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 <fcntl.h>
  #include <inttypes.h>
  #include <limits.h>
  #include <stdbool.h>
  #include <string.h>
  
  #include "auto/config.h"
  #include "nvim/ascii.h"
  #include "nvim/buffer.h"
  #include "nvim/buffer_updates.h"
  #include "nvim/change.h"
  #include "nvim/cursor.h"
  #include "nvim/edit.h"
  #include "nvim/extmark.h"
  #include "nvim/fileio.h"
  #include "nvim/fold.h"
  #include "nvim/garray.h"
  #include "nvim/getchar.h"
  #include "nvim/lib/kvec.h"
  #include "nvim/mark.h"
  #include "nvim/memline.h"
  #include "nvim/memory.h"
  #include "nvim/message.h"
  #include "nvim/option.h"
  #include "nvim/os/input.h"
  #include "nvim/os/os.h"
  #include "nvim/os/time.h"
  #include "nvim/os_unix.h"
  #include "nvim/path.h"
  #include "nvim/pos.h"  // MAXLNUM
  #include "nvim/sha256.h"
  #include "nvim/state.h"
  #include "nvim/strings.h"
  #include "nvim/types.h"
  #include "nvim/undo.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 bool undo_undoes = false;
  
  static int lastmark = 0;
  
  #if defined(U_DEBUG)
  /*
   * Check the undo structures for being valid.  Print a warning when something
   * looks wrong.
   */
  static int seen_b_u_curhead;
  static int seen_b_u_newhead;
  static int header_count;
  
  static void u_check_tree(u_header_T *uhp, u_header_T *exp_uh_next, u_header_T *exp_uh_alt_prev)
  {
    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)) {
      semsg("b_u_newhead invalid: 0x%x", curbuf->b_u_newhead);
    }
    if (curbuf->b_u_curhead != NULL && seen_b_u_curhead == 0) {
      semsg("b_u_curhead invalid: 0x%x", curbuf->b_u_curhead);
    }
    if (header_count != curbuf->b_u_numhead) {
      emsg("b_u_numhead invalid");
      smsg("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 (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(curbuf, 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)
  {
    return u_savecommon(curbuf, lnum - 1, lnum + 1, lnum + 1, false);
  }
  
  /*
   * A new line is inserted before line "lnum" (used by :s command).
   * The line is inserted, so the new bottom line is lnum + 1.
   * Careful: may trigger autocommands that reload the buffer.
   * Returns FAIL when lines could not be saved, OK otherwise.
   */
  int u_inssub(linenr_T lnum)
  {
    return u_savecommon(curbuf, lnum - 1, lnum, lnum + 1, false);
  }
  
  /*
   * Save the lines "lnum" - "lnum" + nlines (used by delete command).
   * The lines are deleted, so the new bottom line is lnum, unless the buffer
   * becomes empty.
   * Careful: may trigger autocommands that reload the buffer.
   * Returns FAIL when lines could not be saved, OK otherwise.
   */
  int u_savedel(linenr_T lnum, long nlines)
  {
    return u_savecommon(curbuf, lnum - 1, lnum + nlines,
                        nlines == curbuf->b_ml.ml_line_count ? 2 : lnum, false);
  }
  
  /// Return true when undo is allowed. Otherwise print an error message and
  /// return false.
  ///
  /// @return true if undo is allowed.
  bool undo_allowed(buf_T *buf)
  {
    // Don't allow changes when 'modifiable' is off.
    if (!MODIFIABLE(buf)) {
      emsg(_(e_modifiable));
      return false;
    }
  
    // In the sandbox it's not allowed to change the text.
    if (sandbox != 0) {
      emsg(_(e_sandbox));
      return false;
    }
  
    // Don't allow changes in the buffer while editing the cmdline.  The
    // caller of getcmdline() may get confused.
    if (textlock != 0) {
      emsg(_(e_secure));
      return false;
    }
  
    return true;
  }
  
  /// Get the 'undolevels' value for the current buffer.
  static long get_undolevel(buf_T *buf)
  {
    if (buf->b_p_ul == NO_LOCAL_UNDOLEVEL) {
      return p_ul;
    }
    return buf->b_p_ul;
  }
  
  static inline void zero_fmark_additional_data(fmark_T *fmarks)
  {
    for (size_t i = 0; i < NMARKS; i++) {
      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(buf_T *buf, 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(buf)) {
        return FAIL;
      }
  
      // Saving text for undo means we are going to make a change.  Give a
      // warning for a read-only file before making the change, so that the
      // FileChangedRO event can replace the buffer with a read-write version
      // (e.g., obtained from a source control system).
      if (buf == curbuf) {
        change_warning(buf, 0);
      }
  
      if (bot > buf->b_ml.ml_line_count + 1) {
        //  This happens when the FileChangedRO autocommand changes the
        //  file in a way it becomes shorter.
        emsg(_("E881: Line count changed unexpectedly"));
        return FAIL;
      }
    }
  
  #ifdef U_DEBUG
    u_check(FALSE);
  #endif
  
    size = bot - top - 1;
  
    // If curbuf->b_u_synced == true make a new header.
    if (buf->b_u_synced) {
      // Need to create new entry in b_changelist.
      buf->b_new_change = true;
  
      if (get_undolevel(buf) >= 0) {
        // Make a new header entry.  Do this first so that we don't mess
        // up the undo info when out of memory.
        uhp = xmalloc(sizeof(u_header_T));
        kv_init(uhp->uh_extmark);
  #ifdef U_DEBUG
        uhp->uh_magic = UH_MAGIC;
  #endif
      } else {
        uhp = NULL;
      }
  
