code: # Copyright 2019 The Magenta 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
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  """Utility functions for working with polyphonic performances."""
  
  from __future__ import division
  
  import abc
  import collections
  import math
  
  from magenta.music import constants
  from magenta.music import events_lib
  from magenta.music import sequences_lib
  from magenta.music.protobuf import music_pb2
  import tensorflow as tf
  
  MAX_MIDI_PITCH = constants.MAX_MIDI_PITCH
  MIN_MIDI_PITCH = constants.MIN_MIDI_PITCH
  
  MAX_MIDI_VELOCITY = constants.MAX_MIDI_VELOCITY
  MIN_MIDI_VELOCITY = constants.MIN_MIDI_VELOCITY
  MAX_NUM_VELOCITY_BINS = MAX_MIDI_VELOCITY - MIN_MIDI_VELOCITY + 1
  
  STANDARD_PPQ = constants.STANDARD_PPQ
  
  DEFAULT_MAX_SHIFT_STEPS = 100
  DEFAULT_MAX_SHIFT_QUARTERS = 4
  
  DEFAULT_PROGRAM = 0
  
  
  class PerformanceEvent(object):
    """Class for storing events in a performance."""
  
    # Start of a new note.
    NOTE_ON = 1
    # End of a note.
    NOTE_OFF = 2
    # Shift time forward.
    TIME_SHIFT = 3
    # Change current velocity.
    VELOCITY = 4
    # Duration of preceding NOTE_ON.
    # For Note-based encoding, used instead of NOTE_OFF events.
    DURATION = 5
  
    def __init__(self, event_type, event_value):
      if event_type in (PerformanceEvent.NOTE_ON, PerformanceEvent.NOTE_OFF):
        if not MIN_MIDI_PITCH <= event_value <= MAX_MIDI_PITCH:
          raise ValueError('Invalid pitch value: %s' % event_value)
      elif event_type == PerformanceEvent.TIME_SHIFT:
        if not 0 <= event_value:
          raise ValueError('Invalid time shift value: %s' % event_value)
      elif event_type == PerformanceEvent.DURATION:
        if not 1 <= event_value:
          raise ValueError('Invalid duration value: %s' % event_value)
      elif event_type == PerformanceEvent.VELOCITY:
        if not 1 <= event_value <= MAX_NUM_VELOCITY_BINS:
          raise ValueError('Invalid velocity value: %s' % event_value)
      else:
        raise ValueError('Invalid event type: %s' % event_type)
  
      self.event_type = event_type
      self.event_value = event_value
  
    def __repr__(self):
      return 'PerformanceEvent(%r, %r)' % (self.event_type, self.event_value)
  
    def __eq__(self, other):
      if not isinstance(other, PerformanceEvent):
        return False
      return (self.event_type == other.event_type and
              self.event_value == other.event_value)
  
  
  def _velocity_bin_size(num_velocity_bins):
    return int(math.ceil(
        (MAX_MIDI_VELOCITY - MIN_MIDI_VELOCITY + 1) / num_velocity_bins))
  
  
  def velocity_to_bin(velocity, num_velocity_bins):
    return ((velocity - MIN_MIDI_VELOCITY) //
            _velocity_bin_size(num_velocity_bins) + 1)
  
  
  def velocity_bin_to_velocity(velocity_bin, num_velocity_bins):
    return (
        MIN_MIDI_VELOCITY + (velocity_bin - 1) *
        _velocity_bin_size(num_velocity_bins))
  
  
  def _program_and_is_drum_from_sequence(sequence, instrument=None):
    """Get MIDI program and is_drum from sequence and (optional) instrument.
  
    Args:
      sequence: The NoteSequence from which MIDI program and is_drum will be
          extracted.
      instrument: The instrument in `sequence` from which MIDI program and
          is_drum will be extracted, or None to consider all instruments.
  
