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,
+    })
+}

src/parser/comment.rs

@@ -1,15 +1,36 @@
+use nom::bytes::complete::is_not;
+use nom::bytes::complete::tag;
+use nom::character::complete::space0;
+use nom::combinator::not;
+use nom::multi::many0;
+use nom::sequence::preceded;
+use nom::sequence::tuple;
+
+use super::util::get_consumed;
 use super::Context;
 use crate::parser::error::Res;
+use crate::parser::parser_with_context::parser_with_context;
+use crate::parser::util::exit_matcher_parser;
 use crate::parser::util::start_of_line;
 use crate::parser::Comment;
 
 #[tracing::instrument(ret, level = "debug")]
 pub fn comment<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, Comment<'s>> {
-    start_of_line(context, input)?;
-    todo!()
+    let comment_line_matcher = parser_with_context!(comment_line)(context);
+    let exit_matcher = parser_with_context!(exit_matcher_parser)(context);
+    let (remaining, first_line) = comment_line_matcher(input)?;
+    let (remaining, remaining_lines) =
+        many0(preceded(not(exit_matcher), comment_line_matcher))(remaining)?;
+    let source = get_consumed(input, remaining);
+    Ok((remaining, Comment { source }))
 }
 
+#[tracing::instrument(ret, level = "debug")]
 fn comment_line<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, &'s str> {
     start_of_line(context, input)?;
-    todo!()
+    let (remaining, _indent) = space0(input)?;
+    let (remaining, (_hash, _leading_whitespace, content)) =
+        tuple((tag("#"), space0, is_not("\r\n")))(remaining)?;
+    let source = get_consumed(input, remaining);
+    Ok((remaining, source))
 }

src/parser/element.rs

@@ -1,3 +1,4 @@
+use super::comment::comment;
 use super::error::Res;
 use super::footnote_definition::footnote_definition;
 use super::greater_block::greater_block;
@@ -90,9 +91,11 @@ pub fn non_paragraph_element<'r, 's>(
     let plain_list_matcher = parser_with_context!(plain_list)(context);
     let greater_block_matcher = parser_with_context!(greater_block)(context);
     let footnote_definition_matcher = parser_with_context!(footnote_definition)(context);
+    let comment_matcher = parser_with_context!(comment)(context);
     alt((
         map(plain_list_matcher, Element::PlainList),
         map(greater_block_matcher, Element::GreaterBlock),
         map(footnote_definition_matcher, Element::FootnoteDefinition),
+        map(comment_matcher, Element::Comment),
     ))(input)
 }