
           buf->b_u_newhead->uh_entry = uep;
         }

         // The executed command may change the line count.
         if (newbot != 0) {
           uep->ue_bot = newbot;
         } else if (bot > buf->b_ml.ml_line_count) {
           uep->ue_bot = 0;
         } else {
           uep->ue_lcount = buf->b_ml.ml_line_count;
           buf->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(buf);
 }

 // add lines in front of entry list
 uep = xmalloc(sizeof(u_entry_T));
 CLEAR_POINTER(uep);
#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 > buf->b_ml.ml_line_count) {
   uep->ue_bot = 0;
 } else {
   uep->ue_lcount = buf->b_ml.ml_line_count;
   buf->b_u_newhead->uh_getbot_entry = uep;
 }

 if (size > 0) {
   uep->ue_array = xmalloc(sizeof(char *) * (size_t)size);
   linenr_T lnum;
   int i;
   for (i = 0, lnum = top + 1; i < size; i++) {
     fast_breakcheck();
     if (got_int) {
       u_freeentry(uep, i);
       return FAIL;
     }
     uep->ue_array[i] = u_save_line_buf(buf, lnum++);
   }
 } else {
   uep->ue_array = NULL;
 }

 uep->ue_next = buf->b_u_newhead->uh_entry;
 buf->b_u_newhead->uh_entry = uep;
 if (reload) {
   // buffer was reloaded, notify text change subscribers
   buf->b_u_newhead->uh_flags |= UH_RELOAD;
 }
 buf->b_u_synced = false;
 undo_undoes = false;

#ifdef U_DEBUG
 u_check(false);
#endif
 return OK;
}

// magic at start of undofile
#define UF_START_MAGIC     "Vim\237UnDo\345"
#define UF_START_MAGIC_LEN     9
// magic at start of header
#define UF_HEADER_MAGIC        0x5fd0
// magic after last header
#define UF_HEADER_END_MAGIC    0xe7aa
// magic at start of entry
#define UF_ENTRY_MAGIC         0xf518
// magic after last entry
#define UF_ENTRY_END_MAGIC     0x3581

// 2-byte undofile version number
#define UF_VERSION             3

// extra fields for header
#define UF_LAST_SAVE_NR        1

// extra fields for uhp
#define UHP_SAVE_NR            1

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

/// Compute the hash for a buffer text into hash[UNDO_HASH_SIZE].
///
/// @param[in] buf The buffer used to compute the hash
/// @param[in] hash Array of size UNDO_HASH_SIZE in which to store the value of
///                 the hash
void u_compute_hash(buf_T *buf, uint8_t *hash)
{
 context_sha256_T ctx;
 sha256_start(&ctx);
 for (linenr_T lnum = 1; lnum <= buf->b_ml.ml_line_count; lnum++) {
   char *p = ml_get_buf(buf, lnum);
   sha256_update(&ctx, (uint8_t *)p, strlen(p) + 1);
 }
 sha256_finish(&ctx, hash);
}

/// Return an allocated string of the full path of the target undofile.
///
/// @param[in]  buf_ffname  Full file name for which undo file location should
///                         be found.
/// @param[in]  reading  If true, find the file to read by traversing all of the
///                      directories in &undodir. If false use the first
///                      existing directory. If none of the directories in
///                      &undodir option exist then last directory in the list
///                      will be automatically created.
///
/// @return [allocated] File name to read from/write to or NULL.
char *u_get_undo_file_name(const char *const buf_ffname, const bool reading)
 FUNC_ATTR_WARN_UNUSED_RESULT
{
 const char *ffname = buf_ffname;

 if (ffname == NULL) {
   return NULL;
 }

#ifdef HAVE_READLINK
 char fname_buf[MAXPATHL];
 // 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

 char dir_name[MAXPATHL + 1];
 char *munged_name = NULL;
 char *undo_file_name = NULL;

 // Loop over 'undodir'.  When reading find the first file that exists.
 // When not reading use the first directory that exists or ".".
 char *dirp = p_udir;
 while (*dirp != NUL) {
   size_t dir_len = copy_option_part(&dirp, dir_name, MAXPATHL, ",");
   if (dir_len == 1 && dir_name[0] == '.') {
     // Use same directory as the ffname,
     // "dir/name" -> "dir/.name.un~"
     const size_t ffname_len = strlen(ffname);
     undo_file_name = xmalloc(ffname_len + 6);
     memmove(undo_file_name, ffname, ffname_len + 1);
     char *const tail = path_tail(undo_file_name);
     const size_t tail_len = strlen(tail);
     memmove(tail + 1, tail, tail_len + 1);
     *tail = '.';
     memmove(tail + tail_len + 1, ".un~", sizeof(".un~"));
   } else {
     dir_name[dir_len] = NUL;

     // Remove trailing pathseps from directory name
     char *p = &dir_name[dir_len - 1];
     while (vim_ispathsep(*p)) {
       *p-- = NUL;
     }

     bool has_directory = os_isdir(dir_name);
     if (!has_directory && *dirp == NUL && !reading) {
       // Last directory in the list does not exist, create it.
       int ret;
       char *failed_dir;
       if ((ret = os_mkdir_recurse(dir_name, 0755, &failed_dir, NULL)) != 0) {
         semsg(_("E5003: Unable to create directory \"%s\" for undo file: %s"),
               failed_dir, os_strerror(ret));
         xfree(failed_dir);
       } else {
         has_directory = true;
       }
     }
     if (has_directory) {
       if (munged_name == NULL) {
         munged_name = xstrdup(ffname);
         for (char *c = munged_name; *c != NUL; MB_PTR_ADV(c)) {
           if (vim_ispathsep(*c)) {
             *c = '%';
           }
         }
       }
       undo_file_name = concat_fnames(dir_name, munged_name, true);
     }
   }

   // When reading check if the file exists.
   if (undo_file_name != NULL
       && (!reading || os_path_exists(undo_file_name))) {
     break;
   }
   XFREE_CLEAR(undo_file_name);
 }

 xfree(munged_name);
 return undo_file_name;
}

/// Display an error for corrupted undo file
///
/// @param[in]  mesg  Identifier of the corruption kind.
/// @param[in]  file_name  File in which error occurred.
static void corruption_error(const char *const mesg, const char *const file_name)
 FUNC_ATTR_NONNULL_ALL
{
 semsg(_("E825: Corrupted undo file (%s): %s"), mesg, file_name);
}

static void u_free_uhp(u_header_T *uhp)
{
 u_entry_T *uep = uhp->uh_entry;
 while (uep != NULL) {
   u_entry_T *nuep = uep->ue_next;
   u_freeentry(uep, uep->ue_size);
   uep = nuep;
 }
 xfree(uhp);
}

/// Writes the undofile header.
///
/// @param bi   The buffer information
/// @param hash The hash of the buffer contents
//
/// @returns false in case of an error.
static bool serialize_header(bufinfo_T *bi, uint8_t *hash)
 FUNC_ATTR_NONNULL_ALL
{
 buf_T *buf = bi->bi_buf;
 FILE *fp = bi->bi_fp;

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

 undo_write_bytes(bi, 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 (!undo_write(bi, hash, UNDO_HASH_SIZE)) {
   return false;
 }

 // Write buffer-specific data.
 undo_write_bytes(bi, (uintmax_t)buf->b_ml.ml_line_count, 4);
 size_t len = buf->b_u_line_ptr ? strlen(buf->b_u_line_ptr) : 0;
 undo_write_bytes(bi, len, 4);
 if (len > 0 && !undo_write(bi, (uint8_t *)buf->b_u_line_ptr, len)) {
   return false;
 }
 undo_write_bytes(bi, (uintmax_t)buf->b_u_line_lnum, 4);
 undo_write_bytes(bi, (uintmax_t)buf->b_u_line_colnr, 4);

