#!/usr/bin/env python3
# Copyright (c) Meta Platforms, Inc. and affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
r"""Box decomposition algorithms.
References
.. [Lacour17]
R. Lacour, K. Klamroth, C. Fonseca. A box decomposition algorithm to
compute the hypervolume indicator. Computers & Operations Research,
Volume 79, 2017.
"""
from __future__ import annotations
from abc import ABC, abstractmethod
import torch
from botorch.exceptions.errors import BotorchError
from botorch.utils.multi_objective.box_decompositions.utils import (
_expand_ref_point,
_pad_batch_pareto_frontier,
update_local_upper_bounds_incremental,
)
from botorch.utils.multi_objective.pareto import is_non_dominated
from torch import Tensor
from torch.nn import Module
[docs]
class BoxDecomposition(Module, ABC):
r"""An abstract class for box decompositions.
Note: Internally, we store the negative reference point (minimization).
"""
def __init__(self, ref_point: Tensor, sort: bool, Y: Tensor | None = None) -> None:
"""Initialize BoxDecomposition.
Args:
ref_point: A `m`-dim tensor containing the reference point.
sort: A boolean indicating whether to sort the Pareto frontier.
Y: A `(batch_shape) x n x m`-dim tensor of outcomes.
"""
super().__init__()
self._neg_ref_point = -ref_point
self.sort = torch.tensor(sort, dtype=torch.bool)
self.num_outcomes = ref_point.shape[-1]
self.register_buffer("hypercell_bounds", None)
if Y is not None:
if Y.isnan().any():
raise ValueError(
"NaN inputs are not supported. Got Y with "
f"{Y.isnan().sum()} NaN values."
)
self._neg_Y = -Y
self._validate_inputs()
self._neg_pareto_Y = self._compute_pareto_Y()
self.partition_space()
else:
self._neg_Y = None
self._neg_pareto_Y = None
@property
def pareto_Y(self) -> Tensor:
r"""This returns the non-dominated set.
Returns:
A `n_pareto x m`-dim tensor of outcomes.
"""
if self._neg_pareto_Y is not None:
return -self._neg_pareto_Y
raise BotorchError("pareto_Y has not been initialized")
@property
def ref_point(self) -> Tensor:
r"""Get the reference point.
Returns:
A `m`-dim tensor of outcomes.
"""
return -self._neg_ref_point
@property
def Y(self) -> Tensor:
r"""Get the raw outcomes.
Returns:
A `n x m`-dim tensor of outcomes.
"""
if self._neg_Y is not None:
return -self._neg_Y
raise BotorchError("Y data has not been initialized")
def _compute_pareto_Y(self) -> Tensor:
if self._neg_Y is None:
raise BotorchError("Y data has not been initialized")
# is_non_dominated assumes maximization
if self._neg_Y.shape[-2] == 0:
return self._neg_Y
# assumes maximization
pareto_Y = -_pad_batch_pareto_frontier(
Y=self.Y,
ref_point=_expand_ref_point(
ref_point=self.ref_point, batch_shape=self.batch_shape
),
)
if not self.sort:
return pareto_Y
# sort by first objective
if len(self.batch_shape) > 0:
pareto_Y = pareto_Y.gather(
index=torch.argsort(pareto_Y[..., :1], dim=-2).expand(pareto_Y.shape),
dim=-2,
)
else:
pareto_Y = pareto_Y[torch.argsort(pareto_Y[:, 0])]
return pareto_Y
def _reset_pareto_Y(self) -> bool:
r"""Update the non-dominated front.
Returns:
A boolean indicating whether the Pareto frontier has changed.
