text: // This is an open source non-commercial project. Dear PVS-Studio, please check
  // it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
  
  /*
   * 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 <inttypes.h>
  #include <limits.h>
  #include <stdbool.h>
  #include <stdint.h>
  #include <string.h>
  #include <fcntl.h>
  
  #include "nvim/vim.h"
  #include "nvim/ascii.h"
  #include "nvim/undo.h"
  #include "nvim/macros.h"
  #include "nvim/cursor.h"
  #include "nvim/edit.h"
  #include "nvim/eval.h"
  #include "nvim/fileio.h"
  #include "nvim/fold.h"
  #include "nvim/mark.h"
  #include "nvim/memline.h"
  #include "nvim/message.h"
  #include "nvim/misc1.h"
  #include "nvim/memory.h"
  #include "nvim/garray.h"
  #include "nvim/option.h"
  #include "nvim/os_unix.h"
  #include "nvim/path.h"
  #include "nvim/quickfix.h"
  #include "nvim/screen.h"
  #include "nvim/sha256.h"
  #include "nvim/state.h"
  #include "nvim/strings.h"
  #include "nvim/types.h"
  #include "nvim/os/os.h"
  #include "nvim/os/time.h"
  
  #ifdef INCLUDE_GENERATED_DECLARATIONS
  # include "undo.c.generated.h"
  #endif
  
  /* used in undo_end() to report number of added and deleted lines */
  static long u_newcount, u_oldcount;
  
  /*
   * When 'u' flag included in 'cpoptions', we behave like vi.  Need to remember
   * the action that "u" should do.
   */
  static int undo_undoes = FALSE;
  
  static int lastmark = 0;
  
  #if defined(U_DEBUG)
  /*
   * Check the undo structures for being valid.  Print a warning when something
   * looks wrong.
   */
  static int seen_b_u_curhead;
  static int seen_b_u_newhead;
  static int header_count;
  
  static void u_check_tree(u_header_T *uhp,
      u_header_T *exp_uh_next,
      u_header_T *exp_uh_alt_prev) {
    u_entry_T *uep;
  
    if (uhp == NULL)
      return;
    ++header_count;
    if (uhp == curbuf->b_u_curhead && ++seen_b_u_curhead > 1) {
      EMSG("b_u_curhead found twice (looping?)");
      return;
    }
    if (uhp == curbuf->b_u_newhead && ++seen_b_u_newhead > 1) {
      EMSG("b_u_newhead found twice (looping?)");
      return;
    }
  
    if (uhp->uh_magic != UH_MAGIC)
      EMSG("uh_magic wrong (may be using freed memory)");
    else {
      /* Check pointers back are correct. */
      if (uhp->uh_next.ptr != exp_uh_next) {
        EMSG("uh_next wrong");
        smsg("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("expected: 0x%x, actual: 0x%x",
            exp_uh_alt_prev, uhp->uh_alt_prev.ptr);
      }
  
      /* Check the undo tree at this header. */
      for (uep = uhp->uh_entry; uep != NULL; uep = uep->ue_next) {
        if (uep->ue_magic != UE_MAGIC) {
          EMSG("ue_magic wrong (may be using freed memory)");
          break;
        }
      }
  
      /* Check the next alt tree. */
      u_check_tree(uhp->uh_alt_next.ptr, uhp->uh_next.ptr, uhp);
  
      /* Check the next header in this branch. */
      u_check_tree(uhp->uh_prev.ptr, uhp, NULL);
    }
  }
  
  static void u_check(int newhead_may_be_NULL)                 {
    seen_b_u_newhead = 0;
    seen_b_u_curhead = 0;
    header_count = 0;
  
    u_check_tree(curbuf->b_u_oldhead, NULL, NULL);
  
    if (seen_b_u_newhead == 0 && curbuf->b_u_oldhead != NULL
        && !(newhead_may_be_NULL && curbuf->b_u_newhead == NULL))
      EMSGN("b_u_newhead invalid: 0x%x", curbuf->b_u_newhead);
    if (curbuf->b_u_curhead != NULL && seen_b_u_curhead == 0)
      EMSGN("b_u_curhead invalid: 0x%x", curbuf->b_u_curhead);
    if (header_count != curbuf->b_u_numhead) {
      EMSG("b_u_numhead invalid");
      smsg("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)
  {
    if (undo_off)
      return OK;
  
    if (top >= bot || bot > (curbuf->b_ml.ml_line_count + 1)) {
      return FAIL;        /* rely on caller to do error messages */
    }
  
    if (top + 2 == bot)
      u_saveline((linenr_T)(top + 1));
  
    return u_savecommon(top, bot, (linenr_T)0, 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)
  {
    if (undo_off)
      return OK;
  
    return u_savecommon(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)
  {
    if (undo_off)
      return OK;
  
    return u_savecommon(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, long nlines)
  {
    if (undo_off)
      return OK;
  
    return u_savecommon(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(void)
  {
    /* Don't allow changes when 'modifiable' is off.  */
    if (!MODIFIABLE(curbuf)) {
      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) {
      EMSG(_(e_secure));
      return false;
    }
  
    return true;
  }
  
  /// Get the 'undolevels' value for the current buffer.
  static long get_undolevel(void)
  {
    if (curbuf->b_p_ul == NO_LOCAL_UNDOLEVEL) {
      return p_ul;
    }
    return curbuf->b_p_ul;
  }
  
  static inline void zero_fmark_additional_data(fmark_T *fmarks)
  {
    for (size_t i = 0; i < NMARKS; i++) {
      tv_dict_unref(fmarks[i].additional_data);
      fmarks[i].additional_data = NULL;
    }
  }
  
  /*
   * 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(linenr_T top, linenr_T bot, linenr_T newbot, int reload)
  {
    linenr_T lnum;
    long i;
    u_header_T  *uhp;
    u_header_T  *old_curhead;
    u_entry_T   *uep;
    u_entry_T   *prev_uep;
    long size;
  
    if (!reload) {
      /* When making changes is not allowed return FAIL.  It's a crude way
       * to make all change commands fail. */
      if (!undo_allowed())
        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).
       */
      change_warning(0);
      if (bot > curbuf->b_ml.ml_line_count + 1) {
        /* This happens when the FileChangedRO autocommand changes the
         * file in a way it becomes shorter. */
        EMSG(_("E881: Line count changed unexpectedly"));
        return FAIL;
      }
    }
  
  #ifdef U_DEBUG
    u_check(FALSE);
  #endif
  
    size = bot - top - 1;
  
    /*
     * If curbuf->b_u_synced == true make a new header.
     */
    if (curbuf->b_u_synced) {
      /* Need to create new entry in b_changelist. */
      curbuf->b_new_change = true;
  
      if (get_undolevel() >= 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));
  #ifdef U_DEBUG
        uhp->uh_magic = UH_MAGIC;
  #endif
      } else
        uhp = NULL;
  
      /*
       * If we undid more than we redid, move the entry lists before and
       * including curbuf->b_u_curhead to an alternate branch.
       */
      old_curhead = curbuf->b_u_curhead;
      if (old_curhead != NULL) {
        curbuf->b_u_newhead = old_curhead->uh_next.ptr;
        curbuf->b_u_curhead = NULL;
      }
  
