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+// itf/online-feature-itf.h
+
+// Copyright    2013  Johns Hopkins University (author: Daniel Povey)
+
+// See ../../COPYING for clarification regarding multiple authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+// THIS CODE IS PROVIDED *AS IS* BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED
+// WARRANTIES OR CONDITIONS OF TITLE, FITNESS FOR A PARTICULAR PURPOSE,
+// MERCHANTABLITY OR NON-INFRINGEMENT.
+// See the Apache 2 License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef KALDI_ITF_ONLINE_FEATURE_ITF_H_
+#define KALDI_ITF_ONLINE_FEATURE_ITF_H_ 1
+#include "base/kaldi-common.h"
+#include "matrix/matrix-lib.h"
+
+namespace kaldi {
+/// @ingroup Interfaces
+/// @{
+
+/**
+   OnlineFeatureInterface is an interface for online feature processing (it is
+   also usable in the offline setting, but currently we're not using it for
+   that).  This is for use in the online2/ directory, and it supersedes the
+   interface in ../online/online-feat-input.h.  We have a slighty different
+   model that puts more control in the hands of the calling thread, and won't
+   involve waiting on semaphores in the decoding thread.
+
+   This interface only specifies how the object *outputs* the features.
+   How it obtains the features, e.g. from a previous object or objects of type
+   OnlineFeatureInterface, is not specified in the interface and you will
+   likely define new constructors or methods in the derived type to do that.
+
+   You should appreciate that this interface is designed to allow random
+   access to features, as long as they are ready.  That is, the user
+   can call GetFrame for any frame less than NumFramesReady(), and when
+   implementing a child class you must not make assumptions about the
+   order in which the user makes these calls.
+*/
+   
+class OnlineFeatureInterface {
+ public:
+  virtual int32 Dim() const = 0; /// returns the feature dimension.
+  
+  /// Returns the total number of frames, since the start of the utterance, that
+  /// are now available.  In an online-decoding context, this will likely
+  /// increase with time as more data becomes available.
+  virtual int32 NumFramesReady() const = 0;
+
+  /// Returns true if this is the last frame.  Frame indices are zero-based, so the
+  /// first frame is zero.  IsLastFrame(-1) will return false, unless the file
+  /// is empty (which is a case that I'm not sure all the code will handle, so
+  /// be careful).  This function may return false for some frame if
+  /// we haven't yet decided to terminate decoding, but later true if we decide
+  /// to terminate decoding.  This function exists mainly to correctly handle
+  /// end effects in feature extraction, and is not a mechanism to determine how
+  /// many frames are in the decodable object (as it used to be, and for backward
+  /// compatibility, still is, in the Decodable interface).
+  virtual bool IsLastFrame(int32 frame) const = 0;
+  
+  /// Gets the feature vector for this frame.  Before calling this for a given
+  /// frame, it is assumed that you called NumFramesReady() and it returned a
+  /// number greater than "frame".  Otherwise this call will likely crash with
+  /// an assert failure.  This function is not declared const, in case there is
+  /// some kind of caching going on, but most of the time it shouldn't modify
+  /// the class.
+  virtual void GetFrame(int32 frame, VectorBase<BaseFloat> *feat) = 0;
+
+  /// Virtual destructor.  Note: constructors that take another member of
+  /// type OnlineFeatureInterface are not expected to take ownership of
+  /// that pointer; the caller needs to keep track of that manually.
+  virtual ~OnlineFeatureInterface() { }  
+};
+
+
+/// Add a virtual class for "source" features such as MFCC or PLP or pitch
+/// features.
+class OnlineBaseFeature: public OnlineFeatureInterface {
+ public:
+  /// This would be called from the application, when you get more wave data.
+  /// Note: the sampling_rate is typically only provided so the code can assert
+  /// that it matches the sampling rate expected in the options.
+  virtual void AcceptWaveform(BaseFloat sampling_rate,
+                              const VectorBase<BaseFloat> &waveform) = 0;
+
+  /// InputFinished() tells the class you won't be providing any
+  /// more waveform.  This will help flush out the last few frames
+  /// of delta or LDA features (it will typically affect the return value
+  /// of IsLastFrame.
+  virtual void InputFinished() = 0;
+};
+
+
+/// @}
+}  // namespace Kaldi
+
+#endif  // KALDI_ITF_ONLINE_FEATURE_ITF_H_