-module(lock3).
-export([init/3]).

%lock3: Each lock has a unique identifier: 1,2,3,4 which gives the lock a priority.
%Use of Lamport clocks to give locks in time priority order.

init(MyId, Nodes, MyClock) ->   %when the lock is started it is given a unique ID: MyId
    MyClock = 0,        %The initial value of MyClock is zero
    open(Nodes, MyId, MyClock).


open(Nodes, MyId, MyClock) ->
    receive
        {take, Master} ->   %my worker wants the lock
            Refs = requests(Nodes, MyId, MyClock),  %inform all other locks that i need to be taken
            wait(Nodes, Master, Refs, [], MyId, MyClock, MyClock);  %and enter waiting state (My current clock and my request clock!!!)
        {request, From, Ref, _, Clock} ->   %request from another lock
            NewClock = lists:max([MyClock, Clock]) + 1, %update MyClock
            From ! {ok, Ref, NewClock}, %immediately reply ok (i'm in open state)
            open(Nodes, MyId, NewClock);    %and enter open state again
        stop ->
            ok
    end.

requests(Nodes, MyId, MyClock) ->   %send a request message to all locks
    lists:map(fun(P) -> R = make_ref(), P ! {request, self(), R, MyId, MyClock}, R end, Nodes). %I send my ID together with the request message.

wait(Nodes, Master, [], Waiting, MyId, MyClock, _) ->   %waiting state with empty list of locks -- all of the locks sent me an ok message!!
    Master ! taken,     %the lock is taken
    held(Nodes, Waiting, MyId, MyClock);    %enter the held state

wait(Nodes, Master, Refs, Waiting, MyId, MyClock, MyReqClock) ->    %waiting for ok messages
    receive
    %prepei na allaksw to format tou mhnymatos request, wste na periexei kai to Id tou lock apo to opoio erxetai to request!!!
        {request, From, Ref, Req_Id, Req_Clock} ->
            NewClock = lists:max([MyClock, Req_Clock]) + 1, %update MyClock
            if
                MyReqClock > Req_Clock ->   %The requesting lock has higher clock priority
                From ! {ok, Ref, NewClock}, %send an ok message to it!
                % NOT back to open state -> back to WAIT state!!!
                wait(Nodes, Master, Refs, Waiting, MyId, NewClock, MyReqClock); %and enter waiting state

                MyReqClock == Req_Clock ->  %The req. clock has the same priority as me -> compare process IDs
                if
                    MyId < Req_Id ->    %The requesting lock has higher priority
                    From ! {ok, Ref, NewClock}, %send an ok message to it!
                    wait(Nodes, Master, Refs, Waiting, MyId, NewClock, MyReqClock); %and enter waiting state

                    true ->
                    wait(Nodes, Master, Refs, [{From, Ref}|Waiting], MyId, NewClock, MyReqClock)
                end;

                true -> %I have higher priority: I keep the request and I go on :p
                wait(Nodes, Master, Refs, [{From, Ref}|Waiting], MyId, NewClock, MyReqClock)
            end;

        {ok, Ref, Req_Clock2} ->    %i received an ok message from a lock
            NewClock2 = lists:max([MyClock, Req_Clock2]) + 1,   %update MyClock
            Refs2 = lists:delete(Ref, Refs),    %and I delete it from my list
            wait(Nodes, Master, Refs2, Waiting, MyId, NewClock2, MyReqClock);   %waiting for the rest of the ok messages

        release ->  %I have to release the lock
            ok(Waiting, MyClock),   %I send ok messages to the locks that requested me while I was waiting
            open(Nodes, MyId, MyClock)  %back to open state
    end.

ok(Waiting, MyClock) -> %send ok message
    lists:map(fun({F, R}) -> F ! {ok, R, MyClock} end, Waiting).

held(Nodes, Waiting, MyId, MyClock) ->  %The lock is mine!!!
    receive
        {request, From, Ref, _, Req_Clock} ->   %I keep on accepting requests from other locks to send them ok afterwards
            NewClock = lists:max([MyClock, Req_Clock]) + 1, %update MyClock
            held(Nodes, [{From, Ref}|Waiting], MyId, NewClock);
        release ->  %release message from the worker -> I have to release the lock
            ok(Waiting, MyClock),   %I inform the waiting locks!
            open(Nodes, MyId, MyClock)  %back to open state!!
    end.