    Returns:
      A tuple containing program and is_drum for the sequence and optional
      instrument. If multiple programs are found (or if is_drum is True),
      program will be None. If multiple values of is_drum are found, is_drum
      will be None.
    """
    notes = [note for note in sequence.notes
             if instrument is None or note.instrument == instrument]
    # Only set program for non-drum tracks.
    if all(note.is_drum for note in notes):
      is_drum = True
      program = None
    elif all(not note.is_drum for note in notes):
      is_drum = False
      programs = set(note.program for note in notes)
      program = programs.pop() if len(programs) == 1 else None
    else:
      is_drum = None
      program = None
    return program, is_drum
  
  
  class BasePerformance(events_lib.EventSequence):
    """Stores a polyphonic sequence as a stream of performance events.
  
    Events are PerformanceEvent objects that encode event type and value.
    """
    __metaclass__ = abc.ABCMeta
  
    def __init__(self, start_step, num_velocity_bins, max_shift_steps,
                 program=None, is_drum=None):
      """Construct a BasePerformance.
  
      Args:
        start_step: The offset of this sequence relative to the beginning of the
            source sequence.
        num_velocity_bins: Number of velocity bins to use.
        max_shift_steps: Maximum number of steps for a single time-shift event.
        program: MIDI program used for this performance, or None if not specified.
        is_drum: Whether or not this performance consists of drums, or None if not
            specified.
  
      Raises:
        ValueError: If `num_velocity_bins` is larger than the number of MIDI
            velocity values.
      """
      if num_velocity_bins > MAX_MIDI_VELOCITY - MIN_MIDI_VELOCITY + 1:
        raise ValueError(
            'Number of velocity bins is too large: %d' % num_velocity_bins)
  
      self._start_step = start_step
      self._num_velocity_bins = num_velocity_bins
      self._max_shift_steps = max_shift_steps
      self._program = program
      self._is_drum = is_drum
  
    @property
    def start_step(self):
      return self._start_step
  
    @property
    def max_shift_steps(self):
      return self._max_shift_steps
  
    @property
    def program(self):
      return self._program
  
    @property
    def is_drum(self):
      return self._is_drum
  
    def _append_steps(self, num_steps):
      """Adds steps to the end of the sequence."""
      if (self._events and
          self._events[-1].event_type == PerformanceEvent.TIME_SHIFT and
          self._events[-1].event_value < self._max_shift_steps):
        # Last event is already non-maximal time shift. Increase its duration.
        added_steps = min(num_steps,
                          self._max_shift_steps - self._events[-1].event_value)
        self._events[-1] = PerformanceEvent(
            PerformanceEvent.TIME_SHIFT,
            self._events[-1].event_value + added_steps)
        num_steps -= added_steps
  
      while num_steps >= self._max_shift_steps:
        self._events.append(
            PerformanceEvent(event_type=PerformanceEvent.TIME_SHIFT,
                             event_value=self._max_shift_steps))
        num_steps -= self._max_shift_steps
  
      if num_steps > 0:
        self._events.append(
            PerformanceEvent(event_type=PerformanceEvent.TIME_SHIFT,
                             event_value=num_steps))
  
    def _trim_steps(self, num_steps):
      """Trims a given number of steps from the end of the sequence."""
      steps_trimmed = 0
      while self._events and steps_trimmed < num_steps:
        if self._events[-1].event_type == PerformanceEvent.TIME_SHIFT:
          if steps_trimmed + self._events[-1].event_value > num_steps:
            self._events[-1] = PerformanceEvent(
                event_type=PerformanceEvent.TIME_SHIFT,
                event_value=(self._events[-1].event_value -
                             num_steps + steps_trimmed))
            steps_trimmed = num_steps
          else:
            steps_trimmed += self._events[-1].event_value
            self._events.pop()
        else:
          self._events.pop()
  
    def set_length(self, steps, from_left=False):
      """Sets the length of the sequence to the specified number of steps.
  
      If the event sequence is not long enough, pads with time shifts to make the
      sequence the specified length. If it is too long, it will be truncated to
      the requested length.
  
