Add -r shortcut to toggle absolute/relative line numbers

CHANGELOG.md

@@ -2,6 +2,10 @@ main
 ====
 
 Improvements:
+- [Issue #134]: Add a single-key shortcut `r` to toggle between
+  absolute-only and relative-only line numbers at runtime. The numeric
+  count buffer is cleared on toggle for consistency with other
+  single-key commands. Help text updated accordingly.
 - [Issue #143]: `ctrl-z` will now send jless to the background
 - `:w[rite] <file>` and `:w[rite]! <file>` can be used to write the
   current input to a file

src/app.rs

@@ -463,6 +463,13 @@ impl App {
                         Key::End => Some(Action::FocusBottom),
                         Key::Char('%') => Some(Action::FocusMatchingPair),
                         Key::Char('m') => Some(Action::ToggleMode),
+                        Key::Char('r') => {
+                            crate::screenwriter::toggle_line_number_mode(
+                                &mut self.screen_writer.show_line_numbers,
+                                &mut self.screen_writer.show_relative_line_numbers,
+                            );
+                            None
+                        }
                         Key::Char('<') => {
                             self.screen_writer
                                 .decrease_indentation_level(self.viewer.flatjson.2 as u16);

src/jless.help

@@ -235,6 +235,9 @@
       is from the currently focused line. This makes it easier to use the j
       and k commands with specified counts.
 
+      Press 'r' at any time to toggle between absolute-only and
+      relative-only line numbers.
+
       The appearance of line numbers can be configured via command line flags:
 
   -n, --line-numbers              Show absolute line numbers.

src/screenwriter.rs

@@ -570,3 +570,39 @@ impl ScreenWriter {
         }
     }
 }
+
+// Toggle between absolute-only and relative-only line numbers.
+// If both flags are the same (both true or both false), default to absolute-only.
+pub(crate) fn toggle_line_number_mode(
+    show_absolute_line_numbers: &mut bool,
+    show_relative_line_numbers: &mut bool,
+) {
+    if *show_absolute_line_numbers == *show_relative_line_numbers {
+        // If both are the same (both true or both false), normalize to absolute-only.
+        *show_absolute_line_numbers = true;
+        *show_relative_line_numbers = false;
+    } else {
+        // Otherwise, flip to the other exclusive mode.
+        *show_absolute_line_numbers = !*show_absolute_line_numbers;
+        *show_relative_line_numbers = !*show_relative_line_numbers;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::toggle_line_number_mode;
+
+    #[test]
+    fn toggle_line_number_mode_cases() {
+        for (start_absolute, start_relative, expected_absolute, expected_relative) in vec![
+            (true, false, false, true),
+            (false, true, true, false),
+            (true, true, true, false),
+            (false, false, true, false),
+        ] {
+            let (mut absolute, mut relative) = (start_absolute, start_relative);
+            toggle_line_number_mode(&mut absolute, &mut relative);
+            assert_eq!((absolute, relative), (expected_absolute, expected_relative));
+        }
+    }
+}