# Copyright 2017 The TensorFlow Authors. All Rights Reserved.
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# 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
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#     http://www.apache.org/licenses/LICENSE-2.0
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# Unless required by applicable law or agreed to in writing, software
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# ==============================================================================
"""Operations for linear algebra."""

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

from tensorflow.python.framework import constant_op
from tensorflow.python.framework import dtypes
from tensorflow.python.framework import ops
from tensorflow.python.framework import tensor_shape
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import gen_linalg_ops
from tensorflow.python.ops import linalg_ops
from tensorflow.python.ops import math_ops
from tensorflow.python.ops import special_math_ops
from tensorflow.python.util import dispatch
from tensorflow.python.util.tf_export import tf_export

# Linear algebra ops.
band_part = array_ops.matrix_band_part
cholesky = linalg_ops.cholesky
cholesky_solve = linalg_ops.cholesky_solve
det = linalg_ops.matrix_determinant
slogdet = gen_linalg_ops.log_matrix_determinant
tf_export('linalg.slogdet')(slogdet)
diag = array_ops.matrix_diag
diag_part = array_ops.matrix_diag_part
eigh = linalg_ops.self_adjoint_eig
eigvalsh = linalg_ops.self_adjoint_eigvals
einsum = special_math_ops.einsum
eye = linalg_ops.eye
inv = linalg_ops.matrix_inverse
logm = gen_linalg_ops.matrix_logarithm
lu = gen_linalg_ops.lu
tf_export('linalg.logm')(logm)
lstsq = linalg_ops.matrix_solve_ls
norm = linalg_ops.norm
qr = linalg_ops.qr
set_diag = array_ops.matrix_set_diag
solve = linalg_ops.matrix_solve
sqrtm = linalg_ops.matrix_square_root
svd = linalg_ops.svd
tensordot = math_ops.tensordot
trace = math_ops.trace
transpose = array_ops.matrix_transpose
triangular_solve = linalg_ops.matrix_triangular_solve


@tf_export('linalg.logdet')
@dispatch.add_dispatch_support
def logdet(matrix, name=None):
  """Computes log of the determinant of a hermitian positive definite matrix.

  ```python
  # Compute the determinant of a matrix while reducing the chance of over- or
  underflow:
  A = ... # shape 10 x 10
  det = tf.exp(tf.linalg.logdet(A))  # scalar
  ```

  Args:
    matrix:  A `Tensor`. Must be `float16`, `float32`, `float64`, `complex64`,
      or `complex128` with shape `[..., M, M]`.
    name:  A name to give this `Op`.  Defaults to `logdet`.

  Returns:
    The natural log of the determinant of `matrix`.

  @compatibility(numpy)
  Equivalent to numpy.linalg.slogdet, although no sign is returned since only
  hermitian positive definite matrices are supported.
  @end_compatibility
  """
  # This uses the property that the log det(A) = 2*sum(log(real(diag(C))))
  # where C is the cholesky decomposition of A.
  with ops.name_scope(name, 'logdet', [matrix]):
    chol = gen_linalg_ops.cholesky(matrix)
    return 2.0 * math_ops.reduce_sum(
        math_ops.log(math_ops.real(array_ops.matrix_diag_part(chol))),
        axis=[-1])


@tf_export('linalg.adjoint')
@dispatch.add_dispatch_support
def adjoint(matrix, name=None):
  """Transposes the last two dimensions of and conjugates tensor `matrix`.

  For example:

  ```python
  x = tf.constant([[1 + 1j, 2 + 2j, 3 + 3j],
                   [4 + 4j, 5 + 5j, 6 + 6j]])
  tf.linalg.adjoint(x)  # [[1 - 1j, 4 - 4j],
                        #  [2 - 2j, 5 - 5j],
                        #  [3 - 3j, 6 - 6j]]
  ```

  Args:
    matrix:  A `Tensor`. Must be `float16`, `float32`, `float64`, `complex64`,
      or `complex128` with shape `[..., M, M]`.
    name:  A name to give this `Op` (optional).

