Record line number in OrgSource.

This will be used for elements who have limits on the number of lines inside of them. This includes LaTeX fragment bodies and text markup.

src/parser/org_source.rs

@@ -18,6 +18,7 @@ pub struct OrgSource<'s> {
     end: usize, // exclusive
     start_of_line: usize,
     preceding_character: Option<char>,
+    line_number: usize,
 }
 
 impl<'s> OrgSource<'s> {
@@ -31,6 +32,7 @@ impl<'s> OrgSource<'s> {
             end: input.len(),
             start_of_line: 0,
             preceding_character: None,
+            line_number: 1,
         }
     }
 
@@ -56,6 +58,10 @@ impl<'s> OrgSource<'s> {
         assert!(other.end <= self.end);
         self.slice(..(other.start - self.start))
     }
+
+    pub fn get_line_number(&self) -> usize {
+        self.line_number
+    }
 }
 
 impl<'s> InputTake for OrgSource<'s> {
@@ -119,10 +125,14 @@ where
         }
 
         let skipped_text = &self.full_source[self.start..new_start];
-        let start_of_line = skipped_text
-            .rfind('\n')
-            .map(|idx| self.start + idx + 1)
-            .unwrap_or(self.start_of_line);
+        let mut start_of_line = self.start_of_line;
+        let mut line_number = self.line_number;
+        for (offset, character) in skipped_text.char_indices() {
+            if character == '\n' {
+                start_of_line = self.start + offset + 1;
+                line_number += 1;
+            }
+        }
 
         OrgSource {
             full_source: self.full_source,
@@ -130,6 +140,7 @@ where
             end: new_end,
             start_of_line,
             preceding_character: skipped_text.chars().last(),
+            line_number,
         }
     }
 }
@@ -369,4 +380,13 @@ mod tests {
         assert_eq!(input.get_preceding_character(), None);
         assert_eq!(input.slice(8..).get_preceding_character(), Some('💛'));
     }
+
+    #[test]
+    fn line_number() {
+        let input = OrgSource::new("lorem\nfoo\nbar\nbaz\nipsum");
+        assert_eq!(input.get_line_number(), 1);
+        assert_eq!(input.slice(5..).get_line_number(), 1);
+        assert_eq!(input.slice(6..).get_line_number(), 2);
+        assert_eq!(input.slice(6..).slice(10..).get_line_number(), 4);
+    }
 }