      /*
       * free headers to keep the size right
       */
      while (curbuf->b_u_numhead > get_undolevel()
             && curbuf->b_u_oldhead != NULL) {
        u_header_T      *uhfree = curbuf->b_u_oldhead;
  
        if (uhfree == old_curhead)
          /* Can't reconnect the branch, delete all of it. */
          u_freebranch(curbuf, uhfree, &old_curhead);
        else if (uhfree->uh_alt_next.ptr == NULL)
          /* There is no branch, only free one header. */
          u_freeheader(curbuf, uhfree, &old_curhead);
        else {
          /* Free the oldest alternate branch as a whole. */
          while (uhfree->uh_alt_next.ptr != NULL)
            uhfree = uhfree->uh_alt_next.ptr;
          u_freebranch(curbuf, uhfree, &old_curhead);
        }
  #ifdef U_DEBUG
        u_check(TRUE);
  #endif
      }
  
      if (uhp == NULL) {                  /* no undo at all */
        if (old_curhead != NULL)
          u_freebranch(curbuf, old_curhead, NULL);
        curbuf->b_u_synced = false;
        return OK;
      }
  
      uhp->uh_prev.ptr = NULL;
      uhp->uh_next.ptr = curbuf->b_u_newhead;
      uhp->uh_alt_next.ptr = old_curhead;
      if (old_curhead != NULL) {
        uhp->uh_alt_prev.ptr = old_curhead->uh_alt_prev.ptr;
        if (uhp->uh_alt_prev.ptr != NULL)
          uhp->uh_alt_prev.ptr->uh_alt_next.ptr = uhp;
        old_curhead->uh_alt_prev.ptr = uhp;
        if (curbuf->b_u_oldhead == old_curhead)
          curbuf->b_u_oldhead = uhp;
      } else
        uhp->uh_alt_prev.ptr = NULL;
      if (curbuf->b_u_newhead != NULL)
        curbuf->b_u_newhead->uh_prev.ptr = uhp;
  
      uhp->uh_seq = ++curbuf->b_u_seq_last;
      curbuf->b_u_seq_cur = uhp->uh_seq;
      uhp->uh_time = time(NULL);
      uhp->uh_save_nr = 0;
      curbuf->b_u_time_cur = uhp->uh_time + 1;
  
      uhp->uh_walk = 0;
      uhp->uh_entry = NULL;
      uhp->uh_getbot_entry = NULL;
      uhp->uh_cursor = curwin->w_cursor;          /* save cursor pos. for undo */
      if (virtual_active() && 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 = (curbuf->b_changed ? UH_CHANGED : 0) +
                      ((curbuf->b_ml.ml_flags & ML_EMPTY) ? UH_EMPTYBUF : 0);
  
      /* save named marks and Visual marks for undo */
      zero_fmark_additional_data(curbuf->b_namedm);
      memmove(uhp->uh_namedm, curbuf->b_namedm,
              sizeof(curbuf->b_namedm[0]) * NMARKS);
      uhp->uh_visual = curbuf->b_visual;
  
      curbuf->b_u_newhead = uhp;
      if (curbuf->b_u_oldhead == NULL)
        curbuf->b_u_oldhead = uhp;
      ++curbuf->b_u_numhead;
    } else {
      if (get_undolevel() < 0)            /* no undo at all */
        return OK;
  
      /*
       * When saving a single line, and it has been saved just before, it
       * doesn't make sense saving it again.  Saves a lot of memory when
       * making lots of changes inside the same line.
       * This is only possible if the previous change didn't increase or
       * decrease the number of lines.
       * Check the ten last changes.  More doesn't make sense and takes too
       * long.
       */
      if (size == 1) {
        uep = u_get_headentry();
        prev_uep = NULL;
        for (i = 0; i < 10; ++i) {
          if (uep == NULL)
            break;
  
          /* If lines have been inserted/deleted we give up.
           * Also when the line was included in a multi-line save. */
          if ((curbuf->b_u_newhead->uh_getbot_entry != uep
               ? (uep->ue_top + uep->ue_size + 1
                  != (uep->ue_bot == 0
                      ? curbuf->b_ml.ml_line_count + 1
                      : uep->ue_bot))
               : uep->ue_lcount != curbuf->b_ml.ml_line_count)
              || (uep->ue_size > 1
                  && top >= uep->ue_top
                  && top + 2 <= uep->ue_top + uep->ue_size + 1))
            break;
  
          /* If it's the same line we can skip saving it again. */
          if (uep->ue_size == 1 && uep->ue_top == top) {
            if (i > 0) {
              /* It's not the last entry: get ue_bot for the last
               * entry now.  Following deleted/inserted lines go to
               * the re-used entry. */
              u_getbot();
              curbuf->b_u_synced = false;
  
              /* Move the found entry to become the last entry.  The
               * order of undo/redo doesn't matter for the entries
               * we move it over, since they don't change the line
               * count and don't include this line.  It does matter
               * for the found entry if the line count is changed by
               * the executed command. */
              prev_uep->ue_next = uep->ue_next;
              uep->ue_next = curbuf->b_u_newhead->uh_entry;
              curbuf->b_u_newhead->uh_entry = uep;
            }
  
            /* The executed command may change the line count. */
            if (newbot != 0)
              uep->ue_bot = newbot;
            else if (bot > curbuf->b_ml.ml_line_count)
              uep->ue_bot = 0;
            else {
              uep->ue_lcount = curbuf->b_ml.ml_line_count;
              curbuf->b_u_newhead->uh_getbot_entry = uep;
            }
            return OK;
          }
          prev_uep = uep;
          uep = uep->ue_next;
        }
      }
  
      /* find line number for ue_bot for previous u_save() */
      u_getbot();
    }
  
    /*
     * add lines in front of entry list
     */
    uep = xmalloc(sizeof(u_entry_T));
    memset(uep, 0, sizeof(u_entry_T));
  #ifdef U_DEBUG
    uep->ue_magic = UE_MAGIC;
  #endif
  
    uep->ue_size = size;
    uep->ue_top = top;
    if (newbot != 0)
      uep->ue_bot = newbot;
    /*
     * Use 0 for ue_bot if bot is below last line.
     * Otherwise we have to compute ue_bot later.
     */
    else if (bot > curbuf->b_ml.ml_line_count)
      uep->ue_bot = 0;
    else {
      uep->ue_lcount = curbuf->b_ml.ml_line_count;
      curbuf->b_u_newhead->uh_getbot_entry = uep;
    }
  
    if (size > 0) {
      uep->ue_array = xmalloc(sizeof(char_u *) * (size_t)size);
      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(lnum++);
      }
    } else
      uep->ue_array = NULL;
    uep->ue_next = curbuf->b_u_newhead->uh_entry;
    curbuf->b_u_newhead->uh_entry = uep;
    curbuf->b_u_synced = false;
    undo_undoes = FALSE;
  
  #ifdef U_DEBUG
    u_check(FALSE);
  #endif
    return OK;
  }
  
  
  # define UF_START_MAGIC     "Vim\237UnDo\345"  /* magic at start of undofile */
  # define UF_START_MAGIC_LEN     9
  # define UF_HEADER_MAGIC        0x5fd0  /* magic at start of header */
  # define UF_HEADER_END_MAGIC    0xe7aa  /* magic after last header */
  # define UF_ENTRY_MAGIC         0xf518  /* magic at start of entry */
  # define UF_ENTRY_END_MAGIC     0x3581  /* magic after last entry */
  # define UF_VERSION             2       /* 2-byte undofile version number */
  