      Args:
        steps: How many quantized steps long the event sequence should be.
        from_left: Whether to add/remove from the left instead of right.
      """
      if from_left:
        raise NotImplementedError('from_left is not supported')
  
      if self.num_steps < steps:
        self._append_steps(steps - self.num_steps)
      elif self.num_steps > steps:
        self._trim_steps(self.num_steps - steps)
  
      assert self.num_steps == steps
  
    def append(self, event):
      """Appends the event to the end of the sequence.
  
      Args:
        event: The performance event to append to the end.
  
      Raises:
        ValueError: If `event` is not a valid performance event.
      """
      if not isinstance(event, PerformanceEvent):
        raise ValueError('Invalid performance event: %s' % event)
      self._events.append(event)
  
    def truncate(self, num_events):
      """Truncates this Performance to the specified number of events.
  
      Args:
        num_events: The number of events to which this performance will be
            truncated.
      """
      self._events = self._events[:num_events]
  
    def __len__(self):
      """How many events are in this sequence.
  
      Returns:
        Number of events as an integer.
      """
      return len(self._events)
  
    def __getitem__(self, i):
      """Returns the event at the given index."""
      return self._events[i]
  
    def __iter__(self):
      """Return an iterator over the events in this sequence."""
      return iter(self._events)
  
    def __str__(self):
      strs = []
      for event in self:
        if event.event_type == PerformanceEvent.NOTE_ON:
          strs.append('(%s, ON)' % event.event_value)
        elif event.event_type == PerformanceEvent.NOTE_OFF:
          strs.append('(%s, OFF)' % event.event_value)
        elif event.event_type == PerformanceEvent.TIME_SHIFT:
          strs.append('(%s, SHIFT)' % event.event_value)
        elif event.event_type == PerformanceEvent.VELOCITY:
          strs.append('(%s, VELOCITY)' % event.event_value)
        else:
          raise ValueError('Unknown event type: %s' % event.event_type)
      return '\n'.join(strs)
  
    @property
    def end_step(self):
      return self.start_step + self.num_steps
  
    @property
    def num_steps(self):
      """Returns how many steps long this sequence is.
  
      Returns:
        Length of the sequence in quantized steps.
      """
      steps = 0
      for event in self:
        if event.event_type == PerformanceEvent.TIME_SHIFT:
          steps += event.event_value
      return steps
  
    @property
    def steps(self):
      """Return a Python list of the time step at each event in this sequence."""
      step = self.start_step
      result = []
      for event in self:
        result.append(step)
        if event.event_type == PerformanceEvent.TIME_SHIFT:
          step += event.event_value
      return result
  
    @staticmethod
    def _from_quantized_sequence(quantized_sequence, start_step,
                                 num_velocity_bins, max_shift_steps,
                                 instrument=None):
      """Extract a list of events from the given quantized NoteSequence object.
  
      Within a step, new pitches are started with NOTE_ON and existing pitches are
      ended with NOTE_OFF. TIME_SHIFT shifts the current step forward in time.
      VELOCITY changes the current velocity value that will be applied to all
      NOTE_ON events.
  
      Args:
        quantized_sequence: A quantized NoteSequence instance.
        start_step: Start converting the sequence at this time step.
        num_velocity_bins: Number of velocity bins to use. If 0, velocity events
            will not be included at all.
        max_shift_steps: Maximum number of steps for a single time-shift event.
        instrument: If not None, extract only the specified instrument. Otherwise,
            extract all instruments into a single event list.
  