      /*
       * If we undid more than we redid, move the entry lists before and
       * including curbuf->b_u_curhead to an alternate branch.
       */
      old_curhead = buf->b_u_curhead;
      if (old_curhead != NULL) {
        buf->b_u_newhead = old_curhead->uh_next.ptr;
        buf->b_u_curhead = NULL;
      }
  
      /*
       * free headers to keep the size right
       */
      while (buf->b_u_numhead > get_undolevel(buf)
             && buf->b_u_oldhead != NULL) {
        u_header_T *uhfree = buf->b_u_oldhead;
  
        if (uhfree == old_curhead) {
          // Can't reconnect the branch, delete all of it.
          u_freebranch(buf, uhfree, &old_curhead);
        } else if (uhfree->uh_alt_next.ptr == NULL) {
          // There is no branch, only free one header.
          u_freeheader(buf, uhfree, &old_curhead);
        } else {
          // Free the oldest alternate branch as a whole.
          while (uhfree->uh_alt_next.ptr != NULL) {
            uhfree = uhfree->uh_alt_next.ptr;
          }
          u_freebranch(buf, uhfree, &old_curhead);
        }
  #ifdef U_DEBUG
        u_check(TRUE);
  #endif
      }
  
      if (uhp == NULL) {  // no undo at all
        if (old_curhead != NULL) {
          u_freebranch(buf, old_curhead, NULL);
        }
        buf->b_u_synced = false;
        return OK;
      }
  
      uhp->uh_prev.ptr = NULL;
      uhp->uh_next.ptr = buf->b_u_newhead;
      uhp->uh_alt_next.ptr = old_curhead;
      if (old_curhead != NULL) {
        uhp->uh_alt_prev.ptr = old_curhead->uh_alt_prev.ptr;
  
        if (uhp->uh_alt_prev.ptr != NULL) {
          uhp->uh_alt_prev.ptr->uh_alt_next.ptr = uhp;
        }
  
        old_curhead->uh_alt_prev.ptr = uhp;
  
        if (buf->b_u_oldhead == old_curhead) {
          buf->b_u_oldhead = uhp;
        }
      } else {
        uhp->uh_alt_prev.ptr = NULL;
      }
  
      if (buf->b_u_newhead != NULL) {
        buf->b_u_newhead->uh_prev.ptr = uhp;
      }
  
      uhp->uh_seq = ++buf->b_u_seq_last;
      buf->b_u_seq_cur = uhp->uh_seq;
      uhp->uh_time = time(NULL);
      uhp->uh_save_nr = 0;
      buf->b_u_time_cur = uhp->uh_time + 1;
  
      uhp->uh_walk = 0;
      uhp->uh_entry = NULL;
      uhp->uh_getbot_entry = NULL;
      uhp->uh_cursor = curwin->w_cursor;          // save cursor pos. for undo
      if (virtual_active() && curwin->w_cursor.coladd > 0) {
        uhp->uh_cursor_vcol = getviscol();
      } else {
        uhp->uh_cursor_vcol = -1;
      }
  
      // save changed and buffer empty flag for undo
      uhp->uh_flags = (buf->b_changed ? UH_CHANGED : 0) +
                      ((buf->b_ml.ml_flags & ML_EMPTY) ? UH_EMPTYBUF : 0);
  
      // save named marks and Visual marks for undo
      zero_fmark_additional_data(buf->b_namedm);
      memmove(uhp->uh_namedm, buf->b_namedm,
              sizeof(buf->b_namedm[0]) * NMARKS);
      uhp->uh_visual = buf->b_visual;
  
      buf->b_u_newhead = uhp;
  
      if (buf->b_u_oldhead == NULL) {
        buf->b_u_oldhead = uhp;
      }
      buf->b_u_numhead++;
    } else {
      if (get_undolevel(buf) < 0) {  // no undo at all
        return OK;
      }
  
      /*
       * When saving a single line, and it has been saved just before, it
       * doesn't make sense saving it again.  Saves a lot of memory when
       * making lots of changes inside the same line.
       * This is only possible if the previous change didn't increase or
       * decrease the number of lines.
       * Check the ten last changes.  More doesn't make sense and takes too
       * long.
       */
      if (size == 1) {
        uep = u_get_headentry(buf);
        prev_uep = NULL;
        for (i = 0; i < 10; ++i) {
          if (uep == NULL) {
            break;
          }
  
          // If lines have been inserted/deleted we give up.
          // Also when the line was included in a multi-line save.
          if ((buf->b_u_newhead->uh_getbot_entry != uep
               ? (uep->ue_top + uep->ue_size + 1
                  != (uep->ue_bot == 0
                      ? buf->b_ml.ml_line_count + 1
                      : uep->ue_bot))
                      : uep->ue_lcount != buf->b_ml.ml_line_count)
              || (uep->ue_size > 1
                  && top >= uep->ue_top
                  && top + 2 <= uep->ue_top + uep->ue_size + 1)) {
            break;
          }
  
          // If it's the same line we can skip saving it again.
          if (uep->ue_size == 1 && uep->ue_top == top) {
            if (i > 0) {
              // It's not the last entry: get ue_bot for the last
              // entry now.  Following deleted/inserted lines go to
              // the re-used entry.
              u_getbot(buf);
              buf->b_u_synced = false;
  
              // Move the found entry to become the last entry.  The
              // order of undo/redo doesn't matter for the entries
              // we move it over, since they don't change the line
              // count and don't include this line.  It does matter
              // for the found entry if the line count is changed by
              // the executed command.
              prev_uep->ue_next = uep->ue_next;
              uep->ue_next = buf->b_u_newhead->uh_entry;
              buf->b_u_newhead->uh_entry = uep;
            }
  