  /* extra fields for header */
  # define UF_LAST_SAVE_NR        1
  
  /* extra fields for uhp */
  # define UHP_SAVE_NR            1
  
  static char_u e_not_open[] = N_("E828: Cannot open undo file for writing: %s");
  
  /*
   * Compute the hash for the current buffer text into hash[UNDO_HASH_SIZE].
   */
  void u_compute_hash(char_u *hash)
  {
    context_sha256_T ctx;
    linenr_T lnum;
    char_u              *p;
  
    sha256_start(&ctx);
    for (lnum = 1; lnum <= curbuf->b_ml.ml_line_count; ++lnum) {
      p = ml_get(lnum);
      sha256_update(&ctx, p, (uint32_t)(STRLEN(p) + 1));
    }
    sha256_finish(&ctx, hash);
  }
  
  /// Return an allocated string of the full path of the target undofile.
  ///
  /// @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
  {
    char *dirp;
    char dir_name[MAXPATHL + 1];
    char *munged_name = NULL;
    char *undo_file_name = NULL;
    const char *ffname = buf_ffname;
  #ifdef HAVE_READLINK
    char fname_buf[MAXPATHL];
  #endif
  
    if (ffname == NULL) {
      return NULL;
    }
  
  #ifdef HAVE_READLINK
    // Expand symlink in the file name, so that we put the undo file with the
    // actual file instead of with the symlink.
    if (resolve_symlink((const char_u *)ffname, (char_u *)fname_buf) == OK) {
      ffname = fname_buf;
    }
  #endif
  
    // Loop over 'undodir'.  When reading find the first file that exists.
    // When not reading use the first directory that exists or ".".
    dirp = (char *) p_udir;
    while (*dirp != NUL) {
      size_t dir_len = copy_option_part((char_u **)&dirp, (char_u *)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 = (char *) path_tail((char_u *) 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;
        bool has_directory = os_isdir((char_u *)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)) != 0) {
            EMSG3(_("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 *p = munged_name; *p != NUL; mb_ptr_adv(p)) {
              if (vim_ispathsep(*p)) {
                *p = '%';
              }
            }
          }
          undo_file_name = concat_fnames(dir_name, munged_name, true);
        }
      }
  
      // When reading check if the file exists.
      if (undo_file_name != NULL
          && (!reading || os_path_exists((char_u *)undo_file_name))) {
        break;
      }
      xfree(undo_file_name);
      undo_file_name = NULL;
    }
  
    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
  {
    EMSG3(_("E825: Corrupted undo file (%s): %s"), mesg, file_name);
  }
  
  static void u_free_uhp(u_header_T *uhp)
  {
    u_entry_T   *nuep;
    u_entry_T   *uep;
  
    uep = uhp->uh_entry;
    while (uep != NULL) {
      nuep = uep->ue_next;
      u_freeentry(uep, uep->ue_size);
      uep = nuep;
    }
    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, char_u *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, 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);
    return true;
  }
  
  static u_header_T *unserialize_uhp(bufinfo_T *bi,
                                     const char *file_name)
  {
    u_header_T *uhp = xmalloc(sizeof(u_header_T));
    memset(uhp, 0, sizeof(u_header_T));
  #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.
    for (;; ) {
      int len = undo_read_byte(bi);
  
      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) {
          (void)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;
    }
  
    return uhp;
  }
  
  /// 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, 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));
    memset(uep, 0, sizeof(u_entry_T));
  #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_u **array = NULL;
    if (uep->ue_size > 0) {
      if ((size_t)uep->ue_size < SIZE_MAX / sizeof(char_u *)) {
        array = xmalloc(sizeof(char_u *) * (size_t)uep->ue_size);
        memset(array, 0, sizeof(char_u *) * (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_u *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);
    if (pos->lnum < 0) {
      pos->lnum = 0;
    }
    pos->col = undo_read_4c(bi);
    if (pos->col < 0) {
      pos->col = 0;
    }
    pos->coladd = undo_read_4c(bi);
    if (pos->coladd < 0) {
      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,
                    char_u *const hash)
    FUNC_ATTR_NONNULL_ARG(3, 4)
  {
    u_header_T  *uhp;
    char *file_name;
    int mark;
  #ifdef U_DEBUG
    int headers_written = 0;
  #endif
    int fd;
    FILE        *fp = NULL;
    int perm;
    bool write_ok = false;
    bufinfo_T bi;
  
    if (name == NULL) {
      file_name = u_get_undo_file_name((char *) buf->b_ffname, false);
      if (file_name == NULL) {
        if (p_verbose > 0) {
          verbose_enter();
          smsg(_("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.
     */
    perm = 0600;
    if (buf->b_ffname != NULL) {
      perm = os_getperm((const char *)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((char_u *)file_name)) {
      if (name == NULL || !forceit) {
        /* Check we can read it and it's an undo file. */
        fd = os_open(file_name, O_RDONLY, 0);
        if (fd < 0) {
          if (name != NULL || p_verbose > 0) {
            if (name == NULL)
              verbose_enter();
            smsg(_("Will not overwrite with undo file, cannot read: %s"),
                 file_name);
            if (name == NULL)
              verbose_leave();
          }
          goto theend;
        } else {
          char_u mbuf[UF_START_MAGIC_LEN];
          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(_("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((char_u *)_("Skipping undo file write, nothing to undo"));
      goto theend;
    }
  
    fd = os_open(file_name, O_CREAT|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
    if (fd < 0) {
      EMSG2(_(e_not_open), file_name);
      goto theend;
    }
    (void)os_setperm(file_name, perm);
    if (p_verbose > 0) {
      verbose_enter();
      smsg(_("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((char *)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));
    }
  # ifdef HAVE_SELINUX
    if (buf->b_ffname != NULL)
      mch_copy_sec(buf->b_ffname, file_name);
  # endif
  #endif
  
    fp = fdopen(fd, "w");
    if (fp == NULL) {
      EMSG2(_(e_not_open), file_name);
      close(fd);
      os_remove(file_name);
      goto theend;
    }
  
    /* Undo must be synced. */
    u_sync(TRUE);
  
    /*
     * Write the header.
     */
    bi.bi_buf = buf;
    bi.bi_fp = fp;
    if (!serialize_header(&bi, hash)) {
      goto write_error;
    }
  
    /*
     * Iteratively serialize UHPs and their UEPs from the top down.
     */
    mark = ++lastmark;
    uhp = buf->b_u_oldhead;
    while (uhp != NULL) {
      /* Serialize current UHP if we haven't seen it */
      if (uhp->uh_walk != mark) {
        uhp->uh_walk = mark;
  #ifdef U_DEBUG
        ++headers_written;
  #endif
        if (!serialize_uhp(&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) {
      EMSGN("Written %" PRId64 " headers, ...", headers_written);
      EMSGN("... but numhead is %" PRId64, buf->b_u_numhead);
    }
  #endif
  
  write_error:
    fclose(fp);
    if (!write_ok)
      EMSG2(_("E829: write error in undo file: %s"), file_name);
  
