# Copyright 2018 The TensorFlow Authors. All Rights Reserved.
#
# 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.
# ==============================================================================
"""Control flow statements: loops, conditionals, etc.

Note: most of these operators accept pairs of get_state/set_state functions, to
capture mutations that the corresponding code blocks might make. These
mutations only need to be captured when staging the control flow, and they just
work when reverting to Python behavior.

__Examples__

```
while cond:
  self.x += i
```

When the functionalized version is executed as a Python loop, it just works:

```
def loop_body():
  self.x += i     # works as expected for Python loops
```

But it won't work for TF loops:

```
def loop_body():
  self.x += i     # self.x has the wrong value!
```

get_state/set_state allow piping the mutations through the loop variables as
well, in effect changing the loop body:

```
def loop_body(self_x):
  self.x = self_x  # self.x now has the proper value
  self.x += i      # the original block
  self_x = self.x  # write self.x back into the loop vars
  return self_x

self_x = tf.while_loop(...)
self.x = self_x    # the result is not properly captured
```
"""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import functools
import traceback

import numpy as np

from tensorflow.python.autograph.operators import py_builtins
from tensorflow.python.autograph.operators import variables
from tensorflow.python.autograph.utils import ag_logging
from tensorflow.python.autograph.utils import misc
from tensorflow.python.autograph.utils import tensors
from tensorflow.python.data.experimental.ops import scan_ops
from tensorflow.python.data.experimental.ops import take_while_ops
from tensorflow.python.data.ops import dataset_ops
from tensorflow.python.data.ops import iterator_ops
from tensorflow.python.framework import constant_op
from tensorflow.python.framework import dtypes
from tensorflow.python.framework import func_graph
from tensorflow.python.framework import ops
from tensorflow.python.framework import tensor_util
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import control_flow_util
from tensorflow.python.ops import math_ops
from tensorflow.python.ops import tensor_array_ops
from tensorflow.python.ops.ragged import ragged_tensor
from tensorflow.python.types import distribute
from tensorflow.python.util import nest


PYTHON_MAX_ITERATIONS = 100000000  # Fails in about one minute for empty loops.
WARN_INEFFICIENT_UNROLL = True
INEFFICIENT_UNROLL_MIN_ITERATIONS = 50000
INEFFICIENT_UNROLL_MIN_OPS = 1


# TODO(mdan): Use the custom operator pattern instead of type dispatch.
# An example of this pattern is found in the implementation of distributed
# datasets. Before it can be used though, we need to standardize the interface.


def _is_none_or_undef(value):
  """Tests whether a value is None or undefined.

  AutoGraph represents undefined symbols using special objects of type Undefined
  or UndefinedReturnValue.

  Args:
    value: value to test
  Returns:
    Boolean
  """
  return ((value is None)
          or isinstance(value, variables.UndefinedReturnValue)
          or isinstance(value, variables.Undefined))


def _verify_tf_condition(cond, tag):
  """Ensures that the condition can be used in a TF control flow."""
  extra_hint = 'to check for None, use `is not None`'
  cond = ops.convert_to_tensor_v2(cond)

  if cond.dtype != dtypes.bool:
    raise ValueError(
        'condition of {} expected to be `tf.bool` scalar, got {}'
        '; to use as boolean Tensor, use `tf.cast`'
        '; {}'.format(tag, cond, extra_hint))

  if cond.shape is None or cond.shape.ndims is None:
    # TODO(mdan): Consider a explicit size check, if not too slow.
    cond = array_ops.reshape(cond, ())

  elif cond.shape.ndims > 0:
    known_dims = [d for d in cond.shape.as_list() if d is not None]
    if np.prod(known_dims) > 1:
      raise ValueError(
          'condition of {} expected to be `tf.bool` scalar, got {}'
          '; {}'.format(tag, cond, extra_hint))
    else:
      cond = array_ops.reshape(cond, ())

  return cond


def _verify_loop_init_vars(init_vars, symbol_names, first_iter_vars=None):
  """Ensures that all values in the state are valid to use in a TF loop.

  The init_vars may contain placeholder values derived from first_iter_vars.

  Args:
    init_vars: initial loop variables (as taken before entering the loop)
    symbol_names: corresponding names of the initial loop variables
    first_iter_vars: loop variables after one iteration of the loop
  """
  if not symbol_names:
    return
  if first_iter_vars is None:
    first_iter_vars = (None,) * len(symbol_names)

  assert len(symbol_names) == len(init_vars)
  assert len(symbol_names) == len(first_iter_vars)
  for name, val, fi_val in zip(symbol_names, init_vars, first_iter_vars):
    if isinstance(val, variables.UndefinedReturnValue):
      if fi_val:
        raise ValueError(
            'the return value from a TensorFlow loop may only be a {}; got {}'
            .format(LEGAL_LOOP_TYPES, type(fi_val)))
      else:
        # TODO(mdan): This can be handled by removing the return value.
        raise NotImplementedError(
            'a return statement cannot be placed inside this TensorFlow loop;'
            ' this may happen if a return statement depends on a'
            ' static Python condition such as a hyperparameter')

    error_msg = None
    if val is None:
      error_msg = "'{}' may not be None before the loop".format(name)
    elif isinstance(val, variables.Undefined):
      error_msg = "'{}' must be defined before the loop".format(name)

