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a248ec75e2
Imagine we have this linked chunk: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] ['d', 'e', 'f']); ``` Before the patch, when we were running: ```rust let position_of_d = linked_chunk.item_position(|item| *item == 'd').unwrap(); linked_chunk.insert_gap_at((), position_of_d)?; ``` it was taking `['d', 'e', 'f']` to split it at index 0, so `[]` + `['d', 'e', 'f']`, and then was inserting a gap in between, thus resulting into: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] [] [-] ['d', 'e', 'f']); ``` The problem is that it creates a useless empty chunk. It's a waste of space, and rapidly, of computation. When used with `EventCache`, this problem occurs every time a backpagination occurs (because it executes a `replace_gap_at` to insert the new item, and then executes a `insert_gap_at` if the backpagination contains another `prev_batch` token). With this patch, the result of the operation is now: ```rust assert_items_eq!(linked_chunk, ['a'] [-] ['b', 'c'] [-] ['d', 'e', 'f']); ``` The `assert_items_eq!` macro has been updated to be able to assert empty chunks. The `test_insert_gap_at` has been updated to test all edge cases.