First stab at implementing comments.

src/compare/diff.rs

@@ -1,5 +1,6 @@
 use super::sexp::Token;
 use crate::compare::util::get_offsets;
+use crate::parser::Comment;
 use crate::parser::Document;
 use crate::parser::DocumentElement;
 use crate::parser::Element;
@@ -216,7 +217,7 @@ fn compare_element<'s>(
         Element::PlainList(obj) => compare_plain_list(source, emacs, obj),
         Element::GreaterBlock(obj) => compare_greater_block(source, emacs, obj),
         Element::FootnoteDefinition(obj) => compare_footnote_definition(source, emacs, obj),
-        Element::Comment(obj) => todo!(),
+        Element::Comment(obj) => compare_comment(source, emacs, obj),
     }
 }
 
@@ -454,3 +455,44 @@ fn compare_footnote_definition<'s>(
         children: child_status,
     })
 }
+
+fn compare_comment<'s>(
+    source: &'s str,
+    emacs: &'s Token<'s>,
+    rust: &'s Comment<'s>,
+) -> Result<DiffResult, Box<dyn std::error::Error>> {
+    let children = emacs.as_list()?;
+    let first_child = children
+        .first()
+        .ok_or("Should have at least one child.")?
+        .as_atom()?;
+    if first_child != "comment" {
+        return Err("Comment should correspond to a comment cell.".into());
+    }
+    let mut child_status = Vec::new();
+    let mut this_status = DiffStatus::Good;
+
+    let attributes_child = children
+        .iter()
+        .nth(1)
+        .ok_or("Should have an attributes child.")?;
+    let attributes_map = attributes_child.as_map()?;
+    let begin = attributes_map
+        .get(":begin")
+        .ok_or("Missing :begin attribute.")?
+        .as_atom()?;
+    let end = attributes_map
+        .get(":end")
+        .ok_or("Missing :end attribute.")?
+        .as_atom()?;
+    let (rust_begin, rust_end) = get_offsets(source, rust);
+    if (rust_begin + 1).to_string() != begin || (rust_end + 1).to_string() != end {
+        this_status = DiffStatus::Bad;
+    }
+
+    Ok(DiffResult {
+        status: this_status,
+        name: "comment".to_owned(),
+        children: child_status,
+    })
+}