  #ifdef HAVE_ACL
    if (buf->b_ffname != NULL) {
      vim_acl_T acl;
  
      /* For systems that support ACL: get the ACL from the original file. */
      acl = mch_get_acl(buf->b_ffname);
      mch_set_acl((char_u *)file_name, acl);
      mch_free_acl(acl);
    }
  #endif
  
  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, char_u *hash, char_u *orig_name)
    FUNC_ATTR_NONNULL_ARG(2)
  {
    u_header_T **uhp_table = NULL;
    char_u *line_ptr = NULL;
  
    char *file_name;
    if (name == NULL) {
      file_name = u_get_undo_file_name((char *) 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((char *)orig_name, &file_info_orig)
          && os_fileinfo((char *)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(_("Not reading undo file, owner differs: %s"),
               file_name);
          verbose_leave();
        }
        return;
      }
  #endif
    } else {
      file_name = (char *) name;
    }
  
    if (p_verbose > 0) {
      verbose_enter();
      smsg(_("Reading undo file: %s"), file_name);
      verbose_leave();
    }
  
    FILE *fp = mch_fopen(file_name, "r");
    if (fp == NULL) {
      if (name != NULL || p_verbose > 0) {
        EMSG2(_("E822: Cannot open undo file for reading: %s"), file_name);
      }
      goto error;
    }
  
    bufinfo_T bi;
    bi.bi_buf = curbuf;
    bi.bi_fp = fp;
  
    // Read the undo file header.
    char_u 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) {
      EMSG2(_("E823: Not an undo file: %s"), file_name);
      goto error;
    }
    int version = get2c(fp);
    if (version != UF_VERSION) {
      EMSG2(_("E824: Incompatible undo file: %s"), file_name);
      goto error;
    }
  
    char_u 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((char_u *)
            _("File contents changed, cannot use undo info"), 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.
    long last_save_nr = 0;
    for (;; ) {
      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) {
            (void)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));
      }
    }
  
    long 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.
    short old_idx = -1, new_idx = -1, 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 <= SHRT_MAX);
        old_idx = (short)i;
        SET_FLAG(i);
      }
      if (new_header_seq > 0 && new_idx < 0 && uhp->uh_seq == new_header_seq) {
        assert(i <= SHRT_MAX);
        new_idx = (short)i;
        SET_FLAG(i);
      }
      if (cur_header_seq > 0 && cur_idx < 0 && uhp->uh_seq == cur_header_seq) {
        assert(i <= SHRT_MAX);
        cur_idx = (short)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) {
        EMSGN("uhp_table entry %" PRId64 " not used, leaking memory", i);
      }
    }
    xfree(uhp_table_used);
    u_check(TRUE);
  #endif
  
    if (name != NULL) {
      smsg(_("Finished reading undo file %s"), file_name);
    }
    goto theend;
  
  error:
    xfree(line_ptr);
    if (uhp_table != NULL) {
      for (long 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)
  {
    return fread(buffer, size, 1, bi->bi_fp) == 1;
  }
  
  /// 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 uint8_t *undo_read_string(bufinfo_T *bi, size_t len)
  {
    uint8_t *ptr = xmallocz(len);
    if (len > 0 && !undo_read(bi, 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);
  }
  
  /*
   * 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);
  }
  
  /// Undo and remove the branch from the undo tree.
  /// Also moves the cursor (as a "normal" undo would).
  bool u_undo_and_forget(int count)
  {
    if (curbuf->b_u_synced == false) {
      u_sync(true);
      count = 1;
    }
    undo_undoes = true;
    u_doit(count, true);
  
    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.
  static void u_doit(int startcount, bool quiet)
  {
    int count = startcount;
  
    if (!undo_allowed())
      return;
  
    u_newcount = 0;
    u_oldcount = 0;
    if (curbuf->b_ml.ml_flags & ML_EMPTY)
      u_oldcount = -1;
    while (count--) {
      /* Do the change warning now, so that it triggers FileChangedRO when
       * needed.  This may cause the file to be reloaded, that must happen
       * before we do anything, because it may change curbuf->b_u_curhead
       * and more. */
      change_warning(0);
  
      if (undo_undoes) {
        if (curbuf->b_u_curhead == NULL)                  /* first undo */
          curbuf->b_u_curhead = curbuf->b_u_newhead;
        else if (get_undolevel() > 0)                     /* multi level undo */
          /* get next undo */
          curbuf->b_u_curhead = curbuf->b_u_curhead->uh_next.ptr;
        /* nothing to undo */
        if (curbuf->b_u_numhead == 0 || curbuf->b_u_curhead == NULL) {
          /* stick curbuf->b_u_curhead at end */
          curbuf->b_u_curhead = curbuf->b_u_oldhead;
          beep_flush();
          if (count == startcount - 1) {
            MSG(_("Already at oldest change"));
            return;
          }
          break;
        }
  
        u_undoredo(true);
      } else {
        if (curbuf->b_u_curhead == NULL || get_undolevel() <= 0) {
          beep_flush();           /* nothing to redo */
          if (count == startcount - 1) {
            MSG(_("Already at newest change"));
            return;
          }
          break;
        }
  
        u_undoredo(FALSE);
  
        /* Advance for next redo.  Set "newhead" when at the end of the
         * redoable changes. */
        if (curbuf->b_u_curhead->uh_prev.ptr == NULL)
          curbuf->b_u_newhead = curbuf->b_u_curhead;
        curbuf->b_u_curhead = curbuf->b_u_curhead->uh_prev.ptr;
      }
    }
    u_undo_end(undo_undoes, false, 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(long step, int sec, int file, int absolute)
  {
    long target;
    long closest;
    long closest_start;
    long closest_seq = 0;
    long val;
    u_header_T      *uhp;
    u_header_T      *last;
    int mark;
    int nomark;
    int round;
    int dosec = sec;
    int dofile = file;
    int above = FALSE;
    int did_undo = TRUE;
  
    /* First make sure the current undoable change is synced. */
    if (curbuf->b_u_synced == false)
      u_sync(TRUE);
  
    u_newcount = 0;
    u_oldcount = 0;
    if (curbuf->b_ml.ml_flags & ML_EMPTY)
      u_oldcount = -1;
  
    /* "target" is the node below which we want to be.
     * Init "closest" to a value we can't reach. */
    if (absolute) {
      if (step == 0) {
        // target 0 does not exist, got to 1 and above it.
        target = 1;
        above = true;
      } else {
        target = step;
      }
      closest = -1;
    } else {
      if (dosec) {
        target = (long)(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) {
        if (target < 0)
          target = 0;
        closest = -1;
      } else {
        if (dosec) {
          closest = (long)(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;
        }
      }
    }
    closest_start = closest;
    closest_seq = curbuf->b_u_seq_cur;
  