      Returns:
        A list of events.
      """
      notes = [note for note in quantized_sequence.notes
               if note.quantized_start_step >= start_step
               and (instrument is None or note.instrument == instrument)]
      sorted_notes = sorted(notes, key=lambda note: (note.start_time, note.pitch))
  
      # Sort all note start and end events.
      onsets = [(note.quantized_start_step, idx, False)
                for idx, note in enumerate(sorted_notes)]
      offsets = [(note.quantized_end_step, idx, True)
                 for idx, note in enumerate(sorted_notes)]
      note_events = sorted(onsets + offsets)
  
      current_step = start_step
      current_velocity_bin = 0
      performance_events = []
  
      for step, idx, is_offset in note_events:
        if step > current_step:
          # Shift time forward from the current step to this event.
          while step > current_step + max_shift_steps:
            # We need to move further than the maximum shift size.
            performance_events.append(
                PerformanceEvent(event_type=PerformanceEvent.TIME_SHIFT,
                                 event_value=max_shift_steps))
            current_step += max_shift_steps
          performance_events.append(
              PerformanceEvent(event_type=PerformanceEvent.TIME_SHIFT,
                               event_value=int(step - current_step)))
          current_step = step
  
        # If we're using velocity and this note's velocity is different from the
        # current velocity, change the current velocity.
        if num_velocity_bins:
          velocity_bin = velocity_to_bin(
              sorted_notes[idx].velocity, num_velocity_bins)
          if not is_offset and velocity_bin != current_velocity_bin:
            current_velocity_bin = velocity_bin
            performance_events.append(
                PerformanceEvent(event_type=PerformanceEvent.VELOCITY,
                                 event_value=current_velocity_bin))
  
        # Add a performance event for this note on/off.
        event_type = (
            PerformanceEvent.NOTE_OFF if is_offset else PerformanceEvent.NOTE_ON)
        performance_events.append(
            PerformanceEvent(event_type=event_type,
                             event_value=sorted_notes[idx].pitch))
  
      return performance_events
  
    @abc.abstractmethod
    def to_sequence(self, velocity, instrument, program, max_note_duration=None):
      """Converts the Performance to NoteSequence proto.
  
      Args:
        velocity: MIDI velocity to give each note. Between 1 and 127 (inclusive).
            If the performance contains velocity events, those will be used
            instead.
        instrument: MIDI instrument to give each note.
        program: MIDI program to give each note, or None to use the program
            associated with the Performance (or the default program if none
            exists).
        max_note_duration: Maximum note duration in seconds to allow. Notes longer
            than this will be truncated. If None, notes can be any length.
  
      Returns:
        A NoteSequence proto.
      """
      pass
  
    def _to_sequence(self, seconds_per_step, velocity, instrument, program,
                     max_note_duration=None):
      sequence_start_time = self.start_step * seconds_per_step
  
      sequence = music_pb2.NoteSequence()
      sequence.ticks_per_quarter = STANDARD_PPQ
  
      step = 0
  
      if program is None:
        # Use program associated with the performance (or default program).
        program = self.program if self.program is not None else DEFAULT_PROGRAM
      is_drum = self.is_drum if self.is_drum is not None else False
  
      # Map pitch to list because one pitch may be active multiple times.
      pitch_start_steps_and_velocities = collections.defaultdict(list)
      for i, event in enumerate(self):
        if event.event_type == PerformanceEvent.NOTE_ON:
          pitch_start_steps_and_velocities[event.event_value].append(
              (step, velocity))
        elif event.event_type == PerformanceEvent.NOTE_OFF:
          if not pitch_start_steps_and_velocities[event.event_value]:
            tf.logging.debug(
                'Ignoring NOTE_OFF at position %d with no previous NOTE_ON' % i)
          else:
            # Create a note for the pitch that is now ending.
            pitch_start_step, pitch_velocity = pitch_start_steps_and_velocities[
                event.event_value][0]
            pitch_start_steps_and_velocities[event.event_value] = (
                pitch_start_steps_and_velocities[event.event_value][1:])
            if step == pitch_start_step:
              tf.logging.debug(
                  'Ignoring note with zero duration at step %d' % step)
              continue
            note = sequence.notes.add()
            note.start_time = (pitch_start_step * seconds_per_step +
                               sequence_start_time)
            note.end_time = step * seconds_per_step + sequence_start_time
            if (max_note_duration and
                note.end_time - note.start_time > max_note_duration):
              note.end_time = note.start_time + max_note_duration
            note.pitch = event.event_value
            note.velocity = pitch_velocity
            note.instrument = instrument
            note.program = program
            note.is_drum = is_drum
            if note.end_time > sequence.total_time:
              sequence.total_time = note.end_time
        elif event.event_type == PerformanceEvent.TIME_SHIFT:
          step += event.event_value
        elif event.event_type == PerformanceEvent.VELOCITY:
          assert self._num_velocity_bins
          velocity = velocity_bin_to_velocity(
              event.event_value, self._num_velocity_bins)
        else:
          raise ValueError('Unknown event type: %s' % event.event_type)
  