 // Write undo structures header data.
 put_header_ptr(bi, buf->b_u_oldhead);
 put_header_ptr(bi, buf->b_u_newhead);
 put_header_ptr(bi, buf->b_u_curhead);

 undo_write_bytes(bi, (uintmax_t)buf->b_u_numhead, 4);
 undo_write_bytes(bi, (uintmax_t)buf->b_u_seq_last, 4);
 undo_write_bytes(bi, (uintmax_t)buf->b_u_seq_cur, 4);
 uint8_t time_buf[8];
 time_to_bytes(buf->b_u_time_cur, time_buf);
 undo_write(bi, time_buf, sizeof(time_buf));

 // Write optional fields.
 undo_write_bytes(bi, 4, 1);
 undo_write_bytes(bi, UF_LAST_SAVE_NR, 1);
 undo_write_bytes(bi, (uintmax_t)buf->b_u_save_nr_last, 4);

 // Write end marker.
 undo_write_bytes(bi, 0, 1);

 return true;
}

/// Writes an undo header.
///
/// @param bi  The buffer information
/// @param uhp The undo header to write
//
/// @returns false in case of an error.
static bool serialize_uhp(bufinfo_T *bi, u_header_T *uhp)
{
 if (!undo_write_bytes(bi, (uintmax_t)UF_HEADER_MAGIC, 2)) {
   return false;
 }

 put_header_ptr(bi, uhp->uh_next.ptr);
 put_header_ptr(bi, uhp->uh_prev.ptr);
 put_header_ptr(bi, uhp->uh_alt_next.ptr);
 put_header_ptr(bi, uhp->uh_alt_prev.ptr);
 undo_write_bytes(bi, (uintmax_t)uhp->uh_seq, 4);
 serialize_pos(bi, uhp->uh_cursor);
 undo_write_bytes(bi, (uintmax_t)uhp->uh_cursor_vcol, 4);
 undo_write_bytes(bi, (uintmax_t)uhp->uh_flags, 2);
 // Assume NMARKS will stay the same.
 for (size_t i = 0; i < (size_t)NMARKS; i++) {
   serialize_pos(bi, uhp->uh_namedm[i].mark);
 }
 serialize_visualinfo(bi, &uhp->uh_visual);
 uint8_t time_buf[8];
 time_to_bytes(uhp->uh_time, time_buf);
 undo_write(bi, time_buf, sizeof(time_buf));

 // Write optional fields.
 undo_write_bytes(bi, 4, 1);
 undo_write_bytes(bi, UHP_SAVE_NR, 1);
 undo_write_bytes(bi, (uintmax_t)uhp->uh_save_nr, 4);

 // Write end marker.
 undo_write_bytes(bi, 0, 1);

 // Write all the entries.
 for (u_entry_T *uep = uhp->uh_entry; uep; uep = uep->ue_next) {
   undo_write_bytes(bi, (uintmax_t)UF_ENTRY_MAGIC, 2);
   if (!serialize_uep(bi, uep)) {
     return false;
   }
 }
 undo_write_bytes(bi, (uintmax_t)UF_ENTRY_END_MAGIC, 2);

 // Write all extmark undo objects
 for (size_t i = 0; i < kv_size(uhp->uh_extmark); i++) {
   if (!serialize_extmark(bi, kv_A(uhp->uh_extmark, i))) {
     return false;
   }
 }
 undo_write_bytes(bi, (uintmax_t)UF_ENTRY_END_MAGIC, 2);

 return true;
}

static u_header_T *unserialize_uhp(bufinfo_T *bi, const char *file_name)
{
 u_header_T *uhp = xmalloc(sizeof(u_header_T));
 CLEAR_POINTER(uhp);
#ifdef U_DEBUG
 uhp->uh_magic = UH_MAGIC;
#endif
 uhp->uh_next.seq = undo_read_4c(bi);
 uhp->uh_prev.seq = undo_read_4c(bi);
 uhp->uh_alt_next.seq = undo_read_4c(bi);
 uhp->uh_alt_prev.seq = undo_read_4c(bi);
 uhp->uh_seq = undo_read_4c(bi);
 if (uhp->uh_seq <= 0) {
   corruption_error("uh_seq", file_name);
   xfree(uhp);
   return NULL;
 }
 unserialize_pos(bi, &uhp->uh_cursor);
 uhp->uh_cursor_vcol = undo_read_4c(bi);
 uhp->uh_flags = undo_read_2c(bi);
 const Timestamp cur_timestamp = os_time();
 for (size_t i = 0; i < (size_t)NMARKS; i++) {
   unserialize_pos(bi, &uhp->uh_namedm[i].mark);
   uhp->uh_namedm[i].timestamp = cur_timestamp;
   uhp->uh_namedm[i].fnum = 0;
 }
 unserialize_visualinfo(bi, &uhp->uh_visual);
 uhp->uh_time = undo_read_time(bi);

 // Unserialize optional fields.
 while (true) {
   int len = undo_read_byte(bi);

   if (len == EOF) {
     corruption_error("truncated", file_name);
     u_free_uhp(uhp);
     return NULL;
   }
   if (len == 0) {
     break;
   }
   int what = undo_read_byte(bi);
   switch (what) {
   case UHP_SAVE_NR:
     uhp->uh_save_nr = undo_read_4c(bi);
     break;
   default:
     // Field not supported, skip it.
     while (--len >= 0) {
       undo_read_byte(bi);
     }
   }
 }

 // Unserialize the uep list.
 u_entry_T *last_uep = NULL;
 int c;
 while ((c = undo_read_2c(bi)) == UF_ENTRY_MAGIC) {
   bool error = false;
   u_entry_T *uep = unserialize_uep(bi, &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;
 }

 // Unserialize all extmark undo information
 kv_init(uhp->uh_extmark);

 while ((c = undo_read_2c(bi)) == UF_ENTRY_MAGIC) {
   bool error = false;
   ExtmarkUndoObject *extup = unserialize_extmark(bi, &error, file_name);
   if (error) {
     kv_destroy(uhp->uh_extmark);
     xfree(extup);
     return NULL;
   }
   kv_push(uhp->uh_extmark, *extup);
   xfree(extup);
 }
 if (c != UF_ENTRY_END_MAGIC) {
   corruption_error("entry end", file_name);
   u_free_uhp(uhp);
   return NULL;
 }

 return uhp;
}

static bool serialize_extmark(bufinfo_T *bi, ExtmarkUndoObject extup)
{
 if (extup.type == kExtmarkSplice) {
   undo_write_bytes(bi, (uintmax_t)UF_ENTRY_MAGIC, 2);
   undo_write_bytes(bi, (uintmax_t)extup.type, 4);
   if (!undo_write(bi, (uint8_t *)&(extup.data.splice),
                   sizeof(ExtmarkSplice))) {
     return false;
   }
 } else if (extup.type == kExtmarkMove) {
   undo_write_bytes(bi, (uintmax_t)UF_ENTRY_MAGIC, 2);
   undo_write_bytes(bi, (uintmax_t)extup.type, 4);
   if (!undo_write(bi, (uint8_t *)&(extup.data.move), sizeof(ExtmarkMove))) {
     return false;
   }
 }
 // Note: We do not serialize ExtmarkSavePos information, since
 // buffer marktrees are not retained when closing/reopening a file
 return true;
}

static ExtmarkUndoObject *unserialize_extmark(bufinfo_T *bi, bool *error, const char *filename)
{
 uint8_t *buf = NULL;

