Move some functions into util.

src/parser/util.rs

@@ -23,3 +23,48 @@ pub fn immediate_in_section<'r, 's, 'x>(context: Context<'r, 's>, section_name:
     }
     false
 }
+
+/// Get one character from before the current position.
+pub fn get_one_before<'s>(document: &'s str, current_position: &'s str) -> Option<&'s str> {
+    assert!(is_slice_of(document, current_position));
+    if document.as_ptr() as usize == current_position.as_ptr() as usize {
+        return None;
+    }
+    let offset = current_position.as_ptr() as usize - document.as_ptr() as usize;
+    let previous_character_offset = document.floor_char_boundary(offset - 1);
+    Some(&document[previous_character_offset..offset])
+}
+
+/// Check if the child string slice is a slice of the parent string slice.
+fn is_slice_of(parent: &str, child: &str) -> bool {
+    let parent_start = parent.as_ptr() as usize;
+    let parent_end = parent_start + parent.len();
+    let child_start = child.as_ptr() as usize;
+    let child_end = child_start + child.len();
+    child_start >= parent_start && child_end <= parent_end
+}
+
+/// Get a slice of the string that was consumed in a parser using the original input to the parser and the remaining input after the parser.
+pub fn get_consumed<'s>(input: &'s str, remaining: &'s str) -> &'s str {
+    assert!(is_slice_of(input, remaining));
+    let source = {
+        let offset = remaining.as_ptr() as usize - input.as_ptr() as usize;
+        &input[..offset]
+    };
+    source
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn get_one_before_unicode() {
+        let input = "🧡💛💚💙💜";
+        let (green_heart_index, _) = input.char_indices().skip(2).next().unwrap();
+        let starting_with_green_heart = &input[green_heart_index..];
+        let yellow_heart = get_one_before(input, starting_with_green_heart).unwrap();
+        assert!(is_slice_of(input, yellow_heart));
+        assert_eq!(yellow_heart, "💛");
+    }
+}