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)
}