 ExtmarkUndoObject *extup = xmalloc(sizeof(ExtmarkUndoObject));

 UndoObjectType type = (UndoObjectType)undo_read_4c(bi);
 extup->type = type;
 if (type == kExtmarkSplice) {
   size_t n_elems = (size_t)sizeof(ExtmarkSplice) / sizeof(uint8_t);
   buf = xcalloc(n_elems, sizeof(uint8_t));
   if (!undo_read(bi, buf, n_elems)) {
     goto error;
   }
   extup->data.splice = *(ExtmarkSplice *)buf;
 } else if (type == kExtmarkMove) {
   size_t n_elems = (size_t)sizeof(ExtmarkMove) / sizeof(uint8_t);
   buf = xcalloc(n_elems, sizeof(uint8_t));
   if (!undo_read(bi, buf, n_elems)) {
     goto error;
   }
   extup->data.move = *(ExtmarkMove *)buf;
 } else {
   goto error;
 }

 xfree(buf);

 return extup;

error:
 xfree(extup);
 if (buf) {
   xfree(buf);
 }
 *error = true;
 return NULL;
}

/// Serializes "uep".
///
/// @param bi  The buffer information
/// @param uep The undo entry to write
//
/// @returns false in case of an error.
static bool serialize_uep(bufinfo_T *bi, u_entry_T *uep)
{
 undo_write_bytes(bi, (uintmax_t)uep->ue_top, 4);
 undo_write_bytes(bi, (uintmax_t)uep->ue_bot, 4);
 undo_write_bytes(bi, (uintmax_t)uep->ue_lcount, 4);
 undo_write_bytes(bi, (uintmax_t)uep->ue_size, 4);

 for (size_t i = 0; i < (size_t)uep->ue_size; i++) {
   size_t len = strlen(uep->ue_array[i]);
   if (!undo_write_bytes(bi, len, 4)) {
     return false;
   }
   if (len > 0 && !undo_write(bi, (uint8_t *)uep->ue_array[i], len)) {
     return false;
   }
 }
 return true;
}

static u_entry_T *unserialize_uep(bufinfo_T *bi, bool *error, const char *file_name)
{
 u_entry_T *uep = xmalloc(sizeof(u_entry_T));
 CLEAR_POINTER(uep);
#ifdef U_DEBUG
 uep->ue_magic = UE_MAGIC;
#endif
 uep->ue_top = undo_read_4c(bi);
 uep->ue_bot = undo_read_4c(bi);
 uep->ue_lcount = undo_read_4c(bi);
 uep->ue_size = undo_read_4c(bi);

 char **array = NULL;
 if (uep->ue_size > 0) {
   if ((size_t)uep->ue_size < SIZE_MAX / sizeof(char *)) {
     array = xmalloc(sizeof(char *) * (size_t)uep->ue_size);
     memset(array, 0, sizeof(char *) * (size_t)uep->ue_size);
   }
 }
 uep->ue_array = array;

 for (size_t i = 0; i < (size_t)uep->ue_size; i++) {
   int line_len = undo_read_4c(bi);
   char *line;
   if (line_len >= 0) {
     line = undo_read_string(bi, (size_t)line_len);
   } else {
     line = NULL;
     corruption_error("line length", file_name);
   }
   if (line == NULL) {
     *error = true;
     return uep;
   }
   array[i] = line;
 }
 return uep;
}

/// Serializes "pos".
static void serialize_pos(bufinfo_T *bi, pos_T pos)
{
 undo_write_bytes(bi, (uintmax_t)pos.lnum, 4);
 undo_write_bytes(bi, (uintmax_t)pos.col, 4);
 undo_write_bytes(bi, (uintmax_t)pos.coladd, 4);
}

/// Unserializes the pos_T at the current position.
static void unserialize_pos(bufinfo_T *bi, pos_T *pos)
{
 pos->lnum = undo_read_4c(bi);
 pos->lnum = MAX(pos->lnum, 0);
 pos->col = undo_read_4c(bi);
 pos->col = MAX(pos->col, 0);
 pos->coladd = undo_read_4c(bi);
 pos->coladd = MAX(pos->coladd, 0);
}

/// Serializes "info".
static void serialize_visualinfo(bufinfo_T *bi, visualinfo_T *info)
{
 serialize_pos(bi, info->vi_start);
 serialize_pos(bi, info->vi_end);
 undo_write_bytes(bi, (uintmax_t)info->vi_mode, 4);
 undo_write_bytes(bi, (uintmax_t)info->vi_curswant, 4);
}

/// Unserializes the visualinfo_T at the current position.
static void unserialize_visualinfo(bufinfo_T *bi, visualinfo_T *info)
{
 unserialize_pos(bi, &info->vi_start);
 unserialize_pos(bi, &info->vi_end);
 info->vi_mode = undo_read_4c(bi);
 info->vi_curswant = undo_read_4c(bi);
}

/// Write the undo tree in an undo file.
///
/// @param[in]  name  Name of the undo file or NULL if this function needs to
///                   generate the undo file name based on buf->b_ffname.
/// @param[in]  forceit  True for `:wundo!`, false otherwise.
/// @param[in]  buf  Buffer for which undo file is written.
/// @param[in]  hash  Hash value of the buffer text. Must have #UNDO_HASH_SIZE
///                   size.
void u_write_undo(const char *const name, const bool forceit, buf_T *const buf, uint8_t *const hash)
 FUNC_ATTR_NONNULL_ARG(3, 4)
{
 char *file_name;
#ifdef U_DEBUG
 int headers_written = 0;
#endif
 FILE *fp = NULL;
 bool write_ok = false;

 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(0, "%s", _("Cannot write undo file in any directory in 'undodir'"));
       verbose_leave();
     }
     return;
   }
 } else {
   file_name = (char *)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.
 int perm = 0600;
 if (buf->b_ffname != NULL) {
   perm = os_getperm(buf->b_ffname);
   if (perm < 0) {
     perm = 0600;
   }
 }

 // Strip any sticky and executable bits.
 perm = perm & 0666;

 // If the undo file already exists, verify that it actually is an undo
 // file, and delete it.
 if (os_path_exists(file_name)) {
   if (name == NULL || !forceit) {
     // Check we can read it and it's an undo file.
     int fd = os_open(file_name, O_RDONLY, 0);
     if (fd < 0) {
       if (name != NULL || p_verbose > 0) {
         if (name == NULL) {
           verbose_enter();
         }
         smsg(0, _("Will not overwrite with undo file, cannot read: %s"),
              file_name);
         if (name == NULL) {
           verbose_leave();
         }
       }
       goto theend;
     } else {
       char mbuf[UF_START_MAGIC_LEN];
       ssize_t len = read_eintr(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(0, _("Will not overwrite, this is not an undo file: %s"),
                file_name);
           if (name == NULL) {
             verbose_leave();
           }
         }
         goto theend;
       }
     }
   }
   os_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(_("Skipping undo file write, nothing to undo"));
   }
   goto theend;
 }

 int fd = os_open(file_name, O_CREAT|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
 if (fd < 0) {
   semsg(_(e_not_open), file_name);
   goto theend;
 }
 os_setperm(file_name, perm);
 if (p_verbose > 0) {
   verbose_enter();
   smsg(0, _("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.
 FileInfo file_info_old;
 FileInfo file_info_new;
 if (buf->b_ffname != NULL
     && os_fileinfo(buf->b_ffname, &file_info_old)
     && os_fileinfo(file_name, &file_info_new)
     && file_info_old.stat.st_gid != file_info_new.stat.st_gid
     && os_fchown(fd, (uv_uid_t)-1, (uv_gid_t)file_info_old.stat.st_gid)) {
   os_setperm(file_name, (perm & 0707) | ((perm & 07) << 3));
 }
#endif

