# 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.
# ==============================================================================
"""Functional operations for RaggedTensors."""

from tensorflow.python.ops.ragged import ragged_tensor
from tensorflow.python.util import nest
from tensorflow.python.util.lazy_loader import LazyLoader


map_fn_lib = LazyLoader(
    "map_fn_lib", globals(),
    "tensorflow.python.ops.map_fn")


def map_fn(fn,
           elems,
           dtype=None,
           parallel_iterations=None,
           back_prop=True,
           swap_memory=False,
           infer_shape=True,
           name=None):
  """map on the list of tensors unpacked from `elems` on dimension 0.

  The simplest version of `map_fn` repeatedly applies the callable `fn` to a
  sequence of elements from first to last. The elements are made of the
  tensors unpacked from `elems`. `dtype` is the data type of the return
  value of `fn`. Users must provide `dtype` if it is different from
  the data type of `elems`.

  Suppose that `elems` is unpacked into `values`, a list of tensors. The shape
  of the result tensor is `[values.shape[0]] + fn(values[0]).shape`.

  This method also allows multi-arity `elems` and output of `fn`.  If `elems`
  is a (possibly nested) list or tuple of tensors, then each of these tensors
  must have a matching first (unpack) dimension.  The signature of `fn` may
  match the structure of `elems`.  That is, if `elems` is
  `(t1, [t2, t3, [t4, t5]])`, then an appropriate signature for `fn` is:
  `fn = lambda (t1, [t2, t3, [t4, t5]]):`.

  Furthermore, `fn` may emit a different structure than its input.  For example,
  `fn` may look like: `fn = lambda t1: return (t1 + 1, t1 - 1)`.  In this case,
  the `dtype` parameter is not optional: `dtype` must be a type or (possibly
  nested) tuple of types matching the output of `fn`.

  To apply a functional operation to the nonzero elements of a SparseTensor
  one of the following methods is recommended. First, if the function is
  expressible as TensorFlow ops, use

  ```python
    result = SparseTensor(input.indices, fn(input.values), input.dense_shape)
  ```

  If, however, the function is not expressible as a TensorFlow op, then use

  ```python
  result = SparseTensor(
    input.indices, map_fn(fn, input.values), input.dense_shape)
  ```

  instead.

  When executing eagerly, map_fn does not execute in parallel even if
  `parallel_iterations` is set to a value > 1. You can still get the
  performance benefits of running a function in parallel by using the
  `tf.contrib.eager.defun` decorator,

  ```python
  # Assume the function being used in map_fn is fn.
  # To ensure map_fn calls fn in parallel, use the defun decorator.
  @tf.contrib.eager.defun
  def func(tensor):
    return tf.map_fn(fn, tensor)
  ```

  Note that if you use the defun decorator, any non-TensorFlow Python code
  that you may have written in your function won't get executed. See
  `tf.contrib.eager.defun` for more details. The recommendation would be to
  debug without defun but switch to defun to get performance benefits of
  running map_fn in parallel.

  Args:
    fn: The callable to be performed.  It accepts one argument, which will have
      the same (possibly nested) structure as `elems`.  Its output must have the
      same structure as `dtype` if one is provided, otherwise it must have the
      same structure as `elems`.
    elems: A tensor or (possibly nested) sequence of tensors, each of which will
      be unpacked along their first dimension.  The nested sequence of the
      resulting slices will be applied to `fn`.
    dtype: (optional) The output type(s) of `fn`.  If `fn` returns a structure
      of Tensors differing from the structure of `elems`, then `dtype` is not
      optional and must have the same structure as the output of `fn`. Use
      `RaggedTensorType` to declare an output of type `RaggedTensor`.
    parallel_iterations: (optional) The number of iterations allowed to run in
      parallel. When graph building, the default value is 10. While executing
      eagerly, the default value is set to 1.
    back_prop: (optional) True enables support for back propagation.
    swap_memory: (optional) True enables GPU-CPU memory swapping.
    infer_shape: (optional) False disables tests for consistent output shapes.
    name: (optional) Name prefix for the returned tensors.

  Returns:
    A possibly nested sequence of potentially ragged tensors.  Each
    tensor packs the results of applying `fn` to tensors unpacked from `elems`
    along the first dimension, from first to last.

  Raises:
    TypeError: if `fn` is not callable or the structure of the output of
      `fn` and `dtype` do not match, or if elems is a SparseTensor.
    ValueError: if the lengths of the output of `fn` and `dtype` do not match.

  #### Examples:

    ```python
    elems = np.array([1, 2, 3, 4, 5, 6])
    squares = map_fn(lambda x: x * x, elems)
    # squares == [1, 4, 9, 16, 25, 36]
    ```

    ```python
    elems = (np.array([1, 2, 3]), np.array([-1, 1, -1]))
    alternate = map_fn(lambda x: x[0] * x[1], elems, dtype=tf.int64)
    # alternate == [-1, 2, -3]
    ```

    ```python
    elems = np.array([1, 2, 3])
    alternates = map_fn(lambda x: (x, -x), elems, dtype=(tf.int64, tf.int64))
    # alternates[0] == [1, 2, 3]
    # alternates[1] == [-1, -2, -3]
    ```

    ```python
    elems=ragged.constant([[1, 2, 3], [4, 5], [6, 7]])
    mean = map_fn(tf.reduce_mean, elems)
    # mean == [2, 4, 6]
    ```

    ```python
    elems=ragged.constant([[1, 2, 3], [4, 5], [6, 7]], dtype=tf.int64)
    out = map_fn(fn=lambda x: x+1, elems,
      dtype=ragged.RaggedTensorType(type=tf.int64, ragged_rank=0))
    # out = tf.ragged.constant([[2, 3, 4], [5, 6], [7, 8]])
    ```
  """
  if dtype is None:
    dtype = nest.map_structure(lambda e: e.dtype, elems)
  dtype = nest.map_structure(_ragged_type_to_spec, dtype)
  return map_fn_lib.map_fn(fn,
                           elems,
                           dtype,
                           parallel_iterations,
                           back_prop,
                           swap_memory,
                           infer_shape,
                           name)


def _ragged_type_to_spec(t):
  if isinstance(t, ragged_tensor.RaggedTensorType):
    # Note: need to adjust ragged_rank by 1, since RaggedTensorSpec gives the
    # type for the mapped `fn` output, but RaggedTensorType gives the type for
    # the result of stacking the mapped `fn` outputs.
    return ragged_tensor.RaggedTensorSpec(
        None, t.dtype, t.ragged_rank - 1, t.row_splits_dtype)
  else:
    return t