Function pack 

pub fn pack<C: Chunk>(
    input: &mut VecDeque<C>,
    done: bool,
    out: &mut VecDeque<C>,
    combine: impl FnMut(Vec<C>) -> C,
    split: impl FnMut(C, usize) -> (C, C),
    seal: impl FnMut(C) -> C,
)

Maximal-packing driver an implementor’s Chunk::settle may delegate to.

Holds a carry run of chunks, grown until it reaches TARGET (then emitted as one chunk) and emitted early when the next chunk can’t be absorbed without exceeding TARGET; over-sized chunks are peeled with split. Each committed chunk is passed through seal (the compress / spill hook — use the identity closure when there’s nothing to do). The closures are the only layout-specific pieces:

• combine(run) — one chunk from a run of at least two (caller guarantees their lengths sum to at most TARGET, and that they form one sorted, consolidated chain). A run is combined once, when emitted, so packing stays linear even when combining copies its inputs.
• split(chunk, n) — the first n updates and the remaining len - n.
• seal(chunk) — commit a chunk (e.g. compress or spill); identity to keep it.