If you're doing batches to reduce the advantage of being fast, you'd have to treat all orders that come in during a batch tick as simultaneous. Resting orders

If you're doing batches to reduce the advantage of being fast, you'd have to treat all orders that come in during a batch tick as simultaneous. Resting orders from previous batches could have priority, if you want. You'd probably end up doing something with assignment of equal priority orders that looks like option assignment, basically random selection of shares among the pool of orders. Personally, I'd fill unconditional market orders first, then market all or nothing (if fillable), then sort limit orders by price and from within limit orders of the same price, unconditional first, then all or nothing, then all or nothing + fill or kill. I don't know if I would assign shares proportional to orders or to shares in orders. Probably shares in orders. Might be gamed, but putting in a really big order because you want to capture a couple shares is risky.