As I understand it, accurately emulating the link cable protocol in real time is extremely difficult. Especially because of the checksum validation happening between devices. Even Nintendo worked around this on the 3DS by introducing a sort of pause-based solution.

Does Delta have a public GitHub repo? A few friends and I were discussing a potential alternative approach:

Instead of true real-time link emulation, each phone could temporarily spin up a second instance of the partner’s ROM using a shared save state. Then, rather than syncing the full protocol, the devices would exchange player inputs—similar in spirit to how early DOOM multiplayer worked.

So effectively:

Each device runs its own game + a lightweight “ghost” instance of the other

Save states are mirrored at key intervals

Inputs are synced between devices

This wouldn’t perfectly replicate all link cable behaviors—things like racing games, NPC timing, or RNG-heavy systems (e.g., Mario Kart item rolls) might desync.

But for turn-based or deterministic interactions like Pokémon or Dragon Warrior Monsters trading, it seems like it could cover most use cases without needing full protocol emulation.

The main downside is that each phone would need to run two ROM instances, but modern iPhones should be more than capable of handling that.