  Returns:
    The adjoint (a.k.a. Hermitian transpose a.k.a. conjugate transpose) of
    matrix.
  """
  with ops.name_scope(name, 'adjoint', [matrix]):
    matrix = ops.convert_to_tensor(matrix, name='matrix')
    return array_ops.matrix_transpose(matrix, conjugate=True)


# This section is ported nearly verbatim from Eigen's implementation:
# https://eigen.tuxfamily.org/dox/unsupported/MatrixExponential_8h_source.html
def _matrix_exp_pade3(matrix):
  """3rd-order Pade approximant for matrix exponential."""
  b = [120.0, 60.0, 12.0]
  b = [constant_op.constant(x, matrix.dtype) for x in b]
  ident = linalg_ops.eye(
      array_ops.shape(matrix)[-2],
      batch_shape=array_ops.shape(matrix)[:-2],
      dtype=matrix.dtype)
  matrix_2 = math_ops.matmul(matrix, matrix)
  tmp = matrix_2 + b[1] * ident
  matrix_u = math_ops.matmul(matrix, tmp)
  matrix_v = b[2] * matrix_2 + b[0] * ident
  return matrix_u, matrix_v


def _matrix_exp_pade5(matrix):
  """5th-order Pade approximant for matrix exponential."""
  b = [30240.0, 15120.0, 3360.0, 420.0, 30.0]
  b = [constant_op.constant(x, matrix.dtype) for x in b]
  ident = linalg_ops.eye(
      array_ops.shape(matrix)[-2],
      batch_shape=array_ops.shape(matrix)[:-2],
      dtype=matrix.dtype)
  matrix_2 = math_ops.matmul(matrix, matrix)
  matrix_4 = math_ops.matmul(matrix_2, matrix_2)
  tmp = matrix_4 + b[3] * matrix_2 + b[1] * ident
  matrix_u = math_ops.matmul(matrix, tmp)
  matrix_v = b[4] * matrix_4 + b[2] * matrix_2 + b[0] * ident
  return matrix_u, matrix_v


def _matrix_exp_pade7(matrix):
  """7th-order Pade approximant for matrix exponential."""
  b = [17297280.0, 8648640.0, 1995840.0, 277200.0, 25200.0, 1512.0, 56.0]
  b = [constant_op.constant(x, matrix.dtype) for x in b]
  ident = linalg_ops.eye(
      array_ops.shape(matrix)[-2],
      batch_shape=array_ops.shape(matrix)[:-2],
      dtype=matrix.dtype)
  matrix_2 = math_ops.matmul(matrix, matrix)
  matrix_4 = math_ops.matmul(matrix_2, matrix_2)
  matrix_6 = math_ops.matmul(matrix_4, matrix_2)
  tmp = matrix_6 + b[5] * matrix_4 + b[3] * matrix_2 + b[1] * ident
  matrix_u = math_ops.matmul(matrix, tmp)
  matrix_v = b[6] * matrix_6 + b[4] * matrix_4 + b[2] * matrix_2 + b[0] * ident
  return matrix_u, matrix_v


def _matrix_exp_pade9(matrix):
  """9th-order Pade approximant for matrix exponential."""
  b = [
      17643225600.0, 8821612800.0, 2075673600.0, 302702400.0, 30270240.0,
      2162160.0, 110880.0, 3960.0, 90.0
  ]
  b = [constant_op.constant(x, matrix.dtype) for x in b]
  ident = linalg_ops.eye(
      array_ops.shape(matrix)[-2],
      batch_shape=array_ops.shape(matrix)[:-2],
      dtype=matrix.dtype)
  matrix_2 = math_ops.matmul(matrix, matrix)
  matrix_4 = math_ops.matmul(matrix_2, matrix_2)
  matrix_6 = math_ops.matmul(matrix_4, matrix_2)
  matrix_8 = math_ops.matmul(matrix_6, matrix_2)
  tmp = (
      matrix_8 + b[7] * matrix_6 + b[5] * matrix_4 + b[3] * matrix_2 +
      b[1] * ident)
  matrix_u = math_ops.matmul(matrix, tmp)
  matrix_v = (
      b[8] * matrix_8 + b[6] * matrix_6 + b[4] * matrix_4 + b[2] * matrix_2 +
      b[0] * ident)
  return matrix_u, matrix_v


