Detect duplicate headings within a page

This is an issue because the duplicate headings will have identical
ids, and all links to the lower heading will instead link to the upper
heading.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/collect_files.rs b/src/collect_files.rs
index d9cfb37..49f5cc0 100644
--- a/src/collect_files.rs
+++ b/src/collect_files.rs
@@ -96,15 +96,23 @@ impl Website {
                 "md" => {
                     let markdown = std::fs::read_to_string(&source_path).unwrap();
                     let document = MarkdownDocument::from_str(&markdown);
+                    let mut heading_set = HashSet::new();
+                    let mut duplicates = HashSet::new();
                     let headings = document.blocks.iter()
                         .filter_map(|block| if let Block::Heading { line, level } = block {
                             let name = line.to_string();
                             let url = make_url_safe(&name);
                             let level = level.to_owned();
+                            if !heading_set.insert(url.clone()) {
+                                duplicates.insert(url.clone());
+                            }
                             Some(Heading { name, url, level })
                         } else {
                             None
                         }).collect();
+                    for url in duplicates {
+                        warn!("Page {name:?} contains multiple headings with ID \"#{url}\"");
+                    }
                     if name_url == "+index" {
                         if parents.is_empty() {
                             // This is the index file for the whole site.