QUBOInstance

QUBOInstance(coefficients=None, device='cpu', dtype=torch.float32)

Represents a single instance of a Quadratic Unconstrained Binary Optimization (QUBO) problem.

ATTRIBUTE	DESCRIPTION
coefficients	Tensor of shape (size, size), representing the QUBO coefficients. TYPE: Tensor
device	Device where tensors are allocated (e.g., "cpu" or "cuda"). TYPE: str
dtype	Data type of the tensors (e.g., torch.float32). TYPE: dtype
solution	Solution to the QUBO problem, if available. TYPE: QUBOSolution | None
density	Fraction of non-zero entries in the coefficient matrix. TYPE: float | None
density_type	Classification of the density (e.g., sparse, dense). TYPE: DensityType | None

Initializes a QUBOInstance.

PARAMETER	DESCRIPTION
coefficients	Coefficients of the QUBO problem. Can be a dictionary, array-like object, or None. TYPE: dict | ArrayLike | None DEFAULT: None
device	Device where tensors are allocated (default: "cpu"). TYPE: str DEFAULT: 'cpu'
dtype	Data type of the tensors (default: torch.float32). TYPE: dtype DEFAULT: float32

Source code in qubosolver/qubo_instance.py

def __init__(
    self,
    coefficients: dict | ArrayLike | None = None,
    device: str = "cpu",
    dtype: torch.dtype = torch.float32,
):
    """
    Initializes a QUBOInstance.

    Args:
        coefficients (dict | ArrayLike | None):
            Coefficients of the QUBO problem. Can be a dictionary, array-like object, or None.
        device (str):
            Device where tensors are allocated (default: "cpu").
        dtype (torch.dtype):
            Data type of the tensors (default: torch.float32).
    """
    self._coefficients: torch.Tensor = torch.zeros([0, 0], dtype=dtype, device=device)
    self.solution: QUBOSolution | None = None
    self.density: float | None = None
    self.density_type: DensityType | None = None

    if coefficients is None:
        self.coefficients = self._coefficients
    else:
        self.coefficients = coefficients

coefficients property writable

Getter for the QUBO coefficient matrix.

RETURNS	DESCRIPTION
Tensor	torch.Tensor: Tensor of shape (size, size) representing the QUBO coefficients.

device property

Get the device on which the coefficient tensor is stored.

RETURNS	DESCRIPTION
device	torch.device: The device (e.g., device("cpu") or device("cuda:0")).

dtype property

Get the data type of the coefficient tensor.

RETURNS	DESCRIPTION
dtype	torch.dtype: The dtype (e.g., torch.float32).

normalized_coefficients property

Return the coefficient matrix normalised by the maximum off-diagonal value.

All coefficients are divided by _max_off_diag so that the largest off-diagonal entry becomes 1.0. Useful for embedding algorithms that require unit-scaled interactions.

RETURNS	DESCRIPTION
Tensor	torch.Tensor: Normalised coefficient matrix of shape (size, size).

size property

Get the size of the QUBO matrix (number of variables).

RETURNS	DESCRIPTION
int	Size of the QUBO matrix. TYPE: int

__repr__()

Returns a string representation of the QUBOInstance.

RETURNS	DESCRIPTION
str	A dictionary-like string summarizing the instance. TYPE: str

Source code in qubosolver/qubo_instance.py

def __repr__(self) -> str:
    """
    Returns a string representation of the QUBOInstance.

    Returns:
        str: A dictionary-like string summarizing the instance.
    """
    return repr(
        f"QUBOInstance of size = {self.size},"
        f"density = {round(self.density, 2) if self.density else None},"
    )

evaluate_solution(solution)

Evaluates a solution for the QUBO problem.

PARAMETER	DESCRIPTION
solution	Solution vector to evaluate. TYPE: list | tuple | ArrayLike

RETURNS	DESCRIPTION
float	The cost of the given solution. TYPE: float

RAISES	DESCRIPTION
ValueError	If the solution size does not match the QUBO size.

Source code in qubosolver/qubo_instance.py

def evaluate_solution(self, solution: list | tuple | ArrayLike) -> float:
    """
    Evaluates a solution for the QUBO problem.

    Args:
        solution (list | tuple | ArrayLike):
            Solution vector to evaluate.

    Returns:
        float:
            The cost of the given solution.

    Raises:
        ValueError: If the solution size does not match the QUBO size.
    """
    solution_tensor = convert_to_tensor(solution, device=self.device, dtype=self.dtype)  # type: ignore[arg-type]
    if solution_tensor.size(0) != self.size:
        raise ValueError("Solution size does not match the QUBO problem size.")
    cost = torch.matmul(
        solution_tensor, torch.matmul(self._coefficients, solution_tensor)
    ).item()
    solution = solution_tensor
    return float(cost)

load(file_like) staticmethod

Deserialises a QUBOInstance from a binary file-like object.

Reconstructs the instance from a file previously created with QUBOInstance.save.

PARAMETER	DESCRIPTION
file_like	A file-like object opened in binary read mode containing the serialised QUBOInstance data. TYPE: FileLike[bytes]

RETURNS	DESCRIPTION
QUBOInstance	A new QUBOInstance reconstructed from the file. TYPE: QUBOInstance

Example

Source code in qubosolver/qubo_instance.py

@staticmethod
def load(file_like: io_utils.FileLike[bytes]) -> QUBOInstance:
    """
    Deserialises a QUBOInstance from a binary file-like object.

    Reconstructs the instance from a file previously created with
    `QUBOInstance.save`.

    Args:
        file_like (io_utils.FileLike[bytes]): A file-like object opened in
            binary read mode containing the serialised QUBOInstance data.

    Returns:
        QUBOInstance: A new QUBOInstance reconstructed from the file.

    Example:
        >>> with open("instance.bin", "rb") as f:
        ...     instance = QUBOInstance.load(f)
    """
    with io_utils.open(file_like, "rb") as f:
        # torch.load might consume too much of the src buffer.
        # Use a dedicated limited buffer.
        buffer = io.BytesIO(io_utils.load_sized_buffer(f))
        Q = torch.load(buffer, weights_only=True)

    return QUBOInstance(Q)

save(file_like, instance) staticmethod

Serialises a QUBOInstance to a binary file-like object.

Uses torch.save internally to persist the coefficient tensor.

PARAMETER	DESCRIPTION
file_like	A file-like object opened in binary write mode (e.g., an open file or a BytesIO buffer). TYPE: FileLike[bytes]
instance	The QUBOInstance to serialise. TYPE: QUBOInstance

RETURNS	DESCRIPTION
None	None

Example

Source code in qubosolver/qubo_instance.py

@staticmethod
def save(file_like: io_utils.FileLike[bytes], instance: QUBOInstance) -> None:
    """
    Serialises a QUBOInstance to a binary file-like object.

    Uses `torch.save` internally to persist the coefficient tensor.

    Args:
        file_like (io_utils.FileLike[bytes]): A file-like object opened in
            binary write mode (e.g., an open file or a BytesIO buffer).
        instance (QUBOInstance): The QUBOInstance to serialise.

    Returns:
        None

    Example:
        >>> with open("instance.bin", "wb") as f:
        ...     QUBOInstance.save(f, my_instance)
    """
    with io_utils.open(file_like, "wb") as f:
        buffer = io.BytesIO()
        torch.save(instance.coefficients, buffer)
        io_utils.save_sized_buffer(f, buffer.getbuffer())

set_coefficients(new_coefficients=None)

Updates the coefficients of the QUBO problem.

PARAMETER	DESCRIPTION
new_coefficients	Dictionary of new coefficients to set. Keys are (row, column) tuples. TYPE: dict[tuple[int, int], float] | None DEFAULT: None

Source code in qubosolver/qubo_instance.py

def set_coefficients(
    self, new_coefficients: dict[tuple[int, int], float] | None = None
) -> None:
    """
    Updates the coefficients of the QUBO problem.

    Args:
        new_coefficients (dict[tuple[int, int], float] | None):
            Dictionary of new coefficients to set. Keys are (row, column) tuples.
    """
    if not new_coefficients:
        return

    max_index = max(max(i, j) for i, j in new_coefficients.keys())
    if max_index >= self.size:
        self._expand_size(max_index + 1)

    indices = torch.tensor(list(new_coefficients.keys()), dtype=torch.long, device=self.device)
    values = torch.tensor(list(new_coefficients.values()), dtype=self.dtype, device=self.device)
    self._coefficients[indices[:, 0], indices[:, 1]] = values
    off_diagonal_mask = indices[:, 0] != indices[:, 1]
    symmetric_indices = indices[off_diagonal_mask]
    self._coefficients[symmetric_indices[:, 1], symmetric_indices[:, 0]] = values[
        off_diagonal_mask
    ]

    self.update_metrics()

update_metrics()

Updates the density metrics of the QUBO problem.

Source code in qubosolver/qubo_instance.py

def update_metrics(self) -> None:
    """
    Updates the density metrics of the QUBO problem.
    """
    if self.size > 0:
        self.density = calculate_density(self._coefficients, self.size)
        self.density_type = classify_density(self.density)
    else:
        self.density = self.density_type = None