def _matrix_exp_pade13(matrix):
  """13th-order Pade approximant for matrix exponential."""
  b = [
      64764752532480000.0, 32382376266240000.0, 7771770303897600.0,
      1187353796428800.0, 129060195264000.0, 10559470521600.0, 670442572800.0,
      33522128640.0, 1323241920.0, 40840800.0, 960960.0, 16380.0, 182.0
  ]
  b = [constant_op.constant(x, matrix.dtype) for x in b]
  ident = linalg_ops.eye(
      array_ops.shape(matrix)[-2],
      batch_shape=array_ops.shape(matrix)[:-2],
      dtype=matrix.dtype)
  matrix_2 = math_ops.matmul(matrix, matrix)
  matrix_4 = math_ops.matmul(matrix_2, matrix_2)
  matrix_6 = math_ops.matmul(matrix_4, matrix_2)
  tmp_u = (
      math_ops.matmul(matrix_6, matrix_6 + b[11] * matrix_4 + b[9] * matrix_2) +
      b[7] * matrix_6 + b[5] * matrix_4 + b[3] * matrix_2 + b[1] * ident)
  matrix_u = math_ops.matmul(matrix, tmp_u)
  tmp_v = b[12] * matrix_6 + b[10] * matrix_4 + b[8] * matrix_2
  matrix_v = (
      math_ops.matmul(matrix_6, tmp_v) + b[6] * matrix_6 + b[4] * matrix_4 +
      b[2] * matrix_2 + b[0] * ident)
  return matrix_u, matrix_v


@tf_export('linalg.expm')
def matrix_exponential(input, name=None):  # pylint: disable=redefined-builtin
  r"""Computes the matrix exponential of one or more square matrices.

  exp(A) = \sum_{n=0}^\infty A^n/n!

  The exponential is computed using a combination of the scaling and squaring
  method and the Pade approximation. Details can be found in:
  Nicholas J. Higham, "The scaling and squaring method for the matrix
  exponential revisited," SIAM J. Matrix Anal. Applic., 26:1179-1193, 2005.

  The input is a tensor of shape `[..., M, M]` whose inner-most 2 dimensions
  form square matrices. The output is a tensor of the same shape as the input
  containing the exponential for all input submatrices `[..., :, :]`.

  Args:
    input: A `Tensor`. Must be `float16`, `float32`, `float64`, `complex64`, or
      `complex128` with shape `[..., M, M]`.
    name:  A name to give this `Op` (optional).

  Returns:
    the matrix exponential of the input.

  Raises:
    ValueError: An unsupported type is provided as input.

  @compatibility(scipy)
  Equivalent to scipy.linalg.expm
  @end_compatibility
  """
  with ops.name_scope(name, 'matrix_exponential', [input]):
    matrix = ops.convert_to_tensor(input, name='input')
    if matrix.shape[-2:] == [0, 0]:
      return matrix
    batch_shape = matrix.shape[:-2]
    if not batch_shape.is_fully_defined():
      batch_shape = array_ops.shape(matrix)[:-2]

    # reshaping the batch makes the where statements work better
    matrix = array_ops.reshape(
        matrix, array_ops.concat(([-1], array_ops.shape(matrix)[-2:]), axis=0))
    l1_norm = math_ops.reduce_max(
        math_ops.reduce_sum(
            math_ops.abs(matrix),
            axis=array_ops.size(array_ops.shape(matrix)) - 2),
        axis=-1)
    const = lambda x: constant_op.constant(x, l1_norm.dtype)

    def _nest_where(vals, cases):
      assert len(vals) == len(cases) - 1
      if len(vals) == 1:
        return array_ops.where(
            math_ops.less(l1_norm, const(vals[0])), cases[0], cases[1])
      else:
        return array_ops.where(
            math_ops.less(l1_norm, const(vals[0])), cases[0],
            _nest_where(vals[1:], cases[1:]))