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

 // Undo must be synced.
 u_sync(true);

 // Write the header.
 bufinfo_T bi = {
   .bi_buf = buf,
   .bi_fp = fp,
 };
 if (!serialize_header(&bi, hash)) {
   goto write_error;
 }

 // Iteratively serialize UHPs and their UEPs from the top down.
 int mark = ++lastmark;
 u_header_T *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(&bi, uhp)) {
       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 (undo_write_bytes(&bi, (uintmax_t)UF_HEADER_END_MAGIC, 2)) {
   write_ok = true;
 }
#ifdef U_DEBUG
 if (headers_written != buf->b_u_numhead) {
   semsg("Written %" PRId64 " headers, ...", (int64_t)headers_written);
   semsg("... but numhead is %" PRId64, (int64_t)buf->b_u_numhead);
 }
#endif

 if ((buf->b_p_fs >= 0 ? buf->b_p_fs : p_fs) && fflush(fp) == 0
     && os_fsync(fd) != 0) {
   write_ok = false;
 }

write_error:
 fclose(fp);
 if (!write_ok) {
   semsg(_(e_write_error_in_undo_file_str), file_name);
 }

 if (buf->b_ffname != NULL) {
   // For systems that support ACL: get the ACL from the original file.
   vim_acl_T acl = os_get_acl(buf->b_ffname);
   os_set_acl(file_name, acl);
   os_free_acl(acl);
 }

theend:
 if (file_name != name) {
   xfree(file_name);
 }
}

/// Loads 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(char *name, const uint8_t *hash, const char *orig_name FUNC_ATTR_UNUSED)
 FUNC_ATTR_NONNULL_ARG(2)
{
 u_header_T **uhp_table = NULL;
 char *line_ptr = NULL;

 char *file_name;
 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 or equal to the current user.
   FileInfo file_info_orig;
   FileInfo file_info_undo;
   if (os_fileinfo(orig_name, &file_info_orig)
       && os_fileinfo(file_name, &file_info_undo)
       && file_info_orig.stat.st_uid != file_info_undo.stat.st_uid
       && file_info_undo.stat.st_uid != getuid()) {
     if (p_verbose > 0) {
       verbose_enter();
       smsg(0, _("Not reading undo file, owner differs: %s"),
            file_name);
       verbose_leave();
     }
     return;
   }
#endif
 } else {
   file_name = name;
 }

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

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

 bufinfo_T bi = {
   .bi_buf = curbuf,
   .bi_fp = fp,
 };

 // Read the undo file header.
 char magic_buf[UF_START_MAGIC_LEN];
 if (fread(magic_buf, UF_START_MAGIC_LEN, 1, fp) != 1
     || memcmp(magic_buf, UF_START_MAGIC, UF_START_MAGIC_LEN) != 0) {
   semsg(_("E823: Not an undo file: %s"), file_name);
   goto error;
 }
 int version = get2c(fp);
 if (version != UF_VERSION) {
   semsg(_("E824: Incompatible undo file: %s"), file_name);
   goto error;
 }

 uint8_t read_hash[UNDO_HASH_SIZE];
 if (!undo_read(&bi, read_hash, UNDO_HASH_SIZE)) {
   corruption_error("hash", file_name);
   goto error;
 }
 linenr_T line_count = (linenr_T)undo_read_4c(&bi);
 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(_("File contents changed, cannot use undo info"), true, true);
     if (name == NULL) {
       verbose_leave();
     }
   }
   goto error;
 }

 // Read undo data for "U" command.
 int str_len = undo_read_4c(&bi);
 if (str_len < 0) {
   goto error;
 }

 if (str_len > 0) {
   line_ptr = undo_read_string(&bi, (size_t)str_len);
 }
 linenr_T line_lnum = (linenr_T)undo_read_4c(&bi);
 colnr_T line_colnr = (colnr_T)undo_read_4c(&bi);
 if (line_lnum < 0 || line_colnr < 0) {
   corruption_error("line lnum/col", file_name);
   goto error;
 }

 // Begin general undo data
 int old_header_seq = undo_read_4c(&bi);
 int new_header_seq = undo_read_4c(&bi);
 int cur_header_seq = undo_read_4c(&bi);
 int num_head = undo_read_4c(&bi);
 int seq_last = undo_read_4c(&bi);
 int seq_cur = undo_read_4c(&bi);
 time_t seq_time = undo_read_time(&bi);

 // Optional header fields.
 int last_save_nr = 0;
 while (true) {
   int len = undo_read_byte(&bi);

   if (len == 0 || len == EOF) {
     break;
   }
   int what = undo_read_byte(&bi);
   switch (what) {
   case UF_LAST_SAVE_NR:
     last_save_nr = undo_read_4c(&bi);
     break;

   default:
     // field not supported, skip
     while (--len >= 0) {
       undo_read_byte(&bi);
     }
   }
 }

 // 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) {
   if ((size_t)num_head < SIZE_MAX / sizeof(*uhp_table)) {
     uhp_table = xmalloc((size_t)num_head * sizeof(*uhp_table));
   }
 }

 int num_read_uhps = 0;

 int c;
 while ((c = undo_read_2c(&bi)) == UF_HEADER_MAGIC) {
   if (num_read_uhps >= num_head) {
     corruption_error("num_head too small", file_name);
     goto error;
   }

   u_header_T *uhp = unserialize_uhp(&bi, file_name);
   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
 size_t amount = num_head * sizeof(int) + 1;
 int *uhp_table_used = xmalloc(amount);
 memset(uhp_table_used, 0, amount);
# 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.
 int16_t old_idx = -1;
 int16_t new_idx = -1;
 int16_t cur_idx = -1;
 for (int i = 0; i < num_head; i++) {
   u_header_T *uhp = uhp_table[i];
   if (uhp == NULL) {
     continue;
   }
   for (int 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 (int 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 (int 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 (int 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 (int 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) {
     assert(i <= INT16_MAX);
     old_idx = (int16_t)i;
     SET_FLAG(i);
   }
   if (new_header_seq > 0 && new_idx < 0 && uhp->uh_seq == new_header_seq) {
     assert(i <= INT16_MAX);
     new_idx = (int16_t)i;
     SET_FLAG(i);
   }
   if (cur_header_seq > 0 && cur_idx < 0 && uhp->uh_seq == cur_header_seq) {
     assert(i <= INT16_MAX);
     cur_idx = (int16_t)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_save_nr_cur = last_save_nr;

 curbuf->b_u_synced = true;
 xfree(uhp_table);

#ifdef U_DEBUG
 for (int i = 0; i < num_head; i++) {
   if (uhp_table_used[i] == 0) {
     semsg("uhp_table entry %" PRId64 " not used, leaking memory", (int64_t)i);
   }
 }
 xfree(uhp_table_used);
 u_check(true);
#endif

 if (name != NULL) {
   smsg(0, _("Finished reading undo file %s"), file_name);
 }
 goto theend;

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

theend:
 if (fp != NULL) {
   fclose(fp);
 }
 if (file_name != name) {
   xfree(file_name);
 }
}

/// Writes a sequence of bytes to the undo file.
///
/// @param bi  The buffer info
/// @param ptr The byte buffer to write
/// @param len The number of bytes to write
///
/// @returns false in case of an error.
static bool undo_write(bufinfo_T *bi, uint8_t *ptr, size_t len)
 FUNC_ATTR_NONNULL_ARG(1)
{
 return fwrite(ptr, len, 1, bi->bi_fp) == 1;
}

