TIconList = class
strict private
  type
    TIconSize = record
      Width: Word;
      Height: Word;
    end;
    THash = type Integer;
    TIconBucket = TDictionary<THash, Integer>;
  class var
    FCurIdx: Integer;
strict private
  FIcons: TObjectDictionary<TIconSize, TIconBucket>;
strict private
  class function CalcHash(AData: Pointer; ADataSize: Integer): THash; static;
public
  constructor Create;
  destructor Destroy; override;
  function TryAddIcon(AData: Pointer; ADataSize, AWidth, AHeight: Integer): Integer;
end;

{ TIconList }

constructor TIconList.Create;
begin
  inherited Create;
  FIcons := TObjectDictionary<TIconSize, TIconBucket>.Create([doOwnsValues]);
end;

destructor TIconList.Destroy;
begin
  FIcons.Free;
  inherited Destroy;
end;

class function TIconList.CalcHash(AData: Pointer; ADataSize: Integer): THash;
begin
  // Result := 0;
end;

function TIconList.TryAddIcon(AData: Pointer; ADataSize, AWidth,
  AHeight: Integer): Integer;
var
  LSize: TIconSize;
  LBucket: TIconBucket;
  LHash: THash;
begin
  LSize.Width := AWidth;
  LSize.Height := AHeight;
  if not FIcons.TryGetValue(LSize, LBucket) then begin
    LBucket := TIconBucket.Create;
    FIcons.Add(LSize, LBucket);
  end;
  LHash := CalcHash(AData, ADataSize);
  if not LBucket.TryGetValue(LHash, Result) then begin
    Result := FCurIdx;
    Inc(FCurIdx);
    LBucket.Add(LHash, Result);
  end;
end;

var
  LMemStrm: TMemoryStream;
  LIStrm: IStream;
begin
  LMemStrm := TMemoryStream.Create;
  try
    LIStrm := TStreamAdapter.Create(LMemStrm, soReference);
    GPCheck(GdipSaveImageToStream(Handle, LIStrm, @TGDIPlusManager.PngEncoder, nil));
    Idx := IconList.TryAddIcon(LMemStrm.Memory, LMemStrm.Size, FIconWidth, FIconHight);
  finally
    LMemStrm.Free;
  end;
end;