History

Discovery

Inventory Slot Transfer (IST) was discovered in June 2022 by zxrobbin ( Video 1, Video 2 ). It initially manifested as an innocent glitch to quickly duplicate items in Breath of the Wild. Little did we know that IST will become the most complicated glitch to ever exist in BOTW.

Around June 17, 2022, as the news spread, glitch hunters from the BOTW speedrunning community started to investigate and quickly found some patterns to the glitch.

Hundo Duplication Simulator

Meanwhile, speedrunners of the 100-percent category started to incorporate it into the route. However, even the simplest form of this glitch required tracking the exact state of the player’s inventory for every action executed, such as picking up or dropping an item. The rules and patterns of this glitch then needs to be applied at even of these steps. It was a painstaking process to manually track all the information.

Luckily, tasks that are difficult for humans are often easy for computers. On June 18, 2022, the Hundo Duplication Simulator was born. Users could type in actions such as get 1 core, and the app will display the inventory state after each step. This was a massive help for optimizing setups for speedruns because people can change one step and the app will re-calculate the state for every step afterwards. This model is still used by the simulator today.

The IST Simulator - V2

As more applications of IST are found, the glitch has found its use in categories outside of 100-percent, for example in All Shrines and All Dungeons. One of such application is Direct Inventory Corruption. However, the simulator was built in one day and did not support any cases other than the very basic rules that were initially discovered.

Now that this glitch is not specific to 100-percent. The project was renamed to IST Simulator. V2 of the simulator added GameData Inventory and more items to simulate the inventory state closer to how the game handles it. Direct Inventory Corruption support was added. People were happy.

Weapon Modifier Corruption - V3

…Until more applications of IST are found again, and the simulator being a simulator, did not support these applications because I couldn’t predict the future.

Every time a new discovery is made, it almost certainly invalidates assumptions previously made when coding the simulator, and the core has to be re-designed to incorporate the new knowledge. In November 2022, I started re-designing the core by referencing the BOTW decompilation project. This massively improved the accuracy of the simulator and supported new applications such as Weapon Modifier Corruption. The UI was also revamped, adding syntax highlighting to commands and displaying modifier information for items.

Skybook - V4

After the V3 update, a few small fixes were submitted, but no big re-architecture was made for 2 years. During this time, some minor inconsistencies were found, mostly related to GameData flags (such as champion ability and discovered tabs).

Even though the remaining edge cases are rarely hit, it was clear that we could not keep chasing these cases. If the simulator was to be improved again, it needs to fix all these cases entirely. During this time, I had theorized a new version of the simulator that uses the game itself to execute the actions. In Spring 2024, I submitted a proposal to make a POC of this new simulator as a senior project to my college, and we got a student team to work on it with me as the advisor. Meanwhile, I started working on the UI of the new simulator around December 2024. The project finished successfully and it was integrated to the project as the core for the new V4 simulator.