# Copyright 2016 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
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# ==============================================================================

"""## Functions for working with arbitrarily nested sequences of elements.

This module is used to perform any operations on nested structures, which can be
specified as sequences that contain non-sequence elements or other sequences.
The utilities here assume (and do not check) that the nested structures form a
'tree', i.e. no references in the structure of the input of these functions
should be recursive.

@@assert_same_structure
@@is_sequence
@@flatten
@@flatten_dict_items
@@pack_sequence_as
@@map_structure
@@assert_shallow_structure
@@flatten_up_to
@@map_structure_up_to
"""

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

import collections

import six


def _sequence_like(instance, args):
  """Converts the sequence `args` to the same type as `instance`.

  Args:
    instance: an instance of `tuple`, `list`, or a `namedtuple` class.
    args: elements to be converted to a sequence.

  Returns:
    `args` with the type of `instance`.
  """
  if (isinstance(instance, tuple) and
      hasattr(instance, "_fields") and
      isinstance(instance._fields, collections.Sequence) and
      all(isinstance(f, six.string_types) for f in instance._fields)):
    # This is a namedtuple
    return type(instance)(*args)
  else:
    # Not a namedtuple
    return type(instance)(args)


def _yield_flat_nest(nest):
  for n in nest:
    if is_sequence(n):
      for ni in _yield_flat_nest(n):
        yield ni
    else:
      yield n


def is_sequence(seq):
  """Returns a true if its input is a collections.Sequence (except strings).

  Args:
    seq: an input sequence.

  Returns:
    True if the sequence is a not a string and is a collections.Sequence.
  """
  return (isinstance(seq, collections.Sequence)
          and not isinstance(seq, six.string_types))


def flatten(nest):
  """Returns a flat sequence from a given nested structure.

  If `nest` is not a sequence, this returns a single-element list: `[nest]`.

  Args:
    nest: an arbitrarily nested structure or a scalar object.
      Note, numpy arrays are considered scalars.

  Returns:
    A Python list, the flattened version of the input.
  """
  return list(_yield_flat_nest(nest)) if is_sequence(nest) else [nest]


def _recursive_assert_same_structure(nest1, nest2):
  is_sequence_nest1 = is_sequence(nest1)
  if is_sequence_nest1 != is_sequence(nest2):
    raise ValueError(
        "The two structures don't have the same nested structure. "
        "First structure: %s, second structure: %s." % (nest1, nest2))

  if is_sequence_nest1:
    type_nest1 = type(nest1)
    type_nest2 = type(nest2)
    if type_nest1 != type_nest2:
      raise TypeError(
          "The two structures don't have the same sequence type. First "
          "structure has type %s, while second structure has type %s."
          % (type_nest1, type_nest2))

    for n1, n2 in zip(nest1, nest2):
      _recursive_assert_same_structure(n1, n2)


def assert_same_structure(nest1, nest2):
  """Asserts that two structures are nested in the same way.

  Args:
    nest1: an arbitrarily nested structure.
    nest2: an arbitrarily nested structure.

  Raises:
    ValueError: If the two structures do not have the same number of elements or
      if the two structures are not nested in the same way.
    TypeError: If the two structures differ in the type of sequence in any of
      their substructures.
  """
  len_nest1 = len(flatten(nest1)) if is_sequence(nest1) else 1
  len_nest2 = len(flatten(nest2)) if is_sequence(nest2) else 1
  if len_nest1 != len_nest2:
    raise ValueError("The two structures don't have the same number of "
                     "elements. First structure: %s, second structure: %s."
                     % (nest1, nest2))
  _recursive_assert_same_structure(nest1, nest2)


def flatten_dict_items(dictionary):
  """Returns a dictionary with flattened keys and values.

  This function flattens the keys and values of a dictionary, which can be
  arbitrarily nested structures, and returns the flattened version of such
  structures:

  ```python
  example_dictionary = {(4, 5, (6, 8)): ("a", "b", ("c", "d"))}
  result = {4: "a", 5: "b", 6: "c", 8: "d"}
  flatten_dict_items(example_dictionary) == result
  ```

  The input dictionary must satisfy two properties:

  1. Its keys and values should have the same exact nested structure.
  2. The set of all flattened keys of the dictionary must not contain repeated
     keys.

