Load the sidebar toc from a shared JS file

Before this change, the Rust `unstable-book` is 88MiB.
With this change, it becomes 15MiB. Other pages might not be
as extreme, but it's expected to help any book like this.

This change is so drastic because, if every chapter has a link to
every other chapter, the result is *O*(n<sup>2</sup>) text output.

src/renderer/html_handlebars/hbs_renderer.rs

@@ -528,6 +528,9 @@ impl Renderer for HtmlHandlebars {
         debug!("Register the header handlebars template");
         handlebars.register_partial("header", String::from_utf8(theme.header.clone())?)?;
 
+        debug!("Register the toc handlebars template");
+        handlebars.register_template_string("toc", String::from_utf8(theme.toc.clone())?)?;
+
         debug!("Register handlebars helpers");
         self.register_hbs_helpers(&mut handlebars, &html_config);
 
@@ -583,6 +586,13 @@ impl Renderer for HtmlHandlebars {
             debug!("Creating print.html ✓");
         }
 
+        debug!("Render toc.js");
+        {
+            let rendered_toc = handlebars.render("toc", &data)?;
+            utils::fs::write_file(destination, "toc.js", rendered_toc.as_bytes())?;
+            debug!("Creating toc.js ✓");
+        }
+
         debug!("Copy static files");
         self.copy_static_files(destination, &theme, &html_config)
             .with_context(|| "Unable to copy across static files")?;

src/renderer/html_handlebars/helpers/toc.rs

@@ -1,8 +1,7 @@
 use std::path::Path;
 use std::{cmp::Ordering, collections::BTreeMap};
 
-use crate::utils;
-use crate::utils::bracket_escape;
+use crate::utils::special_escape;
 
 use handlebars::{
     Context, Handlebars, Helper, HelperDef, Output, RenderContext, RenderError, RenderErrorReason,
@@ -32,21 +31,6 @@ impl HelperDef for RenderToc {
                     RenderErrorReason::Other("Could not decode the JSON data".to_owned()).into()
                 })
         })?;
-        let current_path = rc
-            .evaluate(ctx, "@root/path")?
-            .as_json()
-            .as_str()
-            .ok_or_else(|| {
-                RenderErrorReason::Other("Type error for `path`, string expected".to_owned())
-            })?
-            .replace('\"', "");
-
-        let current_section = rc
-            .evaluate(ctx, "@root/section")?
-            .as_json()
-            .as_str()
-            .map(str::to_owned)
-            .unwrap_or_default();
 
         let fold_enable = rc
             .evaluate(ctx, "@root/fold_enable")?
@@ -67,28 +51,17 @@ impl HelperDef for RenderToc {
         out.write("<ol class=\"chapter\">")?;
 
         let mut current_level = 1;
-        // The "index" page, which has this attribute set, is supposed to alias the first chapter in
-        // the book, i.e. the first link. There seems to be no easy way to determine which chapter
-        // the "index" is aliasing from within the renderer, so this is used instead to force the
-        // first link to be active. See further below.
-        let mut is_first_chapter = ctx.data().get("is_index").is_some();
 
         for item in chapters {
-            let (section, level) = if let Some(s) = item.get("section") {
+            let (_section, level) = if let Some(s) = item.get("section") {
                 (s.as_str(), s.matches('.').count())
             } else {
                 ("", 1)
             };
 
-            let is_expanded =
-                if !fold_enable || (!section.is_empty() && current_section.starts_with(section)) {
-                    // Expand if folding is disabled, or if the section is an
-                    // ancestor or the current section itself.
-                    true
-                } else {
-                    // Levels that are larger than this would be folded.
-                    level - 1 < fold_level as usize
-                };
+            // Expand if folding is disabled, or if levels that are larger than this would not
+            // be folded.
+            let is_expanded = !fold_enable || level - 1 < (fold_level as usize);
 
             match level.cmp(&current_level) {
                 Ordering::Greater => {
@@ -121,7 +94,7 @@ impl HelperDef for RenderToc {
             // Part title
             if let Some(title) = item.get("part") {
                 out.write("<li class=\"part-title\">")?;
-                out.write(&bracket_escape(title))?;
+                out.write(&special_escape(title))?;
                 out.write("</li>")?;
                 continue;
             }
@@ -139,16 +112,8 @@ impl HelperDef for RenderToc {
                         .replace('\\', "/");
 
