# Copyright 2017 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.
# ==============================================================================
"""Utility ops shared across tf.contrib.signal."""

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

from tensorflow.python.framework import ops
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import math_ops


def gcd(a, b, name=None):
  """Returns the greatest common divisor via Euclid's algorithm.

  Args:
    a: The dividend. A scalar integer `Tensor`.
    b: The divisor. A scalar integer `Tensor`.
    name: An optional name for the operation.

  Returns:
    A scalar `Tensor` representing the greatest common divisor between `a` and
    `b`.

  Raises:
    ValueError: If `a` or `b` are not scalar integers.
  """
  with ops.name_scope(name, 'gcd', [a, b]):
    a = ops.convert_to_tensor(a)
    b = ops.convert_to_tensor(b)

    a.shape.assert_has_rank(0)
    b.shape.assert_has_rank(0)

    if not a.dtype.is_integer:
      raise ValueError('a must be an integer type. Got: %s' % a.dtype)
    if not b.dtype.is_integer:
      raise ValueError('b must be an integer type. Got: %s' % b.dtype)

    cond = lambda _, b: math_ops.greater(b, array_ops.zeros_like(b))
    body = lambda a, b: [b, math_ops.mod(a, b)]
    a, b = control_flow_ops.while_loop(cond, body, [a, b], back_prop=False)
    return a