    # This only happens when we could not infer a placeholder for the
    # variable. The canonical case when that happens is when _placeholder_value
    # couldnot infer a placeholder for it. That means it's of an unknown type
    # or it's still undefined after staging one iteration.
    if error_msg is not None:
      if fi_val:
        error_msg += (", unless it's a {}; got {}".format(
            LEGAL_LOOP_TYPES, type(fi_val)))
      else:
        # TODO(mdan): This can be handled by removing the loop var.
        error_msg += '.'
      raise ValueError(error_msg)


def _is_subshape(left, right):
  """Returns True if left shape is at least as specific as right shape."""
  # TODO(mdan): This code should be in TensorShape.
  # Note: this is not the same as TensorShape.is_compatible_with, which is
  # symmetric.
  # This code also duplicates _ShapeLessThanOrEqual from  control_flow_ops.py.
  if right.dims is None:
    return True
  if left.ndims != right.ndims:
    return False
  for ldim, rdim in zip(left.dims, right.dims):
    if rdim.value is not None and ldim.value != rdim.value:
      return False
  return True


# TODO(mdan): Remove these verifications once TF ops can properly report names.
def _verify_single_loop_var(
    name, check_shape, init, entry, exit_, shape_invariant):
  """Verifies whether the initial, entry and exit values are consistent."""
  assert entry is not None, "no TF op should set '{}' to None?".format(name)
  if exit_ is None:
    raise ValueError("'{}' is None at the end of the iteration.".format(name))

  if isinstance(init, (bool, int, float, str, np.ndarray)):
    init = ops.convert_to_tensor_v2(init)
  if isinstance(entry, (bool, int, float, str, np.ndarray)):
    entry = ops.convert_to_tensor_v2(entry)
  if isinstance(exit_, (bool, int, float, str, np.ndarray)):
    exit_ = ops.convert_to_tensor_v2(exit_)

  if (not tensor_util.is_tf_type(entry) or
      not tensor_util.is_tf_type(exit_)):
    return

  # TODO(mdan): Properly account for CompositeTensors.
  if (not hasattr(entry, 'dtype') or
      not hasattr(exit_, 'dtype')):
    return
  if (not hasattr(entry, 'shape') or
      not hasattr(exit_, 'shape')):
    return

  if entry.dtype != exit_.dtype:
    raise TypeError(
        "'{}' has dtype {} before the loop, but dtype {} after one"
        ' iteration'.format(
            name,
            entry.dtype.name,
            exit_.dtype.name,
        ))
  if check_shape:
    exit_shape = exit_.shape
    if shape_invariant is None:
      entry_shape = entry.shape
      if not _is_subshape(exit_shape, entry_shape):
        raise ValueError(
            "'{}' has shape {} before the loop, but shape {} after one"
            ' iteration. Use tf.autograph.experimental.set_loop_options to set'
            ' shape invariants.'.format(name, entry_shape, exit_shape))
    else:
      init_shape = init.shape
      if not _is_subshape(init_shape, shape_invariant):
        raise ValueError(
            "'{}' has shape {} before the loop, which does not conform with"
            ' the shape invariant {}.'.format(name, init_shape,
                                              shape_invariant))
      if not _is_subshape(exit_shape, shape_invariant):
        raise ValueError(
            "'{}' has shape {} after one iteration, which does not conform with"
            ' the shape invariant {}.'.format(
                name, exit_shape, shape_invariant))


def _verify_tf_loop_vars(init_vars,
                         iter_entry_vars,
                         iter_exit_vars,
                         symbol_names,
                         opts,
                         check_shapes=True):
  """Verifies loop variables for consistency."""
  if check_shapes and 'shape_invariants' in opts:
    shape_invariants = opts['shape_invariants']
  else:
    shape_invariants = nest.map_structure(lambda _: None, iter_entry_vars)

  assert len(symbol_names) == len(shape_invariants)
  assert len(symbol_names) == len(init_vars)
  assert len(symbol_names) == len(iter_entry_vars)
  assert len(symbol_names) == len(iter_exit_vars)

  for i in range(len(symbol_names)):
    name = symbol_names[i]
    init = init_vars[i]
    entry = iter_entry_vars[i]
    exit_ = iter_exit_vars[i]
    invariant = shape_invariants[i]

    try:
      nest.assert_same_structure(init, entry, expand_composites=True)
      nest.assert_same_structure(entry, exit_, expand_composites=True)
    except (ValueError, TypeError) as e:
      raise TypeError("'{}' does not have the same nested structure after one"
                      ' iteration.\n\n{}'.format(name, e))
    if invariant is not None:
      try:
        nest.assert_same_structure(init, invariant, expand_composites=False)
      except (ValueError, TypeError) as e:
        raise TypeError("'{}' does not have the same nested structure as its"
                        ' corresponding shape invariant.\n\n{}'.format(name, e))

    nest.map_structure(
        functools.partial(_verify_single_loop_var, name, check_shapes), init,
        entry, exit_, invariant)


def verify_single_cond_var(name, body_var, orelse_var):
  """Verifies whether body_var and orelse_var are consistent."""
  if body_var is None:
    raise ValueError("'{}' is None at the end of the main branch.".format(name))
  if orelse_var is None:
    raise ValueError(
        "'{}' is None at the end of the else branch.".format(name))

  if isinstance(body_var, (bool, int, float, str, np.ndarray)):
    body_var = ops.convert_to_tensor_v2(body_var)

  if isinstance(orelse_var, (bool, int, float, str, np.ndarray)):
    orelse_var = ops.convert_to_tensor_v2(orelse_var)

  if (not tensor_util.is_tf_type(body_var) or
      not tensor_util.is_tf_type(orelse_var)):
    return

