unit Myc.Core.Signals;

interface

uses
    System.SysUtils,
    Myc.Core.Notifier,
    Myc.Signals;

type
    TMycState = class abstract( TInterfacedObject, IMycState )
    strict private
    class var
        FNull: IMycState;
        class constructor ClassCreate;
    protected
        function GetIsSet: Boolean; virtual; abstract;
    public
        function Subscribe(Subscriber: IMycSubscriber): TMycSubscription; virtual; abstract;
        procedure Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag); virtual; abstract;
        property IsSet: Boolean read GetIsSet;
        // Provides access to the singleton null latch instance (always set).
        class property Null: IMycState read FNull;
    end;

    // TMycNullState implements a "null object" pattern for IMycState.
    // This state is always considered "set".
    TMycNullState = class(TInterfacedObject, IMycState)
    protected
        // IMycState
        function GetIsSet: Boolean;
        function Subscribe(Subscriber: IMycSubscriber): TMycSubscription;
        procedure Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag);
    public
        constructor Create;
    end;

    // TMycLatch implements a countdown latch.
    // It is initialized with a count. Calls to its Notify method (as an IMycSubscriber)
    // decrement this count. When the count reaches zero, the latch transitions to the "set"
    // state (IsSet becomes true), notifies its current subscribers once, and then remains set.
    TMycLatch = class(TMycState, IMycLatch)
    strict private
        FSubscribers: TMycNotifyList<IMycSubscriber>; // List of subscribers waiting for this latch to be set.
    class var
        FNull: IMycLatch; // Singleton instance of a latch that is always set.
        class constructor ClassCreate; // Class constructor to initialize FNull.
    private
        [volatile] FCount: Integer; // The internal countdown value for the latch.
        function GetState: IMycState; // Implementation for IMycLatch.GetState and IMycFlag.GetState.
    protected
        function GetIsSet: Boolean; override; final;  // Implementation for IMycState.GetIsSet.
    public
        // Creates the latch with an initial count.
        // If ACount is 0 or less, the latch is effectively pre-set (delegates to FNull via CreateLatch factory).
        constructor Create(ACount: Integer);
        destructor Destroy; override;

        // Factory method: creates a new countdown latch or returns the Null latch if Count <= 0.
        class function CreateLatch(Count: Integer): IMycLatch; static;

        // IMycSignal implementation
        function Subscribe(Subscriber: IMycSubscriber): TMycSubscription; override;
        procedure Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag); override;

        // IMycSubscriber implementation for TMycLatch itself.
        // Decrements the internal count. If the count reaches zero,
        // registered subscribers are notified, and the latch becomes permanently set.
        function Notify: Boolean;

        // Provides access to the singleton null latch instance (always set).
        class property Null: IMycLatch read FNull;
    end;

    // TMycDirty implements a resettable "dirty flag".
    // It can be explicitly set to dirty (via Notify) or reset to clean (via Reset).
    // Subscribers are notified of relevant state changes.
    TMycDirty = class(TMycState, IMycDirty)
    strict private
        FSubscribers: TMycNotifyList<IMycSubscriber>; // List of subscribers interested in state changes of this dirty flag.
    private
        [volatile] FFlag: Boolean; // Internal state: true if dirty/set, false if clean/reset.
        function GetState: IMycState; // Implementation for IMycFlag.GetState.
    protected
        function GetIsSet: Boolean; override; final;  // Implementation for IMycState.GetIsSet (true if dirty).
    public
        // Creates a new dirty flag, initially set to dirty (true).
        constructor Create;
        destructor Destroy; override;

        // Factory method to create a new TMycDirty instance.
        class function CreateDirty: IMycDirty; static;

        // IMycSignal implementation
        function Subscribe(Subscriber: IMycSubscriber): TMycSubscription; override;
        procedure Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag); override;

        // IMycSubscriber implementation for TMycDirty.
        // Sets this flag to dirty (true). Notifies subscribers if it transitioned from clean to dirty.
        function Notify: Boolean;

        // IMycDirty implementation. Resets the flag to clean (false).
        function Reset: Boolean;
    end;

implementation

uses
    System.SyncObjs; // For TInterlocked

class constructor TMycState.ClassCreate;
begin
    // Create a singleton null latch instance that is initially (and always) set.
    FNull := TMycNullState.Create(); // Calls the instance constructor
end;

{ TMycNullState }

constructor TMycNullState.Create;
begin
    inherited Create;
end;

function TMycNullState.GetIsSet: Boolean;
begin
    Result := true; // Null state is always set.
end;

function TMycNullState.Subscribe(Subscriber: IMycSubscriber): TMycSubscription;
begin
    // Since the state is always set, notify the subscriber immediately.
    if Assigned(Subscriber) then
    begin
        Subscriber.Notify;
    end;
    // No ongoing subscription is necessary or meaningful for a null state.
    Result.Create(Self, nil); // Return an empty subscription.
end;

procedure TMycNullState.Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag);
begin
    // Unsubscribing from a null state has no effect.
end;