            // The executed command may change the line count.
            if (newbot != 0) {
              uep->ue_bot = newbot;
            } else if (bot > buf->b_ml.ml_line_count) {
              uep->ue_bot = 0;
            } else {
              uep->ue_lcount = buf->b_ml.ml_line_count;
              buf->b_u_newhead->uh_getbot_entry = uep;
            }
            return OK;
          }
          prev_uep = uep;
          uep = uep->ue_next;
        }
      }
  
      // find line number for ue_bot for previous u_save()
      u_getbot(buf);
    }
  
    /*
     * add lines in front of entry list
     */
    uep = xmalloc(sizeof(u_entry_T));
    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 > buf->b_ml.ml_line_count) {
      uep->ue_bot = 0;
    } else {
      uep->ue_lcount = buf->b_ml.ml_line_count;
      buf->b_u_newhead->uh_getbot_entry = uep;
    }
  
    if (size > 0) {
      uep->ue_array = xmalloc(sizeof(char_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_buf(buf, lnum++);
      }
    } else {
      uep->ue_array = NULL;
    }
  
    uep->ue_next = buf->b_u_newhead->uh_entry;
    buf->b_u_newhead->uh_entry = uep;
    if (reload) {
      // buffer was reloaded, notify text change subscribers
      curbuf->b_u_newhead->uh_flags |= UH_RELOAD;
    }
    buf->b_u_synced = false;
    undo_undoes = false;
  
  #ifdef U_DEBUG
    u_check(FALSE);
  #endif
    return OK;
  }
  
  
  // magic at start of undofile
  #define UF_START_MAGIC     "Vim\237UnDo\345"
  #define UF_START_MAGIC_LEN     9
  // magic at start of header
  #define UF_HEADER_MAGIC        0x5fd0
  // magic after last header
  #define UF_HEADER_END_MAGIC    0xe7aa
  // magic at start of entry
  #define UF_ENTRY_MAGIC         0xf518
  // magic after last entry
  #define UF_ENTRY_END_MAGIC     0x3581
  
  // 2-byte undofile version number
  #define UF_VERSION             3
  
  // extra fields for header
  #define UF_LAST_SAVE_NR        1
  
  // extra fields for uhp
  #define UHP_SAVE_NR            1
  
  static char_u e_not_open[] = N_("E828: Cannot open undo file for writing: %s");
  
  /// Compute the hash for a buffer text into hash[UNDO_HASH_SIZE].
  ///
  /// @param[in] buf The buffer used to compute the hash
  /// @param[in] hash Array of size UNDO_HASH_SIZE in which to store the value of
  ///                 the hash
  void u_compute_hash(buf_T *buf, char_u *hash)
  {
    context_sha256_T ctx;
    linenr_T lnum;
    char_u *p;
  
    sha256_start(&ctx);
    for (lnum = 1; lnum <= buf->b_ml.ml_line_count; lnum++) {
      p = ml_get_buf(buf, lnum, false);
      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
    char *p;
  
    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;
  
        // Remove trailing pathseps from directory name
        p = &dir_name[dir_len - 1];
        while (vim_ispathsep(*p)) {
          *p-- = 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) {
            semsg(_("E5003: Unable to create directory \"%s\" for undo file: %s"),
                  failed_dir, os_strerror(ret));
            xfree(failed_dir);
          } else {
            has_directory = true;
          }
        }
        if (has_directory) {
          if (munged_name == NULL) {
            munged_name = xstrdup(ffname);
            for (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_CLEAR(undo_file_name);
    }
  
    xfree(munged_name);
    return undo_file_name;
  }
  
  /// Display an error for corrupted undo file
  ///
  /// @param[in]  mesg  Identifier of the corruption kind.
  /// @param[in]  file_name  File in which error occurred.
  static void corruption_error(const char *const mesg, const char *const file_name)
    FUNC_ATTR_NONNULL_ALL
  {
    semsg(_("E825: Corrupted undo file (%s): %s"), mesg, file_name);
  }
  
  static void u_free_uhp(u_header_T *uhp)
  {
    u_entry_T *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);
  
    // Write all extmark undo objects
    for (size_t i = 0; i < kv_size(uhp->uh_extmark); i++) {
      if (!serialize_extmark(bi, kv_A(uhp->uh_extmark, i))) {
        return false;
      }
    }
    undo_write_bytes(bi, (uintmax_t)UF_ENTRY_END_MAGIC, 2);
  
    return true;
  }
  
  static u_header_T *unserialize_uhp(bufinfo_T *bi, const char *file_name)
  {
    u_header_T *uhp = xmalloc(sizeof(u_header_T));
    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 == EOF) {
        corruption_error("truncated", file_name);
        u_free_uhp(uhp);
        return NULL;
      }
      if (len == 0) {
        break;
      }
      int what = undo_read_byte(bi);
      switch (what) {
      case UHP_SAVE_NR:
        uhp->uh_save_nr = undo_read_4c(bi);
        break;
      default:
        // Field not supported, skip it.
        while (--len >= 0) {
          (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;
    }
  