      # There could be remaining pitches that were never ended. End them now
      # and create notes.
      for pitch in pitch_start_steps_and_velocities:
        for pitch_start_step, pitch_velocity in pitch_start_steps_and_velocities[
            pitch]:
          if step == pitch_start_step:
            tf.logging.debug(
                'Ignoring note with zero duration at step %d' % step)
            continue
          note = sequence.notes.add()
          note.start_time = (pitch_start_step * seconds_per_step +
                             sequence_start_time)
          note.end_time = step * seconds_per_step + sequence_start_time
          if (max_note_duration and
              note.end_time - note.start_time > max_note_duration):
            note.end_time = note.start_time + max_note_duration
          note.pitch = pitch
          note.velocity = pitch_velocity
          note.instrument = instrument
          note.program = program
          note.is_drum = is_drum
          if note.end_time > sequence.total_time:
            sequence.total_time = note.end_time
  
      return sequence
  
  
  class Performance(BasePerformance):
    """Performance with absolute timing and unknown meter."""
  
    def __init__(self, quantized_sequence=None, steps_per_second=None,
                 start_step=0, num_velocity_bins=0,
                 max_shift_steps=DEFAULT_MAX_SHIFT_STEPS, instrument=None,
                 program=None, is_drum=None):
      """Construct a Performance.
  
      Either quantized_sequence or steps_per_second should be supplied.
  
      Args:
        quantized_sequence: A quantized NoteSequence proto.
        steps_per_second: Number of quantized time steps per second, if using
            absolute quantization.
        start_step: The offset of this sequence relative to the
            beginning of the source sequence. If a quantized sequence is used as
            input, only notes starting after this step will be considered.
        num_velocity_bins: Number of velocity bins to use. If 0, velocity events
            will not be included at all.
        max_shift_steps: Maximum number of steps for a single time-shift event.
        instrument: If not None, extract only the specified instrument from
            `quantized_sequence`. Otherwise, extract all instruments.
        program: MIDI program used for this performance, or None if not specified.
            Ignored if `quantized_sequence` is provided.
        is_drum: Whether or not this performance consists of drums, or None if not
            specified. Ignored if `quantized_sequence` is provided.
  
      Raises:
        ValueError: If both or neither of `quantized_sequence` or
            `steps_per_second` is specified.
      """
      if (quantized_sequence, steps_per_second).count(None) != 1:
        raise ValueError(
            'Must specify exactly one of quantized_sequence or steps_per_second')
  
      if quantized_sequence:
        sequences_lib.assert_is_absolute_quantized_sequence(quantized_sequence)
        self._steps_per_second = (
            quantized_sequence.quantization_info.steps_per_second)
        self._events = self._from_quantized_sequence(
            quantized_sequence, start_step, num_velocity_bins,
            max_shift_steps=max_shift_steps, instrument=instrument)
        program, is_drum = _program_and_is_drum_from_sequence(
            quantized_sequence, instrument)
  
      else:
        self._steps_per_second = steps_per_second
        self._events = []
  
      super(Performance, self).__init__(
          start_step=start_step,
          num_velocity_bins=num_velocity_bins,
          max_shift_steps=max_shift_steps,
          program=program,
          is_drum=is_drum)
  
    @property
    def steps_per_second(self):
      return self._steps_per_second
  
    def to_sequence(self,
                    velocity=100,
                    instrument=0,
                    program=None,
                    max_note_duration=None):
      """Converts the Performance to NoteSequence proto.
  