  Args:
    dictionary: the dictionary to zip

  Returns:
    The zipped dictionary.

  Raises:
    TypeError: If the input is not a dictionary.
    ValueError: If any key and value have not the same structure, or if keys are
      not unique.
  """
  if not isinstance(dictionary, dict):
    raise TypeError("input must be a dictionary")
  flat_dictionary = {}
  for i, v in six.iteritems(dictionary):
    if not is_sequence(i):
      if i in flat_dictionary:
        raise ValueError(
            "Could not flatten dictionary: key %s is not unique." % i)
      flat_dictionary[i] = v
    else:
      flat_i = flatten(i)
      flat_v = flatten(v)
      if len(flat_i) != len(flat_v):
        raise ValueError(
            "Could not flatten dictionary. Key had %d elements, but value had "
            "%d elements. Key: %s, value: %s."
            % (len(flat_i), len(flat_v), flat_i, flat_v))
      for new_i, new_v in zip(flat_i, flat_v):
        if new_i in flat_dictionary:
          raise ValueError(
              "Could not flatten dictionary: key %s is not unique."
              % (new_i))
        flat_dictionary[new_i] = new_v
  return flat_dictionary


def _packed_nest_with_indices(structure, flat, index):
  """Helper function for pack_nest_as.

  Args:
    structure: Substructure (tuple of elements and/or tuples) to mimic
    flat: Flattened values to output substructure for.
    index: Index at which to start reading from flat.

  Returns:
    The tuple (new_index, child), where:
      * new_index - the updated index into `flat` having processed `structure`.
      * packed - the subset of `flat` corresponding to `structure`,
                 having started at `index`, and packed into the same nested
                 format.

  Raises:
    ValueError: if `structure` contains more elements than `flat`
      (assuming indexing starts from `index`).
  """
  packed = []
  for s in structure:
    if is_sequence(s):
      new_index, child = _packed_nest_with_indices(s, flat, index)
      packed.append(_sequence_like(s, child))
      index = new_index
    else:
      packed.append(flat[index])
      index += 1
  return index, packed


def pack_sequence_as(structure, flat_sequence):
  """Returns a given flattened sequence packed into a nest.

  If `structure` is a scalar, `flat_sequence` must be a single-element list;
  in this case the return value is `flat_sequence[0]`.

  Args:
    structure: tuple or list constructed of scalars and/or other tuples/lists,
      or a scalar.  Note: numpy arrays are considered scalars.
    flat_sequence: flat sequence to pack.

  Returns:
    packed: `flat_sequence` converted to have the same recursive structure as
      `structure`.

  Raises:
    ValueError: If nest and structure have different element counts.
  """
  if not is_sequence(flat_sequence):
    raise TypeError("flat_sequence must be a sequence")

  if not is_sequence(structure):
    if len(flat_sequence) != 1:
      raise ValueError("Structure is a scalar but len(flat_sequence) == %d > 1"
                       % len(flat_sequence))
    return flat_sequence[0]

  flat_structure = flatten(structure)
  if len(flat_structure) != len(flat_sequence):
    raise ValueError(
        "Could not pack sequence. Structure had %d elements, but flat_sequence "
        "had %d elements.  Structure: %s, flat_sequence: %s."
        % (len(flat_structure), len(flat_sequence), structure, flat_sequence))

  _, packed = _packed_nest_with_indices(structure, flat_sequence, 0)
  return _sequence_like(structure, packed)


def map_structure(func, *structure):
  """Applies `func` to each entry in `structure` and returns a new structure.

  Applies `func(x[0], x[1], ...)` where x[i] is an entry in
  `structure[i]`.  All structures in `structure` must have the same arity,
  and the return value will contain the results in the same structure.

  Args:
    func: A callable that acceps as many arguments are there are structures.
    *structure: scalar, or tuple or list of constructed scalars and/or other
      tuples/lists, or scalars.  Note: numpy arrays are considered scalars.

  Returns:
    A new structure with the same arity as `structure`, whose values correspond
    to `func(x[0], x[1], ...)` where `x[i]` is a value in the corresponding
    location in `structure[i]`.

