comparing begin and end offsets for top-level sections and headlines.

src/compare/util.rs

@@ -0,0 +1,21 @@
+use crate::parser::Source;
+
+/// 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 the offset into source that the rust object exists at.
+///
+/// These offsets are zero-based unlike the elisp ones.
+pub fn get_offsets<'s, S: Source<'s>>(source: &'s str, rust_object: &'s S) -> (usize, usize) {
+    let rust_object_source = rust_object.get_source();
+    assert!(is_slice_of(source, rust_object_source));
+    let offset = rust_object_source.as_ptr() as usize - source.as_ptr() as usize;
+    let end = offset + rust_object_source.len();
+    (offset, end)
+}