    // Unserialize all extmark undo information
    ExtmarkUndoObject *extup;
    kv_init(uhp->uh_extmark);
  
    while ((c = undo_read_2c(bi)) == UF_ENTRY_MAGIC) {
      bool error = false;
      extup = unserialize_extmark(bi, &error, file_name);
      if (error) {
        kv_destroy(uhp->uh_extmark);
        xfree(extup);
        return NULL;
      }
      kv_push(uhp->uh_extmark, *extup);
      xfree(extup);
    }
    if (c != UF_ENTRY_END_MAGIC) {
      corruption_error("entry end", file_name);
      u_free_uhp(uhp);
      return NULL;
    }
  
    return uhp;
  }
  
  static bool serialize_extmark(bufinfo_T *bi, ExtmarkUndoObject extup)
  {
    if (extup.type == kExtmarkSplice) {
      undo_write_bytes(bi, (uintmax_t)UF_ENTRY_MAGIC, 2);
      undo_write_bytes(bi, (uintmax_t)extup.type, 4);
      if (!undo_write(bi, (uint8_t *)&(extup.data.splice),
                      sizeof(ExtmarkSplice))) {
        return false;
      }
    } else if (extup.type == kExtmarkMove) {
      undo_write_bytes(bi, (uintmax_t)UF_ENTRY_MAGIC, 2);
      undo_write_bytes(bi, (uintmax_t)extup.type, 4);
      if (!undo_write(bi, (uint8_t *)&(extup.data.move), sizeof(ExtmarkMove))) {
        return false;
      }
    }
    // Note: We do not serialize ExtmarkSavePos information, since
    // buffer marktrees are not retained when closing/reopening a file
    return true;
  }
  
  static ExtmarkUndoObject *unserialize_extmark(bufinfo_T *bi, bool *error, const char *filename)
  {
    UndoObjectType type;
    uint8_t *buf = NULL;
    size_t n_elems;
  
    ExtmarkUndoObject *extup = xmalloc(sizeof(ExtmarkUndoObject));
  
    type = (UndoObjectType)undo_read_4c(bi);
    extup->type = type;
    if (type == kExtmarkSplice) {
      n_elems = (size_t)sizeof(ExtmarkSplice) / sizeof(uint8_t);
      buf = xcalloc(n_elems, sizeof(uint8_t));
      if (!undo_read(bi, buf, n_elems)) {
        goto error;
      }
      extup->data.splice = *(ExtmarkSplice *)buf;
    } else if (type == kExtmarkMove) {
      n_elems = (size_t)sizeof(ExtmarkMove) / sizeof(uint8_t);
      buf = xcalloc(n_elems, sizeof(uint8_t));
      if (!undo_read(bi, buf, n_elems)) {
        goto error;
      }
      extup->data.move = *(ExtmarkMove *)buf;
    } else {
      goto error;
    }
  
    xfree(buf);
  
    return extup;
  
  error:
    xfree(extup);
    if (buf) {
      xfree(buf);
    }
    *error = true;
    return NULL;
  }
  
  /// Serializes "uep".
  ///
  /// @param bi  The buffer information
  /// @param uep The undo entry to write
  //
  /// @returns false in case of an error.
  static bool serialize_uep(bufinfo_T *bi, u_entry_T *uep)
  {
    undo_write_bytes(bi, (uintmax_t)uep->ue_top, 4);
    undo_write_bytes(bi, (uintmax_t)uep->ue_bot, 4);
    undo_write_bytes(bi, (uintmax_t)uep->ue_lcount, 4);
    undo_write_bytes(bi, (uintmax_t)uep->ue_size, 4);
  
    for (size_t i = 0; i < (size_t)uep->ue_size; i++) {
      size_t len = STRLEN(uep->ue_array[i]);
      if (!undo_write_bytes(bi, len, 4)) {
        return false;
      }
      if (len > 0 && !undo_write(bi, 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 *)) {  // -V547
        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("%s", _("Cannot write undo file in any directory in 'undodir'"));
          verbose_leave();
        }
        return;
      }
    } else {
      file_name = (char *)name;
    }
  
    /*
     * Decide about the permission to use for the undo file.  If the buffer
     * has a name use the permission of the original file.  Otherwise only
     * allow the user to access the undo file.
     */
    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(_("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) {
      semsg(_(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));
    }
  #endif
  
    fp = fdopen(fd, "w");
    if (fp == NULL) {
      semsg(_(e_not_open), file_name);
      close(fd);
      os_remove(file_name);
      goto theend;
    }
  
    // Undo must be synced.
    u_sync(true);
  
    /*
     * Write the header.
     */
    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) {
      semsg("Written %" PRId64 " headers, ...", (int64_t)headers_written);
      semsg("... but numhead is %" PRId64, (int64_t)buf->b_u_numhead);
    }
  #endif
  
  write_error:
    fclose(fp);
    if (!write_ok) {
      semsg(_("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, const char_u *hash, const char_u *orig_name FUNC_ATTR_UNUSED)
    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((const char *)orig_name, &file_info_orig)
          && os_fileinfo(file_name, &file_info_undo)
          && file_info_orig.stat.st_uid != file_info_undo.stat.st_uid
          && file_info_undo.stat.st_uid != getuid()) {
        if (p_verbose > 0) {
          verbose_enter();
          smsg(_("Not reading undo file, owner differs: %s"),
               file_name);
          verbose_leave();
        }
        return;
      }
  #endif
    } else {
      file_name = name;
    }
  
    if (p_verbose > 0) {
      verbose_enter();
      smsg(_("Reading undo file: %s"), file_name);
      verbose_leave();
    }
  
    FILE *fp = os_fopen(file_name, "r");
    if (fp == NULL) {
      if (name != NULL || p_verbose > 0) {
        semsg(_("E822: Cannot open undo file for reading: %s"), file_name);
      }
      goto error;
    }
  
    bufinfo_T bi;
    bi.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) {
      semsg(_("E823: Not an undo file: %s"), file_name);
      goto error;
    }
    int version = get2c(fp);
    if (version != UF_VERSION) {
      semsg(_("E824: Incompatible undo file: %s"), file_name);
      goto error;
    }
  
    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)) {  // -V547
        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) {
        semsg("uhp_table entry %" PRId64 " not used, leaking memory", (int64_t)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)
  {
    const bool retval = fread(buffer, size, 1, bi->bi_fp) == 1;
    if (!retval) {
      // Error may be checked for only later.  Fill with zeros,
      // so that the reader won't use garbage.
      memset(buffer, 0, size);
    }
    return retval;
  }
  
  /// Reads a string of length "len" from "bi->bi_fd" and appends a zero to it.
  ///
  /// @param len can be zero to allocate an empty line.
  ///
  /// @returns a pointer to allocated memory or NULL in case of an error.
  static 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, true);
  }
  
  /*
   * If 'cpoptions' contains 'u': Repeat the previous undo or redo.
   * If 'cpoptions' does not contain 'u': Always redo.
   */
  void u_redo(int count)
  {
    if (vim_strchr(p_cpo, CPO_UNDO) == NULL) {
      undo_undoes = false;
    }
  
    u_doit(count, false, true);
  }
  
  /// Undo and remove the branch from the undo tree.
  /// Also moves the cursor (as a "normal" undo would).
  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,
           // Don't send nvim_buf_lines_event for u_undo_and_forget().
           false);
  
    if (curbuf->b_u_curhead == NULL) {
      // nothing was undone.
      return false;
    }
  
    // Delete the current redo header
    // set the redo header to the next alternative branch (if any)
    // otherwise we will be in the leaf state
    u_header_T *to_forget = curbuf->b_u_curhead;
    curbuf->b_u_newhead = to_forget->uh_next.ptr;
    curbuf->b_u_curhead = to_forget->uh_alt_next.ptr;
    if (curbuf->b_u_curhead) {
      to_forget->uh_alt_next.ptr = NULL;
      curbuf->b_u_curhead->uh_alt_prev.ptr = to_forget->uh_alt_prev.ptr;
      curbuf->b_u_seq_cur = curbuf->b_u_curhead->uh_next.ptr ?
                            curbuf->b_u_curhead->uh_next.ptr->uh_seq : 0;
    } else if (curbuf->b_u_newhead) {
      curbuf->b_u_seq_cur = curbuf->b_u_newhead->uh_seq;
    }
    if (to_forget->uh_alt_prev.ptr) {
      to_forget->uh_alt_prev.ptr->uh_alt_next.ptr = curbuf->b_u_curhead;
    }
    if (curbuf->b_u_newhead) {
      curbuf->b_u_newhead->uh_prev.ptr = curbuf->b_u_curhead;
    }
    if (curbuf->b_u_seq_last == to_forget->uh_seq) {
      curbuf->b_u_seq_last--;
    }
    u_freebranch(curbuf, to_forget, NULL);
    return true;
  }
  
  /// Undo or redo, depending on `undo_undoes`, `count` times.
  ///
  /// @param startcount How often to undo or redo
  /// @param quiet If `true`, don't show messages
  /// @param do_buf_event If `true`, send the changedtick with the buffer updates
  static void u_doit(int startcount, bool quiet, bool do_buf_event)
  {
    int count = startcount;
  
    if (!undo_allowed(curbuf)) {
      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(curbuf, 0);
  
      if (undo_undoes) {
        if (curbuf->b_u_curhead == NULL) {  // first undo
          curbuf->b_u_curhead = curbuf->b_u_newhead;
        } else if (get_undolevel(curbuf) > 0) {  // multi level undo
          // get next undo
          curbuf->b_u_curhead = curbuf->b_u_curhead->uh_next.ptr;
        }
        // nothing to undo
        if (curbuf->b_u_numhead == 0 || curbuf->b_u_curhead == NULL) {
          // stick curbuf->b_u_curhead at end
          curbuf->b_u_curhead = curbuf->b_u_oldhead;
          beep_flush();
          if (count == startcount - 1) {
            msg(_("Already at oldest change"));
            return;
          }
          break;
        }
  
        u_undoredo(true, do_buf_event);
      } else {
        if (curbuf->b_u_curhead == NULL || get_undolevel(curbuf) <= 0) {
          beep_flush();  // nothing to redo
          if (count == startcount - 1) {
            msg(_("Already at newest change"));
            return;
          }
          break;
        }
  
        u_undoredo(false, do_buf_event);
  
        // Advance for next redo.  Set "newhead" when at the end of the
        // redoable changes.
        if (curbuf->b_u_curhead->uh_prev.ptr == NULL) {
          curbuf->b_u_newhead = curbuf->b_u_curhead;
        }
        curbuf->b_u_curhead = curbuf->b_u_curhead->uh_prev.ptr;
      }
    }
    u_undo_end(undo_undoes, false, quiet);
  }
  
  // Undo or redo over the timeline.
  // When "step" is negative go back in time, otherwise goes forward in time.
  // When "sec" is false make "step" steps, when "sec" is true use "step" as
  // seconds.
  // When "file" is true use "step" as a number of file writes.
  // When "absolute" is true use "step" as the sequence number to jump to.
  // "sec" must be false then.
  void undo_time(long step, bool sec, bool file, bool absolute)
  {
    long target;
    long closest;
    long closest_start;
    long closest_seq = 0;
    long val;
    u_header_T *uhp = NULL;
    u_header_T *last;
    int mark;
    int nomark = 0;  // shut up compiler
    int round;
    bool dosec = sec;
    bool dofile = file;
    bool above = false;
    bool did_undo = true;
  
    // First make sure the current undoable change is synced.
    if (curbuf->b_u_synced == false) {
      u_sync(true);
    }
  
    u_newcount = 0;
    u_oldcount = 0;
    if (curbuf->b_ml.ml_flags & ML_EMPTY) {
      u_oldcount = -1;
    }
  
    // "target" is the node below which we want to be.
    // Init "closest" to a value we can't reach.
    if (absolute) {
      target = step;
      closest = -1;
    } else {
      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;
  
    // When "target" is 0; Back to origin.
    if (target == 0) {
      mark = lastmark;  // avoid that GCC complains
      goto target_zero;
    }
  
    /*
     * May do this twice:
     * 1. Search for "target", update "closest" to the best match found.
     * 2. If "target" not found search for "closest".
     *
     * When using the closest time we use the sequence number in the second
     * round, because there may be several entries with the same time.
     */
    for (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;
        } else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
                   // go up in the tree if we haven't been there and we are at the
                   // start of alternate branches
                   && uhp->uh_next.ptr->uh_walk != nomark
                   && uhp->uh_next.ptr->uh_walk != mark) {
          // If still at the start we don't go through this change.
          if (uhp == curbuf->b_u_curhead) {
            uhp->uh_walk = nomark;
          }
          uhp = uhp->uh_next.ptr;
        } else {
          // need to backtrack; mark this node as useless
          uhp->uh_walk = nomark;
          if (uhp->uh_alt_prev.ptr != NULL) {
            uhp = uhp->uh_alt_prev.ptr;
          } else {
            uhp = uhp->uh_next.ptr;
          }
        }
      }
  
      if (uhp != NULL) {      // found it
        break;
      }
  
      if (absolute) {
        semsg(_("E830: Undo number %" PRId64 " not found"), (int64_t)step);
        return;
      }
  
      if (closest == closest_start) {
        if (step < 0) {
          msg(_("Already at oldest change"));
        } else {
          msg(_("Already at newest change"));
        }
        return;
      }
  
      target = closest_seq;
      dosec = false;
      dofile = false;
      if (step < 0) {
        above = true;             // stop above the header
      }
    }
  
  target_zero:
    // If we found it: Follow the path to go to where we want to be.
    if (uhp != NULL || target == 0) {
      // First go up the tree as much as needed.
      while (!got_int) {
        // Do the change warning now, for the same reason as above.
        change_warning(curbuf, 0);
  
        uhp = curbuf->b_u_curhead;
        if (uhp == NULL) {
          uhp = curbuf->b_u_newhead;
        } else {
          uhp = uhp->uh_next.ptr;
        }
        if (uhp == NULL
            || (target > 0 && uhp->uh_walk != mark)
            || (uhp->uh_seq == target && !above)) {
          break;
        }
        curbuf->b_u_curhead = uhp;
        u_undoredo(true, true);
        if (target > 0) {
          uhp->uh_walk = nomark;          // don't go back down here
        }
      }
  
      // When back to origin, redo is not needed.
      if (target > 0) {
        // And now go down the tree (redo), branching off where needed.
        while (!got_int) {
          // Do the change warning now, for the same reason as above.
          change_warning(curbuf, 0);
  
          uhp = curbuf->b_u_curhead;
          if (uhp == NULL) {
            break;
          }
  
          // Go back to the first branch with a mark.
          while (uhp->uh_alt_prev.ptr != NULL
                 && uhp->uh_alt_prev.ptr->uh_walk == mark) {
            uhp = uhp->uh_alt_prev.ptr;
          }
  
          // Find the last branch with a mark, that's the one.
          last = uhp;
          while (last->uh_alt_next.ptr != NULL
                 && last->uh_alt_next.ptr->uh_walk == mark) {
            last = last->uh_alt_next.ptr;
          }
          if (last != uhp) {
            // Make the used branch the first entry in the list of
            // alternatives to make "u" and CTRL-R take this branch.
            while (uhp->uh_alt_prev.ptr != NULL) {
              uhp = uhp->uh_alt_prev.ptr;
            }
            if (last->uh_alt_next.ptr != NULL) {
              last->uh_alt_next.ptr->uh_alt_prev.ptr = last->uh_alt_prev.ptr;
            }
            last->uh_alt_prev.ptr->uh_alt_next.ptr = last->uh_alt_next.ptr;
            last->uh_alt_prev.ptr = NULL;
            last->uh_alt_next.ptr = uhp;
            uhp->uh_alt_prev.ptr = last;
  
            if (curbuf->b_u_oldhead == uhp) {
              curbuf->b_u_oldhead = last;
            }
            uhp = last;
            if (uhp->uh_next.ptr != NULL) {
              uhp->uh_next.ptr->uh_prev.ptr = uhp;
            }
          }
          curbuf->b_u_curhead = uhp;
  
          if (uhp->uh_walk != mark) {
            break;            // must have reached the target
          }
  
          // Stop when going backwards in time and didn't find the exact
          // header we were looking for.
          if (uhp->uh_seq == target && above) {
            curbuf->b_u_seq_cur = target - 1;
            break;
          }
  
          u_undoredo(false, true);
  
          // Advance "curhead" to below the header we last used.  If it
          // becomes NULL then we need to set "newhead" to this leaf.
          if (uhp->uh_prev.ptr == NULL) {
            curbuf->b_u_newhead = uhp;
          }
          curbuf->b_u_curhead = uhp->uh_prev.ptr;
          did_undo = false;
  
          if (uhp->uh_seq == target) {    // found it!
            break;
          }
  
          uhp = uhp->uh_prev.ptr;
          if (uhp == NULL || uhp->uh_walk != mark) {
            // Need to redo more but can't find it...
            internal_error("undo_time()");
            break;
          }
        }
      }
    }
    u_undo_end(did_undo, absolute, false);
  }
  
  /// u_undoredo: common code for undo and redo
  ///
  /// The lines in the file are replaced by the lines in the entry list at
  /// curbuf->b_u_curhead. The replaced lines in the file are saved in the entry
  /// list for the next undo/redo.
  ///
  /// @param undo If `true`, go up the tree. Down if `false`.
  /// @param do_buf_event If `true`, send buffer updates.
  static void u_undoredo(int undo, bool do_buf_event)
  {
    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;
    bool 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)
                | (old_flags & UH_RELOAD);
    setpcmark();
  