      Args:
        velocity: MIDI velocity to give each note. Between 1 and 127 (inclusive).
            If the performance contains velocity events, those will be used
            instead.
        instrument: MIDI instrument to give each note.
        program: MIDI program to give each note, or None to use the program
            associated with the Performance (or the default program if none
            exists).
        max_note_duration: Maximum note duration in seconds to allow. Notes longer
            than this will be truncated. If None, notes can be any length.
  
      Returns:
        A NoteSequence proto.
      """
      seconds_per_step = 1.0 / self.steps_per_second
      return self._to_sequence(
          seconds_per_step=seconds_per_step,
          velocity=velocity,
          instrument=instrument,
          program=program,
          max_note_duration=max_note_duration)
  
  
  class MetricPerformance(BasePerformance):
    """Performance with quarter-note relative timing."""
  
    def __init__(self, quantized_sequence=None, steps_per_quarter=None,
                 start_step=0, num_velocity_bins=0,
                 max_shift_quarters=DEFAULT_MAX_SHIFT_QUARTERS, instrument=None,
                 program=None, is_drum=None):
      """Construct a MetricPerformance.
  
      Either quantized_sequence or steps_per_quarter should be supplied.
  
      Args:
        quantized_sequence: A quantized NoteSequence proto.
        steps_per_quarter: Number of quantized time steps per quarter note, if
            using metric quantization.
        start_step: The offset of this sequence relative to the
            beginning of the source sequence. If a quantized sequence is used as
            input, only notes starting after this step will be considered.
        num_velocity_bins: Number of velocity bins to use. If 0, velocity events
            will not be included at all.
        max_shift_quarters: Maximum number of quarter notes for a single time-
            shift event.
        instrument: If not None, extract only the specified instrument from
            `quantized_sequence`. Otherwise, extract all instruments.
        program: MIDI program used for this performance, or None if not specified.
            Ignored if `quantized_sequence` is provided.
        is_drum: Whether or not this performance consists of drums, or None if not
            specified. Ignored if `quantized_sequence` is provided.
  
      Raises:
        ValueError: If both or neither of `quantized_sequence` or
            `steps_per_quarter` is specified.
      """
      if (quantized_sequence, steps_per_quarter).count(None) != 1:
        raise ValueError(
            'Must specify exactly one of quantized_sequence or steps_per_quarter')
  
      if quantized_sequence:
        sequences_lib.assert_is_relative_quantized_sequence(quantized_sequence)
        self._steps_per_quarter = (
            quantized_sequence.quantization_info.steps_per_quarter)
        self._events = self._from_quantized_sequence(
            quantized_sequence, start_step, num_velocity_bins,
            max_shift_steps=self._steps_per_quarter * max_shift_quarters,
            instrument=instrument)
        program, is_drum = _program_and_is_drum_from_sequence(
            quantized_sequence, instrument)
  
      else:
        self._steps_per_quarter = steps_per_quarter
        self._events = []
  
      super(MetricPerformance, self).__init__(
          start_step=start_step,
          num_velocity_bins=num_velocity_bins,
          max_shift_steps=self._steps_per_quarter * max_shift_quarters,
          program=program,
          is_drum=is_drum)
  
    @property
    def steps_per_quarter(self):
      return self._steps_per_quarter
  
    def to_sequence(self,
                    velocity=100,
                    instrument=0,
                    program=None,
                    max_note_duration=None,
                    qpm=120.0):
      """Converts the Performance to NoteSequence proto.
  
      Args:
        velocity: MIDI velocity to give each note. Between 1 and 127 (inclusive).
            If the performance contains velocity events, those will be used
            instead.
        instrument: MIDI instrument to give each note.
        program: MIDI program to give each note, or None to use the program
            associated with the Performance (or the default program if none
            exists).
        max_note_duration: Maximum note duration in seconds to allow. Notes longer
            than this will be truncated. If None, notes can be any length.
        qpm: The tempo to use, in quarter notes per minute.
  
