[troff]: Implement new `.nn` register

...to report the remaining count of lines to have their numbering
suppressed.

* src/roff/troff/env.h (class environment): Declare new member function
  `get_no_number_count()`.

* src/roff/troff/env.cpp (get_no_number_count): Define new member
  function, returning value of member variable `no_number_count`.

  (init_env_requests): Define new ".nn" register and attach it to
  `get_no_number_count()`.

* doc/groff.texi (Miscellaneous): Document it.  Include example of use
  to determine whether current output line will be numbered.  Also
  clarify meaning of register; `.nn` is not decremented except when
  output line numbering is enabled.

* man/groff.7.man (Read-only registers): Document it.

* src/roff/groff/tests/dot-nn_register_works.sh: Test it.
* src/roff/groff/groff.am (groff_TESTS): Run test.

* NEWS: Add item.

See <https://lists.gnu.org/archive/html/groff/2022-11/msg00153.html>.

4 files changed, 85 insertions, 0 deletions

diff --git a/src/roff/groff/groff.am b/src/roff/groff/groff.am
index 874400887..8937d4ced 100644
--- a/src/roff/groff/groff.am
+++ b/src/roff/groff/groff.am
@@ -43,6 +43,7 @@ groff_TESTS = \
   src/roff/groff/tests/do_not_loop_infinitely_when_breaking_cjk.sh \
   src/roff/groff/tests/dot-cp_register_works.sh \
   src/roff/groff/tests/dot-nm_register_works.sh \
+  src/roff/groff/tests/dot-nn_register_works.sh \
   src/roff/groff/tests/evc_produces_no_output_if_invalid.sh \
   src/roff/groff/tests/fp_should_not_traverse_directories.sh \
   src/roff/groff/tests/handle_special_input_code_points.sh \
diff --git a/src/roff/groff/tests/dot-nn_register_works.sh b/src/roff/groff/tests/dot-nn_register_works.sh
new file mode 100755
index 000000000..0998cb05a
--- /dev/null
+++ b/src/roff/groff/tests/dot-nn_register_works.sh
@@ -0,0 +1,77 @@
+#!/bin/sh
+#
+# Copyright (C) 2022 Free Software Foundation, Inc.
+#
+# This file is part of groff.
+#
+# groff is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# groff is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+# for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+groff="${abs_top_builddir:-.}/test-groff"
+
+# Unit test .nn register.
+
+fail=
+
+wail () {
+  echo ...FAILED >&2
+  fail=YES
+}
+
+input='.ec @
+.de is-numbered
+.  nop This line
+.  ie (@@n[.nm] & (1-@@n[.nn])) IS
+.  el                           ISN'"'"'T
+.  nop numbered.
+.  br
+..
+Test line numbering.
+.is-numbered
+.nm 1
+.nn 2
+.is-numbered
+.is-numbered
+.is-numbered
+.nm
+.is-numbered
+.pl @n[nl]u'
+
+# Apply line numbers to the output externally for easy grepping.
+output=$(echo "$input" | $groff -Tascii | nl)
+echo "$output"
+
+echo "verifying that line 1 isn't numbered" >&2
+echo "$output" | \
+  grep -Eq "[[:space:]]+1[[:space:]]+Test line numbering\." || wail
+
+echo "verifying that line 2 isn't numbered" >&2
+echo "$output" | \
+  grep -Eq "[[:space:]]+2[[:space:]]+This line ISN'T" || wail
+
+echo "verifying that line 3 isn't numbered" >&2
+echo "$output" | \
+  grep -Eq "[[:space:]]+3[[:space:]]+This line ISN'T" || wail
+
+echo "verifying that line 4 is numbered" >&2
+echo "$output" | \
+  grep -Eq "[[:space:]]+4[[:space:]]+1 +This line IS numbered" || wail
+
+echo "verifying that line 5 isn't numbered" >&2
+echo "$output" | \
+  grep -Eq "[[:space:]]+5[[:space:]]+This line ISN'T" || wail
+
+test -z "$fail"
+
+# vim:set autoindent expandtab shiftwidth=2 tabstop=2 textwidth=72:
diff --git a/src/roff/troff/env.cpp b/src/roff/troff/env.cpp
index 71bb88581..68aafaa0c 100644
--- a/src/roff/troff/env.cpp
+++ b/src/roff/troff/env.cpp
@@ -417,6 +417,11 @@ int environment::get_right_justify_lines()
   return right_justify_lines;
 }
 
+int environment::get_no_number_count()
+{
+  return no_number_count;
+}
+
 void environment::add_italic_correction()
 {
   if (current_tab) {
@@ -3493,6 +3498,7 @@ void init_env_requests()
   init_string_env_reg(".m", get_glyph_color_string);
   init_hunits_env_reg(".n", get_prev_text_length);
   init_int_env_reg(".nm", get_numbering_nodes);
+  init_int_env_reg(".nn", get_no_number_count);
   init_int_env_reg(".ps", get_point_size);
   init_int_env_reg(".psr", get_requested_point_size);
   init_vunits_env_reg(".pvs", get_post_vertical_spacing);
diff --git a/src/roff/troff/env.h b/src/roff/troff/env.h
index c7c6e2039..f6c1d211c 100644
--- a/src/roff/troff/env.h
+++ b/src/roff/troff/env.h
@@ -311,6 +311,7 @@ public:
   hunits get_hyphenation_margin();
   int get_center_lines();
   int get_right_justify_lines();
+  int get_no_number_count();
   int get_prev_line_interrupted() { return prev_line_interrupted; }
   color *get_fill_color();
   color *get_glyph_color();