fn midpoint_rank(existing_ranks: &[Option<i64>], pos: usize) -> Option<i64>Expand description
Computes the rank to assign to a new book being inserted at position pos
in a list of existing ranks (which may be None for books whose ranks have
not yet been computed).
Returns None when the gap between the two neighbours has been fully
exhausted (they differ by ≤ 1), signalling that a full recompute is needed.