                     // Add link
-                    out.write(&utils::fs::path_to_root(&current_path))?;
                     out.write(&tmp)?;
-                    out.write("\"")?;
-
-                    if path == &current_path || is_first_chapter {
-                        is_first_chapter = false;
-                        out.write(" class=\"active\"")?;
-                    }
-
-                    out.write(">")?;
+                    out.write("\">")?;
                     path_exists = true;
                 }
                 _ => {
@@ -167,7 +132,7 @@ impl HelperDef for RenderToc {
             }
 
             if let Some(name) = item.get("name") {
-                out.write(&bracket_escape(name))?
+                out.write(&special_escape(name))?
             }
 
             if path_exists {

src/theme/index.hbs

@@ -109,35 +109,14 @@
         </script>
 
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
-            <div class="sidebar-scrollbox">
-                {{#toc}}{{/toc}}
-            </div>
+            <!-- populated by js -->
+            <div class="sidebar-scrollbox"></div>
             <div id="sidebar-resize-handle" class="sidebar-resize-handle">
                 <div class="sidebar-resize-indicator"></div>
             </div>
         </nav>
 
-        <!-- Track and set sidebar scroll position -->
-        <script>
-            var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
-            sidebarScrollbox.addEventListener('click', function(e) {
-                if (e.target.tagName === 'A') {
-                    sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
-                }
-            }, { passive: true });
-            var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
-            sessionStorage.removeItem('sidebar-scroll');
-            if (sidebarScrollTop) {
-                // preserve sidebar scroll position when navigating via links within sidebar
-                sidebarScrollbox.scrollTop = sidebarScrollTop;
-            } else {
-                // scroll sidebar to current active section when navigating via "next/previous chapter" buttons
-                var activeSection = document.querySelector('#sidebar .active');
-                if (activeSection) {
-                    activeSection.scrollIntoView({ block: 'center' });
-                }
-            }
-        </script>
+        <script async src="{{ path_to_root }}toc.js"></script>
 
         <div id="page-wrapper" class="page-wrapper">
 

src/theme/mod.rs

@@ -17,6 +17,7 @@ pub static INDEX: &[u8] = include_bytes!("index.hbs");
 pub static HEAD: &[u8] = include_bytes!("head.hbs");
 pub static REDIRECT: &[u8] = include_bytes!("redirect.hbs");
 pub static HEADER: &[u8] = include_bytes!("header.hbs");
+pub static TOC: &[u8] = include_bytes!("toc.js.hbs");
 pub static CHROME_CSS: &[u8] = include_bytes!("css/chrome.css");
 pub static GENERAL_CSS: &[u8] = include_bytes!("css/general.css");
 pub static PRINT_CSS: &[u8] = include_bytes!("css/print.css");
@@ -50,6 +51,7 @@ pub struct Theme {
     pub head: Vec<u8>,
     pub redirect: Vec<u8>,
     pub header: Vec<u8>,
+    pub toc: Vec<u8>,
     pub chrome_css: Vec<u8>,
     pub general_css: Vec<u8>,
     pub print_css: Vec<u8>,
@@ -85,6 +87,7 @@ impl Theme {
                 (theme_dir.join("head.hbs"), &mut theme.head),
                 (theme_dir.join("redirect.hbs"), &mut theme.redirect),
                 (theme_dir.join("header.hbs"), &mut theme.header),
+                (theme_dir.join("toc.js.hbs"), &mut theme.toc),
                 (theme_dir.join("book.js"), &mut theme.js),
                 (theme_dir.join("css/chrome.css"), &mut theme.chrome_css),
                 (theme_dir.join("css/general.css"), &mut theme.general_css),
@@ -174,6 +177,7 @@ impl Default for Theme {
             head: HEAD.to_owned(),
             redirect: REDIRECT.to_owned(),
             header: HEADER.to_owned(),
+            toc: TOC.to_owned(),
             chrome_css: CHROME_CSS.to_owned(),
             general_css: GENERAL_CSS.to_owned(),
             print_css: PRINT_CSS.to_owned(),
@@ -232,6 +236,7 @@ mod tests {
             "head.hbs",
             "redirect.hbs",
             "header.hbs",
+            "toc.js.hbs",
             "favicon.png",
             "favicon.svg",
             "css/chrome.css",
@@ -263,6 +268,7 @@ mod tests {
             head: Vec::new(),
             redirect: Vec::new(),
             header: Vec::new(),
+            toc: Vec::new(),
             chrome_css: Vec::new(),
             general_css: Vec::new(),
             print_css: Vec::new(),

