Parse Obsidian-style comments

In addition to normal HTML comment style, Obsidian supports comments
enclosed by "%%".

Author: Johan Förberg

src/main.rs

@@ -3,7 +3,9 @@ use std::path::PathBuf;
 
 use eyre::{eyre, Result};
 use gumdrop::Options;
-use obsidian_export::postprocessors::{filter_by_tags, softbreaks_to_hardbreaks};
+use obsidian_export::postprocessors::{
+    filter_by_tags, parse_obsidian_comments, softbreaks_to_hardbreaks,
+};
 use obsidian_export::{ExportError, Exporter, FrontmatterStrategy, WalkOptions};
 
 const VERSION: &str = env!("CARGO_PKG_VERSION");
@@ -107,6 +109,8 @@ fn main() {
     exporter.preserve_mtime(args.preserve_mtime);
     exporter.walk_options(walk_options);
 
+    exporter.add_postprocessor(&parse_obsidian_comments);
+
     if args.hard_linebreaks {
         exporter.add_postprocessor(&softbreaks_to_hardbreaks);
     }

src/postprocessors.rs

@@ -19,6 +19,62 @@ pub fn softbreaks_to_hardbreaks(
     PostprocessorResult::Continue
 }
 
+/// In addition to HTML comments "<!-- my comment -->", Obsidian syntax includes a second comment
+/// syntax "%% my comment %%". This function converts Obsidian-style comments to the standard HTML
+/// style.
+///
+/// Because this step is done when the document is already parsed from markdown, any embedded
+/// markup inside comments will be elided from the output.
+pub fn parse_obsidian_comments(
+    context: &mut Context,
+    events: &mut MarkdownEvents<'_>,
+) -> PostprocessorResult {
+    let mut in_comment = false;
+    let mut comment_acc = String::new();
+    let input = std::mem::take(events);
+    let mut output = MarkdownEvents::new();
+
+    for event in input {
+        match event {
+            Event::Text(s) => {
+                for (idx, text) in s.split("%%").enumerate() {
+                    if idx > 0 {
+                        if in_comment && !comment_acc.is_empty() {
+                            output.push(Event::InlineHtml(format!("<!--{comment_acc}-->").into()));
+                            comment_acc.clear();
+                        }
+
+                        in_comment = !in_comment;
+                    }
+
+                    if !text.is_empty() {
+                        if in_comment {
+                            comment_acc.push_str(text);
+                        } else {
+                            output.push(Event::Text(text.to_owned().into()));
+                        }
+                    }
+                }
+            }
+            _ => {
+                if !in_comment {
+                    output.push(event);
+                }
+            }
+        }
+    }
+
+    assert!(
+        !in_comment,
+        "Unmatched comment delimiter in {}",
+        context.destination.display()
+    );
+
+    std::mem::swap(events, &mut output);
+
+    PostprocessorResult::Continue
+}
+
 pub fn filter_by_tags(
     skip_tags: Vec<String>,
     only_tags: Vec<String>,

tests/export_test.rs

@@ -6,6 +6,7 @@ use std::io::prelude::*;
 use std::os::unix::fs::PermissionsExt;
 use std::path::PathBuf;
 
+use obsidian_export::postprocessors::parse_obsidian_comments;
 use obsidian_export::{ExportError, Exporter, FrontmatterStrategy};
 use pretty_assertions::assert_eq;
 use tempfile::TempDir;
@@ -459,3 +460,20 @@ fn test_same_filename_different_directories() {
     let actual = read_to_string(tmp_dir.path().join(PathBuf::from("Note.md"))).unwrap();
     assert_eq!(expected, actual);
 }
+
+#[test]
+fn test_parse_obsidian_comments() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/comments/"),
+        tmp_dir.path().to_path_buf(),
+    );
+    exporter.add_postprocessor(&parse_obsidian_comments);
+    exporter.run().expect("exporter returned error");
+
+    assert_eq!(
+        read_to_string("tests/testdata/expected/comments/note.md").unwrap(),
+        read_to_string(tmp_dir.path().join(PathBuf::from("note.md"))).unwrap(),
+    );
+}

tests/testdata/expected/comments/note.md

@@ -0,0 +1,8 @@
+Test
+
+<!-- A comment -->
+
+Test2
+<!-- A comment containing embedded markup -->
+
+This sentence is <!-- not --> true.

tests/testdata/input/comments/note.md

@@ -0,0 +1,8 @@
+Test
+
+%% A comment %%
+
+Test2
+%% A comment containing [[embedded]] **markup** %%
+
+This sentence is %% not %% true.