Implement offsets for absolute line numbers

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-offset>:
New variable to add an offset to absolute line numbers.
(syms_of_xdisp) <display-line-numbers>: Mention it in docstring.
(maybe_produce_line_number): Use it.

* doc/emacs/display.texi (Display Custom): Document it.

* etc/NEWS (value): Announce it.

* lisp/frame.el: Add `display-line-numbers-offset' to list of
variables which should trigger redisplay of the current buffer.

4 files changed, 48 insertions, 7 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index 406feb8c127..cb37ef448e8 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1855,6 +1855,13 @@ the variable @code{display-line-numbers-widen} to a non-@code{nil}
 value, line numbers will disregard any narrowing and will start at the
 first character of the buffer.
 
+@vindex display-line-numbers-offset
+If the value of @code{display-line-numbers-offset} is non-zero, it is
+added to each absolute line number, and lines are counted from the
+beginning of the buffer, as if @code{display-line-numbers-widen} were
+non-@code{nil}.  It has no effect when set to zero, or when line
+numbers are not absolute.
+
 @vindex display-line-numbers-width-start
 @vindex display-line-numbers-grow-only
 @vindex display-line-numbers-width
diff --git a/etc/NEWS b/etc/NEWS
index 2ca681ff9b9..49aa7f6007b 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -560,11 +560,15 @@ now prompts the user for the directory containing the desktop file.
 
 +++
 ** display-line-numbers-mode
+
 *** New faces 'line-number-major-tick' and 'line-number-minor-tick',
 and customizable variables 'display-line-numbers-major-tick' and
 'display-line-numbers-minor-tick' can be used to highlight the line
 numbers of lines multiple of certain numbers.
 
+*** New variable `display-line-numbers-offset', when non-zero, adds
+an offset to absolute line numbers.
+
 +++
 ** winner
 *** A new variable, 'winner-boring-buffers-regexp', has been added.
diff --git a/lisp/frame.el b/lisp/frame.el
index 51b3b621ff4..018c2f578e4 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2726,6 +2726,7 @@ See also `toggle-frame-maximized'."
         display-line-numbers-widen
         display-line-numbers-major-tick
         display-line-numbers-minor-tick
+        display-line-numbers-offset
         display-fill-column-indicator
         display-fill-column-indicator-column
         display-fill-column-indicator-character
diff --git a/src/xdisp.c b/src/xdisp.c
index 52275a11b86..893ce9269c1 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -22512,10 +22512,22 @@ maybe_produce_line_number (struct it *it)
   ptrdiff_t start_from, bytepos;
   ptrdiff_t this_line;
   bool first_time = false;
-  ptrdiff_t beg_byte = display_line_numbers_widen ? BEG_BYTE : BEGV_BYTE;
-  ptrdiff_t z_byte = display_line_numbers_widen ? Z_BYTE : ZV_BYTE;
+  ptrdiff_t beg_byte;
+  ptrdiff_t z_byte;
+  bool line_numbers_wide;
   void *itdata = bidi_shelve_cache ();
 
+  if (display_line_numbers_offset
+      && !display_line_numbers_widen
+      && !EQ (Vdisplay_line_numbers, Qvisual)
+      && !EQ (Vdisplay_line_numbers, Qrelative))
+    line_numbers_wide = true;
+  else
+    line_numbers_wide = display_line_numbers_widen;
+
+  beg_byte = line_numbers_wide ? BEG_BYTE : BEGV_BYTE;
+  z_byte = line_numbers_wide ? Z_BYTE : ZV_BYTE;
+
   if (EQ (Vdisplay_line_numbers, Qvisual))
     this_line = display_count_lines_visually (it);
   else
@@ -22530,7 +22542,7 @@ maybe_produce_line_number (struct it *it)
                  numbers, so we cannot use its data if the user wants
                  line numbers that disregard narrowing, or if the
                  buffer's narrowing has just changed.  */
-              && !(display_line_numbers_widen
+              && !(line_numbers_wide
                    && (BEG_BYTE != BEGV_BYTE || Z_BYTE != ZV_BYTE))
               && !current_buffer->clip_changed)
             {
@@ -22620,6 +22632,8 @@ maybe_produce_line_number (struct it *it)
     lnum_offset = it->pt_lnum;
   else if (EQ (Vdisplay_line_numbers, Qvisual))
     lnum_offset = 0;
+  else if (display_line_numbers_offset)
+    lnum_offset -= display_line_numbers_offset;
 
   /* Under 'relative', display the absolute line number for the
      current line, unless the user requests otherwise.  */
@@ -34711,12 +34725,18 @@ To add a prefix to continuation lines, use `wrap-prefix'.  */);
 
   DEFVAR_LISP ("display-line-numbers", Vdisplay_line_numbers,
     doc: /* Non-nil means display line numbers.
+
 If the value is t, display the absolute number of each line of a buffer
 shown in a window.  Absolute line numbers count from the beginning of
-the current narrowing, or from buffer beginning.  If the value is
-`relative', display for each line not containing the window's point its
-relative number instead, i.e. the number of the line relative to the
-line showing the window's point.
+the current narrowing, or from buffer beginning.  The variable
+`display-line-numbers-offset', if non-zero, is a signed offset added
+to each absolute line number; it also forces line numbers to be counted
+from the beginning of the buffer, as if `display-line-numbers-wide'
+were non-nil.  It has no effect when line numbers are not absolute.
+
+If the value is `relative', display for each line not containing the
+window's point its relative number instead, i.e. the number of the line
+relative to the line showing the window's point.
 
 In either case, line numbers are displayed at the beginning of each
 non-continuation line that displays buffer text, i.e. after each newline
@@ -34757,6 +34777,15 @@ either `relative' or `visual'.  */);
   DEFSYM (Qdisplay_line_numbers_widen, "display-line-numbers-widen");
   Fmake_variable_buffer_local (Qdisplay_line_numbers_widen);
 
+  DEFVAR_INT ("display-line-numbers-offset", display_line_numbers_offset,
+    doc: /* A signed integer added to each absolute line number.
+When this variable is non-zero, line numbers are always counted from
+the beginning of the buffer even if `display-line-numbers-widen' is nil.
+It has no effect when set to 0, or when line numbers are not absolute.  */);
+  display_line_numbers_offset = 0;
+  DEFSYM (Qdisplay_line_numbers_offset, "display-line-numbers-offset");
+  Fmake_variable_buffer_local (Qdisplay_line_numbers_offset);
+
   DEFVAR_BOOL ("display-fill-column-indicator", Vdisplay_fill_column_indicator,
     doc: /* Non-nil means display the fill column indicator.  */);
   Vdisplay_fill_column_indicator = false;