    if matrix.dtype in [dtypes.float16, dtypes.float32, dtypes.complex64]:
      maxnorm = const(3.925724783138660)
      squarings = math_ops.maximum(
          math_ops.floor(
              math_ops.log(l1_norm / maxnorm) / math_ops.log(const(2.0))), 0)
      u3, v3 = _matrix_exp_pade3(matrix)
      u5, v5 = _matrix_exp_pade5(matrix)
      u7, v7 = _matrix_exp_pade7(matrix / math_ops.pow(
          constant_op.constant(2.0, dtype=matrix.dtype),
          math_ops.cast(
              squarings,
              matrix.dtype))[..., array_ops.newaxis, array_ops.newaxis])
      conds = (4.258730016922831e-001, 1.880152677804762e+000)
      u = _nest_where(conds, (u3, u5, u7))
      v = _nest_where(conds, (v3, v5, v7))
    elif matrix.dtype in [dtypes.float64, dtypes.complex128]:
      maxnorm = const(5.371920351148152)
      squarings = math_ops.maximum(
          math_ops.floor(
              math_ops.log(l1_norm / maxnorm) / math_ops.log(const(2.0))), 0)
      u3, v3 = _matrix_exp_pade3(matrix)
      u5, v5 = _matrix_exp_pade5(matrix)
      u7, v7 = _matrix_exp_pade7(matrix)
      u9, v9 = _matrix_exp_pade9(matrix)
      u13, v13 = _matrix_exp_pade13(matrix / math_ops.pow(
          constant_op.constant(2.0, dtype=matrix.dtype),
          math_ops.cast(
              squarings,
              matrix.dtype))[..., array_ops.newaxis, array_ops.newaxis])
      conds = (1.495585217958292e-002, 2.539398330063230e-001,
               9.504178996162932e-001, 2.097847961257068e+000)
      u = _nest_where(conds, (u3, u5, u7, u9, u13))
      v = _nest_where(conds, (v3, v5, v7, v9, v13))
    else:
      raise ValueError('tf.linalg.expm does not support matrices of type %s' %
                       matrix.dtype)
    numer = u + v
    denom = -u + v
    result = linalg_ops.matrix_solve(denom, numer)
    max_squarings = math_ops.reduce_max(squarings)

    i = const(0.0)
    c = lambda i, r: math_ops.less(i, max_squarings)

    def b(i, r):
      return i + 1, array_ops.where(
          math_ops.less(i, squarings), math_ops.matmul(r, r), r)

    _, result = control_flow_ops.while_loop(c, b, [i, result])
    if not matrix.shape.is_fully_defined():
      return array_ops.reshape(
          result,
          array_ops.concat((batch_shape, array_ops.shape(result)[-2:]), axis=0))
    return array_ops.reshape(result, batch_shape.concatenate(result.shape[-2:]))


@tf_export('linalg.tridiagonal_solve')
def tridiagonal_solve(diagonals,
                      rhs,
                      diagonals_format='compact',
                      transpose_rhs=False,
                      conjugate_rhs=False,
                      name=None,
                      partial_pivoting=True):
  r"""Solves tridiagonal systems of equations.

  The input can be supplied in various formats: `matrix`, `sequence` and
  `compact`, specified by the `diagonals_format` arg.

  In `matrix` format, `diagonals` must be a tensor of shape `[..., M, M]`, with
  two inner-most dimensions representing the square tridiagonal matrices.
  Elements outside of the three diagonals will be ignored.

  In `sequence` format, `diagonals` are supplied as a tuple or list of three
  tensors of shapes `[..., N]`, `[..., M]`, `[..., N]` representing
  superdiagonals, diagonals, and subdiagonals, respectively. `N` can be either
  `M-1` or `M`; in the latter case, the last element of superdiagonal and the
  first element of subdiagonal will be ignored.

  In `compact` format the three diagonals are brought together into one tensor
  of shape `[..., 3, M]`, with last two dimensions containing superdiagonals,
  diagonals, and subdiagonals, in order. Similarly to `sequence` format,
  elements `diagonals[..., 0, M-1]` and `diagonals[..., 2, 0]` are ignored.

