"""Operations for clipping (gradient, weight) tensors to min/max values."""

import collections

from tensorflow.python.framework import ops
from tensorflow.python.framework import types
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import constant_op
from tensorflow.python.ops import math_ops


def clip_by_value(t, clip_value_min, clip_value_max,
                  name=None):
  """Clips tensor values to a specified min and max.

  Given a tensor `t`, this operation returns a tensor of the same type and
  shape as `t` with its values clipped to `clip_value_min` and `clip_value_max`.
  Any values less than `clip_value_min` are set to `clip_value_min`. Any values
  greater than `clip_value_max` are set to `clip_value_max`.

  Args:
    t: A `Tensor`.
    clip_value_min: A 0-D (scalar) `Tensor`. The minimum value to clip by.
    clip_value_max: A 0-D (scalar) `Tensor`. The maximum value to clip by.
    name: A name for the operation (optional).

  Returns:
    A clipped `Tensor`.
  """
  with ops.op_scope([t, clip_value_min, clip_value_max], name,
                   "clip_by_value") as name:
    t = ops.convert_to_tensor(t, name="t")

    # Go through list of tensors, for each value in each tensor clip
    t_min = math_ops.minimum(
        t, array_ops.fill(array_ops.shape(t), clip_value_max))
    t_max = math_ops.maximum(
        t_min, array_ops.fill(array_ops.shape(t), clip_value_min),
        name=name)

  return t_max


def clip_by_norm(t, clip_norm, name=None):
  """Clips tensor values to a maximum L2-norm.

  Given a tensor `t`, and a maximum clip value `clip_norm`, this operation
  normalizes `t` so that its L2-norm is less than or equal to `clip_norm'.
  Specifically, if the L2-norm is already less than or equal to `clip_norm`,
  then `t` is not modified. If the L2-norm is greater than `clip_norm`, then
  this operation returns a tensor of the same type and shape as `t` with its
  values set to:

  `t * clip_norm / l2norm(t)`

  In this case, the L2-norm of the output tensor is `clip_norm`.

  This operation is typically used to clip gradients before applying them with
  an optimizer.

  Args:
    t: A `Tensor`.
    clip_norm: A 0-D (scalar) `Tensor` > 0. A maximum clipping value.
    name: A name for the operation (optional).

  Returns:
    A clipped `Tensor`.
  """
  with ops.op_scope([t, clip_norm], name, "clip_by_norm") as name:
    t = ops.convert_to_tensor(t, name="t")

    # Calculate L2-norm, clip elements by ratio of clip_norm to L2-norm
    l2norm_inv = math_ops.rsqrt(
        math_ops.reduce_sum(t * t, math_ops.range(0, array_ops.rank(t))))
    tclip = array_ops.identity(t * clip_norm * math_ops.minimum(
        l2norm_inv, constant_op.constant(1.0 / clip_norm)), name=name)

  return tclip

def global_norm(t_list, name=None):
  """Computes the global norm of multiple tensors.

  Given a tuple or list of tensors `t_list`, this operation returns the
  global norm of the elements in all tensors in `t_list`. The global norm is
  computed as:

  `global_norm = sqrt(sum([l2norm(t)**2 for t in t_list]))`

  Any entries in `t_list` that are of type None are ignored.

  Args:
    t_list: A tuple or list of mixed `Tensors`, `IndexedSlices`, or None.
    name: A name for the operation (optional).

  Returns:
    A 0-D (scalar) `Tensor` of type `float`.

  Raises:
    TypeError: If `t_list` is not a sequence.
  """
  if (not isinstance(t_list, collections.Sequence)
      or isinstance(t_list, basestring)):
    raise TypeError("t_list should be a sequence")
  t_list = list(t_list)
  with ops.op_scope(t_list, name, "global_norm") as name:
    values = [
        ops.convert_to_tensor(
            t.values if isinstance(t, ops.IndexedSlices) else t,
            name="t_%d" % i)
        if t is not None else t
        for i, t in enumerate(t_list)]
    squared_norms = array_ops.pack(
        [math_ops.reduce_sum(v * v) for v in values if v])

    norm = math_ops.sqrt(
        math_ops.reduce_sum(squared_norms), name="global_norm")

  return norm

def clip_by_global_norm(t_list, clip_norm, use_norm=None, name=None):
  """Clips values of multiple tensors by the ratio of the sum of their norms.

