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/main.rs

@@ -0,0 +1,67 @@
+use std::io;
+use std::process;
+
+use clap::{Arg, ArgMatches, Command};
+use mdbook::errors::Error;
+use mdbook::preprocess::{CmdPreprocessor, Preprocessor};
+use semver::{Version, VersionReq};
+
+use mdbook_exercise_linker::ExerciseLinker;
+
+pub fn make_app() -> Command {
+    Command::new("exercise-linker").subcommand(
+        Command::new("supports")
+            .arg(Arg::new("renderer").required(true))
+            .about("Check whether a renderer is supported by this preprocessor"),
+    )
+}
+
+fn main() {
+    let matches = make_app().get_matches();
+
+    // Users will want to construct their own preprocessor here
+    let preprocessor = ExerciseLinker::new();
+
+    if let Some(sub_args) = matches.subcommand_matches("supports") {
+        handle_supports(&preprocessor, sub_args);
+    } else if let Err(e) = handle_preprocessing(&preprocessor) {
+        eprintln!("{}", e);
+        process::exit(1);
+    }
+}
+
+fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<(), Error> {
+    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())?;
+
+    let book_version = Version::parse(&ctx.mdbook_version)?;
+    let version_req = VersionReq::parse(mdbook::MDBOOK_VERSION)?;
+
+    if !version_req.matches(&book_version) {
+        eprintln!(
+            "Warning: The {} plugin was built against version {} of mdbook, \
+             but we're being called from version {}",
+            pre.name(),
+            mdbook::MDBOOK_VERSION,
+            ctx.mdbook_version
+        );
+    }
+
+    let processed_book = pre.run(&ctx, book)?;
+    serde_json::to_writer(io::stdout(), &processed_book)?;
+
+    Ok(())
+}
+
+fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
+    let renderer = sub_args
+        .get_one::<String>("renderer")
+        .expect("Required argument");
+    let supported = pre.supports_renderer(renderer);
+
+    // Signal whether the renderer is supported by exiting with 1 or 0.
+    if supported {
+        process::exit(0);
+    } else {
+        process::exit(1);
+    }
+}