  Raises:
    TypeError: If `func` is not callable or if the structures do not match
      each other by depth tree.
    ValueError: If no structure is provided or if the structures do not match
      each other by type.
  """
  if not callable(func):
    raise TypeError("func must be callable, got: %s" % func)

  if not structure:
    raise ValueError("Must provide at least one structure")

  for other in structure[1:]:
    assert_same_structure(structure[0], other)

  flat_structure = [flatten(s) for s in structure]
  entries = zip(*flat_structure)

  return pack_sequence_as(
      structure[0], [func(*x) for x in entries])


def _yield_flat_up_to(shallow_tree, input_tree):
  """Yields elements `input_tree` partially flattened up to `shallow_tree`."""
  if is_sequence(shallow_tree):
    for shallow_branch, input_branch in zip(shallow_tree, input_tree):
      for input_leaf in _yield_flat_up_to(shallow_branch, input_branch):
        yield input_leaf
  else:
    yield input_tree


def assert_shallow_structure(shallow_tree, input_tree):
  """Asserts that `shallow_tree` is a shallow structure of `input_tree`.

  That is, this function tests if the `input_tree` structure can be created from
  the `shallow_tree` structure by replacing its leaf nodes with deeper
  tree structures.

  Examples:

  The following code will raise an exception:
  ```python
    shallow_tree = ["a", "b"]
    input_tree = ["c", ["d", "e"], "f"]
    assert_shallow_structure(shallow_tree, input_tree)
  ```

  The following code will not raise an exception:
  ```python
    shallow_tree = ["a", "b"]
    input_tree = ["c", ["d", "e"]]
    assert_shallow_structure(shallow_tree, input_tree)
  ```

  Args:
    shallow_tree: an arbitrarily nested structure.
    input_tree: an arbitrarily nested structure.

  Raises:
    TypeError: If `shallow_tree` is a sequence but `input_tree` is not.
    TypeError: If the sequence types of `shallow_tree` are different from
      `input_tree`.
    ValueError: If the sequence lengths of `shallow_tree` are different from
      `input_tree`.
  """
  if is_sequence(shallow_tree):
    if not is_sequence(input_tree):
      raise TypeError(
          "If shallow structure is a sequence, input must also be a sequence. "
          "Input has type: %s." % type(input_tree))

    if not isinstance(input_tree, type(shallow_tree)):
      raise TypeError(
          "The two structures don't have the same sequence type. Input "
          "structure has type %s, while shallow structure has type %s."
          % (type(input_tree), type(shallow_tree)))

    if len(input_tree) != len(shallow_tree):
      raise ValueError(
          "The two structures don't have the same sequence length. Input "
          "structure has length %s, while shallow structure has length %s."
          % (len(input_tree), len(shallow_tree)))

    for shallow_branch, input_branch in zip(shallow_tree, input_tree):
      assert_shallow_structure(shallow_branch, input_branch)


def flatten_up_to(shallow_tree, input_tree):
  """Flattens `input_tree` up to `shallow_tree`.

  Any further depth in structure in `input_tree` is retained as elements in the
  partially flatten output.

  If `shallow_tree` and `input_tree` are not sequences, this returns a
  single-element list: `[input_tree]`.

  Use Case:

  Sometimes we may wish to partially flatten a nested sequence, retaining some
  of the nested structure. We achieve this by specifying a shallow structure,
  `shallow_tree`, we wish to flatten up to.

  The input, `input_tree`, can be thought of as having the same structure as
  `shallow_tree`, but with leaf nodes that are themselves tree structures.

  Examples:

  ```python
  input_tree = [[[2, 2], [3, 3]], [[4, 9], [5, 5]]]
  shallow_tree = [[True, True], [False, True]]

  flattened_input_tree = flatten_up_to(shallow_tree, input_tree)
  flattened_shallow_tree = flatten_up_to(shallow_tree, shallow_tree)

  # Output is:
  # [[2, 2], [3, 3], [4, 9], [5, 5]]
  # [True, True, False, True]
  ```

  ```python
  input_tree = [[('a', 1), [('b', 2), [('c', 3), [('d', 4)]]]]]
  shallow_tree = [['level_1', ['level_2', ['level_3', ['level_4']]]]]

  input_tree_flattened_as_shallow_tree = flatten_up_to(shallow_tree, input_tree)
  input_tree_flattened = flatten(input_tree)

  # Output is:
  # [('a', 1), ('b', 2), ('c', 3), ('d', 4)]
  # ['a', 1, 'b', 2, 'c', 3, 'd', 4]
  ```

