Add zero-pad option for numbered chapter titles

A "Zero-pad #" checkbox pads chapter titles that default to their
position number with leading zeros to a uniform width (the digit count
of the largest chapter number, e.g. 03. of 12). Padding never applies to
"# Title:" headers or manual overrides. The setting persists in config.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -69,7 +69,9 @@ Terminal=false
    auto-derived one (the field shows the automatic title as a hint). Leaving it
    blank falls back to a `# Title:` line in the header, and if there is none, to
    the chapter's **position in the left pane** followed by a period (`1.`, `2.`,
-   …). Titles are saved to `titles.json` and sync with git.
+   …). Tick **Zero-pad #** in the top bar to pad those numbered titles with
+   leading zeros to a uniform width (e.g. `03.` when there are 12 chapters).
+   Titles are saved to `titles.json` and sync with git.
 5. Set the **Export** path (type it, or click **📂** to choose the `.odt` file in a
    native save dialog) and press **Export ODT** to produce the concatenated
    document. Each file becomes a chapter headed by its title, optionally followed

src/app.rs

@@ -294,17 +294,28 @@ impl App {
         };
     }
 
+    /// Digits to zero-pad a defaulted chapter number to: the width of the largest
+    /// chapter number when padding is enabled, otherwise 1 (no padding).
+    fn index_pad_width(&self) -> usize {
+        if self.config.zero_pad_index {
+            self.files.len().to_string().len().max(1)
+        } else {
+            1
+        }
+    }
+
     /// The title a chapter at `idx` would get without a manual override: its
     /// `# Title:` header, or its 1-based position (e.g. "3."). Shown as the hint
     /// in the chapter-title field.
     fn auto_title(&self, idx: usize, markdown: &str) -> String {
         let header = crate::preprocess::parse(markdown, &self.config.draft_marker);
-        resolve_chapter_title(None, header.title.as_deref(), idx)
+        resolve_chapter_title(None, header.title.as_deref(), idx, self.index_pad_width())
     }
 
     fn export_odt(&mut self) {
         self.save_current();
         let marker = self.config.draft_marker.clone();
+        let pad_width = self.index_pad_width();
         let mut chapters = Vec::new();
         for (i, name) in self.files.iter().enumerate() {
             let raw = std::fs::read_to_string(self.path_for(name)).unwrap_or_default();
@@ -318,6 +329,7 @@ impl App {
                 self.titles.get(name).map(String::as_str),
                 header.title.as_deref(),
                 i,
+                pad_width,
             );
             chapters.push(Chapter {
                 title,
@@ -471,6 +483,17 @@ impl App {
                 if marker_resp.lost_focus() {
                     self.config.save();
                 }
+                ui.separator();
+                if ui
+                    .checkbox(&mut self.config.zero_pad_index, "Zero-pad #")
+                    .on_hover_text(
+                        "When a chapter title defaults to its number, pad it with \
+                         leading zeros (e.g. 03. of 12).",
+                    )
+                    .changed()
+                {
+                    self.config.save();
+                }
             });
             ui.add_space(4.0);
         });
@@ -815,18 +838,20 @@ fn render_preview(ui: &mut egui::Ui, markdown: &str) {
 }
 
 /// Resolve a chapter's title: a non-empty manual `override_title` wins, then the
-/// `# Title:` header value, otherwise the chapter's 1-based position (e.g. "3.").
+/// `# Title:` header value, otherwise the chapter's 1-based position followed by
+/// a period (e.g. "3."), zero-padded to `pad_width` digits (`1` = no padding).
 fn resolve_chapter_title(
     override_title: Option<&str>,
     header_title: Option<&str>,
     index: usize,
+    pad_width: usize,
 ) -> String {
     override_title
         .map(str::trim)
         .filter(|t| !t.is_empty())
         .or_else(|| header_title.map(str::trim).filter(|t| !t.is_empty()))
         .map(|t| t.to_string())
-        .unwrap_or_else(|| format!("{}.", index + 1))
+        .unwrap_or_else(|| format!("{:0width$}.", index + 1, width = pad_width))
 }
 
 /// Count words in a string: runs of non-whitespace separated by whitespace.
@@ -873,19 +898,33 @@ mod tests {
 
     #[test]
     fn chapter_title_falls_back_to_position() {
-        // No override, no header title -> 1-based index with a period.
-        assert_eq!(resolve_chapter_title(None, None, 0), "1.");
-        assert_eq!(resolve_chapter_title(None, None, 2), "3.");
+        // No override, no header title -> 1-based index with a period (width 1).
+        assert_eq!(resolve_chapter_title(None, None, 0, 1), "1.");
+        assert_eq!(resolve_chapter_title(None, None, 2, 1), "3.");
         // Header title is used when present.
-        assert_eq!(resolve_chapter_title(None, Some("The Gate"), 4), "The Gate");
+        assert_eq!(resolve_chapter_title(None, Some("The Gate"), 4, 1), "The Gate");
         // Override wins over everything, even a header title.
         assert_eq!(
-            resolve_chapter_title(Some("My Override"), Some("The Gate"), 4),
+            resolve_chapter_title(Some("My Override"), Some("The Gate"), 4, 1),
             "My Override"
         );
         // Blank/whitespace override or header title are ignored.
-        assert_eq!(resolve_chapter_title(Some("   "), None, 1), "2.");
-        assert_eq!(resolve_chapter_title(Some(""), Some("  "), 6), "7.");
+        assert_eq!(resolve_chapter_title(Some("   "), None, 1, 1), "2.");
+        assert_eq!(resolve_chapter_title(Some(""), Some("  "), 6, 1), "7.");
+    }
+
+    #[test]
+    fn chapter_number_zero_pads_to_width() {
+        // Width 2 pads single digits; wider numbers are unaffected.
+        assert_eq!(resolve_chapter_title(None, None, 0, 2), "01.");
+        assert_eq!(resolve_chapter_title(None, None, 8, 2), "09.");
+        assert_eq!(resolve_chapter_title(None, None, 11, 2), "12.");
+        assert_eq!(resolve_chapter_title(None, None, 4, 3), "005.");
+        // Padding never applies to a real title.
+        assert_eq!(
+            resolve_chapter_title(None, Some("The Gate"), 0, 3),
+            "The Gate"
+        );
     }
 
     #[test]

src/config.rs

@@ -16,6 +16,10 @@ pub struct Config {
     /// An empty value disables header splitting.
     #[serde(default = "default_marker")]
     pub draft_marker: String,
+    /// When a chapter title defaults to its position number, pad it with leading
+    /// zeros to the width of the largest chapter number (e.g. "03." of 12).
+    #[serde(default)]
+    pub zero_pad_index: bool,
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
@@ -32,6 +36,7 @@ impl Default for Config {
             export_path,
             show_preview: false,
             draft_marker: default_marker(),
+            zero_pad_index: false,
         }
     }
 }