  # TODO(mdan): Properly account for CompositeTensors.
  if (not hasattr(body_var, 'dtype') or
      not hasattr(orelse_var, 'dtype')):
    return

  if body_var.dtype != orelse_var.dtype:
    raise TypeError(
        "'{}' has dtype {} in the main branch, but dtype {} in the else"
        ' branch'.format(name, body_var.dtype.name,
                         orelse_var.dtype.name))


def _verify_tf_cond_branch_vars(vars_, symbol_names, branch_name):
  """Verifies variables output by a conditional branch for consistency."""
  for name, var_ in zip(symbol_names, vars_):
    if isinstance(var_, variables.Undefined):
      raise ValueError(
          "'{}' must also be initialized in the {} branch".format(
              name, branch_name))
    if isinstance(var_, variables.UndefinedReturnValue):
      raise ValueError(
          'the {} branch must also have a return statement.'.format(
              branch_name))


def _verify_tf_cond_vars(body_vars, orelse_vars, symbol_names):
  """Verifies variables manipulated by a conditional for consistency."""
  named_vars = zip(symbol_names, body_vars, orelse_vars)

  for name, body_var, orelse_var in named_vars:
    try:
      nest.assert_same_structure(body_var, orelse_var, expand_composites=True)
    except (ValueError, TypeError) as e:
      raise TypeError(
          "'{}' must have the same nested structure in the main and else"
          ' branches:\n\n{}'.format(name, str(e)))
    nest.map_structure(
        functools.partial(verify_single_cond_var, name), body_var, orelse_var)


def for_stmt(iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Functional form of a for statement.

  The loop operates on a state, which includes all symbols that are
  variant across loop iterations, excluding the variables local to the loop.

  For example, given the loop below that calculates the geometric and
  arithmetic means or some numbers:

  ```
    geo_mean = 1
    arith_mean = 0
    for i in range(n):
      a = numbers[i]
      geo_mean *= a
      arith_mean += a
  ```

  The state is represented by the variables geo_mean and arith_mean. The
  `extra_test`, `body`, `get_state` and `set_state` functions must bind to the
  original `geo_mean` and `arith_mean` symbols, using `nonlocal`.

  The inputs and outputs of the callables representing the loop blocks are not
  explicit - instead, these functions must use nonlocal/global for side effects.
  The inputs and outputs are instead controlled by the set_state/get_state
  functions.

  Args:
    iter_: The entity being iterated over.
    extra_test: Callable with boolean return type.
      An additional loop condition.
    body: Callable representing the actual loop body.
    get_state: Additional callable which can capture additional state (such as
      the values of composite symbols). This is only useful when staging the
      loop.
    set_state: Additional callable which save values captured by get_state back
      into the Python environment. This is only useful when staging the loop.
    symbol_names: Tuple containing names of the loop variables returned by
      get_state.
    opts: Optional dict of extra loop parameters.
  """
  if tensor_util.is_tf_type(iter_):
    if tensors.is_range_tensor(iter_):
      _tf_range_for_stmt(iter_, extra_test, body, get_state, set_state,
                         symbol_names, opts)
    elif isinstance(iter_, ragged_tensor.RaggedTensor):
      _tf_ragged_for_stmt(iter_, extra_test, body, get_state, set_state,
                          symbol_names, opts)
    else:
      _known_len_tf_for_stmt(
          iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  elif isinstance(iter_, dataset_ops.DatasetV2):
    _tf_dataset_for_stmt(
        iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  elif isinstance(iter_, iterator_ops.OwnedIterator):
    _tf_iterator_for_stmt(
        iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  elif isinstance(iter_, ragged_tensor.RaggedTensor):
    _tf_ragged_for_stmt(
        iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  elif isinstance(iter_, distribute.Iterator):
    _tf_iterator_for_stmt(
        iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  elif isinstance(iter_, distribute.Iterable):
    # TODO(b/162250181): Use _tf_iterator_for_stmt(iter(iter_)...
    _tf_distributed_iterable_for_stmt(
        iter_, extra_test, body, get_state, set_state, symbol_names, opts)

  else:
    _py_for_stmt(iter_, extra_test, body, None, None)


def _py_for_stmt(iter_, extra_test, body, get_state, set_state):
  """Overload of for_stmt that executes a Python for loop."""
  del get_state, set_state

  if __debug__:
    checker = _PythonLoopChecker()
    before_iteration = checker.before_iteration
    after_iteration = checker.after_iteration
    before_iteration()

    original_body = body
    def protected_body(protected_iter):
      original_body(protected_iter)
      after_iteration()
      before_iteration()
    body = protected_body

  if extra_test is not None:
    if extra_test():
      for target in iter_:
        body(target)
        if not extra_test():
          break

  else:
    for target in iter_:
      body(target)


def _add_max_iterations_hint(opts, n):
  # TODO(b/159186914): Remove the safeguard, and always set maximum_iterations.
  if control_flow_util.GraphOrParentsInXlaContext(ops.get_default_graph()):
    opts['maximum_iterations'] = n


def _known_len_tf_for_stmt(
    iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of for_stmt that iterates over TF entities that admit a length."""
  n = py_builtins.len_(iter_)