  The `compact` format is recommended as the one with best performance. In case
  you need to cast a tensor into a compact format manually, use `tf.gather_nd`.
  An example for a tensor of shape [m, m]:

  ```python
  rhs = tf.constant([...])
  matrix = tf.constant([[...]])
  m = matrix.shape[0]
  dummy_idx = [0, 0]  # An arbitrary element to use as a dummy
  indices = [[[i, i + 1] for i in range(m - 1)] + [dummy_idx],  # Superdiagonal
           [[i, i] for i in range(m)],                          # Diagonal
           [dummy_idx] + [[i + 1, i] for i in range(m - 1)]]    # Subdiagonal
  diagonals=tf.gather_nd(matrix, indices)
  x = tf.linalg.tridiagonal_solve(diagonals, rhs)
  ```

  Regardless of the `diagonals_format`, `rhs` is a tensor of shape `[..., M]` or
  `[..., M, K]`. The latter allows to simultaneously solve K systems with the
  same left-hand sides and K different right-hand sides. If `transpose_rhs`
  is set to `True` the expected shape is `[..., M]` or `[..., K, M]`.

  The batch dimensions, denoted as `...`, must be the same in `diagonals` and
  `rhs`.

  The output is a tensor of the same shape as `rhs`: either `[..., M]` or
  `[..., M, K]`.

  The op isn't guaranteed to raise an error if the input matrix is not
  invertible. `tf.debugging.check_numerics` can be applied to the output to
  detect invertibility problems.

  **Note**: with large batch sizes, the computation on the GPU may be slow, if
  either `partial_pivoting=True` or there are multiple right-hand sides
  (`K > 1`). If this issue arises, consider if it's possible to disable pivoting
  and have `K = 1`, or, alternatively, consider using CPU.

  On CPU, solution is computed via Gaussian elimination with or without partial
  pivoting, depending on `partial_pivoting` parameter. On GPU, Nvidia's cuSPARSE
  library is used: https://docs.nvidia.com/cuda/cusparse/index.html#gtsv

  Args:
    diagonals: A `Tensor` or tuple of `Tensor`s describing left-hand sides. The
      shape depends of `diagonals_format`, see description above. Must be
      `float32`, `float64`, `complex64`, or `complex128`.
    rhs: A `Tensor` of shape [..., M] or [..., M, K] and with the same dtype as
      `diagonals`.
    diagonals_format: one of `matrix`, `sequence`, or `compact`. Default is
      `compact`.
    transpose_rhs: If `True`, `rhs` is transposed before solving (has no effect
      if the shape of rhs is [..., M]).
    conjugate_rhs: If `True`, `rhs` is conjugated before solving.
    name:  A name to give this `Op` (optional).
    partial_pivoting: whether to perform partial pivoting. `True` by default.
      Partial pivoting makes the procedure more stable, but slower. Partial
      pivoting is unnecessary in some cases, including diagonally dominant and
      symmetric positive definite matrices (see e.g. theorem 9.12 in [1]).

  Returns:
    A `Tensor` of shape [..., M] or [..., M, K] containing the solutions.

  Raises:
    ValueError: An unsupported type is provided as input, or when the input
    tensors have incorrect shapes.

  [1] Nicholas J. Higham (2002). Accuracy and Stability of Numerical Algorithms:
  Second Edition. SIAM. p. 175. ISBN 978-0-89871-802-7.

  """
  if diagonals_format == 'compact':
    return _tridiagonal_solve_compact_format(diagonals, rhs, transpose_rhs,
                                             conjugate_rhs, partial_pivoting,
                                             name)

  if diagonals_format == 'sequence':
    if not isinstance(diagonals, (tuple, list)) or len(diagonals) != 3:
      raise ValueError('Expected diagonals to be a sequence of length 3.')

    superdiag, maindiag, subdiag = diagonals
    if (not subdiag.shape[:-1].is_compatible_with(maindiag.shape[:-1]) or
        not superdiag.shape[:-1].is_compatible_with(maindiag.shape[:-1])):
      raise ValueError(
          'Tensors representing the three diagonals must have the same shape,'
          'except for the last dimension, got {}, {}, {}'.format(
              subdiag.shape, maindiag.shape, superdiag.shape))

    m = tensor_shape.dimension_value(maindiag.shape[-1])

    def pad_if_necessary(t, name, last_dim_padding):
      n = tensor_shape.dimension_value(t.shape[-1])
      if not n or n == m:
        return t
      if n == m - 1:
        paddings = ([[0, 0] for _ in range(len(t.shape) - 1)] +
                    [last_dim_padding])
        return array_ops.pad(t, paddings)
      raise ValueError('Expected {} to be have length {} or {}, got {}.'.format(
          name, m, m - 1, n))