{ TMycLatch }

constructor TMycLatch.Create(ACount: Integer);
begin
    inherited Create;
    Assert( ACount >= 0 );
    FCount := ACount;
    FSubscribers.Create;
end;

class constructor TMycLatch.ClassCreate;
begin
    // Create a singleton null latch instance that is initially (and always) set.
    FNull := TMycLatch.Create(0); // Calls the instance constructor
end;

destructor TMycLatch.Destroy;
begin
    FSubscribers.Destroy;
    inherited;
end;

class function TMycLatch.CreateLatch(Count: Integer): IMycLatch;
begin
    // Factory method: returns a new latch or the shared Null latch if Count <= 0.
    if Count > 0 then
        Result := TMycLatch.Create(Count) // Instance constructor
    else
        Result := FNull;
end;

function TMycLatch.Notify: Boolean;
var
    currentCountAfterDecrement: Integer;
    shouldNotifySubscribers: Boolean;
begin
    FSubscribers.Lock;
    try
        currentCountAfterDecrement := TInterlocked.Decrement(FCount);

        if currentCountAfterDecrement < -MaxInt div 2 then
            TInterlocked.Increment(FCount); // Defend against extreme underflow.

        shouldNotifySubscribers := (currentCountAfterDecrement = 0); // Notify only on transition to zero.

        if shouldNotifySubscribers then
        begin
            FSubscribers.Notify(
                function(Subscriber: IMycSubscriber): Boolean
                begin
                    Result := Subscriber.Notify;
                end);
        end;

        // Returns true if the latch has not yet been set by this Notify call (count > 0).
        Result := currentCountAfterDecrement > 0;

        // If the latch is now set (or was already set), unadvise all subscribers.
        // This ensures one-shot notification for the current set of subscribers.
        if currentCountAfterDecrement <= 0 then
            FSubscribers.UnadviseAll;
    finally
        FSubscribers.Release;
    end;
end;

function TMycLatch.GetIsSet: Boolean;
begin
    // The latch is set if its count has reached zero or less.
    Result := FCount <= 0;
end;

function TMycLatch.GetState: IMycState;
begin
    // TMycLatch itself implements IMycState.
    exit(Self);
end;

function TMycLatch.Subscribe(Subscriber: IMycSubscriber): TMycSubscription;
var
    alreadySet: Boolean;
begin
    if not Assigned(Subscriber) then
        exit;

    FSubscribers.Lock;
    try
        alreadySet := (FCount <= 0); // Check if latch is already set.

        if alreadySet then
        begin
            // If already set, notify immediately; no persistent subscription needed.
            Subscriber.Notify;
        end
        else
        begin
            // If not yet set, advise the subscriber.
            Result.Create(Self, FSubscribers.Advise(Subscriber));
        end;
    finally
        FSubscribers.Release;
    end;
end;

procedure TMycLatch.Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag);
begin
    if Tag = nil then
        exit;

    FSubscribers.Lock;
    try
        FSubscribers.Unadvise(Tag);
    finally
        FSubscribers.Release;
    end;
end;

{ TMycDirty }

constructor TMycDirty.Create;
begin
    inherited Create;
    FFlag := true; // A new dirty flag is initially considered dirty.
    FSubscribers.Create;
end;

class function TMycDirty.CreateDirty: IMycDirty;
begin
    // Factory method to create a new TMycDirty instance.
    Result := TMycDirty.Create;
end;

destructor TMycDirty.Destroy;
begin
    FSubscribers.Destroy; // Clean up the subscriber list.
    inherited;
end;

function TMycDirty.Reset: Boolean;
begin
    // Sets the flag to false (clean) and returns true if it was previously true (dirty).
    Result := TInterlocked.Exchange(FFlag, false);
end;

function TMycDirty.GetIsSet: Boolean;
begin
    // IsSet is true if the flag is dirty (FFlag = true).
    Result := FFlag;
end;

function TMycDirty.GetState: IMycState;
begin
    // TMycDirty itself implements IMycState.
    exit(Self);
end;

function TMycDirty.Notify: Boolean;
var
    wasPreviouslyClean: Boolean;
begin
    FSubscribers.Lock;
    try
        // Set the flag to true (dirty) and check if it was previously false (clean).
        wasPreviouslyClean := not TInterlocked.Exchange(FFlag, true);

        if wasPreviouslyClean then // Only notify subscribers if state changed from clean to dirty.
        begin
            FSubscribers.Notify(
                function(Subscriber: IMycSubscriber): Boolean
                begin
                    Result := Subscriber.Notify;
                end);
        end;
        // The return value of this Notify (as an IMycSubscriber) indicates if this
        // TMycDirty instance itself would want more notifications if it were subscribed to something.
        // Here, it reflects whether a state change to dirty occurred due to this call.
        Result := wasPreviouslyClean;
    finally
        FSubscribers.Release;
    end;
end;

function TMycDirty.Subscribe(Subscriber: IMycSubscriber): TMycSubscription;
begin
    if not Assigned(Subscriber) then
        exit;

    // For TMycDirty, we always add the subscriber and then check if we need to notify immediately.
    // Ref counting for the subscriber interface is assumed to be handled by TMycNotifyList.Advise/Unadvise.
    // Or, if explicit management is needed like in TMycLatch, it should be added.
    // For consistency with TMycLatch, let's add AddRef/Release here too.
    FSubscribers.Lock;
    try
        // Add subscriber regardless of current state.
        Result.Create(Self, FSubscribers.Advise(Subscriber));
        if FFlag then // If currently dirty, notify the new subscriber immediately.
        begin
            Subscriber.Notify;
        end;
    finally
        FSubscribers.Release;
    end;
end;

procedure TMycDirty.Unsubscribe(Tag: TMycNotifyList<IMycSubscriber>.TTag);
begin
    if Tag = nil then
        exit;

    FSubscribers.Lock;
    try
        FSubscribers.Unadvise(Tag);
    finally
        FSubscribers.Release;
    end;
end;

end.