Implement text since line break.

src/main.rs

@@ -31,6 +31,12 @@ impl<'s> WrappedInput<'s> {
             preceding_line_break: None,
         }
     }
+
+    /// Get the text since the line break preceding the start of this WrappedInput.
+    pub fn text_since_line_break(&self) -> &'s str {
+        let start = self.preceding_line_break.unwrap_or(0);
+        &self.full_source[start..self.start]
+    }
 }
 
 impl<'s> InputTake for WrappedInput<'s> {
@@ -87,12 +93,17 @@ where
             panic!("Attempted to extend past the end of the WrappedInput.")
         }
 
+        let skipped_text = &self.full_source[self.start..new_start];
+        let last_line_feed = skipped_text
+            .rfind('\n')
+            .map(|idx| idx + self.preceding_line_break.unwrap_or(0) + 1);
+
         // TODO: calculate updated values for WrappedInput
         WrappedInput {
             full_source: self.full_source,
             start: new_start,
             end: new_end,
-            preceding_line_break: None,
+            preceding_line_break: last_line_feed,
         }
     }
 }
@@ -158,4 +169,21 @@ mod tests {
         let first_cut = input.slice(6..17);
         first_cut.slice(4..14);
     }
+
+    #[test]
+    fn line_break() {
+        let input = WrappedInput::new("lorem\nfoo\nbar\nbaz\nipsum");
+        assert_eq!(input.slice(5..).preceding_line_break, None);
+        assert_eq!(input.slice(6..).preceding_line_break, Some(6));
+        assert_eq!(input.slice(6..).slice(10..).preceding_line_break, Some(14));
+    }
+
+    #[test]
+    fn text_since_line_break() {
+        let input = WrappedInput::new("lorem\nfoo\nbar\nbaz\nipsum");
+        assert_eq!(input.text_since_line_break(), "");
+        assert_eq!(input.slice(5..).text_since_line_break(), "lorem");
+        assert_eq!(input.slice(6..).text_since_line_break(), "");
+        assert_eq!(input.slice(6..).slice(10..).text_since_line_break(), "ba");
+    }
 }