    /*
     * May do this twice:
     * 1. Search for "target", update "closest" to the best match found.
     * 2. If "target" not found search for "closest".
     *
     * When using the closest time we use the sequence number in the second
     * round, because there may be several entries with the same time.
     */
    for (round = 1; round <= 2; ++round) {
      /* Find the path from the current state to where we want to go.  The
       * desired state can be anywhere in the undo tree, need to go all over
       * it.  We put "nomark" in uh_walk where we have been without success,
       * "mark" where it could possibly be. */
      mark = ++lastmark;
      nomark = ++lastmark;
  
      if (curbuf->b_u_curhead == NULL)            /* at leaf of the tree */
        uhp = curbuf->b_u_newhead;
      else
        uhp = curbuf->b_u_curhead;
  
      while (uhp != NULL) {
        uhp->uh_walk = mark;
        if (dosec) {
          val = (long)(uhp->uh_time);
        } else if (dofile) {
          val = uhp->uh_save_nr;
        } else {
          val = uhp->uh_seq;
        }
  
        if (round == 1 && !(dofile && val == 0)) {
          /* Remember the header that is closest to the target.
           * It must be at least in the right direction (checked with
           * "b_u_seq_cur").  When the timestamp is equal find the
           * highest/lowest sequence number. */
          if ((step < 0 ? uhp->uh_seq <= curbuf->b_u_seq_cur
               : uhp->uh_seq > curbuf->b_u_seq_cur)
              && ((dosec && val == closest)
                  ? (step < 0
                     ? uhp->uh_seq < closest_seq
                                     : uhp->uh_seq > closest_seq)
                  : closest == closest_start
                  || (val > target
                      ? (closest > target
                         ? val - target <= closest - target
                         : val - target <= target - closest)
                      : (closest > target
                         ? target - val <= closest - target
                         : target - val <= target - closest)))) {
            closest = val;
            closest_seq = uhp->uh_seq;
          }
        }
  
        /* Quit searching when we found a match.  But when searching for a
         * time we need to continue looking for the best uh_seq. */
        if (target == val && !dosec) {
          target = uhp->uh_seq;
          break;
        }
  
        /* go down in the tree if we haven't been there */
        if (uhp->uh_prev.ptr != NULL && uhp->uh_prev.ptr->uh_walk != nomark
            && uhp->uh_prev.ptr->uh_walk != mark)
          uhp = uhp->uh_prev.ptr;
  
        /* go to alternate branch if we haven't been there */
        else if (uhp->uh_alt_next.ptr != NULL
                 && uhp->uh_alt_next.ptr->uh_walk != nomark
                 && uhp->uh_alt_next.ptr->uh_walk != mark)
          uhp = uhp->uh_alt_next.ptr;
  
        /* go up in the tree if we haven't been there and we are at the
         * start of alternate branches */
        else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
                 && uhp->uh_next.ptr->uh_walk != nomark
                 && uhp->uh_next.ptr->uh_walk != mark) {
          /* If still at the start we don't go through this change. */
          if (uhp == curbuf->b_u_curhead)
            uhp->uh_walk = nomark;
          uhp = uhp->uh_next.ptr;
        } else {
          /* need to backtrack; mark this node as useless */
          uhp->uh_walk = nomark;
          if (uhp->uh_alt_prev.ptr != NULL)
            uhp = uhp->uh_alt_prev.ptr;
          else
            uhp = uhp->uh_next.ptr;
        }
      }
  
      if (uhp != NULL)        /* found it */
        break;
  
      if (absolute) {
        EMSGN(_("E830: Undo number %" PRId64 " not found"), step);
        return;
      }
  
      if (closest == closest_start) {
        if (step < 0)
          MSG(_("Already at oldest change"));
        else
          MSG(_("Already at newest change"));
        return;
      }
  
      target = closest_seq;
      dosec = FALSE;
      dofile = FALSE;
      if (step < 0)
        above = TRUE;             /* stop above the header */
    }
  
    /* If we found it: Follow the path to go to where we want to be. */
    if (uhp != NULL) {
      /*
       * First go up the tree as much as needed.
       */
      while (!got_int) {
        /* Do the change warning now, for the same reason as above. */
        change_warning(0);
  
        uhp = curbuf->b_u_curhead;
        if (uhp == NULL)
          uhp = curbuf->b_u_newhead;
        else
          uhp = uhp->uh_next.ptr;
        if (uhp == NULL || uhp->uh_walk != mark
            || (uhp->uh_seq == target && !above))
          break;
        curbuf->b_u_curhead = uhp;
        u_undoredo(TRUE);
        uhp->uh_walk = nomark;            /* don't go back down here */
      }
  
      /*
       * And now go down the tree (redo), branching off where needed.
       */
      while (!got_int) {
        /* Do the change warning now, for the same reason as above. */
        change_warning(0);
  
        uhp = curbuf->b_u_curhead;
        if (uhp == NULL)
          break;
  
        /* Go back to the first branch with a mark. */
        while (uhp->uh_alt_prev.ptr != NULL
               && uhp->uh_alt_prev.ptr->uh_walk == mark)
          uhp = uhp->uh_alt_prev.ptr;
  
        /* Find the last branch with a mark, that's the one. */
        last = uhp;
        while (last->uh_alt_next.ptr != NULL
               && last->uh_alt_next.ptr->uh_walk == mark)
          last = last->uh_alt_next.ptr;
        if (last != uhp) {
          /* Make the used branch the first entry in the list of
           * alternatives to make "u" and CTRL-R take this branch. */
          while (uhp->uh_alt_prev.ptr != NULL)
            uhp = uhp->uh_alt_prev.ptr;
          if (last->uh_alt_next.ptr != NULL)
            last->uh_alt_next.ptr->uh_alt_prev.ptr =
              last->uh_alt_prev.ptr;
          last->uh_alt_prev.ptr->uh_alt_next.ptr = last->uh_alt_next.ptr;
          last->uh_alt_prev.ptr = NULL;
          last->uh_alt_next.ptr = uhp;
          uhp->uh_alt_prev.ptr = last;
  
          if (curbuf->b_u_oldhead == uhp)
            curbuf->b_u_oldhead = last;
          uhp = last;
          if (uhp->uh_next.ptr != NULL)
            uhp->uh_next.ptr->uh_prev.ptr = uhp;
        }
        curbuf->b_u_curhead = uhp;
  
        if (uhp->uh_walk != mark)
          break;              /* must have reached the target */
  
        /* Stop when going backwards in time and didn't find the exact
         * header we were looking for. */
        if (uhp->uh_seq == target && above) {
          curbuf->b_u_seq_cur = target - 1;
          break;
        }
  
        u_undoredo(FALSE);
  
        /* Advance "curhead" to below the header we last used.  If it
        * becomes NULL then we need to set "newhead" to this leaf. */
        if (uhp->uh_prev.ptr == NULL)
          curbuf->b_u_newhead = uhp;
        curbuf->b_u_curhead = uhp->uh_prev.ptr;
        did_undo = FALSE;
  
        if (uhp->uh_seq == target)        /* found it! */
          break;
  
        uhp = uhp->uh_prev.ptr;
        if (uhp == NULL || uhp->uh_walk != mark) {
          // Need to redo more but can't find it...
          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.
   *
   * When "undo" is TRUE we go up in the tree, when FALSE we go down.
   */
  static void u_undoredo(int undo)
  {
    char_u      **newarray = NULL;
    linenr_T oldsize;
    linenr_T newsize;
    linenr_T top, bot;
    linenr_T lnum;
    linenr_T newlnum = MAXLNUM;
    long i;
    u_entry_T   *uep, *nuep;
    u_entry_T   *newlist = NULL;
    int old_flags;
    int new_flags;
    fmark_T namedm[NMARKS];
    visualinfo_T visualinfo;
    int empty_buffer;                         /* buffer became empty */
    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
    old_flags = curhead->uh_flags;
    new_flags = (curbuf->b_changed ? UH_CHANGED : 0) +
                ((curbuf->b_ml.ml_flags & ML_EMPTY) ? UH_EMPTYBUF : 0);
    setpcmark();
  