      Returns:
        A NoteSequence proto.
      """
      seconds_per_step = 60.0 / (self.steps_per_quarter * qpm)
      sequence = self._to_sequence(
          seconds_per_step=seconds_per_step,
          velocity=velocity,
          instrument=instrument,
          program=program,
          max_note_duration=max_note_duration)
      sequence.tempos.add(qpm=qpm)
      return sequence
  
  
  class NotePerformanceError(Exception):
    pass
  
  
  class TooManyTimeShiftStepsError(NotePerformanceError):
    pass
  
  
  class TooManyDurationStepsError(NotePerformanceError):
    pass
  
  
  class NotePerformance(BasePerformance):
    """Stores a polyphonic sequence as a stream of performance events.
  
    Events are PerformanceEvent objects that encode event type and value.
    In this version, the performance is encoded in 4-event tuples:
    TIME_SHIFT, NOTE_ON, VELOCITY, DURATION.
    """
  
    def __init__(self, quantized_sequence, num_velocity_bins, instrument=0,
                 start_step=0, max_shift_steps=1000, max_duration_steps=1000):
      """Construct a NotePerformance.
  
      Args:
        quantized_sequence: A quantized NoteSequence proto.
        num_velocity_bins: Number of velocity bins to use.
        instrument: If not None, extract only the specified instrument from
            `quantized_sequence`. Otherwise, extract all instruments.
        start_step: The offset of this sequence relative to the beginning of the
            source sequence.
        max_shift_steps: Maximum number of steps for a time-shift event.
        max_duration_steps: Maximum number of steps for a duration event.
  
      Raises:
        ValueError: If `num_velocity_bins` is larger than the number of MIDI
            velocity values.
      """
      program, is_drum = _program_and_is_drum_from_sequence(
          quantized_sequence, instrument)
  
      super(NotePerformance, self).__init__(
          start_step=start_step,
          num_velocity_bins=num_velocity_bins,
          max_shift_steps=max_shift_steps,
          program=program,
          is_drum=is_drum)
  
      self._max_duration_steps = max_duration_steps
  
      sequences_lib.assert_is_absolute_quantized_sequence(quantized_sequence)
      self._steps_per_second = (
          quantized_sequence.quantization_info.steps_per_second)
      self._events = self._from_quantized_sequence(
          quantized_sequence, instrument)
  
    @property
    def steps_per_second(self):
      return self._steps_per_second
  
    def set_length(self, steps, from_left=False):
      # This is not actually implemented, but to avoid raising exceptions during
      # generation just return instead of raising NotImplementedError.
      # TODO(fjord): Implement this.
      return
  
    def append(self, event):
      """Appends the event to the end of the sequence.
  
      Args:
        event: The performance event tuple to append to the end.
  
      Raises:
        ValueError: If `event` is not a valid performance event tuple.
      """
      if not isinstance(event, tuple):
        raise ValueError('Invalid performance event tuple: %s' % event)
      self._events.append(event)
  
    def __str__(self):
      strs = []
      for event in self:
        strs.append('TIME_SHIFT<%s>, NOTE_ON<%s>, VELOCITY<%s>, DURATION<%s>' % (
            event[0].event_value, event[1].event_value, event[2].event_value,
            event[3].event_value))
      return '\n'.join(strs)
  
    @property
    def num_steps(self):
      """Returns how many steps long this sequence is.
  
      Returns:
        Length of the sequence in quantized steps.
      """
      steps = 0
      for event in self._events:
        steps += event[0].event_value
      if self._events:
        steps += self._events[-1][3].event_value
      return steps
  
    @property
    def steps(self):
      """Return a Python list of the time step at each event in this sequence."""
      step = self.start_step
      result = []
      for event in self:
        step += event[0].event_value
        result.append(step)
      return result
  
    def _from_quantized_sequence(self, quantized_sequence, instrument):
      """Extract a list of events from the given quantized NoteSequence object.
  
