% prolog_backend.pl — Topology checker
% Preservation, reachability, floating ports, conduction soundness

:- module(topology_checker, [
  check_preservation/1,
  check_reachability/3,
  check_floating/1,
  check_conduction/1
]).

% Preservation: state before → state after (invariant holds)
check_preservation(Kernel) :-
  atom(Kernel),
  \+ \+ (
    state_before(S1),
    execute_kernel(Kernel, S1, S2),
    state_after(S2),
    invariant_holds(S2)
  ).

% Reachability: can reach goal from start?
check_reachability(Graph, Start, Goal) :-
  reachable(Graph, Start, Goal, []).

reachable(_, Goal, Goal, _) :- !.
reachable(Graph, Current, Goal, Visited) :-
  edge(Graph, Current, Next),
  \+ member(Next, Visited),
  reachable(Graph, Next, Goal, [Current|Visited]).

% Floating ports: all ports connected or grounded
check_floating(Circuit) :-
  forall(
    port(Circuit, Port),
    (connected(Port) ; grounded(Port))
  ).

% Conduction soundness: voltage transitions valid
check_conduction(Levels) :-
  forall(
    transition(Levels, From, To),
    valid_transition(From, To)
  ).

valid_transition(low, high).
valid_transition(high, low).
valid_transition(X, X).

% Dynamic facts
:- dynamic state_before/1.
:- dynamic state_after/1.
:- dynamic invariant_holds/1.
:- dynamic edge/3.
:- dynamic port/2.
:- dynamic connected/1.
:- dynamic grounded/1.
:- dynamic transition/4.

% Stub implementations
execute_kernel(_, State, State).