  Given a tuple or list of tensors `t_list`, and a clipping ratio `clip_norm`,
  this operation returns a list of clipped tensors `list_clipped`
  and the global norm (`global_norm`) of all tensors in `t_list`. Optionally,
  if you've already computed the global norm for `t_list`, you can specify
  the global norm with `use_norm`.

  To perform the clipping, the values t_list[i] are set to:

  `t_list[i] * clip_norm / max(global_norm, clip_norm)`

  where:

  `global_norm = sqrt(sum([l2norm(t)**2 for t in t_list]))`

  If `clip_norm > global_norm` then the entries in `t_list` remain as they are,
  otherwise they're all shrunk by the global ratio.

  Any of the entries of `t_list` that are of type None are ignored.

  This is the correct way to perform gradient clipping (for example, see
  R. Pascanu, T. Mikolov, and Y. Bengio, "On the difficulty of training
  Recurrent Neural Networks".  http://arxiv.org/abs/1211.5063)

  However, it is slower than `clip_by_norm()` because all the parameters must be
  ready before the clipping operation can be performed.

  Args:
    t_list: A tuple or list of mixed `Tensors`, `IndexedSlices`, or None.
    clip_norm: A 0-D (scalar) `Tensor` > 0. The clipping ratio.
    use_norm: A 0-D (scalar) `Tensor` of type `float` (optional). The global
      norm to use. If not provided, `global_norm()` is used to compute the norm.
    name: A name for the operation (optional).

  Returns:
    list_clipped: A list of `Tensors` of the same type as `list_t`.
    global_norm: A 0-D (scalar) `Tensor` representing the global norm.

  Raises:
    TypeError: If `t_list` is not a sequence.
  """
  if (not isinstance(t_list, collections.Sequence)
      or isinstance(t_list, basestring)):
    raise TypeError("t_list should be a sequence")
  t_list = list(t_list)
  if use_norm is None:
    use_norm = global_norm(t_list, name)

  with ops.op_scope(t_list + [clip_norm], name, "clip_by_global_norm") as name:
    # Calculate L2-norm, clip elements by ratio of clip_norm to L2-norm
    scale = clip_norm * math_ops.minimum(
        1.0 / use_norm, constant_op.constant(1.0 / clip_norm))

    values = [
        ops.convert_to_tensor(
            t.values if isinstance(t, ops.IndexedSlices) else t,
            name="t_%d" % i)
        if t is not None else t
        for i, t in enumerate(t_list)]

    values_clipped = [
        array_ops.identity(v * scale, name="%s_%d" % (name, i))
        if v is not None else None
        for i, v in enumerate(values)]

    list_clipped = [
        ops.IndexedSlices(c_v, t.indices)
        if isinstance(t, ops.IndexedSlices)
        else c_v
        for (c_v, t) in zip(values_clipped, t_list)]

  return list_clipped, use_norm


def clip_by_average_norm(t, clip_norm, name=None):
  """Clips tensor values to a maximum average L2-norm.

  Given a tensor `t`, and a maximum clip value `clip_norm`, this operation
  normalizes `t` so that its average L2-norm is less than or equal to
  `clip_norm'. Specifically, if the average L2-norm is already less than or
  equal to `clip_norm`, then `t` is not modified. If the average L2-norm is
  greater than `clip_norm`, then this operation returns a tensor of the same
  type and shape as `t` with its values set to:

  `t * clip_norm / l2norm_avg(t)`

  In this case, the average L2-norm of the output tensor is `clip_norm`.

  This operation is typically used to clip gradients before applying them with
  an optimizer.

  Args:
    t: A `Tensor`.
    clip_norm: A 0-D (scalar) `Tensor` > 0. A maximum clipping value.
    name: A name for the operation (optional).

  Returns:
    A clipped `Tensor`.
  """
  with ops.op_scope([t, clip_norm], name, "clip_by_average_norm") as name:
    t = ops.convert_to_tensor(t, name="t")

    # Calculate L2-norm per element, clip elements by ratio of clip_norm to
    # L2-norm per element
    n_element = math_ops.cast(array_ops.size(t), types.float32)
    l2norm_inv = math_ops.rsqrt(
        math_ops.reduce_sum(t * t, math_ops.range(0, array_ops.rank(t))))
    tclip = array_ops.identity(
        t * clip_norm * math_ops.minimum(
            l2norm_inv * n_element, constant_op.constant(1.0 / clip_norm)),
        name=name)

  return tclip