  # TODO(b/117628877): Revisit performance once XLA has the necessary support.
  # Note: using a TensorArray creates an extra copy, but can calculate
  # gradients more efficiently than StridedSlice.
  ta = tensor_array_ops.TensorArray(iter_.dtype, size=n)
  iter_ = ta.unstack(iter_)

  iterate_index = 0

  def aug_get_state():
    return (iterate_index,) + get_state()

  def aug_set_state(aug_loop_vars):
    nonlocal iterate_index
    # TODO(b/171479293): Drop the lint override.
    iterate_index, *loop_vars = aug_loop_vars  # pylint:disable=unused-variable
    # The iteration index is not "output" by the for loop. If the iterate
    # is used outside the loop, it will appear in the loop vars separately.
    set_state(loop_vars)

  def aug_body():
    nonlocal iterate_index
    body(iter_.read(iterate_index))
    iterate_index += 1

  def aug_test():
    main_test = iterate_index < n
    if extra_test is not None:
      return control_flow_ops.cond(main_test, extra_test, lambda: False)
    return main_test

  _add_max_iterations_hint(opts, n)

  _tf_while_stmt(
      aug_test,
      aug_body,
      aug_get_state,
      aug_set_state,
      ('<internal iterate>',) + symbol_names,
      opts,
  )


def _tf_ragged_for_stmt(
    iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of for_stmt that iterates over TF ragged tensors."""
  init_vars = get_state()
  _verify_loop_init_vars(init_vars, symbol_names)

  # TODO(mdan): Move this into len()? Requires eager support.
  if iter_.shape and iter_.shape[0] is not None:
    n = iter_.shape[0]
  else:
    n = iter_.row_lengths()[0]

  iterate_index = 0

  def aug_get_state():
    return (iterate_index,) + get_state()

  def aug_set_state(aug_loop_vars):
    nonlocal iterate_index
    # TODO(b/171479293): Drop the lint override.
    iterate_index, *loop_vars = aug_loop_vars  # pylint:disable=unused-variable
    # The iteration index is not "output" by the for loop. If the iterate
    # is used outside the loop, it will appear in the loop vars separately.
    set_state(loop_vars)

  def aug_body():
    nonlocal iterate_index
    body(iter_[iterate_index])
    iterate_index += 1

  def aug_test():
    main_test = iterate_index < n
    if extra_test is not None:
      return control_flow_ops.cond(main_test, extra_test, lambda: False)
    return main_test

  _add_max_iterations_hint(opts, n)

  _tf_while_stmt(
      aug_test,
      aug_body,
      aug_get_state,
      aug_set_state,
      ('<internal iterate>',) + symbol_names,
      opts)


def _tf_range_for_stmt(
    iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of for_stmt that iterates over a TF range (and elides it)."""
  start, limit, delta = iter_.op.inputs

  iterate = start

  def _value_or(name, var, default):
    if (name == opts['iterate_names'] and isinstance(var, variables.Undefined)):
      return default
    return var

  def aug_get_state():
    state_vars = get_state()
    state_vars = tuple(
        _value_or(name, var, iterate)
        for name, var in zip(symbol_names, state_vars))
    return (iterate,) + state_vars

  def aug_set_state(aug_loop_vars):
    nonlocal iterate
    # TODO(b/171479293): Drop the lint override.
    iterate, *loop_vars = aug_loop_vars  # pylint:disable=unused-variable
    # The iteration index is not "output" by the for loop. If the iterate
    # is used outside the loop, it will appear in the loop vars separately.
    set_state(loop_vars)

  def aug_body():
    nonlocal iterate
    body(iterate)
    iterate += delta

  def aug_test():
    # TODO(b/159713842): Remove once constant folding works.
    const_delta = tensor_util.constant_value(delta)
    if const_delta is not None:
      if const_delta >= 0:
        main_test = iterate < limit
      else:
        main_test = iterate > limit
    else:
      main_test = math_ops.logical_or(
          math_ops.logical_and(delta >= 0, iterate < limit),
          math_ops.logical_and(delta < 0, iterate > limit))

    if extra_test is not None:
      main_test = control_flow_ops.cond(main_test, extra_test, lambda: False)
    return main_test

  _add_max_iterations_hint(
      opts,
      math_ops.cast(misc.get_range_len(start, limit, delta), dtypes.int32))

