# Source code for tensorly.tt_tensor

```"""
Core operations on tensors in Tensor-Train (TT) format, also known as Matrix-Product-State (MPS)
"""

import tensorly as tl
from ._factorized_tensor import FactorizedTensor
import numpy as np
import warnings

def _validate_tt_tensor(tt_tensor):
factors = tt_tensor
n_factors = len(factors)

if isinstance(tt_tensor, TTTensor):
# it's already been validated at creation
return tt_tensor.shape, tt_tensor.rank
elif isinstance(tt_tensor, (float, int)):  # 0-order tensor
return 0, 0

rank = []
shape = []
for index, factor in enumerate(factors):
current_rank, current_shape, next_rank = tl.shape(factor)

# Check that factors are third order tensors
if not tl.ndim(factor) == 3:
raise ValueError(
"TT expresses a tensor as third order factors (tt-cores).\n"
f"However, tl.ndim(factors[{index}]) = {tl.ndim(factor)}"
)
# Consecutive factors should have matching ranks
if index and tl.shape(factors[index - 1])[2] != current_rank:
raise ValueError(
"Consecutive factors should have matching ranks\n"
" -- e.g. tl.shape(factors[0])[2]) == tl.shape(factors[1])[0])\n"
f"However, tl.shape(factor[{index-1}])[2] == {tl.shape(factors[index - 1])[2]} but"
f" tl.shape(factor[{index}])[0] == {current_rank} "
)
# Check for boundary conditions
if (index == 0) and current_rank != 1:
raise ValueError(
"Boundary conditions dictate factor[0].shape[0] == 1."
f"However, got factor[0].shape[0] = {current_rank}."
)
if (index == n_factors - 1) and next_rank != 1:
raise ValueError(
"Boundary conditions dictate factor[-1].shape[2] == 1."
f"However, got factor[{n_factors}].shape[2] = {next_rank}."
)

shape.append(current_shape)
rank.append(current_rank)

# Add last rank (boundary condition)
rank.append(next_rank)

return tuple(shape), tuple(rank)

[docs]
def tt_to_tensor(factors):
"""Returns the full tensor whose TT decomposition is given by 'factors'

Re-assembles 'factors', which represent a tensor in TT/Matrix-Product-State format
into the corresponding full tensor

Parameters
----------
factors : list of 3D-arrays
TT factors (TT-cores)

Returns
-------
output_tensor : ndarray
tensor whose TT/MPS decomposition was given by 'factors'
"""
if isinstance(factors, (float, int)):  # 0-order tensor
return factors

full_shape = [f.shape[1] for f in factors]
full_tensor = tl.reshape(factors[0], (full_shape[0], -1))

for factor in factors[1:]:
rank_prev, _, rank_next = factor.shape
factor = tl.reshape(factor, (rank_prev, -1))
full_tensor = tl.dot(full_tensor, factor)
full_tensor = tl.reshape(full_tensor, (-1, rank_next))

return tl.reshape(full_tensor, full_shape)

[docs]
def tt_to_unfolded(factors, mode):
"""Returns the unfolding matrix of a tensor given in TT (or Tensor-Train) format

Reassembles a full tensor from 'factors' and returns its unfolding matrix
with mode given by 'mode'

Parameters
----------
factors: list of 3D-arrays
TT factors
mode: int
unfolding matrix to be computed along this mode

Returns
-------
2-D array
unfolding matrix at mode given by 'mode'
"""
return tl.unfold(tt_to_tensor(factors), mode)

[docs]
def tt_to_vec(factors):
"""Returns the tensor defined by its TT format ('factors') into
its vectorized format

Parameters
----------
factors: list of 3D-arrays
TT factors

Returns
-------
1-D array
vectorized format of tensor defined by 'factors'
"""
return tl.tensor_to_vec(tt_to_tensor(factors))

def _tt_n_param(tensor_shape, rank):
"""Number of parameters of a MPS decomposition for a given `rank` and full `tensor_shape`.

Parameters
----------
tensor_shape : int tuple
shape of the full tensor to decompose (or approximate)

rank : tuple
rank of the MPS decomposition

Returns
-------
n_params : int
Number of parameters of a MPS decomposition of rank `rank` of a full tensor of shape `tensor_shape`
"""
factor_params = []
for i, s in enumerate(tensor_shape):
factor_params.append(rank[i] * s * rank[i + 1])
return np.sum(factor_params)

def validate_tt_rank(
tensor_shape,
rank="same",
constant_rank=False,
rounding="round",
allow_overparametrization=True,
):
"""Returns the rank of a TT Decomposition

Parameters
----------
tensor_shape : tupe
shape of the tensor to decompose
rank : {'same', float, tuple, int}, default is same
way to determine the rank, by default 'same'
if 'same': rank is computed to keep the number of parameters (at most) the same
if float, computes a rank so as to keep rank percent of the original number of parameters
if int or tuple, just returns rank
constant_rank : bool, default is False
* if True, the *same* rank will be chosen for each modes
* if False (default), the rank of each mode will be proportional to the corresponding tensor_shape

*used only if rank == 'same' or 0 < rank <= 1*

rounding = {'round', 'floor', 'ceil'}

allow_overparametrization : bool, default is True
if False, the rank must be realizable through iterative application of SVD
(used in tensorly.decomposition.tensor_train)

Returns
-------
rank : int tuple
rank of the decomposition
"""
if rounding == "ceil":
rounding_fun = np.ceil
elif rounding == "floor":
rounding_fun = np.floor
elif rounding == "round":
rounding_fun = np.round
else:
raise ValueError(f"Rounding should be round, floor or ceil, but got {rounding}")

if rank == "same":
rank = float(1)

if isinstance(rank, float) and constant_rank:
# Choose the *same* rank for each mode
n_param_tensor = np.prod(tensor_shape) * rank
order = len(tensor_shape)

if order == 2:
rank = (1, n_param_tensor / (tensor_shape[0] + tensor_shape[1]), 1)
warnings.warn(
f"Determining the tt-rank for the trivial case of a matrix (order 2 tensor) of shape {tensor_shape}, not a higher-order tensor."