    /*
     * save marks before undo/redo
     */
    zero_fmark_additional_data(curbuf->b_namedm);
    memmove(namedm, curbuf->b_namedm, sizeof(curbuf->b_namedm[0]) * NMARKS);
    visualinfo = 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, kExtmarkNOOP);
        if (curbuf->b_op_start.lnum > top + oldsize) {
          curbuf->b_op_start.lnum += newsize - oldsize;
        }
        if (curbuf->b_op_end.lnum > top + oldsize) {
          curbuf->b_op_end.lnum += newsize - oldsize;
        }
      }
  
      changed_lines(top + 1, 0, bot, newsize - oldsize, do_buf_event);
  
      // 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;
    }
  
    // Adjust Extmarks
    ExtmarkUndoObject undo_info;
    if (undo) {
      for (i = (int)kv_size(curhead->uh_extmark) - 1; i > -1; i--) {
        undo_info = kv_A(curhead->uh_extmark, i);
        extmark_apply_undo(undo_info, undo);
      }
      // redo
    } else {
      for (i = 0; i < (int)kv_size(curhead->uh_extmark); i++) {
        undo_info = kv_A(curhead->uh_extmark, i);
        extmark_apply_undo(undo_info, undo);
      }
    }
    if (curhead->uh_flags & UH_RELOAD) {
      // TODO(bfredl): this is a bit crude. When 'undoreload' is used we
      // should have all info to send a buffer-reloaing on_lines/on_bytes event
      buf_updates_unload(curbuf, true);
    }
    // finish Adjusting extmarks
  
  
    curhead->uh_entry = newlist;
    curhead->uh_flags = new_flags;
    if ((old_flags & UH_EMPTYBUF) && buf_is_empty(curbuf)) {
      curbuf->b_ml.ml_flags |= ML_EMPTY;
    }
    if (old_flags & UH_CHANGED) {
      changed();
    } else {
      unchanged(curbuf, false, true);
    }
  
    // because the calls to changed()/unchanged() above will bump changedtick
    // again, we need to send a nvim_buf_lines_event with just the new value of
    // b:changedtick
    if (do_buf_event) {
      buf_updates_changedtick(curbuf);
    }
  
    /*
     * restore marks from before undo/redo
     */
    for (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).
  ///
  /// @param did_undo  just did an undo
  /// @param absolute  used ":undo N"
  static void u_undo_end(bool did_undo, bool absolute, 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 {
      undo_fmt_time(msgbuf, sizeof(msgbuf), uhp->uh_time);
    }
  
    {
      FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
        if (wp->w_buffer == curbuf && wp->w_p_cole > 0) {
          redraw_later(wp, NOT_VALID);
        }
      }
    }
  
    smsg_attr_keep(0,
                   _("%" PRId64 " %s; %s #%" PRId64 "  %s"),
                   u_oldcount < 0 ? (int64_t)-u_oldcount : (int64_t)u_oldcount,
                   _(msgstr),
                   did_undo ? _("before") : _("after"),
                   uhp == NULL ? (int64_t)0L : (int64_t)uhp->uh_seq,
                   msgbuf);
  }
  
  /// Put the timestamp of an undo header in "buf[buflen]" in a nice format.
  void undo_fmt_time(char_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 {
      int64_t seconds = time(NULL) - tt;
      vim_snprintf((char *)buf, buflen,
                   NGETTEXT("%" PRId64 " second ago",
                            "%" PRId64 " seconds ago", (uint32_t)seconds),
                   seconds);
    }
  }
  
  /// u_sync: stop adding to the current entry list
  ///
  /// @param force  if true, also sync when no_u_sync is set.
  void u_sync(bool force)
  {
    // Skip it when already synced or syncing is disabled.
    if (curbuf->b_u_synced || (!force && no_u_sync > 0)) {
      return;
    }
  
    if (get_undolevel(curbuf) < 0) {
      curbuf->b_u_synced = true;  // no entries, nothing to do
    } else {
      u_getbot(curbuf);  // compute ue_bot of previous u_save
      curbuf->b_u_curhead = NULL;
    }
  }
  
  /*
   * ":undolist": List the leafs of the undo tree
   */
  void ex_undolist(exarg_T *eap)
  {
    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 %7d  ", uhp->uh_seq, changes);
        undo_fmt_time(IObuff + STRLEN(IObuff), IOSIZE - STRLEN(IObuff), uhp->uh_time);
        if (uhp->uh_save_nr > 0) {
          while (STRLEN(IObuff) < 33) {
            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;
      } else if (uhp->uh_next.ptr != NULL && uhp->uh_alt_prev.ptr == NULL
                 // go up in the tree if we haven't been there and we are at the
                 // start of alternate branches
                 && uhp->uh_next.ptr->uh_walk != nomark
                 && uhp->uh_next.ptr->uh_walk != mark) {
        uhp = uhp->uh_next.ptr;
        --changes;
      } else {
        // need to backtrack; mark this node as done
        uhp->uh_walk = nomark;
        if (uhp->uh_alt_prev.ptr != NULL) {
          uhp = uhp->uh_alt_prev.ptr;
        } else {
          uhp = uhp->uh_next.ptr;
          --changes;
        }
      }
    }
  