  _tf_while_stmt(
      aug_test,
      aug_body,
      aug_get_state,
      aug_set_state,
      ('<internal iterate>',) + symbol_names,
      opts)


def _tf_iterator_for_stmt(
    iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of for_stmt that iterates over TF Iterators. See for_loop."""
  symbol_names = ('<internal has_next>',) + symbol_names
  has_next = True

  def aug_get_state():
    return (has_next,) + get_state()

  def aug_set_state(aug_loop_vars):
    nonlocal has_next
    # TODO(b/171479293): Drop the lint override.
    has_next, *loop_vars = aug_loop_vars  # pylint:disable=unused-variable
    set_state(loop_vars)

  init_vars = aug_get_state()
  _verify_loop_init_vars(init_vars, symbol_names)

  def aug_body():
    """Main body passed to _tf_while_stmt."""
    nonlocal has_next
    opt_iterate = iter_.get_next_as_optional()
    has_next = opt_iterate.has_value()
    loop_vars = aug_get_state()  # updated by set_state() in _tf_while_loop.

    def main_path():
      body(opt_iterate.get_value())
      new_loop_vars = aug_get_state()
      # Note: this verification duplicates the one performed in tf_while_stmt,
      # but needs to be done earlier to prevent the tf.cond from blowing up
      # first.
      _verify_tf_loop_vars(
          init_vars, loop_vars, new_loop_vars, symbol_names, opts)
      return new_loop_vars

    def noop_path():
      return loop_vars

    # TODO(mdan): If tf.while_loop supported Optional, this could be avoided.
    # Calling set_state so that get_state() _tf_while_loop sees the conditional
    # tensors.
    aug_set_state(
        control_flow_ops.cond(has_next, main_path, noop_path))

  def aug_test():
    # This value takes a complicated path to get here:
    #   prev_iteration_body -> get_state -> tf.while_loop (as loop var)
    #   -> current_iteration_body -> set_state -> has_next
    main_test = has_next
    if extra_test is not None:
      return control_flow_ops.cond(main_test, extra_test, lambda: False)
    return main_test

  _tf_while_stmt(
      aug_test,
      aug_body,
      aug_get_state,
      aug_set_state,
      symbol_names,
      opts)


def _general_purpose_scan(ds, init_state, body):
  """Variant of Dataset.scan with semantics of general-purpose computation."""
  # Datasets are typically intended for data preprocessing. However, in
  # autograph loops they usually appear as general-purpose computations (for
  # example, a custom training loop). These two use cases require significantly
  # different optimization policies, the most important of which is the device
  # placement. The flag override for use_default_device below instructs the
  # runtime to treat the computation as general-purpose, rather than data
  # preprocessing.
  # TODO(mdan): s/use_default_device/specialize_for_input_pipeline.
  # TODO(mdan): Don't use private symbols.
  return scan_ops._ScanDataset(ds, init_state, body, use_default_device=False)  # pylint:disable=protected-access


def _tf_dataset_for_stmt(
    ds, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of _dataset_for_stmt with early stopping. See for_stmt."""
  # Note: This is easier to follow with the insight that the computations in
  # a dataset pipeline are transposed (aka fused).
  # For example, given a pipeline input -> scan -> take_while -> reduce,
  # and a dataset with input [1, 2, 3], the computations occur in the following
  # order:
  #  reduce(take_while(scan(1)))
  #  reduce(take_while(scan(2)))
  #  reduce(take_while(scan(3)))

  init_vars = get_state()
  _verify_loop_init_vars(init_vars, symbol_names)

  # Workaround for Dataset.reduce not allowing empty state tensors - create
  # a dummy state variable that remains unused.
  # TODO(mdan): reduce should allow and match empty structures.
  if not init_vars:
    init_vars = (constant_op.constant(0),)
    symbol_names = ('<internal dummy>',)

    def dummy_set_state(unused_dummy):
      pass

    def dummy_get_state():
      return (constant_op.constant(0),)

    get_state, set_state = dummy_get_state, dummy_set_state

  def scan_body(scan_state, scan_inputs):
    """Main body of the Dataset.scan."""
    loop_vars, iterate = scan_state, scan_inputs
    set_state(loop_vars)

    def main_path():
      body(iterate)
      new_loop_vars = get_state()
      _verify_tf_loop_vars(
          init_vars, loop_vars, new_loop_vars, symbol_names, opts,
          check_shapes=False)
      return new_loop_vars

    if extra_test is not None:
      extra_cond = extra_test()
      new_loop_vars = control_flow_ops.cond(
          extra_cond, main_path, lambda: loop_vars)
    else:
      # TODO(mdan): the optimizer should be able to remove an invariant cond?
      extra_cond = (constant_op.constant(True),)  # dummy value, unused
      new_loop_vars = main_path()

    scan_outputs = new_loop_vars, extra_cond
    new_scan_state = new_loop_vars
    return new_scan_state, scan_outputs

  def take_while_predicate(unused_loop_vars, extra_cond):
    return extra_cond

  def reduce_body(unused_reduce_state, scan_outputs):
    output_loop_vars, unused_extra_cond = scan_outputs
    new_reduce_state = output_loop_vars
    return new_reduce_state

  ds = _general_purpose_scan(ds, init_vars, scan_body)
  if extra_test is not None:
    ds = ds.apply(take_while_ops.take_while(take_while_predicate))
  final_loop_vars = ds.reduce(init_vars, reduce_body)
  set_state(final_loop_vars)


def _tf_distributed_iterable_for_stmt(
    iter_, extra_test, body, get_state, set_state, symbol_names, opts):
  """Overload of for_stmt that iterates over TF distributed datasets."""