"""
pareto_Y = self._compute_pareto_Y()
if (self._neg_pareto_Y is None) or not torch.equal(
pareto_Y, self._neg_pareto_Y
):
self._neg_pareto_Y = pareto_Y
return True
return False
[docs]
def partition_space(self) -> None:
r"""Compute box decomposition."""
if self.num_outcomes == 2:
try:
self._partition_space_2d()
except NotImplementedError:
self._partition_space()
else:
self._partition_space()
def _partition_space_2d(self) -> None:
r"""Compute box decomposition for 2 objectives."""
raise NotImplementedError
@abstractmethod
def _partition_space(self) -> None:
r"""Partition the non-dominated space into disjoint hypercells.
This method supports an arbitrary number of outcomes, but is
less efficient than `partition_space_2d` for the 2-outcome case.
"""
[docs]
@abstractmethod
def get_hypercell_bounds(self) -> Tensor:
r"""Get the bounds of each hypercell in the decomposition.
Returns:
A `2 x num_cells x num_outcomes`-dim tensor containing the
lower and upper vertices bounding each hypercell.
"""
def _update_neg_Y(self, Y: Tensor) -> bool:
r"""Update the set of outcomes.
Returns:
A boolean indicating if _neg_Y was initialized.
"""
if Y.isnan().any():
raise ValueError(
"NaN inputs are not supported. Got Y with "
f"{Y.isnan().sum()} NaN values."
)
# multiply by -1, since internally we minimize.
if self._neg_Y is not None:
self._neg_Y = torch.cat([self._neg_Y, -Y], dim=-2)
return False
self._neg_Y = -Y
return True
[docs]
def update(self, Y: Tensor) -> None:
r"""Update non-dominated front and decomposition.
By default, the partitioning is recomputed. Subclasses can override
this functionality.
Args:
Y: A `(batch_shape) x n x m`-dim tensor of new, incremental outcomes.
"""
self._update_neg_Y(Y=Y)
self.reset()
def _validate_inputs(self) -> None:
self.batch_shape = self.Y.shape[:-2]
self.num_outcomes = self.Y.shape[-1]
if len(self.batch_shape) > 1:
raise NotImplementedError(
f"{type(self).__name__} only supports a single "
f"batch dimension, but got {len(self.batch_shape)} "
"batch dimensions."
)
elif len(self.batch_shape) > 0 and self.num_outcomes > 2:
raise NotImplementedError(
f"{type(self).__name__} only supports a batched box "
f"decompositions in the 2-objective setting."
)
[docs]
def reset(self) -> None:
r"""Reset non-dominated front and decomposition."""
self._validate_inputs()
is_new_pareto = self._reset_pareto_Y()
# Update decomposition if the Pareto front changed
if is_new_pareto:
self.partition_space()
@abstractmethod
def _compute_hypervolume_if_y_has_data(self) -> Tensor:
"""Compute hypervolume for the case that there is data in self._neg_pareto_Y."""
[docs]
def compute_hypervolume(self) -> Tensor:
r"""Compute hypervolume that is dominated by the Pareto Froniter.
Returns:
A `(batch_shape)`-dim tensor containing the hypervolume dominated by
each Pareto frontier.
"""
if self._neg_pareto_Y is None:
return torch.tensor(0.0)
if self._neg_pareto_Y.shape[-2] == 0:
return torch.zeros(
self._neg_pareto_Y.shape[:-2],
dtype=self._neg_pareto_Y.dtype,
device=self._neg_pareto_Y.device,
)
return self._compute_hypervolume_if_y_has_data()
[docs]
class FastPartitioning(BoxDecomposition, ABC):
r"""A class for partitioning the (non-)dominated space into hyper-cells.
Note: this assumes maximization. Internally, it multiplies outcomes by -1
and performs the decomposition under minimization.
This class is abstract to support to two applications of Alg 1 from
[Lacour17]_: 1) partitioning the space that is dominated by the Pareto
frontier and 2) partitioning the space that is not dominated by the
Pareto frontier.
"""
def __init__(
self,
ref_point: Tensor,
Y: Tensor | None = None,
) -> None:
"""
Args:
ref_point: A `m`-dim tensor containing the reference point.
