Automatically add exercise links to sections. (#52)

We use an mdbook preprocessor to automatically generate links to the relevant exercise for each section.
We remove all existing manual links and refactor the deploy process to push the rendered book to a branch.

helpers/mdbook-exercise-linker/src/lib.rs

@@ -0,0 +1,72 @@
+use anyhow::{Context, Error};
+use mdbook::book::Book;
+use mdbook::preprocess::{Preprocessor, PreprocessorContext};
+use mdbook::BookItem;
+
+/// A no-op preprocessor.
+pub struct ExerciseLinker;
+
+impl ExerciseLinker {
+    pub fn new() -> ExerciseLinker {
+        ExerciseLinker
+    }
+}
+
+impl Preprocessor for ExerciseLinker {
+    fn name(&self) -> &str {
+        "exercise-linker"
+    }
+
+    fn run(&self, ctx: &PreprocessorContext, mut book: Book) -> Result<Book, Error> {
+        let config = ctx
+            .config
+            .get_preprocessor(self.name())
+            .context("Failed to get preprocessor configuration")?;
+        let key = String::from("exercise_root_url");
+        let root_url = config
+            .get(&key)
+            .context("Failed to get `exercise_root_url`")?;
+        let root_url = root_url
+            .as_str()
+            .context("`exercise_root_url` is not a string")?
+            .to_owned();
+
+        book.sections
+            .iter_mut()
+            .for_each(|i| process_book_item(i, &root_url));
+        Ok(book)
+    }
+
+    fn supports_renderer(&self, _renderer: &str) -> bool {
+        true
+    }
+}
+
+fn process_book_item(item: &mut BookItem, root_url: &str) {
+    match item {
+        BookItem::Chapter(chapter) => {
+            chapter.sub_items.iter_mut().for_each(|item| {
+                process_book_item(item, root_url);
+            });
+
+            let Some(source_path) = &chapter.source_path else {
+                return;
+            };
+            let source_path = source_path.display().to_string();
+
+            // Ignore non-exercise chapters
+            if !source_path.chars().take(2).all(|c| c.is_digit(10)) {
+                return;
+            }
+
+            let exercise_path = source_path.strip_suffix(".md").unwrap();
+            let link_section = format!(
+                    "\n## Exercise\n\nThe exercise for this section is located in [`{exercise_path}`]({})",
+                    format!("{}/{}", root_url, exercise_path)
+                );
+            chapter.content.push_str(&link_section);
+        }
+        BookItem::Separator => {}
+        BookItem::PartTitle(_) => {}
+    }
+}