State evaluations with path dependent 
heuristics/evaluators (e.g.
LMCountHeuristic, GEvaluator, 
PrefEvaluator) require information from 
parent states.

Fast Downward already provides 
EvaluationContext that maintains 
information
during a state evaluation phase and makes 
this information available to
different components (heuristics, 
evaluators, open lists). This short term
caching of information saves re-
computation of information used by 
several
components.

At the moment EvaluationContext stores 
the state to evaluate, heuristic values,
current path cost and a preferred flag.

I suggest to additionally store a 
reference to the evaluation context of 
the
parent state in order to enable 
components to have access to parent state 
and
its related information.

In a second step, setting or updating the 
path cost of a state and determining
if a state is preferred can be done in 
g_evaluator respectively in
pref_evaluator instead of doing this in 
the search code.

In a third step, evaluators could cache 
their results themselves analogous to
heuristic.

Note, that decoupling evaluations from 
the search code by providing adding the
parent evaluation context allows for 
implementing many other path dependent
evaluators without the need of touching 
the search code. For example a depth
evaluator (issue641).

I am closing this because I think we are now addressing the same goal with a
different mechanism, namely the notify methods of evaluators. If it later turns
out we need something different/something additional, we can open a new issue or
reopen this one.