    /*
     * save marks before undo/redo
     */
    zero_fmark_additional_data(curbuf->b_namedm);
    memmove(namedm, curbuf->b_namedm, sizeof(curbuf->b_namedm[0]) * NMARKS);
    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 (uep = curhead->uh_entry; uep != NULL; uep = nuep) {
      top = uep->ue_top;
      bot = uep->ue_bot;
      if (bot == 0)
        bot = curbuf->b_ml.ml_line_count + 1;
      if (top > curbuf->b_ml.ml_line_count || top >= bot
          || bot > curbuf->b_ml.ml_line_count + 1) {
        unblock_autocmds();
        IEMSG(_("E438: u_undo: line numbers wrong"));
        changed();                // don't want UNCHANGED now
        return;
      }
  
      oldsize = bot - top - 1;        /* number of lines before undo */
      newsize = uep->ue_size;         /* number of lines after undo */
  
      if (top < newlnum) {
        /* If the saved cursor is somewhere in this undo block, move it to
         * the remembered position.  Makes "gwap" put the cursor back
         * where it was. */
        lnum = curhead->uh_cursor.lnum;
        if (lnum >= top && lnum <= top + newsize + 1) {
          curwin->w_cursor = curhead->uh_cursor;
          newlnum = curwin->w_cursor.lnum - 1;
        } else {
          /* Use the first line that actually changed.  Avoids that
           * undoing auto-formatting puts the cursor in the previous
           * line. */
          for (i = 0; i < newsize && i < oldsize; ++i)
            if (STRCMP(uep->ue_array[i], ml_get(top + 1 + i)) != 0)
              break;
          if (i == newsize && newlnum == MAXLNUM && uep->ue_next == NULL) {
            newlnum = top;
            curwin->w_cursor.lnum = newlnum + 1;
          } else if (i < newsize)   {
            newlnum = top + i;
            curwin->w_cursor.lnum = newlnum + 1;
          }
        }
      }
  
      empty_buffer = FALSE;
  
      /* delete the lines between top and bot and save them in newarray */
      if (oldsize > 0) {
        newarray = xmalloc(sizeof(char_u *) * (size_t)oldsize);
        /* delete backwards, it goes faster in most cases */
        for (lnum = bot - 1, i = oldsize; --i >= 0; --lnum) {
          /* what can we do when we run out of memory? */
          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, FALSE);
        }
      } else
        newarray = NULL;
  
      /* insert the lines in u_array between top and bot */
      if (newsize) {
        for (lnum = top, i = 0; i < newsize; ++i, ++lnum) {
          /*
           * If the file is empty, there is an empty line 1 that we
           * should get rid of, by replacing it with the new line
           */
          if (empty_buffer && lnum == 0)
            ml_replace((linenr_T)1, uep->ue_array[i], TRUE);
          else
            ml_append(lnum, uep->ue_array[i], (colnr_T)0, FALSE);
          xfree(uep->ue_array[i]);
        }
        xfree((char_u *)uep->ue_array);
      }
  
      /* adjust marks */
      if (oldsize != newsize) {
        mark_adjust(top + 1, top + oldsize, (long)MAXLNUM,
                    (long)newsize - (long)oldsize, false);
        if (curbuf->b_op_start.lnum > top + oldsize) {
          curbuf->b_op_start.lnum += newsize - oldsize;
        }
        if (curbuf->b_op_end.lnum > top + oldsize) {
          curbuf->b_op_end.lnum += newsize - oldsize;
        }
      }
  
      changed_lines(top + 1, 0, bot, newsize - oldsize);
  
      /* set '[ and '] mark */
      if (top + 1 < curbuf->b_op_start.lnum)
        curbuf->b_op_start.lnum = top + 1;
      if (newsize == 0 && top + 1 > curbuf->b_op_end.lnum)
        curbuf->b_op_end.lnum = top + 1;
      else if (top + newsize > curbuf->b_op_end.lnum)
        curbuf->b_op_end.lnum = top + newsize;
  
      u_newcount += newsize;
      u_oldcount += oldsize;
      uep->ue_size = oldsize;
      uep->ue_array = newarray;
      uep->ue_bot = top + newsize + 1;
  
      /*
       * insert this entry in front of the new entry list
       */
      nuep = uep->ue_next;
      uep->ue_next = newlist;
      newlist = uep;
    }
  
    curhead->uh_entry = newlist;
    curhead->uh_flags = new_flags;
    if ((old_flags & UH_EMPTYBUF) && BUFEMPTY()) {
      curbuf->b_ml.ml_flags |= ML_EMPTY;
    }
    if (old_flags & UH_CHANGED) {
      changed();
    } else {
      unchanged(curbuf, FALSE);
    }
  
    /*
     * restore marks from before undo/redo
     */
    for (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() && curhead->uh_cursor_vcol >= 0)
          coladvance((colnr_T)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();
  
    /* 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).
  static void u_undo_end(
      int did_undo,     ///< just did an undo
      int absolute,     ///< used ":undo N"
      bool quiet)
  {
    char        *msgstr;
    u_header_T  *uhp;
    char_u msgbuf[80];
  
    if ((fdo_flags & FDO_UNDO) && 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;
    if (u_oldcount == -1)
      msgstr = N_("more line");
    else if (u_oldcount < 0)
      msgstr = N_("more lines");
    else if (u_oldcount == 1)
      msgstr = N_("line less");
    else if (u_oldcount > 1)
      msgstr = N_("fewer lines");
    else {
      u_oldcount = u_newcount;
      if (u_newcount == 1)
        msgstr = N_("change");
      else
        msgstr = N_("changes");
    }
  
    if (curbuf->b_u_curhead != NULL) {
      /* For ":undo N" we prefer a "after #N" message. */
      if (absolute && curbuf->b_u_curhead->uh_next.ptr != NULL) {
        uhp = curbuf->b_u_curhead->uh_next.ptr;
        did_undo = FALSE;
      } else if (did_undo)
        uhp = curbuf->b_u_curhead;
      else
        uhp = curbuf->b_u_curhead->uh_next.ptr;
    } else
      uhp = curbuf->b_u_newhead;
  
    if (uhp == NULL)
      *msgbuf = NUL;
    else
      u_add_time(msgbuf, sizeof(msgbuf), uhp->uh_time);
  
    {
      FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
        if (wp->w_buffer == curbuf && wp->w_p_cole > 0) {
          redraw_win_later(wp, NOT_VALID);
        }
      }
    }
  
    smsg(_("%" PRId64 " %s; %s #%" PRId64 "  %s"),
        u_oldcount < 0 ? (int64_t)-u_oldcount : (int64_t)u_oldcount,
        _(msgstr),
        did_undo ? _("before") : _("after"),
        uhp == NULL ? (int64_t)0L : (int64_t)uhp->uh_seq,
        msgbuf);
  }
  
