Fix bug in split_before and split_after for first and last node edge cases

[morlinbrot]

Fixes #18

The split_before and split_after methods don't handle the edge case of being at the first or last node correctly.

Example split_before:
In the current implementation, output_front will be set to the list's head before checking if there actually is a node before where the cursor currently is, which in the case of being at the first node will not be the case. In this case, we want to return an empty list which can be done by simply not setting a new head.

I tried to implement a fix as unobtrusive to the original implementation as possible, since I assume that the methods are written with readability in mind for the too-many-lists project. In my implementation, I wrote the methods a bit more "cleverly" which may be not as easy to follow but shorter and more focused on all the edge cases that need to be covered, here's a snippet of it:

pub fn split_before(&mut self) -> LinkedList<T> {
    match self.idx {
        None => mem::replace(self.list, LinkedList::new()),
        Some(0) => LinkedList::new(),
        Some(idx) => unsafe {
            let cur = self.cur.unwrap();
            let out_head = self.list.head;
            let out_tail = (*cur.as_ptr()).prev.take();

            if let Some(node) = out_tail {
                (*node.as_ptr()).next = None;
            }

            let old_len = self.list.len;
            let new_len = old_len - idx;

            self.list.head = Some(cur);
            self.list.len = new_len;
            self.idx = Some(0);

            LinkedList {
                head: out_head,
                tail: out_tail,
                len: old_len - new_len,
                _marker: PhantomData,
            }
        },
    }
}

Let me know if you'd be interested in switching to my implementation, I'd be glad to update the PR.

[morlinbrot]

Note the differences in implementation between this and the same fix over at too-many-lists.

[alexkudryavtsev88]

I've checked your new solution - it looks like all stuff works fine:

• cursor.split_before() after first cursor.move_next() returns empty list, the source list stays in initial state;
• also, there is no more attempt to subtract with overflow panic at pop_front that's called upon the list drop (this issue was present in "book" implementation)

[pczarn]

I'll look into this