Extract the line number from the switches.

org_mode_samples/lesser_element/lesser_block/example/line_number_negative.org

@@ -0,0 +1,7 @@
+#+begin_example -n -10
+foo
+#+end_example
+
+#+begin_example +n -15
+bar
+#+end_example

org_mode_samples/lesser_element/lesser_block/example/line_number_zero.org

@@ -0,0 +1,7 @@
+#+begin_example -n 0
+foo
+#+end_example
+
+#+begin_example +n 0
+bar
+#+end_example

src/parser/lesser_block.rs

@@ -6,10 +6,12 @@ use nom::character::complete::space0;
 use nom::character::complete::space1;
 use nom::combinator::consumed;
 use nom::combinator::eof;
+use nom::combinator::map;
 use nom::combinator::opt;
 use nom::combinator::recognize;
 use nom::combinator::verify;
 use nom::multi::many_till;
+use nom::multi::separated_list1;
 use nom::sequence::tuple;
 
 use super::org_source::OrgSource;
@@ -29,9 +31,11 @@ use crate::parser::util::text_until_exit;
 use crate::types::CommentBlock;
 use crate::types::ExampleBlock;
 use crate::types::ExportBlock;
+use crate::types::LineNumber;
 use crate::types::Object;
 use crate::types::PlainText;
 use crate::types::SrcBlock;
+use crate::types::SwitchNumberLines;
 use crate::types::VerseBlock;
 
 #[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
@@ -130,7 +134,7 @@ pub(crate) fn example_block<'b, 'g, 'r, 's>(
     input: OrgSource<'s>,
 ) -> Res<OrgSource<'s>, ExampleBlock<'s>> {
     let (remaining, _name) = lesser_block_begin("example")(context, input)?;
-    let (remaining, parameters) = opt(tuple((space1, data)))(remaining)?;
+    let (remaining, parameters) = opt(tuple((space1, example_switches)))(remaining)?;
     let (remaining, _nl) = recognize(tuple((space0, line_ending)))(remaining)?;
     let lesser_block_end_specialized = lesser_block_end("example");
     let contexts = [
@@ -150,13 +154,20 @@ pub(crate) fn example_block<'b, 'g, 'r, 's>(
     let (remaining, _end) = lesser_block_end_specialized(&parser_context, remaining)?;
 
     let source = get_consumed(input, remaining);
+    let (switches, number_lines) = {
+        if let Some(parameters) = parameters {
+            (Some(parameters.source), parameters.number_lines)
+        } else {
+            (None, None)
+        }
+    };
     Ok((
         remaining,
         ExampleBlock {
             source: source.into(),
             name: source.into(),
-            switches: parameters.map(|parameters| Into::<&str>::into(parameters)),
-            number_lines: None, // TODO
+            switches,
+            number_lines,
             contents: contents.into(),
         },
     ))
@@ -302,3 +313,64 @@ fn _lesser_block_begin<'b, 'g, 'r, 's, 'c>(
     ))(remaining)?;
     Ok((remaining, name))
 }
+
+struct ExampleSwitches<'s> {
+    source: &'s str,
+    number_lines: Option<SwitchNumberLines>,
+}
+
+enum SwitchState {
+    Normal,
+    NewLineNumber,
+    ContinuedLineNumber,
+}
+
+#[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
+fn example_switches<'s>(input: OrgSource<'s>) -> Res<OrgSource<'s>, ExampleSwitches<'s>> {
+    let mut number_lines = None;
+    let (remaining, (source, words)) = consumed(separated_list1(
+        space1,
+        map(is_not(" \t\r\n"), |val| Into::<&str>::into(val)),
+    ))(input)?;
+
+    let mut state = SwitchState::Normal;
+    for word in words {
+        state = match (state, word) {
+            (SwitchState::Normal, "-n") => SwitchState::NewLineNumber,
+            (SwitchState::Normal, "+n") => SwitchState::ContinuedLineNumber,
+            (SwitchState::NewLineNumber, _) => {
+                let val = word
+                    .parse::<LineNumber>()
+                    .expect("TODO: I should be able to return CustomError from nom parsers.");
+                if val < 0 {
+                    number_lines = Some(SwitchNumberLines::New(0));
+                } else {
+                    // Note that this can result in a negative 1 if the val is originally 0.
+                    number_lines = Some(SwitchNumberLines::New(val - 1));
+                }
+                SwitchState::Normal
+            }
+            (SwitchState::ContinuedLineNumber, _) => {
+                let val = word
+                    .parse::<LineNumber>()
+                    .expect("TODO: I should be able to return CustomError from nom parsers.");
+                if val < 0 {
+                    number_lines = Some(SwitchNumberLines::Continued(0));
+                } else {
+                    // Note that this can result in a negative 1 if the val is originally 0.
+                    number_lines = Some(SwitchNumberLines::Continued(val - 1));
+                }
+                SwitchState::Normal
+            }
+            (state @ SwitchState::Normal, _) => state,
+        };
+    }
+
+    Ok((
+        remaining,
+        ExampleSwitches {
+            source: Into::<&str>::into(source),
+            number_lines,
+        },
+    ))
+}

src/types/lesser_element.rs

@@ -108,7 +108,10 @@ pub struct LatexEnvironment<'s> {
     pub source: &'s str,
 }
 
-pub type LineNumber = usize;
+/// A line number used in switches to lesser blocks.
+///
+/// This must be signed because emacs subtracts 1 from the actual value in the org-mode text, which makes a 0 turn into a -1.
+pub type LineNumber = isize;
 
 #[derive(Debug)]
 pub enum SwitchNumberLines {