src/theme/toc.js.hbs

@@ -0,0 +1,54 @@
+// Populate the sidebar
+//
+// This is a script, and not included directly in the page, to control the total size of the book.
+// The TOC contains an entry for each page, so if each page includes a copy of the TOC,
+// the total size of the page becomes O(n**2).
+var sidebarScrollbox = document.querySelector("#sidebar .sidebar-scrollbox");
+sidebarScrollbox.innerHTML = '{{#toc}}{{/toc}}';
+(function() {
+    let current_page = document.location.href.toString();
+    if (current_page.endsWith("/")) {
+        current_page += "index.html";
+    }
+    var links = sidebarScrollbox.querySelectorAll("a");
+    var l = links.length;
+    for (var i = 0; i < l; ++i) {
+        var link = links[i];
+        var href = link.getAttribute("href");
+        if (href && !href.startsWith("#") && !/^(?:[a-z+]+:)?\/\//.test(href)) {
+            link.href = path_to_root + href;
+        }
+        // The "index" page is supposed to alias the first chapter in the book.
+        if (link.href === current_page || (i === 0 && path_to_root === "" && current_page.endsWith("/index.html"))) {
+            link.classList.add("active");
+            var parent = link.parentElement;
+            while (parent) {
+                if (parent.tagName === "LI" && parent.previousElementSibling) {
+                    if (parent.previousElementSibling.classList.contains("chapter-item")) {
+                        parent.previousElementSibling.classList.add("expanded");
+                    }
+                }
+                parent = parent.parentElement;
+            }
+        }
+    }
+})();
+
+// Track and set sidebar scroll position
+sidebarScrollbox.addEventListener('click', function(e) {
+    if (e.target.tagName === 'A') {
+        sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
+    }
+}, { passive: true });
+var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
+sessionStorage.removeItem('sidebar-scroll');
+if (sidebarScrollTop) {
+    // preserve sidebar scroll position when navigating via links within sidebar
+    sidebarScrollbox.scrollTop = sidebarScrollTop;
+} else {
+    // scroll sidebar to current active section when navigating via "next/previous chapter" buttons
+    var activeSection = document.querySelector('#sidebar .active');
+    if (activeSection) {
+        activeSection.scrollIntoView({ block: 'center' });
+    }
+}

src/utils/mod.rs

@@ -265,6 +265,25 @@ pub fn log_backtrace(e: &Error) {
     }
 }
 
+pub(crate) fn special_escape(mut s: &str) -> String {
+    let mut escaped = String::with_capacity(s.len());
+    let needs_escape: &[char] = &['<', '>', '\'', '\\', '&'];
+    while let Some(next) = s.find(needs_escape) {
+        escaped.push_str(&s[..next]);
+        match s.as_bytes()[next] {
+            b'<' => escaped.push_str("&lt;"),
+            b'>' => escaped.push_str("&gt;"),
+            b'\'' => escaped.push_str("&#39;"),
+            b'\\' => escaped.push_str("&#92;"),
+            b'&' => escaped.push_str("&amp;"),
+            _ => unreachable!(),
+        }
+        s = &s[next + 1..];
+    }
+    escaped.push_str(s);
+    escaped
+}
+
 pub(crate) fn bracket_escape(mut s: &str) -> String {
     let mut escaped = String::with_capacity(s.len());
     let needs_escape: &[char] = &['<', '>'];
@@ -283,7 +302,7 @@ pub(crate) fn bracket_escape(mut s: &str) -> String {
 
 #[cfg(test)]
 mod tests {
-    use super::bracket_escape;
+    use super::{bracket_escape, special_escape};
 
     mod render_markdown {
         use super::super::render_markdown;
@@ -506,5 +525,20 @@ more text with spaces
         assert_eq!(bracket_escape("<>"), "&lt;&gt;");
         assert_eq!(bracket_escape("<test>"), "&lt;test&gt;");
         assert_eq!(bracket_escape("a<test>b"), "a&lt;test&gt;b");
+        assert_eq!(bracket_escape("'"), "'");
+        assert_eq!(bracket_escape("\\"), "\\");
+    }
+
+    #[test]
+    fn escaped_special() {
+        assert_eq!(special_escape(""), "");
+        assert_eq!(special_escape("<"), "&lt;");
+        assert_eq!(special_escape(">"), "&gt;");
+        assert_eq!(special_escape("<>"), "&lt;&gt;");
+        assert_eq!(special_escape("<test>"), "&lt;test&gt;");
+        assert_eq!(special_escape("a<test>b"), "a&lt;test&gt;b");
+        assert_eq!(special_escape("'"), "&#39;");
+        assert_eq!(special_escape("\\"), "&#92;");
+        assert_eq!(special_escape("&"), "&amp;");
     }
 }