      Within a step, new pitches are started with NOTE_ON and existing pitches are
      ended with NOTE_OFF. TIME_SHIFT shifts the current step forward in time.
      VELOCITY changes the current velocity value that will be applied to all
      NOTE_ON events.
  
      Args:
        quantized_sequence: A quantized NoteSequence instance.
        instrument: If not None, extract only the specified instrument. Otherwise,
            extract all instruments into a single event list.
  
      Returns:
        A list of events.
  
      Raises:
        TooManyTimeShiftStepsError: If the maximum number of time
          shift steps is exceeded.
        TooManyDurationStepsError: If the maximum number of duration
          shift steps is exceeded.
      """
      notes = [note for note in quantized_sequence.notes
               if note.quantized_start_step >= self.start_step
               and (instrument is None or note.instrument == instrument)]
      sorted_notes = sorted(notes, key=lambda note: (note.start_time, note.pitch))
  
      current_step = self.start_step
      performance_events = []
  
      for note in sorted_notes:
        sub_events = []
  
        # TIME_SHIFT
        time_shift_steps = note.quantized_start_step - current_step
        if time_shift_steps > self._max_shift_steps:
          raise TooManyTimeShiftStepsError(
              'Too many steps for timeshift: %d' % time_shift_steps)
        else:
          sub_events.append(
              PerformanceEvent(event_type=PerformanceEvent.TIME_SHIFT,
                               event_value=time_shift_steps))
        current_step = note.quantized_start_step
  
        # NOTE_ON
        sub_events.append(
            PerformanceEvent(event_type=PerformanceEvent.NOTE_ON,
                             event_value=note.pitch))
  
        # VELOCITY
        velocity_bin = velocity_to_bin(note.velocity, self._num_velocity_bins)
        sub_events.append(
            PerformanceEvent(event_type=PerformanceEvent.VELOCITY,
                             event_value=velocity_bin))
  
        # DURATION
        duration_steps = note.quantized_end_step - note.quantized_start_step
        if duration_steps > self._max_duration_steps:
          raise TooManyDurationStepsError(
              'Too many steps for duration: %s' % note)
        sub_events.append(
            PerformanceEvent(event_type=PerformanceEvent.DURATION,
                             event_value=duration_steps))
  
        performance_events.append(tuple(sub_events))
  
      return performance_events
  
    def to_sequence(self, instrument=0, program=None, max_note_duration=None):
      """Converts the Performance to NoteSequence proto.
  
      Args:
        instrument: MIDI instrument to give each note.
        program: MIDI program to give each note, or None to use the program
            associated with the Performance (or the default program if none
            exists).
        max_note_duration: Not used in this implementation.
  
      Returns:
        A NoteSequence proto.
      """
      seconds_per_step = 1.0 / self.steps_per_second
      sequence_start_time = self.start_step * seconds_per_step
  
      sequence = music_pb2.NoteSequence()
      sequence.ticks_per_quarter = STANDARD_PPQ
  
      step = 0
  
      if program is None:
        # Use program associated with the performance (or default program).
        program = self.program if self.program is not None else DEFAULT_PROGRAM
      is_drum = self.is_drum if self.is_drum is not None else False
  
      for event in self:
        step += event[0].event_value
  
        note = sequence.notes.add()
        note.start_time = step * seconds_per_step + sequence_start_time
        note.end_time = ((step + event[3].event_value) * seconds_per_step +
                         sequence_start_time)
        note.pitch = event[1].event_value
        note.velocity = velocity_bin_to_velocity(
            event[2].event_value, self._num_velocity_bins)
        note.instrument = instrument
        note.program = program
        note.is_drum = is_drum
  
        if note.end_time > sequence.total_time:
          sequence.total_time = note.end_time
  
      return sequence
  
apis: tensorflow.logging.debug