    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);
    }
  }
  
  /*
   * ":undojoin": continue adding to the last entry list
   */
  void ex_undojoin(exarg_T *eap)
  {
    if (curbuf->b_u_newhead == NULL) {
      return;                 // nothing changed before
    }
    if (curbuf->b_u_curhead != NULL) {
      emsg(_("E790: undojoin is not allowed after undo"));
      return;
    }
    if (!curbuf->b_u_synced) {
      return;                 // already unsynced
    }
    if (get_undolevel(curbuf) < 0) {
      return;                 // no entries, nothing to do
    } 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(buf_T *buf)
  {
    if (buf->b_u_newhead == NULL || buf->b_u_newhead->uh_entry == NULL) {
      iemsg(_("E439: undo list corrupt"));
      return NULL;
    }
    return buf->b_u_newhead->uh_entry;
  }
  
  /*
   * u_getbot(): compute the line number of the previous u_save
   *              It is called only when b_u_synced is false.
   */
  static void u_getbot(buf_T *buf)
  {
    u_entry_T *uep;
    linenr_T extra;
  
    uep = u_get_headentry(buf);  // check for corrupt undo list
    if (uep == NULL) {
      return;
    }
  
    uep = buf->b_u_newhead->uh_getbot_entry;
    if (uep != NULL) {
      /*
       * the new ue_bot is computed from the number of lines that has been
       * inserted (0 - deleted) since calling u_save. This is equal to the
       * old line count subtracted from the current line count.
       */
      extra = buf->b_ml.ml_line_count - uep->ue_lcount;
      uep->ue_bot = uep->ue_top + uep->ue_size + 1 + extra;
      if (uep->ue_bot < 1 || uep->ue_bot > buf->b_ml.ml_line_count) {
        iemsg(_("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
      }
  
      buf->b_u_newhead->uh_getbot_entry = NULL;
    }
  
    buf->b_u_synced = true;
  }
  
  /// Free one header "uhp" and its entry list and adjust the pointers.
  ///
  /// @param uhpp  if not NULL reset when freeing this header
  static void u_freeheader(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
  {
    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.
  ///
  /// @param uhpp  if not NULL reset when freeing this header
  static void u_freebranch(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
  {
    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.
  ///
  /// @param uhpp  if not NULL reset when freeing this header
  static void u_freeentries(buf_T *buf, u_header_T *uhp, u_header_T **uhpp)
  {
    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);
    }
  
    kv_destroy(uhp->uh_extmark);
  
  #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_CLEAR(curbuf->b_u_line_ptr);
      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 (curbuf->b_u_line_ptr == NULL
        || curbuf->b_u_line_lnum > curbuf->b_ml.ml_line_count) {
      beep_flush();
      return;
    }
  
    // first save the line for the 'u' command
    if (u_savecommon(curbuf, curbuf->b_u_line_lnum - 1,
                     curbuf->b_u_line_lnum + 1, (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);
    extmark_splice_cols(curbuf, (int)curbuf->b_u_line_lnum-1, 0, (colnr_T)STRLEN(oldp),
                        (colnr_T)STRLEN(curbuf->b_u_line_ptr), kExtmarkUndo);
    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);
  }
  
  /// Allocate memory and copy curbuf line into it.
  ///
  /// @param lnum the line to copy
  static char_u *u_save_line(linenr_T lnum)
  {
    return u_save_line_buf(curbuf, lnum);
  }
  
  /// Allocate memory and copy line into it
  ///
  /// @param lnum line to copy
  /// @param buf buffer to copy from
  static char_u *u_save_line_buf(buf_T *buf, linenr_T lnum)
  {
    return vim_strsave(ml_get_buf(buf, lnum, false));
  }
  
  /// Check if the 'modified' flag is set, or 'ff' has changed (only need to
  /// check the first character, because it can only be "dos", "unix" or "mac").
  /// "nofile" and "scratch" type buffers are considered to always be unchanged.
  ///
  /// @param buf The buffer to check
  ///
  /// @return true if the buffer has changed
  bool bufIsChanged(buf_T *buf)
    FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
  {
    // In a "prompt" buffer we do respect 'modified', so that we can control
    // closing the window by setting or resetting that option.
    return (!bt_dontwrite(buf) || bt_prompt(buf))
           && (buf->b_changed || file_ff_differs(buf, true));
  }
  
  // Return true if any buffer has changes.  Also buffers that are not written.
  bool anyBufIsChanged(void)
    FUNC_ATTR_WARN_UNUSED_RESULT
  {
    FOR_ALL_BUFFERS(buf) {
      if (bufIsChanged(buf)) {
        return true;
      }
    }
    return false;
  }
  
  /// @see bufIsChanged
  /// @return true if the current buffer has changed
  bool curbufIsChanged(void)
    FUNC_ATTR_WARN_UNUSED_RESULT
  {
    return bufIsChanged(curbuf);
  }
  
  /// Append the list of undo blocks to a newly allocated list
  ///
  /// For use in undotree(). Recursive.
  ///
  /// @param[in]  first_uhp  Undo blocks list to start with.
  ///
  /// @return [allocated] List with a representation of undo blocks.
  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;
  }
  
  // Given the buffer, Return the undo header. If none is set, set one first.
  // NULL will be returned if e.g undolevels = -1 (undo disabled)
  u_header_T *u_force_get_undo_header(buf_T *buf)
  {
    u_header_T *uhp = NULL;
    if (buf->b_u_curhead != NULL) {
      uhp = buf->b_u_curhead;
    } else if (buf->b_u_newhead) {
      uhp = buf->b_u_newhead;
    }
    // Create the first undo header for the buffer
    if (!uhp) {
      // Undo is normally invoked in change code, which already has swapped
      // curbuf.
      // Args are tricky: this means replace empty range by empty range..
      u_savecommon(curbuf, 0, 1, 1, true);
  
      uhp = buf->b_u_curhead;
      if (!uhp) {
        uhp = buf->b_u_newhead;
        if (get_undolevel(curbuf) > 0 && !uhp) {
          abort();
        }
      }
    }
    return uhp;
  }
  