  if extra_test is not None:
    raise NotImplementedError(
        'break and return statements are not yet supported in '
        'for ... in distributed input loops.')

  init_vars = get_state()
  _verify_loop_init_vars(init_vars, symbol_names)

  if 'shape_invariants' in opts:
    opts['shape_invariants'] = _shape_invariants_mapping_to_positional_list(
        opts['shape_invariants'], init_vars)

  def reduce_body(loop_vars, iterate):
    set_state(loop_vars)
    body(iterate)
    new_loop_vars = get_state()
    _verify_tf_loop_vars(
        init_vars, loop_vars, new_loop_vars, symbol_names, opts)
    return new_loop_vars

  set_state(iter_.reduce(init_vars, reduce_body))


def while_stmt(test, body, get_state, set_state, symbol_names, opts):
  """Functional form of a while statement.

  The loop operates on a so-called state, which includes all symbols that are
  variant across loop iterations. In what follows we refer to state as either
  a tuple of entities that represent an actual state, or a list of arguments
  of the corresponding types.

  The inputs and outputs of the callables representing the loop blocks are not
  explicit - instead, these functions must use nonlocal/global for side effects.
  The inputs and outputs are instead controlled by the set_state/get_state
  functions.

  Args:
    test: Callable with boolean return type. The loop condition.
    body: Callable representing the actual loop body.
    get_state: Additional callable which can capture additional state (such as
      the values of composite symbols). This is only useful when staging the
      loop.
    set_state: Additional callable which save values captured by get_state back
      into the Python environment. This is only useful when staging the loop.
    symbol_names: Tuple containing the names of all loop variables.
    opts: Optional dict of extra loop parameters.

  Returns:
    Tuple containing the final state.
  """

  # Evaluate the initial test once in order to do the dispatch. The evaluation
  # is isolated to minimize unwanted side effects.
  # TODO(mdan): Do a full iteration - some state types might lower to Tensor.
  with func_graph.FuncGraph('tmp').as_default():
    init_test = test()

  # TensorFlow: Multiple evaluations are acceptable in this case, so we're fine
  # with the re-evaluation of `test` that `_tf_while_stmt` will make.
  if tensors.is_dense_tensor(init_test):
    _tf_while_stmt(test, body, get_state, set_state, symbol_names, opts)
    return

  # Normal Python: We already consumed one evaluation of `test`; consistently,
  # unroll one iteration before dispatching to a normal loop.
  # TODO(mdan): Push the "init_test" value via opts into _py_while_stmt?
  if not init_test:
    return
  body()

  _py_while_stmt(test, body, get_state, set_state, opts)


class _PythonLoopChecker(object):
  """Verifies Python loops for TF-specific limits."""

  __slots__ = (
      'iterations',
      'check_inefficient_unroll',
      'check_op_count_after_iteration',
      'ops_before_iteration',
      )

  def __init__(self):
    self.iterations = 1
    self.check_inefficient_unroll = WARN_INEFFICIENT_UNROLL

    # Triggered when we decided to test the op counts.
    self.check_op_count_after_iteration = False

  def _get_ops(self):
    return ops.get_default_graph().get_operations()

  def _check_unroll_limits(self):
    if self.iterations > PYTHON_MAX_ITERATIONS:
      raise ValueError('iteration limit exceeded')

  def _stop_checking_inefficient_unroll(self):
    self.check_inefficient_unroll = False
    self.check_op_count_after_iteration = False
    self.ops_before_iteration = None

  def _verify_inefficient_unroll(self):
    """Checks for possibly-inefficient creation of ops in a Python loop."""
    assert self.ops_before_iteration is not None
    ops_after_iteration = self._get_ops()
    new_ops = tuple(
        op for op in ops_after_iteration if op not in self.ops_before_iteration)

    if len(new_ops) < INEFFICIENT_UNROLL_MIN_OPS:
      return False

    ag_logging.warn(
        'Large unrolled loop detected. Did you mean to use a TF loop?'
        ' The following ops were created after iteration %s: %s'
        '\nSee'
        ' https://github.com/tensorflow/tensorflow/blob/master/'
        'tensorflow/python/autograph/g3doc/reference/common_errors.md'
        '#warning-large-unrolled-loop-detected'
        '\n'
        'Location:'
        '\n%s'
        '', self.iterations, new_ops, '\n'.join(traceback.format_stack()))
    return True

  def before_iteration(self):
    """Called before each iteration in a Python loop."""
    if (self.check_inefficient_unroll and
        self.iterations > INEFFICIENT_UNROLL_MIN_ITERATIONS):
      self.ops_before_iteration = self._get_ops()
      self.check_op_count_after_iteration = True

  def after_iteration(self):
    """Called after each iteration in a Python loop."""
    self.iterations += 1

    self._check_unroll_limits()

    if self.check_op_count_after_iteration:
      did_warn = self._verify_inefficient_unroll()
      if did_warn:
        self._stop_checking_inefficient_unroll()  # Only warn once.
      elif self.iterations > INEFFICIENT_UNROLL_MIN_ITERATIONS + 3:
        # Once deciding to check the op counts, only do it for a few iterations.
        self._stop_checking_inefficient_unroll()


def _py_while_stmt(test, body, get_state, set_state, opts):
  """Overload of while_stmt that executes a Python while loop."""
  del opts, get_state, set_state

  if __debug__:
    checker = _PythonLoopChecker()
    before_iteration = checker.before_iteration
    after_iteration = checker.after_iteration
    before_iteration()

    original_body = body
    def protected_body():
      original_body()
      after_iteration()
      before_iteration()
    body = protected_body

  while test():
    body()


def _shape_invariants_mapping_to_positional_list(mapping, keys):
  # The keys are not expected to be hashable.
  mapping = {id(k): (k, v) for k, v in mapping}
  result = []
  for k in keys:
    map_key, map_val = mapping.get(id(k), (None, None))
    result.append(map_val if map_key is k else None)
  return tuple(result)


# Textual description of what a legal TF loop variable is. This description
# summarizes types that _placeholder_value below can handle. Keep the two
# together and in sync.
LEGAL_LOOP_TYPES = 'Tensor, int, float, bool or a list, tuple or dict thereof'


def _placeholder_value(like, original=None):
  if isinstance(like, (variables.Undefined, variables.UndefinedReturnValue)):
    return original
  if isinstance(like, (int, float, bool)):
    return type(like)(0)
  if tensor_util.is_tf_type(like):
    return array_ops.zeros(like.shape, like.dtype)
  elif isinstance(like, (list, tuple, dict)):
    return nest.map_structure(_placeholder_value, like)
  return original


def _try_handling_undefineds(
    body, get_state, set_state, init_vars, nulls, symbol_names):
  """Makes a best-effort attempt to substitute undefineds with placeholders.

  Note: this substitution requires two things to happen:
   1. the types of loop variables could be inferred (usually by staging one
       iteration)
   2. these types could be replaced by placeholders (e.g. zero values, for
       tensors.

  Args:
    body: a function representing the loop body. See while_stmt.
    get_state: state getter for the loop statement. See while_stmt.
    set_state: state getter for the loop statement. See while_stmt.
    init_vars: loop variables before entering the loop. See while_stmt.
    nulls: list of boolean flags indicating whether the corresponding loop
        var is None or undefined.
    symbol_names: list of loop variable names. See while_stmt.
  Returns:
    A tuple (success, new_init_vars). success is a boolean flag indicating
    whether types could be successfully inferred (step 1 above). new_init_vars
    contains the loop vars, with None or undefined values replaced by
    placeholders, where possible (step 2 above).
  """
  state_modified = False

  try:
    # Stage an iteration of the loop body in a temporary graph.
    with func_graph.FuncGraph('tmp').as_default():
      # This call to set_state helps report nicer error messages when symbols
      # are inconsistently used.
      set_state(init_vars)
      state_modified = True

      body()
      first_iter_vars = get_state()
  except (UnboundLocalError, TypeError, ValueError, KeyError):
    # Fall back to the old functionality. It will likely result in an input
    # validation failure.
    first_iter_vars = None
  finally:
    if state_modified:
      set_state(init_vars)

  if first_iter_vars is not None:
    # Note: the actual placeholder value doesn't matter, because as the staging
    # proved, it will be replaced by an actual value before being read.
    init_vars = tuple(
        (_placeholder_value(iv, v) if n else v)
        for v, n, iv in zip(init_vars, nulls, first_iter_vars))
    success = True
  else:
    success = False

  # This check runs regardless, in case we captured non-Tensor inputs.
  _verify_loop_init_vars(init_vars, symbol_names, first_iter_vars)

  return success, init_vars


def _runtime_zero_iterations_errmsg(symbol_names, nulls, init_vars):
  """Creates an error message asking for the loop to iterate at least once."""
  var_names = []
  for sn, n, v in zip(symbol_names, nulls, init_vars):
    if not n:
      continue
    if isinstance(v, variables.UndefinedReturnValue):
      var_names.append('the function return value')
    else:
      var_names.append(sn)
  var_names = ', '.join(var_names)
  return 'loop must iterate at least once to initialize {}'.format(var_names)


def _tf_while_stmt(test, body, get_state, set_state, symbol_names, opts):
  """Overload of while_stmt that stages a TF while_stmt."""
  init_vars = get_state()
  orig_init_vars = init_vars

  nulls = tuple(_is_none_or_undef(v) for v in init_vars)
  if any(nulls):
    require_one_iteration, init_vars = _try_handling_undefineds(
        body, get_state, set_state, init_vars, nulls, symbol_names)
  else:
    require_one_iteration = False

  def aug_test(*loop_vars):
    if require_one_iteration:
      loop_vars = loop_vars[1:]

    set_state(loop_vars)
    return _verify_tf_condition(test(), 'while loop')

  def aug_body(*loop_vars):
    if require_one_iteration:
      loop_vars = loop_vars[1:]

    set_state(loop_vars)
    body()
    new_loop_vars = get_state()
    _verify_tf_loop_vars(
        init_vars, loop_vars, new_loop_vars, symbol_names, opts)

    if require_one_iteration:
      new_loop_vars = (True,) + new_loop_vars

    return new_loop_vars

  if 'shape_invariants' in opts:
    opts['shape_invariants'] = _shape_invariants_mapping_to_positional_list(
        opts['shape_invariants'], init_vars)

  while_loop_opts = dict(opts)
  while_loop_opts.pop('iterate_names', None)