Y: A `(batch_shape) x n x m`-dim tensor
"""
super().__init__(ref_point=ref_point, Y=Y, sort=ref_point.shape[-1] == 2)
[docs]
def update(self, Y: Tensor) -> None:
r"""Update non-dominated front and decomposition.
Args:
Y: A `(batch_shape) x n x m`-dim tensor of new, incremental outcomes.
"""
if self._update_neg_Y(Y=Y):
self.reset()
else:
if self.num_outcomes == 2 or self._neg_pareto_Y.shape[-2] == 0:
# If there are two objective, recompute the box decomposition
# because the partitions can be computed analytically.
# If the current pareto set has no points, recompute the box
# decomposition.
self.reset()
else:
# only include points that are better than the reference point
better_than_ref = (Y > self.ref_point).all(dim=-1)
Y = Y[better_than_ref]
Y_all = torch.cat([self._neg_pareto_Y, -Y], dim=-2)
pareto_mask = is_non_dominated(-Y_all)
# determine the number of points in Y that are Pareto optimal
num_new_pareto = pareto_mask[-Y.shape[-2] :].sum()
self._neg_pareto_Y = Y_all[pareto_mask]
if num_new_pareto > 0:
# update local upper bounds for the minimization problem
self._U, self._Z = update_local_upper_bounds_incremental(
# this assumes minimization
new_pareto_Y=self._neg_pareto_Y[-num_new_pareto:],
U=self._U,
Z=self._Z,
)
# use the negative local upper bounds as the new pareto
# frontier for the minimization problem and perform
# box decomposition on dominated space.
self._get_partitioning()
@abstractmethod
def _get_single_cell(self) -> None:
r"""Set the partitioning to be a single cell in the case of no Pareto points.
This method should set self.hypercell_bounds
"""
pass # pragma: no cover
[docs]
def partition_space(self) -> None:
if self._neg_pareto_Y.shape[-2] == 0:
self._get_single_cell()
else:
super().partition_space()
def _partition_space(self):
r"""Partition the non-dominated space into disjoint hypercells.
This method supports an arbitrary number of outcomes, but is
less efficient than `partition_space_2d` for the 2-outcome case.
"""
if len(self.batch_shape) > 0:
# this could be triggered when m=2 outcomes and
# BoxDecomposition._partition_space_2d is not overridden.
raise NotImplementedError(
"_partition_space does not support batch dimensions."
)
# this assumes minimization
# initialize local upper bounds
self.register_buffer("_U", self._neg_ref_point.unsqueeze(-2).clone())
# initialize defining points to be the dummy points \hat{z} that are
# defined in Sec 2.1 in [Lacour17]_. Note that in [Lacour17]_, outcomes
# are assumed to be between [0,1], so they used 0 rather than -inf.
self._Z = torch.zeros(
1,
self.num_outcomes,
self.num_outcomes,
dtype=self.Y.dtype,
device=self.Y.device,
)
for j in range(self.ref_point.shape[-1]):
# use ref point for maximization as the ideal point for minimization.
self._Z[0, j] = float("-inf")
self._Z[0, j, j] = self._U[0, j]
# incrementally update local upper bounds and defining points
# for each new Pareto point
self._U, self._Z = update_local_upper_bounds_incremental(
new_pareto_Y=self._neg_pareto_Y,
U=self._U,
Z=self._Z,
)
self._get_partitioning()
@abstractmethod
def _get_partitioning(self) -> None:
r"""Compute partitioning given local upper bounds for the minimization problem.
This method should set self.hypercell_bounds
"""
pass # pragma: no cover
[docs]
def get_hypercell_bounds(self) -> Tensor:
r"""Get the bounds of each hypercell in the decomposition.
Returns:
A `2 x (batch_shape) x num_cells x m`-dim tensor containing the
lower and upper vertices bounding each hypercell.
"""
return self.hypercell_bounds
