Sly 2:Game state

The DAG (Directed Acyclic Graph) is a data structure used to manage the current state of the game world. It was the subject of a 2005 GDC presentation by Bruce Oberg, the Principal Engineer at Sucker Punch.

Tasks
The DAG is a directed graph of nodes, where each node is called a task. For instance, t1_follow_dmitri_on_streets is the task of tailing Dimitri in the "Follow Dimitri" job (the prefix t1 indicates that it is a task in Episode 1, and follow is the codename for the "Follow Dimitri" job).

Task States
Each task has a state, which is one of the following:


 * 0 – Unavailable
 * 1 – Available
 * 2 – Complete
 * 3 – Final

All tasks in the DAG start off as Unavailable. A task becomes Available once all of its predecessors are Complete. Tasks become Complete through conditions being met in the level scripts, and tasks become Final through missions.

State Forcing
At all times, every task before an Available task must be Complete or Final. Additionally, every task after an Available task must be unavailable. It is possible to force a task to a particular state, which propagates throughout the DAG to ensure those conditions are always met.

Missions
Tasks in the DAG can be grouped into missions. A mission usually refers to a particular job in the game's story. For instance, m1_follow is the job "Follow Dmitri" (the prefix m1 indicates that it is a mission in Episode 1, and follow is the codename for the mission).

If a task belongs to a mission and the exit task in that mission is Completed, then all the tasks in that job become Final.

Checkpoints
A task is considered a checkpoint task if it has some checkpoint data associated with it, including a particular character and location in the game world. When a checkpoint task becomes Available, that checkpoint is set as the latest checkpoint.

Rollbacks
A rollback is when the game rewinds the DAG to an earlier state (such as when the player dies). This is accomplished by:
 * Resetting the world
 * Placing the player at the latest checkpoint
 * Forcing the checkpoint task to Available (which may result in the states of other tasks being updated)