6 files changed, 59 insertions, 4 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index b0b65545ab..497b4ae9a4 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -1176,6 +1176,29 @@ free_exit:
   return 0;
 }
 
+Dictionary nvim__buf_stats(Buffer buffer, Error *err)
+{
+  Dictionary rv = ARRAY_DICT_INIT;
+
+  buf_T *buf = find_buffer_by_handle(buffer, err);
+  if (!buf) {
+    return rv;
+  }
+
+  // Number of times the cached line was flushed.
+  // This should generally not increase while editing the same
+  // line in the same mode.
+  PUT(rv, "flush_count", INTEGER_OBJ(buf->flush_count));
+  // lnum of current line
+  PUT(rv, "current_lnum", INTEGER_OBJ(buf->b_ml.ml_line_lnum));
+  // whether the line has unflushed changes.
+  PUT(rv, "line_dirty", BOOLEAN_OBJ(buf->b_ml.ml_flags & ML_LINE_DIRTY));
+  // NB: this should be zero at any time API functions are called,
+  // this exists to debug issues
+  PUT(rv, "dirty_bytes", INTEGER_OBJ((Integer)buf->deleted_bytes));
+  return rv;
+}
+
 // Check if deleting lines made the cursor position invalid.
 // Changed lines from `lo` to `hi`; added `extra` lines (negative if deleted).
 static void fix_cursor(linenr_T lo, linenr_T hi, linenr_T extra)
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 143737b478..eb26e4ad8e 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -807,6 +807,9 @@ struct file_buffer {
   kvec_t(uint64_t) update_channels;
   kvec_t(BufUpdateCallbacks) update_callbacks;
 
+  size_t deleted_bytes;
+  int flush_count;
+
   int b_diff_failed;    // internal diff failed for this buffer
 };
 
diff --git a/src/nvim/buffer_updates.c b/src/nvim/buffer_updates.c
index 21efda9fd9..7dea8bfac5 100644
--- a/src/nvim/buffer_updates.c
+++ b/src/nvim/buffer_updates.c
@@ -169,6 +169,8 @@ void buf_updates_send_changes(buf_T *buf,
                               int64_t num_removed,
                               bool send_tick)
 {
+  size_t deleted_bytes = ml_flush_deleted_bytes(buf);
+
   if (!buf_updates_active(buf)) {
     return;
   }
@@ -231,8 +233,8 @@ void buf_updates_send_changes(buf_T *buf,
     bool keep = true;
     if (cb.on_lines != LUA_NOREF) {
       Array args = ARRAY_DICT_INIT;
-      Object items[5];
-      args.size = 5;
+      Object items[6];
+      args.size = 6;
       args.items = items;
 
       // the first argument is always the buffer handle
@@ -250,6 +252,8 @@ void buf_updates_send_changes(buf_T *buf,
       // the last line in the updated range
       args.items[4] = INTEGER_OBJ(firstline - 1 + num_added);
 
+      // byte count of previous contents
+      args.items[5] = INTEGER_OBJ((Integer)deleted_bytes);
       textlock++;
       Object res = executor_exec_lua_cb(cb.on_lines, "lines", args, true);
       textlock--;
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index a164cf47d5..2232de8c1e 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -1755,6 +1755,7 @@ failed:
       ml_delete(curbuf->b_ml.ml_line_count, false);
       linecnt--;
     }
+    curbuf->deleted_bytes = 0;
     linecnt = curbuf->b_ml.ml_line_count - linecnt;
     if (filesize == 0)
       linecnt = 0;
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index b027459706..0b16f86416 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -2403,13 +2403,26 @@ int ml_replace(linenr_T lnum, char_u *line, bool copy)
   if (curbuf->b_ml.ml_mfp == NULL && open_buffer(FALSE, NULL, 0) == FAIL)
     return FAIL;
 
+  bool readlen = true;
+
   if (copy) {
     line = vim_strsave(line);
   }
-  if (curbuf->b_ml.ml_line_lnum != lnum)            /* other line buffered */
+  if (curbuf->b_ml.ml_line_lnum != lnum) {           /* other line buffered */
     ml_flush_line(curbuf);                          /* flush it */
-  else if (curbuf->b_ml.ml_flags & ML_LINE_DIRTY)   /* same line allocated */
+  } else if (curbuf->b_ml.ml_flags & ML_LINE_DIRTY) {  /* same line allocated */
+    // TODO FIXME: see other "TODO FIXME"
+    curbuf->deleted_bytes += STRLEN(curbuf->b_ml.ml_line_ptr)+1;
     xfree(curbuf->b_ml.ml_line_ptr);             /* free it */
+    readlen = false; // already read it.
+  }
+
+  if (readlen) {
+    if (true) { // TODO: buffer updates active
+      curbuf->deleted_bytes += STRLEN(ml_get_buf(curbuf, lnum, false))+1;
+    }
+  }
+
   curbuf->b_ml.ml_line_ptr = line;
   curbuf->b_ml.ml_line_lnum = lnum;
   curbuf->b_ml.ml_flags = (curbuf->b_ml.ml_flags | ML_LINE_DIRTY) & ~ML_EMPTY;
@@ -2491,6 +2504,7 @@ static int ml_delete_int(buf_T *buf, linenr_T lnum, bool message)
   else
     line_size = ((dp->db_index[idx - 1]) & DB_INDEX_MASK) - line_start;
 
+  buf->deleted_bytes += line_size;
 
   /*
    * special case: If there is only one line in the data block it becomes empty.
@@ -2676,6 +2690,13 @@ void ml_clearmarked(void)
   return;
 }
 
+size_t ml_flush_deleted_bytes(buf_T *buf)
+{
+  size_t ret = buf->deleted_bytes;
+  buf->deleted_bytes = 0;
+  return ret;
+}
+
 /*
  * flush ml_line if necessary
  */
@@ -2704,6 +2725,8 @@ static void ml_flush_line(buf_T *buf)
       return;
     entered = TRUE;
 
+    buf->flush_count++;
+
     lnum = buf->b_ml.ml_line_lnum;
     new_line = buf->b_ml.ml_line_ptr;
 
diff --git a/src/nvim/misc1.c b/src/nvim/misc1.c
index db0d56b5fd..112ca6f287 100644
--- a/src/nvim/misc1.c
+++ b/src/nvim/misc1.c
@@ -1685,6 +1685,7 @@ int del_bytes(colnr_T count, bool fixpos_arg, bool use_delcombine)
   bool was_alloced = ml_line_alloced();     // check if oldp was allocated
   char_u *newp;
   if (was_alloced) {
+    curbuf->deleted_bytes += (size_t)oldlen+1;
     newp = oldp;                            // use same allocated memory
   } else {                                  // need to allocate a new line
     newp = xmalloc((size_t)(oldlen + 1 - count));