  # Non-v2 while_loop unpacks the results when there is only one return value.
  # This enforces consistency across versions.
  while_loop_opts['return_same_structure'] = True

  if require_one_iteration:
    aug_init_vars = (False,) + init_vars
  else:
    aug_init_vars = init_vars

  final_loop_vars = control_flow_ops.while_loop(
      aug_test, aug_body, aug_init_vars, **while_loop_opts)

  if require_one_iteration:
    with ops.control_dependencies([
        control_flow_ops.Assert(final_loop_vars[0], [
            _runtime_zero_iterations_errmsg(symbol_names, nulls, orig_init_vars)
        ])
    ]):
      final_loop_vars = nest.map_structure(
          lambda v: (array_ops.identity(v) if tensor_util.is_tf_type(v) else v),
          final_loop_vars[1:],
      )

  set_state(final_loop_vars)


def if_stmt(cond, body, orelse, get_state, set_state, symbol_names, nouts):
  """Functional form of an if statement.

  The conditional operates on a state, which includes all symbols whose values
  are a function of the branch taken.

  For example, given the code below that calculates the abs function:

  ```
    x = 1
    if x > 0:
      x = -x
  ```

  The state is represented by the variable `x`. The `body, `orelse` and
  `set_state` functions must bind to the original `x` symbol, using `nonlocal`.

  The inputs and outputs of the callables representing the loop blocks are not
  explicit - instead, these functions must use nonlocal/global for side effects.
  The inputs and outputs are instead controlled by the set_state/get_state
  functions.

  Args:
    cond: Boolean.
    body: Callable representing the main block of the conditional.
    orelse: Callable representing the else block of the conditional.
    get_state: Function that returns a tuple containing the values of all
      composite symbols modified within the conditional. This allows access to
      state that branches may mutate through side effects. This function is not
      needed and should not be called when dispatching to code matching Python's
      default semantics. This is useful for checkpointing to avoid unintended
      side-effects when staging requires evaluating all code-paths.
    set_state: Function to set the values of all composite symbols modified
      within the conditional. This is the complement to get_state, used to
      restore checkpointed values. The single argument a tuple containing values
      for each composite symbol that may be modified in a branch of the
      conditional. The is usually the result of a call to get_state.
    symbol_names: Tuple containing basic loop var names.
    nouts: Number of variables output by the statement. Vars which are
      not outputs will not be passed through staged control flow such as
      tf.cond. This includes variables that are defined before the conditional,
      but are not used after it.
  """
  # Note: tf.cond doesn't support SparseTensor.
  if tensors.is_dense_tensor(cond):
    _tf_if_stmt(cond, body, orelse, get_state, set_state, symbol_names, nouts)
  else:
    _py_if_stmt(cond, body, orelse)


def _tf_if_stmt(
    cond, body, orelse, get_state, set_state, symbol_names, nouts):
  """Overload of if_stmt that stages a TF cond."""
  cond = _verify_tf_condition(cond, 'if statement')

  if not nouts:
    prev_get_state, prev_set_state = get_state, set_state
    # Control flow V1 wants at least one output.
    get_state = lambda: (0,) + prev_get_state()
    set_state = lambda v: prev_set_state(v[1:])
    symbol_names += ('<unused dummy>',)
    nouts = 1

  init_vars = get_state()

  # TODO(mdan): Use nonlocal once we no longer need to support py2.
  new_body_vars_ = [None]
  new_orelse_vars_ = [None]

  def aug_body():
    set_state(init_vars)
    body()
    new_body_vars = get_state()
    new_body_vars = new_body_vars[:nouts]
    new_body_vars_[0] = new_body_vars
    _verify_tf_cond_branch_vars(new_body_vars, symbol_names, 'main')
    if new_orelse_vars_[0] is not None:
      _verify_tf_cond_vars(new_body_vars, new_orelse_vars_[0], symbol_names)
    return new_body_vars

  def aug_orelse():
    set_state(init_vars)
    orelse()
    new_orelse_vars = get_state()
    new_orelse_vars = new_orelse_vars[:nouts]
    new_orelse_vars_[0] = new_orelse_vars
    _verify_tf_cond_branch_vars(new_orelse_vars, symbol_names, 'else')
    if new_body_vars_[0] is not None:
      _verify_tf_cond_vars(new_body_vars_[0], new_orelse_vars, symbol_names)
    return new_orelse_vars

  final_cond_vars = control_flow_ops.cond(
      cond, aug_body, aug_orelse, strict=True)
  final_cond_vars = final_cond_vars + init_vars[nouts:]

  set_state(final_cond_vars)


def _py_if_stmt(cond, body, orelse):
  """Overload of if_stmt that executes a Python if statement."""
  return body() if cond else orelse()