  /*
   * u_sync: stop adding to the current entry list
   */
  void 
  u_sync (
      int force              /* Also sync when no_u_sync is set. */
  )
  {
    /* Skip it when already synced or syncing is disabled. */
    if (curbuf->b_u_synced || (!force && no_u_sync > 0))
      return;
    if (get_undolevel() < 0)
      curbuf->b_u_synced = true;      /* no entries, nothing to do */
    else {
      u_getbot();                     /* compute ue_bot of previous u_save */
      curbuf->b_u_curhead = NULL;
    }
  }
  
  /*
   * ":undolist": List the leafs of the undo tree
   */
  void ex_undolist(exarg_T *eap)
  {
    garray_T ga;
    u_header_T  *uhp;
    int mark;
    int nomark;
    int changes = 1;
  
    /*
     * 1: walk the tree to find all leafs, put the info in "ga".
     * 2: sort the lines
     * 3: display the list
     */
    mark = ++lastmark;
    nomark = ++lastmark;
    ga_init(&ga, (int)sizeof(char *), 20);
  
    uhp = curbuf->b_u_oldhead;
    while (uhp != NULL) {
      if (uhp->uh_prev.ptr == NULL && uhp->uh_walk != nomark
          && uhp->uh_walk != mark) {
        vim_snprintf((char *)IObuff, IOSIZE, "%6ld %7ld  ",
            uhp->uh_seq, changes);
        u_add_time(IObuff + STRLEN(IObuff), IOSIZE - STRLEN(IObuff),
            uhp->uh_time);
        if (uhp->uh_save_nr > 0) {
          while (STRLEN(IObuff) < 33)
            STRCAT(IObuff, " ");
          vim_snprintf_add((char *)IObuff, IOSIZE,
              "  %3ld", uhp->uh_save_nr);
        }
        GA_APPEND(char_u *, &ga, vim_strsave(IObuff));
      }
  
      uhp->uh_walk = mark;
  
      /* go down in the tree if we haven't been there */
      if (uhp->uh_prev.ptr != NULL && uhp->uh_prev.ptr->uh_walk != nomark
          && uhp->uh_prev.ptr->uh_walk != mark) {
        uhp = uhp->uh_prev.ptr;
        ++changes;
      }
      /* go to alternate branch if we haven't been there */
      else if (uhp->uh_alt_next.ptr != NULL
               && uhp->uh_alt_next.ptr->uh_walk != nomark
               && uhp->uh_alt_next.ptr->uh_walk != mark)
        uhp = uhp->uh_alt_next.ptr;
  
      /* go up in the tree if we haven't been there and we are at the
       * start of alternate branches */
      else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
               && uhp->uh_next.ptr->uh_walk != nomark
               && uhp->uh_next.ptr->uh_walk != mark) {
        uhp = uhp->uh_next.ptr;
        --changes;
      } else {
        /* need to backtrack; mark this node as done */
        uhp->uh_walk = nomark;
        if (uhp->uh_alt_prev.ptr != NULL)
          uhp = uhp->uh_alt_prev.ptr;
        else {
          uhp = uhp->uh_next.ptr;
          --changes;
        }
      }
    }
  
    if (GA_EMPTY(&ga))
      MSG(_("Nothing to undo"));
    else {
      sort_strings((char_u **)ga.ga_data, ga.ga_len);
  
      msg_start();
      msg_puts_attr(_("number changes  when               saved"),
                    hl_attr(HLF_T));
      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);
    }
  }
  
  /*
   * Put the timestamp of an undo header in "buf[buflen]" in a nice format.
   */
  static void u_add_time(char_u *buf, size_t buflen, time_t tt)
  {
    struct tm curtime;
  
    if (time(NULL) - tt >= 100) {
      os_localtime_r(&tt, &curtime);
      if (time(NULL) - tt < (60L * 60L * 12L))
        /* within 12 hours */
        (void)strftime((char *)buf, buflen, "%H:%M:%S", &curtime);
      else
        /* longer ago */
        (void)strftime((char *)buf, buflen, "%Y/%m/%d %H:%M:%S", &curtime);
    } else
    vim_snprintf((char *)buf, buflen, _("%" PRId64 " seconds ago"),
        (int64_t)(time(NULL) - tt));
  }
  
  /*
   * ":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() < 0) {
      return;                 // no entries, nothing to do
    } else {
      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;
    u_entry_T   *uep;
    linenr_T lnum;
  
    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. */
    uep = uhp->uh_entry;
    if (uep->ue_top != 0 || uep->ue_bot != 0)
      return;
  
    for (lnum = 1; lnum < curbuf->b_ml.ml_line_count
         && lnum <= uep->ue_size; ++lnum)
      if (STRCMP(ml_get_buf(curbuf, lnum, FALSE),
              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)
  {
    u_header_T  *uhp;
  
    ++buf->b_u_save_nr_last;
    buf->b_u_save_nr_cur = buf->b_u_save_nr_last;
    uhp = buf->b_u_curhead;
    if (uhp != NULL)
      uhp = uhp->uh_next.ptr;
    else
      uhp = buf->b_u_newhead;
    if (uhp != NULL)
      uhp->uh_save_nr = buf->b_u_save_nr_last;
  }
  
  static void u_unch_branch(u_header_T *uhp)
  {
    u_header_T  *uh;
  
    for (uh = uhp; uh != NULL; uh = uh->uh_prev.ptr) {
      uh->uh_flags |= UH_CHANGED;
      if (uh->uh_alt_next.ptr != NULL)
        u_unch_branch(uh->uh_alt_next.ptr);           /* recursive */
    }
  }
  
  /*
   * Get pointer to last added entry.
   * If it's not valid, give an error message and return NULL.
   */
  static u_entry_T *u_get_headentry(void)
  {
    if (curbuf->b_u_newhead == NULL || curbuf->b_u_newhead->uh_entry == NULL) {
      IEMSG(_("E439: undo list corrupt"));
      return NULL;
    }
    return curbuf->b_u_newhead->uh_entry;
  }
  
  /*
   * u_getbot(): compute the line number of the previous u_save
   *		It is called only when b_u_synced is false.
   */
  static void u_getbot(void)
  {
    u_entry_T   *uep;
    linenr_T extra;
  
    uep = u_get_headentry();      /* check for corrupt undo list */
    if (uep == NULL)
      return;
  
    uep = curbuf->b_u_newhead->uh_getbot_entry;
    if (uep != NULL) {
      /*
       * the new ue_bot is computed from the number of lines that has been
       * inserted (0 - deleted) since calling u_save. This is equal to the
       * old line count subtracted from the current line count.
       */
      extra = curbuf->b_ml.ml_line_count - uep->ue_lcount;
      uep->ue_bot = uep->ue_top + uep->ue_size + 1 + extra;
      if (uep->ue_bot < 1 || uep->ue_bot > curbuf->b_ml.ml_line_count) {
        IEMSG(_("E440: undo line missing"));
        uep->ue_bot = uep->ue_top + 1;        // assume all lines deleted, will
                                              // get all the old lines back
                                              // without deleting the current
                                              // ones
      }
  
      curbuf->b_u_newhead->uh_getbot_entry = NULL;
    }
  
    curbuf->b_u_synced = true;
  }
  
  /*
   * Free one header "uhp" and its entry list and adjust the pointers.
   */
  static void 
  u_freeheader (
      buf_T *buf,
      u_header_T *uhp,
      u_header_T **uhpp         /* if not NULL reset when freeing this header */
  )
  {
    u_header_T      *uhap;
  
