Add ChapterMutThin for parallel chapter access

crates/mdbook-core/src/book.rs

@@ -89,6 +89,14 @@ impl Book {
         );
     }
 
+    /// Recursively collect all chapters in the book as mutable thin references,
+    /// allowing you to mutate them in parallel.
+    pub fn chapters_mut_thin(&mut self) -> Vec<ChapterMutThin<'_>> {
+        let mut chapters_thin = Vec::new();
+        chapters_mut_thin(&mut self.items, &mut chapters_thin);
+        chapters_thin
+    }
+
     /// Append a `BookItem` to the `Book`.
     pub fn push_item<I: Into<BookItem>>(&mut self, item: I) -> &mut Self {
         self.items.push(item.into());
@@ -110,6 +118,32 @@ where
     }
 }
 
+/// Collect all chapters in the book.
+pub fn chapters_mut_thin<'a>(items: &'a mut [BookItem], accumulator: &mut Vec<ChapterMutThin<'a>>) {
+    items.iter_mut().for_each(move |item| {
+        if let BookItem::Chapter(Chapter {
+            name,
+            content,
+            number,
+            sub_items,
+            path,
+            source_path,
+            parent_names,
+        }) = item
+        {
+            accumulator.push(ChapterMutThin {
+                name,
+                content,
+                number,
+                path,
+                source_path,
+                parent_names,
+            });
+            chapters_mut_thin(sub_items, accumulator);
+        }
+    })
+}
+
 /// Enum representing any type of item which can be added to a book.
 #[allow(
     clippy::exhaustive_enums,
@@ -206,6 +240,30 @@ impl Chapter {
     }
 }
 
+/// A thin mutable reference to a chapter for parallel book patching.
+#[non_exhaustive]
+pub struct ChapterMutThin<'a> {
+    /// The chapter's name.
+    pub name: &'a mut String,
+    /// The chapter's contents.
+    pub content: &'a mut String,
+    /// The chapter's section number, if it has one.
+    pub number: &'a mut Option<SectionNumber>,
+    /// The chapter's location, relative to the `SUMMARY.md` file.
+    pub path: &'a mut Option<PathBuf>,
+    /// The chapter's source file, relative to the `SUMMARY.md` file.
+    pub source_path: &'a mut Option<PathBuf>,
+    /// An ordered list of the names of each chapter above this one in the hierarchy.
+    pub parent_names: &'a mut Vec<String>,
+}
+
+impl ChapterMutThin<'_> {
+    /// Check if the chapter is a draft chapter, meaning it has no path to a source markdown file.
+    pub fn is_draft_chapter(&self) -> bool {
+        self.path.is_none()
+    }
+}
+
 impl Display for Chapter {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         if let Some(ref section_number) = self.number {

crates/mdbook-core/src/book/tests.rs

@@ -121,3 +121,56 @@ fn iterate_over_nested_book_items() {
 
     assert_eq!(chapter_names, should_be);
 }
+
+#[test]
+fn chapters_mut_thin_visits_nested_chapters() {
+    let items = vec![BookItem::Chapter(Chapter {
+        name: String::from("Chapter 1"),
+        content: String::from("# Chapter 1"),
+        number: None,
+        path: Some(PathBuf::from("Chapter_1/index.md")),
+        source_path: Some(PathBuf::from("Chapter_1/index.md")),
+        parent_names: Vec::new(),
+        sub_items: vec![
+            BookItem::Chapter(Chapter::new(
+                "Hello World",
+                String::from("hello"),
+                "Chapter_1/hello.md",
+                Vec::new(),
+            )),
+            BookItem::Chapter(Chapter::new(
+                "Goodbye World",
+                String::from("goodbye"),
+                "Chapter_1/goodbye.md",
+                Vec::new(),
+            )),
+        ],
+    })];
+    let mut book = Book::new_with_items(items);
+
+    let mut chapters = book.chapters_mut_thin();
+    assert_eq!(chapters.len(), 3);
+
+    chapters[0].name.push_str(" updated");
+    chapters[1].content.push_str(" world");
+
+    drop(chapters);
+
+    let chapter_names: Vec<_> = book
+        .iter()
+        .filter_map(|item| match item {
+            BookItem::Chapter(chapter) => Some(chapter.name.clone()),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(chapter_names[0], "Chapter 1 updated");
+
+    let chapter_contents: Vec<_> = book
+        .iter()
+        .filter_map(|item| match item {
+            BookItem::Chapter(chapter) => Some(chapter.content.clone()),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(chapter_contents[1], "hello world");
+}