    subdiag = pad_if_necessary(subdiag, 'subdiagonal', [1, 0])
    superdiag = pad_if_necessary(superdiag, 'superdiagonal', [0, 1])

    diagonals = array_ops.stack((superdiag, maindiag, subdiag), axis=-2)
    return _tridiagonal_solve_compact_format(diagonals, rhs, transpose_rhs,
                                             conjugate_rhs, partial_pivoting,
                                             name)

  if diagonals_format == 'matrix':
    m1 = tensor_shape.dimension_value(diagonals.shape[-1])
    m2 = tensor_shape.dimension_value(diagonals.shape[-2])
    if m1 and m2 and m1 != m2:
      raise ValueError(
          'Expected last two dimensions of diagonals to be same, got {} and {}'
          .format(m1, m2))
    m = m1 or m2
    if not m:
      raise ValueError('The size of the matrix needs to be known for '
                       'diagonals_format="matrix"')

    # Extract diagonals; use input[..., 0, 0] as "dummy" m-th elements of sub-
    # and superdiagonal.
    # gather_nd slices into first indices, whereas we need to slice into the
    # last two, so transposing back and forth is necessary.
    dummy_idx = [0, 0]
    indices = ([[[1, 0], [0, 0], dummy_idx]] +
               [[[i + 1, i], [i, i], [i - 1, i]] for i in range(1, m - 1)] +
               [[dummy_idx, [m - 1, m - 1], [m - 2, m - 1]]])
    diagonals = array_ops.transpose(
        array_ops.gather_nd(array_ops.transpose(diagonals), indices))
    return _tridiagonal_solve_compact_format(diagonals, rhs, transpose_rhs,
                                             conjugate_rhs, partial_pivoting,
                                             name)

  raise ValueError('Unrecognized diagonals_format: {}'.format(diagonals_format))


def _tridiagonal_solve_compact_format(diagonals, rhs, transpose_rhs,
                                      conjugate_rhs, partial_pivoting, name):
  """Helper function used after the input has been cast to compact form."""
  diags_rank, rhs_rank = len(diagonals.shape), len(rhs.shape)

  if diags_rank < 2:
    raise ValueError(
        'Expected diagonals to have rank at least 2, got {}'.format(diags_rank))
  if rhs_rank != diags_rank and rhs_rank != diags_rank - 1:
    raise ValueError('Expected the rank of rhs to be {} or {}, got {}'.format(
        diags_rank - 1, diags_rank, rhs_rank))
  if diagonals.shape[-2] and diagonals.shape[-2] != 3:
    raise ValueError('Expected 3 diagonals got {}'.format(diagonals.shape[-2]))
  if not diagonals.shape[:-2].is_compatible_with(rhs.shape[:diags_rank - 2]):
    raise ValueError('Batch shapes {} and {} are incompatible'.format(
        diagonals.shape[:-2], rhs.shape[:diags_rank - 2]))

  def check_num_lhs_matches_num_rhs():
    if (diagonals.shape[-1] and rhs.shape[-2] and
        diagonals.shape[-1] != rhs.shape[-2]):
      raise ValueError('Expected number of left-hand sided and right-hand '
                       'sides to be equal, got {} and {}'.format(
                           diagonals.shape[-1], rhs.shape[-2]))

  if rhs_rank == diags_rank - 1:
    # Rhs provided as a vector, ignoring transpose_rhs
    if conjugate_rhs:
      rhs = math_ops.conj(rhs)
    rhs = array_ops.expand_dims(rhs, -1)
    check_num_lhs_matches_num_rhs()
    return array_ops.squeeze(
        linalg_ops.tridiagonal_solve(diagonals, rhs, partial_pivoting, name),
        -1)

  if transpose_rhs:
    rhs = array_ops.matrix_transpose(rhs, conjugate=conjugate_rhs)
  elif conjugate_rhs:
    rhs = math_ops.conj(rhs)

  check_num_lhs_matches_num_rhs()
  result = linalg_ops.tridiagonal_solve(diagonals, rhs, partial_pivoting, name)
  return array_ops.matrix_transpose(result) if transpose_rhs else result


@tf_export('linalg.tridiagonal_matmul')
def tridiagonal_matmul(diagonals, rhs, diagonals_format='compact', name=None):
  r"""Multiplies tridiagonal matrix by matrix.