    /* When there is an alternate redo list free that branch completely,
     * because we can never go there. */
    if (uhp->uh_alt_next.ptr != NULL)
      u_freebranch(buf, uhp->uh_alt_next.ptr, uhpp);
  
    if (uhp->uh_alt_prev.ptr != NULL)
      uhp->uh_alt_prev.ptr->uh_alt_next.ptr = NULL;
  
    /* Update the links in the list to remove the header. */
    if (uhp->uh_next.ptr == NULL)
      buf->b_u_oldhead = uhp->uh_prev.ptr;
    else
      uhp->uh_next.ptr->uh_prev.ptr = uhp->uh_prev.ptr;
  
    if (uhp->uh_prev.ptr == NULL)
      buf->b_u_newhead = uhp->uh_next.ptr;
    else
      for (uhap = uhp->uh_prev.ptr; uhap != NULL;
           uhap = uhap->uh_alt_next.ptr)
        uhap->uh_next.ptr = uhp->uh_next.ptr;
  
    u_freeentries(buf, uhp, uhpp);
  }
  
  /*
   * Free an alternate branch and any following alternate branches.
   */
  static void 
  u_freebranch (
      buf_T *buf,
      u_header_T *uhp,
      u_header_T **uhpp         /* if not NULL reset when freeing this header */
  )
  {
    u_header_T      *tofree, *next;
  
    /* If this is the top branch we may need to use u_freeheader() to update
     * all the pointers. */
    if (uhp == buf->b_u_oldhead) {
      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;
  
    next = uhp;
    while (next != NULL) {
      tofree = next;
      if (tofree->uh_alt_next.ptr != NULL)
        u_freebranch(buf, tofree->uh_alt_next.ptr, uhpp);         /* recursive */
      next = tofree->uh_prev.ptr;
      u_freeentries(buf, tofree, uhpp);
    }
  }
  
  /*
   * Free all the undo entries for one header and the header itself.
   * This means that "uhp" is invalid when returning.
   */
  static void 
  u_freeentries (
      buf_T *buf,
      u_header_T *uhp,
      u_header_T **uhpp         /* if not NULL reset when freeing this header */
  )
  {
    u_entry_T       *uep, *nuep;
  
    /* Check for pointers to the header that become invalid now. */
    if (buf->b_u_curhead == uhp)
      buf->b_u_curhead = NULL;
    if (buf->b_u_newhead == uhp)
      buf->b_u_newhead = NULL;      /* freeing the newest entry */
    if (uhpp != NULL && uhp == *uhpp)
      *uhpp = NULL;
  
    for (uep = uhp->uh_entry; uep != NULL; uep = nuep) {
      nuep = uep->ue_next;
      u_freeentry(uep, uep->ue_size);
    }
  
  #ifdef U_DEBUG
    uhp->uh_magic = 0;
  #endif
    xfree((char_u *)uhp);
    --buf->b_u_numhead;
  }
  
  /*
   * free entry 'uep' and 'n' lines in uep->ue_array[]
   */
  static void u_freeentry(u_entry_T *uep, long n)
  {
    while (n > 0)
      xfree(uep->ue_array[--n]);
    xfree((char_u *)uep->ue_array);
  #ifdef U_DEBUG
    uep->ue_magic = 0;
  #endif
    xfree((char_u *)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;
  }
  
  /*
   * save the line "lnum" for the "U" command
   */
  void u_saveline(linenr_T lnum)
  {
    if (lnum == curbuf->b_u_line_lnum)        /* line is already saved */
      return;
    if (lnum < 1 || lnum > curbuf->b_ml.ml_line_count)   /* should never happen */
      return;
    u_clearline();
    curbuf->b_u_line_lnum = lnum;
    if (curwin->w_cursor.lnum == lnum)
      curbuf->b_u_line_colnr = curwin->w_cursor.col;
    else
      curbuf->b_u_line_colnr = 0;
    curbuf->b_u_line_ptr = u_save_line(lnum);
  }
  
  /*
   * clear the line saved for the "U" command
   * (this is used externally for crossing a line while in insert mode)
   */
  void u_clearline(void)
  {
    if (curbuf->b_u_line_ptr != NULL) {
      xfree(curbuf->b_u_line_ptr);
      curbuf->b_u_line_ptr = NULL;
      curbuf->b_u_line_lnum = 0;
    }
  }
  
  /*
   * Implementation of the "U" command.
   * Differentiation from vi: "U" can be undone with the next "U".
   * We also allow the cursor to be in another line.
   * Careful: may trigger autocommands that reload the buffer.
   */
  void u_undoline(void)
  {
    colnr_T t;
    char_u  *oldp;
  
    if (undo_off)
      return;
  
    if (curbuf->b_u_line_ptr == NULL
        || curbuf->b_u_line_lnum > curbuf->b_ml.ml_line_count) {
      beep_flush();
      return;
    }
  
    /* first save the line for the 'u' command */
    if (u_savecommon(curbuf->b_u_line_lnum - 1,
            curbuf->b_u_line_lnum + 1, (linenr_T)0, FALSE) == FAIL)
      return;
    oldp = u_save_line(curbuf->b_u_line_lnum);
    ml_replace(curbuf->b_u_line_lnum, curbuf->b_u_line_ptr, TRUE);
    changed_bytes(curbuf->b_u_line_lnum, 0);
    xfree(curbuf->b_u_line_ptr);
    curbuf->b_u_line_ptr = oldp;
  
    t = curbuf->b_u_line_colnr;
    if (curwin->w_cursor.lnum == curbuf->b_u_line_lnum)
      curbuf->b_u_line_colnr = curwin->w_cursor.col;
    curwin->w_cursor.col = t;
    curwin->w_cursor.lnum = curbuf->b_u_line_lnum;
    check_cursor_col();
  }
  
  /*
   * Free all allocated memory blocks for the buffer 'buf'.
   */
  void u_blockfree(buf_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);
  }
  
  /*
   * u_save_line(): allocate memory and copy line 'lnum' into it.
   */
  static char_u *u_save_line(linenr_T lnum)
  {
    return vim_strsave(ml_get(lnum));
  }
  
  /// Check if the 'modified' flag is set, or 'ff' has changed (only need to
  /// check the first character, because it can only be "dos", "unix" or "mac").
  /// "nofile" and "scratch" type buffers are considered to always be unchanged.
  ///
  /// @param buf The buffer to check
  ///
  /// @return true if the buffer has changed
  bool bufIsChanged(buf_T *buf)
  {
    return !bt_dontwrite(buf) && (buf->b_changed || file_ff_differs(buf, true));
  }
  
  /// 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.
  ///
  /// @return true if the current buffer has changed
  bool curbufIsChanged(void)
  {
    return (!bt_dontwrite(curbuf)
            && (curbuf->b_changed || file_ff_differs(curbuf, true)));
  }
  
  /// 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.
  list_T *u_eval_tree(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 == curbuf->b_u_newhead) {
        tv_dict_add_nr(dict, S_LEN("newhead"), 1);
      }
      if (uhp == curbuf->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(uhp->uh_alt_next.ptr));
      }
  
      tv_list_append_dict(list, dict);
    }
  
    return list;
  }
  