/// Writes a number, most significant bit first, in "len" bytes.
///
/// Must match with undo_read_?c() functions.
///
/// @param bi  The buffer info
/// @param nr  The number to write
/// @param len The number of bytes to use when writing the number.
///
/// @returns false in case of an error.
static bool undo_write_bytes(bufinfo_T *bi, uintmax_t nr, size_t len)
{
 assert(len > 0);
 uint8_t buf[8];
 for (size_t i = len - 1, bufi = 0; bufi < len; i--, bufi++) {
   buf[bufi] = (uint8_t)(nr >> (i * 8));
 }
 return undo_write(bi, buf, len);
}

/// Writes 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(bufinfo_T *bi, u_header_T *uhp)
{
 assert(uhp == NULL || uhp->uh_seq >= 0);
 undo_write_bytes(bi, (uint64_t)(uhp != NULL ? uhp->uh_seq : 0), 4);
}

static int undo_read_4c(bufinfo_T *bi)
{
 return get4c(bi->bi_fp);
}

static int undo_read_2c(bufinfo_T *bi)
{
 return get2c(bi->bi_fp);
}

static int undo_read_byte(bufinfo_T *bi)
{
 return getc(bi->bi_fp);
}

static time_t undo_read_time(bufinfo_T *bi)
{
 return get8ctime(bi->bi_fp);
}

/// Reads "buffer[size]" from the undo file.
///
/// @param bi     The buffer info
/// @param buffer Character buffer to read data into
/// @param size   The size of the character buffer
///
/// @returns false in case of an error.
static bool undo_read(bufinfo_T *bi, uint8_t *buffer, size_t size)
 FUNC_ATTR_NONNULL_ARG(1)
{
 const bool retval = fread(buffer, size, 1, bi->bi_fp) == 1;
 if (!retval) {
   // Error may be checked for only later.  Fill with zeros,
   // so that the reader won't use garbage.
   memset(buffer, 0, size);
 }
 return retval;
}

/// Reads a string of length "len" from "bi->bi_fd" and appends a zero to it.
///
/// @param len can be zero to allocate an empty line.
///
/// @returns a pointer to allocated memory or NULL in case of an error.
static char *undo_read_string(bufinfo_T *bi, size_t len)
{
 char *ptr = xmallocz(len);
 if (len > 0 && !undo_read(bi, (uint8_t *)ptr, len)) {
   xfree(ptr);
   return NULL;
 }
 return ptr;
}

/// If 'cpoptions' contains 'u': Undo the previous undo or redo (vi compatible).
/// If 'cpoptions' does not contain 'u': Always undo.
void u_undo(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, false, true);
}

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

 u_doit(count, false, true);
}

/// Undo and remove the branch from the undo tree.
/// Also moves the cursor (as a "normal" undo would).
///
/// @param do_buf_event If `true`, send the changedtick with the buffer updates
bool u_undo_and_forget(int count, bool do_buf_event)
{
 if (curbuf->b_u_synced == false) {
   u_sync(true);
   count = 1;
 }
 undo_undoes = true;
 u_doit(count, true, do_buf_event);

 if (curbuf->b_u_curhead == NULL) {
   // nothing was undone.
   return false;
 }

 // Delete the current redo header
 // set the redo header to the next alternative branch (if any)
 // otherwise we will be in the leaf state
 u_header_T *to_forget = curbuf->b_u_curhead;
 curbuf->b_u_newhead = to_forget->uh_next.ptr;
 curbuf->b_u_curhead = to_forget->uh_alt_next.ptr;
 if (curbuf->b_u_curhead) {
   to_forget->uh_alt_next.ptr = NULL;
   curbuf->b_u_curhead->uh_alt_prev.ptr = to_forget->uh_alt_prev.ptr;
   curbuf->b_u_seq_cur = curbuf->b_u_curhead->uh_next.ptr
                         ? curbuf->b_u_curhead->uh_next.ptr->uh_seq : 0;
 } else if (curbuf->b_u_newhead) {
   curbuf->b_u_seq_cur = curbuf->b_u_newhead->uh_seq;
 }
 if (to_forget->uh_alt_prev.ptr) {
   to_forget->uh_alt_prev.ptr->uh_alt_next.ptr = curbuf->b_u_curhead;
 }
 if (curbuf->b_u_newhead) {
   curbuf->b_u_newhead->uh_prev.ptr = curbuf->b_u_curhead;
 }
 if (curbuf->b_u_seq_last == to_forget->uh_seq) {
   curbuf->b_u_seq_last--;
 }
 u_freebranch(curbuf, to_forget, NULL);
 return true;
}

/// Undo or redo, depending on `undo_undoes`, `count` times.
///
/// @param startcount How often to undo or redo
/// @param quiet If `true`, don't show messages
/// @param do_buf_event If `true`, send the changedtick with the buffer updates
static void u_doit(int startcount, bool quiet, bool do_buf_event)
{
 if (!undo_allowed(curbuf)) {
   return;
 }

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

 msg_ext_set_kind("undo");
 int count = startcount;
 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(curbuf, 0);

   if (undo_undoes) {
     if (curbuf->b_u_curhead == NULL) {  // first undo
       curbuf->b_u_curhead = curbuf->b_u_newhead;
     } else if (get_undolevel(curbuf) > 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"), 0);
         return;
       }
       break;
     }

     u_undoredo(true, do_buf_event);
   } else {
     if (curbuf->b_u_curhead == NULL || get_undolevel(curbuf) <= 0) {
       beep_flush();  // nothing to redo
       if (count == startcount - 1) {
         msg(_("Already at newest change"), 0);
         return;
       }
       break;
     }

     u_undoredo(false, do_buf_event);

     // 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, quiet);
}

// 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(int step, bool sec, bool file, bool absolute)
{
 if (text_locked()) {
   text_locked_msg();
   return;
 }

 // 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;
 }

 int target;
 int closest;
 u_header_T *uhp = NULL;
 bool dosec = sec;
 bool dofile = file;
 bool above = false;
 bool did_undo = true;

 // "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 {
   if (dosec) {
     target = (int)curbuf->b_u_time_cur + 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) {
     target = MAX(target, 0);
     closest = -1;
   } else {
     if (dosec) {
       closest = (int)(os_time() + 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;
     }
   }
 }
 int closest_start = closest;
 int closest_seq = curbuf->b_u_seq_cur;
 int mark;
 int nomark = 0;  // shut up compiler

 // When "target" is 0; Back to origin.
 if (target == 0) {
   mark = lastmark;  // avoid that GCC complains
   goto target_zero;
 }

 // 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 (int 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;
     int val = dosec ? (int)(uhp->uh_time)
                     : dofile ? uhp->uh_save_nr
                              : 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;
     } else if (uhp->uh_alt_next.ptr != NULL
                && uhp->uh_alt_next.ptr->uh_walk != nomark
                && uhp->uh_alt_next.ptr->uh_walk != mark) {
       // go to alternate branch if we haven't been there
       uhp = uhp->uh_alt_next.ptr;
     } else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
                // go up in the tree if we haven't been there and we are at the
                // start of alternate branches
                && 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) {
     semsg(_("E830: Undo number %" PRId64 " not found"), (int64_t)step);
     return;
   }