)

# R_k I_k R_{k+1} = R^2 I_k
a = np.sum(tensor_shape[1:-1])

# Border rank of 1, R_0 = R_N = 1
# First and last factor of size I_0 R and I_N R
b = np.sum(tensor_shape[0] + tensor_shape[-1])

# We want the number of params of decomp (=sum of params of factors)
# To be equal to c = \prod_k I_k
c = -n_param_tensor
delta = np.sqrt(b**2 - 4 * a * c)

# We get the non-negative solution
solution = int(rounding_fun((-b + delta) / (2 * a)))
rank = rank = (1,) + (solution,) * (order - 1) + (1,)

elif isinstance(rank, float):
# Choose a rank proportional to the size of each mode
# The method is similar to the above one for constant_rank == True
order = len(tensor_shape)
avg_dim = [
(tensor_shape[i] + tensor_shape[i + 1]) / 2 for i in range(order - 1)
]
if len(avg_dim) > 1:
a = sum(
avg_dim[i - 1] * tensor_shape[i] * avg_dim[i]
for i in range(1, order - 1)
)
else:
warnings.warn(
f"Determining the tt-rank for the trivial case of a matrix (order 2 tensor) of shape {tensor_shape}, not a higher-order tensor."
)
a = avg_dim[0] ** 2 * tensor_shape[0]
b = tensor_shape[0] * avg_dim[0] + tensor_shape[-1] * avg_dim[-1]
c = -np.prod(tensor_shape) * rank
delta = np.sqrt(b**2 - 4 * a * c)

# We get the non-negative solution
fraction_param = (-b + delta) / (2 * a)
rank = tuple([max(int(rounding_fun(d * fraction_param)), 1) for d in avg_dim])
rank = (1,) + rank + (1,)

else:
# Check user input for potential errors
n_dim = len(tensor_shape)
if isinstance(rank, int):
rank = [1] + [rank] * (n_dim - 1) + [1]
elif n_dim + 1 != len(rank):
message = f"Provided incorrect number of ranks. Should verify len(rank) == tl.ndim(tensor)+1, but len(rank) = {len(rank)} while tl.ndim(tensor) + 1  = {n_dim+1}"
raise (ValueError(message))

# Initialization
if rank[0] != 1:
message = "Provided rank[0] == {} but boundary conditions dictate rank[0] == rank[-1] == 1.".format(
rank[0]
)
raise ValueError(message)
if rank[-1] != 1:
message = "Provided rank[-1] == {} but boundary conditions dictate rank[0] == rank[-1] == 1.".format(
rank[-1]
)
raise ValueError(message)

if allow_overparametrization:
return list(rank)
else:
validated_rank = [1]
for i, s in enumerate(tensor_shape[:-1]):
n_row = int(rank[i] * s)
n_column = np.prod(tensor_shape[(i + 1) :])  # n_column of unfolding
validated_rank.append(min(n_row, n_column, rank[i + 1]))
validated_rank.append(1)

return validated_rank

class TTTensor(FactorizedTensor):
def __init__(self, factors, inplace=False):
super().__init__()

# Will raise an error if invalid
shape, rank = _validate_tt_tensor(factors)

self.shape = tuple(shape)
self.rank = tuple(rank)
self.factors = factors

def __getitem__(self, index):
return self.factors[index]

def __setitem__(self, index, value):
self.factors[index] = value

def __iter__(self):
for index in range(len(self)):
yield self[index]

def __len__(self):
return len(self.factors)

def __repr__(self):
message = f"factors list : rank-{self.rank} matrix-product-state tensor of shape {self.shape} "
return message

def to_tensor(self):
return tt_to_tensor(self)

def to_unfolding(self, mode):
return tt_to_unfolded(self, mode)

def to_vec(self):
return tt_to_vec(self)

[docs]
"""Pads the factors of a Tensor-Train so as to increase its rank without changing its reconstruction

The tensor-train (ring) will be padded with 0s to increase its rank only but not the underlying tensor it represents.

Parameters
----------
factor_list : tensor list
n_padding : int, default is 1
how much to increase the rank (bond dimension) by
pad_boundaries : bool, default is False
if True, also pad the boundaries (useful for a tensor-ring)
should be False for a tensor-train to keep the boundary rank to be 1

Returns
-------
"""
new_factors = []
n_factors = len(factor_list)

for i, factor in enumerate(factor_list):
if (i == 0) and not pad_boundaries: