## Sequence Diagram: GTT Expiry and Mode/Uncross Check
### Overview
The image is a sequence diagram illustrating the interaction between different components (Time, GTT Expiry Check, Mode/Uncross Check, and State) during the Good-Till-Time (GTT) order expiry and mode/uncross check process. It shows the flow of events and decisions made at different ticks, particularly focusing on auction mode and market order presence.
### Components/Axes
* **Horizontal Axes (Top)**:
* Time
* GTT Expiry Check
* Mode/Uncross Check
* State
* **Vertical Axis**: Represents the progression of time and events.
* **Nodes**:
* Initial State (yellow box): t = 2699, Auction Mode, uncross_at = 2700, order expires_at = 2700, market_extension = 1
* After Tick 1 (yellow box): t = 2700, Still in auction
* Final State (pink box): Both events occur in the same tick, MIT201 violated
### Detailed Analysis or ### Content Details
* **Initial State**:
* Time (t) is 2699.
* The system is in Auction Mode.
* `uncross_at` is set to 2700.
* `order expires_at` is set to 2700.
* `market_extension` is set to 1.
* **Tick 1: t = 2699 -> 2700**
* **GTT Expiry Check**:
* Checks expiry.
* Asks: "In auction mode?"
* Extends to max(2700, 2700+1) = 2701.
* Checks: "2700 >= 2701? NO"
* Result: "Order NOT expired"
* **Mode/Uncross Check**:
* Checks mode change.
* Checks: "2700 >= 2700? YES"
* Asks: "Market orders present? 2700 >= 2700 + 1? NO"
* Result: "Stay in auction"
* **After Tick 1**:
* Time (t) is 2700.
* The system is still in auction.
* **Tick 2: t = 2700 -> 2701**
* **GTT Expiry Check**:
* Checks expiry.
* Asks: "In auction mode?"
* Extends to max(2700, 2700+1) = 2701.
* Checks: "2701 >= 2701? YES"
* Result: "ORDER EXPIRES"
* **Mode/Uncross Check**:
* Checks mode change.
* Checks: "2701 >= 2700? YES"
* Asks: "Market orders present? 2701 >= 2700 + 1? YES"
* Result: "UNCROSS"
* **Final State**:
* "Both events occur in the same tick"
* "MIT201 violated"
### Key Observations
* The diagram illustrates a scenario where an order is initially set to expire at time 2700.
* In Tick 1, the order does not expire because 2700 is not greater than or equal to 2701. The system stays in auction because market orders are not present.
* In Tick 2, the order expires because 2701 is greater than or equal to 2701. The system uncrosses because market orders are present.
* The final state indicates a violation of MIT201, implying that both the order expiring and the uncross event occurring in the same tick is problematic.
### Interpretation
The sequence diagram depicts a critical race condition or conflict where the order expiry and uncross events occur simultaneously, leading to a violation of the MIT201 rule. This suggests a potential flaw in the system's logic or timing mechanism, where the expiry check and uncross check are not properly synchronized or prioritized. The diagram highlights the importance of ensuring that these events are handled in a specific order or with proper synchronization to avoid the MIT201 violation. The `market_extension` parameter seems to play a role in extending the auction, but the simultaneous expiry and uncross indicate a failure to prevent the conflict.