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

   target = closest_seq;
   dosec = false;
   dofile = false;
   if (step < 0) {
     above = true;             // stop above the header
   }
 }

target_zero:
 // If we found it: Follow the path to go to where we want to be.
 if (uhp != NULL || target == 0) {
   // 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(curbuf, 0);

     uhp = curbuf->b_u_curhead;
     if (uhp == NULL) {
       uhp = curbuf->b_u_newhead;
     } else {
       uhp = uhp->uh_next.ptr;
     }
     if (uhp == NULL
         || (target > 0 && uhp->uh_walk != mark)
         || (uhp->uh_seq == target && !above)) {
       break;
     }
     curbuf->b_u_curhead = uhp;
     u_undoredo(true, true);
     if (target > 0) {
       uhp->uh_walk = nomark;          // don't go back down here
     }
   }

   // When back to origin, redo is not needed.
   if (target > 0) {
     // 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(curbuf, 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.
       u_header_T *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, true);

       // 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...
         internal_error("undo_time()");
         break;
       }
     }
   }
 }
 u_undo_end(did_undo, absolute, false);
}

/// 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.
///
/// @param undo If `true`, go up the tree. Down if `false`.
/// @param do_buf_event If `true`, send buffer updates.
static void u_undoredo(bool undo, bool do_buf_event)
{
 char **newarray = NULL;
 linenr_T newlnum = MAXLNUM;
 pos_T new_curpos = curwin->w_cursor;
 u_entry_T *nuep;
 u_entry_T *newlist = NULL;
 fmark_T namedm[NMARKS];
 u_header_T *curhead = curbuf->b_u_curhead;

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

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

 // save marks before undo/redo
 zero_fmark_additional_data(curbuf->b_namedm);
 memmove(namedm, curbuf->b_namedm, sizeof(curbuf->b_namedm[0]) * NMARKS);
 visualinfo_T visualinfo = curbuf->b_visual;
 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 (u_entry_T *uep = curhead->uh_entry; uep != NULL; uep = nuep) {
   linenr_T top = uep->ue_top;
   linenr_T 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) {
     unblock_autocmds();
     iemsg(_("E438: u_undo: line numbers wrong"));
     changed(curbuf);                // don't want UNCHANGED now
     return;
   }

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

   // Decide about the cursor position, depending on what text changed.
   // Don't set it yet, it may be invalid if lines are going to be added.
   {
     // 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.
     linenr_T lnum = curhead->uh_cursor.lnum;
     if (lnum >= top && lnum <= top + newsize + 1) {
       new_curpos = curhead->uh_cursor;
       // We don't want other entries to override saved cursor
       // position.
       newlnum = -1;
     } else if (top < newlnum) {
       // Use the first line that actually changed.  Avoids that
       // undoing auto-formatting puts the cursor in the previous
       // line.
       int i;
       for (i = 0; i < newsize && i < oldsize; i++) {
         if (strcmp(uep->ue_array[i], ml_get(top + 1 + (linenr_T)i)) != 0) {
           break;
         }
       }
       if (i == newsize && newlnum == MAXLNUM && uep->ue_next == NULL) {
         newlnum = top;
         new_curpos.lnum = newlnum + 1;
       } else if (i < newsize) {
         newlnum = top + (linenr_T)i;
         new_curpos.lnum = newlnum + 1;
       }
     }
   }

   bool empty_buffer = false;

   // Delete the lines between top and bot and save them in newarray.
   if (oldsize > 0) {
     newarray = xmalloc(sizeof(char *) * (size_t)oldsize);
     // delete backwards, it goes faster in most cases
     int i;
     linenr_T lnum;
     for (lnum = bot - 1, i = oldsize; --i >= 0; lnum--) {
       // what can we do when we run out of memory?
       newarray[i] = u_save_line(lnum);
       // 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);  // ML_DEL_UNDO
     }
   } else {
     newarray = NULL;
   }

   // make sure the cursor is on a valid line after the deletions
   check_cursor_lnum(curwin);

   // Insert the lines in u_array between top and bot.
   if (newsize) {
     int i;
     linenr_T lnum;
     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(1, uep->ue_array[i], true);
       } else {
         ml_append_flags(lnum, uep->ue_array[i], 0, 0);  // ML_APPEND_UNDO
       }
       xfree(uep->ue_array[i]);
     }
     xfree(uep->ue_array);
   }

   // Adjust marks
   if (oldsize != newsize) {
     mark_adjust(top + 1, top + oldsize, MAXLNUM, newsize - oldsize, kExtmarkNOOP);
     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;
     }
   }

   if (oldsize > 0 || newsize > 0) {
     changed_lines(curbuf, top + 1, 0, bot, newsize - oldsize, do_buf_event);
     // When text has been changed, possibly the start of the next line
     // may have SpellCap that should be removed or it needs to be
     // displayed.  Schedule the next line for redrawing just in case.
     if (spell_check_window(curwin) && bot <= curbuf->b_ml.ml_line_count) {
       redrawWinline(curwin, bot);
     }
   }

   // Set the '[ mark.
   curbuf->b_op_start.lnum = MIN(curbuf->b_op_start.lnum, top + 1);
   // Set the '] mark.
   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;
 }

 // Ensure the '[ and '] marks are within bounds.
 curbuf->b_op_start.lnum = MIN(curbuf->b_op_start.lnum, curbuf->b_ml.ml_line_count);
 curbuf->b_op_end.lnum = MIN(curbuf->b_op_end.lnum, curbuf->b_ml.ml_line_count);

 // Adjust Extmarks
 if (undo) {
   for (int i = (int)kv_size(curhead->uh_extmark) - 1; i > -1; i--) {
     extmark_apply_undo(kv_A(curhead->uh_extmark, i), undo);
   }
   // redo
 } else {
   for (int i = 0; i < (int)kv_size(curhead->uh_extmark); i++) {
     extmark_apply_undo(kv_A(curhead->uh_extmark, i), undo);
   }
 }
 if (curhead->uh_flags & UH_RELOAD) {
   // TODO(bfredl): this is a bit crude. When 'undoreload' is used we
   // should have all info to send a buffer-reloaing on_lines/on_bytes event
   buf_updates_unload(curbuf, true);
 }
 // Finish adjusting extmarks

 // Set the cursor to the desired position.  Check that the line is valid.
 curwin->w_cursor = new_curpos;
 check_cursor_lnum(curwin);

 curhead->uh_entry = newlist;
 curhead->uh_flags = new_flags;
 if ((old_flags & UH_EMPTYBUF) && buf_is_empty(curbuf)) {
   curbuf->b_ml.ml_flags |= ML_EMPTY;
 }
 if (old_flags & UH_CHANGED) {
   changed(curbuf);
 } else {
   unchanged(curbuf, false, true);
 }

 // because the calls to changed()/unchanged() above will bump changedtick
 // again, we need to send a nvim_buf_lines_event with just the new value of
 // b:changedtick
 if (do_buf_event) {
   buf_updates_changedtick(curbuf);
 }

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

 // 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 (curwin->w_cursor.lnum <= curbuf->b_ml.ml_line_count) {
   if (curhead->uh_cursor.lnum == curwin->w_cursor.lnum) {
     curwin->w_cursor.col = curhead->uh_cursor.col;
     if (virtual_active(curwin) && curhead->uh_cursor_vcol >= 0) {
       coladvance(curwin, curhead->uh_cursor_vcol);
     } else {
       curwin->w_cursor.coladd = 0;
     }
   } else {
     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;
   curwin->w_cursor.coladd = 0;
 }

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

 // 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 = curhead->uh_next.ptr
                         ? curhead->uh_next.ptr->uh_seq : 0;
 }

 // 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;

 unblock_autocmds();
#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).
///
/// @param did_undo  just did an undo
/// @param absolute  used ":undo N"
static void u_undo_end(bool did_undo, bool absolute, bool quiet)
{
 if ((fdo_flags & kOptFdoFlagUndo) && KeyTyped) {
   foldOpenCursor();
 }

 if (quiet
     || global_busy        // no messages 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;
 char *msgstr;
 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");
   }
 }

 u_header_T *uhp;
 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;
 }

 char msgbuf[80];
 if (uhp == NULL) {
   *msgbuf = NUL;
 } else {
   undo_fmt_time(msgbuf, sizeof(msgbuf), uhp->uh_time);
 }

 {
   FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
     if (wp->w_buffer == curbuf && wp->w_p_cole > 0) {
       redraw_later(wp, UPD_NOT_VALID);
     }
   }
 }

 if (VIsual_active) {
   check_pos(curbuf, &VIsual);
 }

 smsg_keep(0, _("%" PRId64 " %s; %s #%" PRId64 "  %s"),
           u_oldcount < 0 ? (int64_t)-u_oldcount : (int64_t)u_oldcount,
           _(msgstr),
           did_undo ? _("before") : _("after"),
           uhp == NULL ? 0 : (int64_t)uhp->uh_seq,
           msgbuf);
}

/// Put the timestamp of an undo header in "buf[buflen]" in a nice format.
void undo_fmt_time(char *buf, size_t buflen, time_t tt)
{
 if (time(NULL) - tt >= 100) {
   struct tm curtime;
   os_localtime_r(&tt, &curtime);
   size_t n;
   if (time(NULL) - tt < (60 * 60 * 12)) {
     // within 12 hours
     n = strftime(buf, buflen, "%H:%M:%S", &curtime);
   } else {
     // longer ago
     n = strftime(buf, buflen, "%Y/%m/%d %H:%M:%S", &curtime);
   }
   if (n == 0) {
     buf[0] = NUL;
   }
 } else {
   int64_t seconds = time(NULL) - tt;
   vim_snprintf(buf, buflen,
                NGETTEXT("%" PRId64 " second ago",
                         "%" PRId64 " seconds ago", (uint32_t)seconds),
                seconds);
 }
}

/// u_sync: stop adding to the current entry list
///
/// @param force  if true, also sync when no_u_sync is set.
void u_sync(bool force)
{
 // Skip it when already synced or syncing is disabled.
 if (curbuf->b_u_synced || (!force && no_u_sync > 0)) {
   return;
 }

 if (get_undolevel(curbuf) < 0) {
   curbuf->b_u_synced = true;  // no entries, nothing to do
 } else {
   u_getbot(curbuf);  // compute ue_bot of previous u_save
   curbuf->b_u_curhead = NULL;
 }
}

/// ":undolist": List the leafs of the undo tree
void ex_undolist(exarg_T *eap)
{
 int changes = 1;

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

 u_header_T *uhp = curbuf->b_u_oldhead;
 while (uhp != NULL) {
   if (uhp->uh_prev.ptr == NULL && uhp->uh_walk != nomark
       && uhp->uh_walk != mark) {
     vim_snprintf(IObuff, IOSIZE, "%6d %7d  ", uhp->uh_seq, changes);
     undo_fmt_time(IObuff + strlen(IObuff), IOSIZE - strlen(IObuff), uhp->uh_time);
     if (uhp->uh_save_nr > 0) {
       while (strlen(IObuff) < 33) {
         xstrlcat(IObuff, " ", IOSIZE);
       }
       vim_snprintf_add(IObuff, IOSIZE, "  %3d", uhp->uh_save_nr);
     }
     GA_APPEND(char *, &ga, xstrdup(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++;
   } else if (uhp->uh_alt_next.ptr != NULL
              && uhp->uh_alt_next.ptr->uh_walk != nomark
              && uhp->uh_alt_next.ptr->uh_walk != mark) {
     // go to alternate branch if we haven't been there
     uhp = uhp->uh_alt_next.ptr;
   } else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
              // go up in the tree if we haven't been there and we are at the
              // start of alternate branches
              && 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--;
     }
   }
 }

 msg_ext_set_kind("list_cmd");
 if (GA_EMPTY(&ga)) {
   msg(_("Nothing to undo"), 0);
 } else {
   sort_strings(ga.ga_data, ga.ga_len);

   msg_start();
   msg_puts_hl(_("number changes  when               saved"), HLF_T, false);
   for (int i = 0; i < ga.ga_len && !got_int; i++) {
     msg_putchar('\n');
     if (got_int) {
       break;
     }
     msg_puts(((const char **)ga.ga_data)[i]);
   }
   msg_end();

   ga_clear_strings(&ga);
 }
}

/// ":undojoin": continue adding to the last entry list
void ex_undojoin(exarg_T *eap)
{
 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 (get_undolevel(curbuf) < 0) {
   return;                 // no entries, nothing to do
 }
 curbuf->b_u_synced = false;  // Append next change to last entry
}

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

/// After reloading a buffer which was saved for 'undoreload': Find the first
/// line that was changed and set the cursor there.
void u_find_first_changed(void)
{
 u_header_T *uhp = curbuf->b_u_newhead;

 if (curbuf->b_u_curhead != NULL || uhp == NULL) {
   return;      // undid something in an autocmd?
 }
 // Check that the last undo block was for the whole file.
 u_entry_T *uep = uhp->uh_entry;
 if (uep->ue_top != 0 || uep->ue_bot != 0) {
   return;
 }

 linenr_T lnum;
 for (lnum = 1; lnum < curbuf->b_ml.ml_line_count
      && lnum <= uep->ue_size; lnum++) {
   if (strcmp(ml_get_buf(curbuf, lnum), uep->ue_array[lnum - 1]) != 0) {
     clearpos(&(uhp->uh_cursor));
     uhp->uh_cursor.lnum = lnum;
     return;
   }
 }
 if (curbuf->b_ml.ml_line_count != uep->ue_size) {
   // lines added or deleted at the end, put the cursor there
   clearpos(&(uhp->uh_cursor));
   uhp->uh_cursor.lnum = lnum;
 }
}

/// Increase the write count, store it in the last undo header, what would be
/// used for "u".
void u_update_save_nr(buf_T *buf)
{
 buf->b_u_save_nr_last++;
 buf->b_u_save_nr_cur = buf->b_u_save_nr_last;
 u_header_T *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(u_header_T *uhp)
{
 for (u_header_T *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(buf_T *buf)
{
 if (buf->b_u_newhead == NULL || buf->b_u_newhead->uh_entry == NULL) {
   iemsg(_(e_undo_list_corrupt));
   return NULL;
 }
 return buf->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(buf_T *buf)
{
 u_entry_T *uep = u_get_headentry(buf);  // check for corrupt undo list
 if (uep == NULL) {
   return;
 }

 uep = buf->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.
   linenr_T extra = buf->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 > buf->b_ml.ml_line_count) {
     iemsg(_(e_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
   }

   buf->b_u_newhead->uh_getbot_entry = NULL;
 }

 buf->b_u_synced = true;
}

/// Free one header "uhp" and its entry list and adjust the pointers.
///
/// @param uhpp  if not NULL reset when freeing this header
static void u_freeheader(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
{
 // 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 (u_header_T *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.
///
/// @param uhpp  if not NULL reset when freeing this header
static void u_freebranch(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
{
 // If this is the top branch we may need to use u_freeheader() to update
 // all the pointers.
 if (uhp == buf->b_u_oldhead) {
   while (buf->b_u_oldhead != NULL) {
     u_freeheader(buf, buf->b_u_oldhead, uhpp);
   }
   return;
 }

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

 u_header_T *next = uhp;
 while (next != NULL) {
   u_header_T *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.
///
/// @param uhpp  if not NULL reset when freeing this header
static void u_freeentries(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
{
 // 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;
 }

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

 kv_destroy(uhp->uh_extmark);

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

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

/// invalidate the undo buffer; called when storage has already been released
void u_clearall(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;
}

/// Free all allocated memory blocks for the buffer 'buf'.
void u_blockfree(buf_T *buf)
{
 while (buf->b_u_oldhead != NULL) {
#ifndef NDEBUG
   u_header_T *previous_oldhead = buf->b_u_oldhead;
#endif

   u_freeheader(buf, buf->b_u_oldhead, NULL);
   assert(buf->b_u_oldhead != previous_oldhead);
 }
 xfree(buf->b_u_line_ptr);
}

/// Free all allocated memory blocks for the buffer 'buf'.
/// and invalidate the undo buffer
void u_clearallandblockfree(buf_T *buf)
{
 u_blockfree(buf);
 u_clearall(buf);
}

/// Save the line "lnum" for the "U" command.
static void u_saveline(buf_T *buf, linenr_T lnum)
{
 if (lnum == buf->b_u_line_lnum) {      // line is already saved
   return;
 }
 if (lnum < 1 || lnum > buf->b_ml.ml_line_count) {  // should never happen
   return;
 }
 u_clearline(buf);
 buf->b_u_line_lnum = lnum;
 if (curwin->w_buffer == buf && curwin->w_cursor.lnum == lnum) {
   buf->b_u_line_colnr = curwin->w_cursor.col;
 } else {
   buf->b_u_line_colnr = 0;
 }
 buf->b_u_line_ptr = u_save_line_buf(buf, lnum);
}

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

 XFREE_CLEAR(buf->b_u_line_ptr);
 buf->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(void)
{
 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, curbuf->b_u_line_lnum - 1,
                  curbuf->b_u_line_lnum + 1, 0, false) == FAIL) {
   return;
 }

 char *oldp = u_save_line(curbuf->b_u_line_lnum);
 ml_replace(curbuf->b_u_line_lnum, curbuf->b_u_line_ptr, true);
 extmark_splice_cols(curbuf, (int)curbuf->b_u_line_lnum - 1, 0, (colnr_T)strlen(oldp),
                     (colnr_T)strlen(curbuf->b_u_line_ptr), kExtmarkUndo);
 changed_bytes(curbuf->b_u_line_lnum, 0);
 xfree(curbuf->b_u_line_ptr);
 curbuf->b_u_line_ptr = oldp;

 colnr_T 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(curwin);
}

/// Allocate memory and copy curbuf line into it.
///
/// @param lnum the line to copy
static char *u_save_line(linenr_T lnum)
{
 return u_save_line_buf(curbuf, lnum);
}

/// Allocate memory and copy line into it
///
/// @param lnum line to copy
/// @param buf buffer to copy from
static char *u_save_line_buf(buf_T *buf, linenr_T lnum)
{
 return xstrdup(ml_get_buf(buf, 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.
///
/// @param buf The buffer to check
///
/// @return true if the buffer has changed
bool bufIsChanged(buf_T *buf)
 FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
{
 // In a "prompt" buffer we do respect 'modified', so that we can control
 // closing the window by setting or resetting that option.
 return (!bt_dontwrite(buf) || bt_prompt(buf))
        && (buf->b_changed || file_ff_differs(buf, true));
}

// Return true if any buffer has changes.  Also buffers that are not written.
bool anyBufIsChanged(void)
 FUNC_ATTR_WARN_UNUSED_RESULT
{
 FOR_ALL_BUFFERS(buf) {
   if (bufIsChanged(buf)) {
     return true;
   }
 }
 return false;
}

/// @see bufIsChanged
/// @return true if the current buffer has changed
bool curbufIsChanged(void)
 FUNC_ATTR_WARN_UNUSED_RESULT
{
 return bufIsChanged(curbuf);
}

/// Append the list of undo blocks to a newly allocated list
///
/// For use in undotree(). Recursive.
///
/// @param[in]  first_uhp  Undo blocks list to start with.
///
/// @return [allocated] List with a representation of undo blocks.
static list_T *u_eval_tree(buf_T *const buf, const u_header_T *const first_uhp)
 FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_RET
{
 list_T *const list = tv_list_alloc(kListLenMayKnow);

 for (const u_header_T *uhp = first_uhp; uhp != NULL; uhp = uhp->uh_prev.ptr) {
   dict_T *const dict = tv_dict_alloc();
   tv_dict_add_nr(dict, S_LEN("seq"), (varnumber_T)uhp->uh_seq);
   tv_dict_add_nr(dict, S_LEN("time"), (varnumber_T)uhp->uh_time);
   if (uhp == buf->b_u_newhead) {
     tv_dict_add_nr(dict, S_LEN("newhead"), 1);
   }
   if (uhp == buf->b_u_curhead) {
     tv_dict_add_nr(dict, S_LEN("curhead"), 1);
   }
   if (uhp->uh_save_nr > 0) {
     tv_dict_add_nr(dict, S_LEN("save"), (varnumber_T)uhp->uh_save_nr);
   }

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

   tv_list_append_dict(list, dict);
 }

 return list;
}

/// "undofile(name)" function
void f_undofile(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
{
 rettv->v_type = VAR_STRING;
 const char *const fname = tv_get_string(&argvars[0]);

 if (*fname == NUL) {
   // If there is no file name there will be no undo file.
   rettv->vval.v_string = NULL;
 } else {
   char *ffname = FullName_save(fname, true);

   if (ffname != NULL) {
     rettv->vval.v_string = u_get_undo_file_name(ffname, false);
   }
   xfree(ffname);
 }
}

/// "undotree(expr)" function
void f_undotree(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
{
 tv_dict_alloc_ret(rettv);

 typval_T *const tv = &argvars[0];
 buf_T *const buf = tv->v_type == VAR_UNKNOWN ? curbuf : get_buf_arg(tv);
 if (buf == NULL) {
   return;
 }

 dict_T *dict = rettv->vval.v_dict;

 tv_dict_add_nr(dict, S_LEN("synced"), (varnumber_T)buf->b_u_synced);
 tv_dict_add_nr(dict, S_LEN("seq_last"), (varnumber_T)buf->b_u_seq_last);
 tv_dict_add_nr(dict, S_LEN("save_last"), (varnumber_T)buf->b_u_save_nr_last);
 tv_dict_add_nr(dict, S_LEN("seq_cur"), (varnumber_T)buf->b_u_seq_cur);
 tv_dict_add_nr(dict, S_LEN("time_cur"), (varnumber_T)buf->b_u_time_cur);
 tv_dict_add_nr(dict, S_LEN("save_cur"), (varnumber_T)buf->b_u_save_nr_cur);

 tv_dict_add_list(dict, S_LEN("entries"), u_eval_tree(buf, buf->b_u_oldhead));
}

// Given the buffer, Return the undo header. If none is set, set one first.
// NULL will be returned if e.g undolevels = -1 (undo disabled)
u_header_T *u_force_get_undo_header(buf_T *buf)
{
 u_header_T *uhp = NULL;
 if (buf->b_u_curhead != NULL) {
   uhp = buf->b_u_curhead;
 } else if (buf->b_u_newhead) {
   uhp = buf->b_u_newhead;
 }
 // Create the first undo header for the buffer
 if (!uhp) {
   // Args are tricky: this means replace empty range by empty range..
   u_savecommon(buf, 0, 1, 1, true);

   uhp = buf->b_u_curhead;
   if (!uhp) {
     uhp = buf->b_u_newhead;
     if (get_undolevel(buf) > 0 && !uhp) {
       abort();
     }
   }
 }
 return uhp;
}