Cache

Copy the value of (value, expr) from the last cache to the current cache

It assumes the value is Hot if it doesn't exist in the last cache.

It updates the current caches if the values change.

The two caches are required because we want to make sure in a new iteration, an expression is evaluated once.

Backup the state of the cache

All the shared data structures must be immutable.

Prepare for checking next class

1. Reset changed flag.

2. Commit current cache to stable cache if not changed.

3. Update stable heap if not changed.

4. Reset last cache.

Prepare cache for the next iteration

1. Reset changed flag.

2. Use current cache as last cache and set current cache to be empty.

3. Revert heap to stable.

Restore state from a backup