%% Backward chaining

-module(backward).
-export([start/0,solveGoal/2,solveRule/2,collect/3,unify/2]).

start() ->
    Query = io:read("Enter query: "),
    solveGoal(element(2, Query), self()),
    receive
        {_, Truth} -> io:format("Answer is: ~w~n", [Truth])
    end.

solveGoal(Goal, Caller) ->
    %% Fetch rules...
    Rule = case Goal of
        g -> [g,a,b,c];
        a -> [a];
        b -> [b];
        c -> [c]
    end,
    Applicable = backward:unify(Goal, Rule),
    if Applicable == true ->
        if
            %% If RHS is null
            tl(Rule) == [] -> Caller ! {Goal, 0.9};
            %% If RHS contains subgoals
            true -> spawn(backward, solveRule, [tl(Rule), self()])
        end
    %% If unify fails do nothing
    end,
    receive
        %% Each message is of the form {goal,truth}
        %% Just accept the 1st answer
        {_, Truth} -> Caller ! {Goal, Truth}
    end.

solveRule(RuleTail, Caller) ->
    N = length(RuleTail),
    Collector = spawn(backward, collect, [[], N, Caller]),
    %% For each subgoal in Rule
    %% Spawn a process to solve each subgoal
    lists:map(fun(Subgoal) -> spawn(backward, solveGoal, [Subgoal, Collector]) end,
        RuleTail).

%% Accumulate results
collect(Truths, N, Caller) ->
    %% If we have N messages send back the answer
    %% Answer = min of truth values, equivalent to fuzzy-logic AND
    if length(Truths) == N -> Caller ! {dont_care, lists:min(Truths)};
       true -> true
    end,
    receive
        {_, Truth} -> collect([Truth | Truths], N, Caller)
    end.

%% Unify: just a stub
unify(Goal,Rule) -> Goal == hd(Rule).