  Non-Sequence Edge Cases:

  ```python
  flatten_up_to(0, 0)  # Output: [0]
  flatten_up_to(0, [0, 1, 2])  # Output: [[0, 1, 2]]
  flatten_up_to([0, 1, 2], 0)  # Output: TypeError
  flatten_up_to([0, 1, 2], [0, 1, 2])  # Output: [0, 1, 2]
  ```

  Args:
    shallow_tree: a possibly pruned structure of input_tree.
    input_tree: an arbitrarily nested structure or a scalar object.
      Note, numpy arrays are considered scalars.

  Returns:
    A Python list, the partially flattened version of `input_tree` according to
    the structure of `shallow_tree`.

  Raises:
    TypeError: If `shallow_tree` is a sequence but `input_tree` is not.
    TypeError: If the sequence types of `shallow_tree` are different from
      `input_tree`.
    ValueError: If the sequence lengths of `shallow_tree` are different from
      `input_tree`.
  """
  assert_shallow_structure(shallow_tree, input_tree)
  return list(_yield_flat_up_to(shallow_tree, input_tree))


def map_structure_up_to(shallow_tree, func, *inputs):
  """Applies a function or op to a number of partially flattened inputs.

  The `inputs` are flattened up to `shallow_tree` before being mapped.

  Use Case:

  Sometimes we wish to apply a function to a partially flattened
  sequence (for example when the function itself takes sequence inputs). We
  achieve this by specifying a shallow structure, `shallow_tree` we wish to
  flatten up to.

  The `inputs`, can be thought of as having the same structure as
  `shallow_tree`, but with leaf nodes that are themselves tree structures.

  This function therefore will return something with the same base structure as
  `shallow_tree`.

  Examples:

  ```python
  ab_tuple = collections.namedtuple("ab_tuple", "a, b")
  op_tuple = collections.namedtuple("op_tuple", "add, mul")
  inp_val = ab_tuple(a=2, b=3)
  inp_ops = ab_tuple(a=op_tuple(add=1, mul=2), b=op_tuple(add=2, mul=3))
  out = map_structure_up_to(inp_val, lambda val, ops: (val + ops.add) * ops.mul,
                            inp_val, inp_ops)

  # Output is: ab_tuple(a=6, b=15)
  ```

  ```python
  data_list = [[2, 4, 6, 8], [[1, 3, 5, 7, 9], [3, 5, 7]]]
  name_list = ['evens', ['odds', 'primes']]
  out = map_structure_up_to(
      name_list,
      lambda name, sec: "first_{}_{}".format(len(sec), name),
      name_list, data_list)

  # Output is: ['first_4_evens', ['first_5_odds', 'first_3_primes']]
  ```

  Args:
    shallow_tree: a shallow tree, common to all the inputs.
    func: callable which will be applied to each input individually.
    *inputs: arbitrarily nested combination of objects that are compatible with
        shallow_tree. The function `func` is applied to corresponding
        partially flattened elements of each input, so the function must support
        arity of `len(inputs)`.

  Raises:
    TypeError: If `shallow_tree` is a sequence but `input_tree` is not.
    TypeError: If the sequence types of `shallow_tree` are different from
      `input_tree`.
    ValueError: If the sequence lengths of `shallow_tree` are different from
      `input_tree`.

  Returns:
    result of repeatedly applying `func`, with same structure as
    `shallow_tree`.
  """
  if not inputs:
    raise ValueError("Cannot map over no sequences")
  for input_tree in inputs:
    assert_shallow_structure(shallow_tree, input_tree)

  # Flatten each input separately, apply the function to corresponding elements,
  # then repack based on the structure of the first input.
  all_flattened_up_to = [flatten_up_to(shallow_tree, input_tree)
                         for input_tree in inputs]
  results = [func(*tensors) for tensors in zip(*all_flattened_up_to)]
  return pack_sequence_as(structure=shallow_tree, flat_sequence=results)