  `diagonals` is representation of 3-diagonal NxN matrix, which depends on
  `diagonals_format`.

  In `matrix` format, `diagonals` must be a tensor of shape `[..., M, M]`, with
  two inner-most dimensions representing the square tridiagonal matrices.
  Elements outside of the three diagonals will be ignored.

  If `sequence` format, `diagonals` is list or tuple of three tensors:
  `[superdiag, maindiag, subdiag]`, each having shape [..., M]. Last element
  of `superdiag` first element of `subdiag` are ignored.

  In `compact` format the three diagonals are brought together into one tensor
  of shape `[..., 3, M]`, with last two dimensions containing superdiagonals,
  diagonals, and subdiagonals, in order. Similarly to `sequence` format,
  elements `diagonals[..., 0, M-1]` and `diagonals[..., 2, 0]` are ignored.

  The `sequence` format is recommended as the one with the best performance.

  `rhs` is matrix to the right of multiplication. It has shape `[..., M, N]`.

  Example:

  ```python
  superdiag = tf.constant([-1, -1, 0], dtype=tf.float64)
  maindiag = tf.constant([2, 2, 2], dtype=tf.float64)
  subdiag = tf.constant([0, -1, -1], dtype=tf.float64)
  diagonals = [superdiag, maindiag, subdiag]
  rhs = tf.constant([[1, 1], [1, 1], [1, 1]], dtype=tf.float64)
  x = tf.linalg.tridiagonal_matmul(diagonals, rhs, diagonals_format='sequence')
  ```

  Args:
    diagonals: A `Tensor` or tuple of `Tensor`s describing left-hand sides. The
      shape depends of `diagonals_format`, see description above. Must be
      `float32`, `float64`, `complex64`, or `complex128`.
    rhs: A `Tensor` of shape [..., M, N] and with the same dtype as `diagonals`.
    diagonals_format: one of `sequence`, or `compact`. Default is `compact`.
    name:  A name to give this `Op` (optional).

  Returns:
    A `Tensor` of shape [..., M, N] containing the result of multiplication.

  Raises:
    ValueError: An unsupported type is provided as input, or when the input
    tensors have incorrect shapes.
  """
  if diagonals_format == 'compact':
    superdiag = diagonals[..., 0, :]
    maindiag = diagonals[..., 1, :]
    subdiag = diagonals[..., 2, :]
  elif diagonals_format == 'sequence':
    superdiag, maindiag, subdiag = diagonals
  elif diagonals_format == 'matrix':
    m1 = tensor_shape.dimension_value(diagonals.shape[-1])
    m2 = tensor_shape.dimension_value(diagonals.shape[-2])
    if not m1 or not m2:
      raise ValueError('The size of the matrix needs to be known for '
                       'diagonals_format="matrix"')
    if m1 != m2:
      raise ValueError(
          'Expected last two dimensions of diagonals to be same, got {} and {}'
          .format(m1, m2))

    # TODO(b/131695260): use matrix_diag_part when it supports extracting
    # arbitrary diagonals.
    maindiag = array_ops.matrix_diag_part(diagonals)
    diagonals = array_ops.transpose(diagonals)
    dummy_index = [0, 0]
    superdiag_indices = [[i + 1, i] for i in range(0, m1 - 1)] + [dummy_index]
    subdiag_indices = [dummy_index] + [[i - 1, i] for i in range(1, m1)]
    superdiag = array_ops.transpose(
        array_ops.gather_nd(diagonals, superdiag_indices))
    subdiag = array_ops.transpose(
        array_ops.gather_nd(diagonals, subdiag_indices))
  else:
    raise ValueError('Unrecognized diagonals_format: %s' % diagonals_format)

  # C++ backend requires matrices.
  # Converting 1-dimensional vectors to matrices with 1 row.
  superdiag = array_ops.expand_dims(superdiag, -2)
  maindiag = array_ops.expand_dims(maindiag, -2)
  subdiag = array_ops.expand_dims(subdiag, -2)

  return linalg_ops.tridiagonal_mat_mul(superdiag, maindiag, subdiag, rhs, name)
