#!/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.
from __future__ import annotations
import math
from collections.abc import Iterable, Iterator
from functools import lru_cache
from math import pi
from numbers import Number
from typing import Any, Callable, Optional, Union
import torch
from botorch.utils.safe_math import logdiffexp
from numpy.polynomial.legendre import leggauss as numpy_leggauss
from torch import BoolTensor, LongTensor, Tensor
CaseNd = tuple[Callable[[], BoolTensor], Callable[[BoolTensor], Tensor]]
_log_2 = math.log(2)
_sqrt_pi = math.sqrt(pi)
_inv_sqrt_pi = 1 / _sqrt_pi
_inv_sqrt_2pi = 1 / math.sqrt(2 * pi)
_inv_sqrt_2 = 1 / math.sqrt(2)
_neg_inv_sqrt_2 = -_inv_sqrt_2
_log_sqrt_2pi = math.log(2 * pi) / 2
STANDARDIZED_RANGE: tuple[float, float] = (-1e6, 1e6)
_log_two_inv_sqrt_2pi = _log_2 - _log_sqrt_2pi # = log(2 / sqrt(2 * pi))
[docs]
def case_dispatcher(
out: Tensor,
cases: Iterable[CaseNd] = (),
default: Callable[[BoolTensor], Tensor] = None,
) -> Tensor:
r"""Basic implementation of a tensorized switching case statement.
Args:
out: Tensor to which case outcomes are written.
cases: Iterable of function pairs (pred, func), where `mask=pred()` specifies
whether `func` is applicable for each entry in `out`. Note that cases are
resolved first-come, first-serve.
default: Optional `func` to which all unclaimed entries of `out` are dispatched.
"""
active = None
for closure, func in cases:
pred = closure()
if not pred.any():
continue
mask = pred if (active is None) else pred & active
if not mask.any():
continue
if mask.all(): # where possible, use Ellipsis to avoid indexing
out[...] = func(...)
return out
out[mask] = func(mask)
if active is None:
active = ~mask
else:
active[mask] = False
if not active.any():
break
if default is not None:
if active is None:
out[...] = default(...)
elif active.any():
out[active] = default(active)
return out
[docs]
@lru_cache(maxsize=None)
def get_constants(
values: Union[Number, Iterator[Number]],
device: Optional[torch.device] = None,
dtype: Optional[torch.dtype] = None,
) -> Union[Tensor, tuple[Tensor, ...]]:
r"""Returns scalar-valued Tensors containing each of the given constants.
Used to expedite tensor operations involving scalar arithmetic. Note that
the returned Tensors should not be modified in-place."""
if isinstance(values, Number):
return torch.full((), values, dtype=dtype, device=device)
return tuple(torch.full((), val, dtype=dtype, device=device) for val in values)
[docs]
def get_constants_like(
values: Union[Number, Iterator[Number]],
ref: Tensor,
) -> Union[Tensor, Iterator[Tensor]]:
return get_constants(values, device=ref.device, dtype=ref.dtype)
[docs]
def gen_positional_indices(
shape: torch.Size,
dim: int,
device: Optional[torch.device] = None,
) -> Iterator[torch.LongTensor]:
ndim = len(shape)
_dim = ndim + dim if dim < 0 else dim
if _dim >= ndim or _dim < 0:
raise ValueError(f"dim={dim} invalid for shape {shape}.")
cumsize = shape[_dim + 1 :].numel()
for i, s in enumerate(reversed(shape[: _dim + 1])):
yield torch.arange(0, s * cumsize, cumsize, device=device)[(...,) + i * (None,)]
cumsize *= s
[docs]
def build_positional_indices(
shape: torch.Size,
dim: int,
device: Optional[torch.device] = None,
) -> LongTensor:
return sum(gen_positional_indices(shape=shape, dim=dim, device=device))
[docs]
@lru_cache(maxsize=None)
def leggauss(deg: int, **tkwargs: Any) -> tuple[Tensor, Tensor]:
x, w = numpy_leggauss(deg)
return torch.as_tensor(x, **tkwargs), torch.as_tensor(w, **tkwargs)
[docs]
def ndtr(x: Tensor) -> Tensor:
r"""Standard normal CDF."""
half, neg_inv_sqrt_2 = get_constants_like((0.5, _neg_inv_sqrt_2), x)
return half * torch.erfc(neg_inv_sqrt_2 * x)
[docs]
def phi(x: Tensor) -> Tensor:
r"""Standard normal PDF."""
inv_sqrt_2pi, neg_half = get_constants_like((_inv_sqrt_2pi, -0.5), x)
return inv_sqrt_2pi * (neg_half * x.square()).exp()
[docs]
def log_phi(x: Tensor) -> Tensor:
r"""Logarithm of standard normal pdf"""
log_sqrt_2pi, neg_half = get_constants_like((_log_sqrt_2pi, -0.5), x)
return neg_half * x.square() - log_sqrt_2pi
[docs]
def log_ndtr(x: Tensor) -> Tensor:
"""Implementation of log_ndtr that remedies problems of torch.special's version
for large negative x, where the torch implementation yields Inf or NaN gradients.
Args:
x: An input tensor with dtype torch.float32 or torch.float64.
Returns:
A tensor of values of the same type and shape as x containing log(ndtr(x)).
"""
if not (x.dtype == torch.float32 or x.dtype == torch.float64):
raise TypeError(
f"log_Phi only supports torch.float32 and torch.float64 "
f"dtypes, but received {x.dtype = }."
)
neg_inv_sqrt_2, log_2 = get_constants_like((_neg_inv_sqrt_2, _log_2), x)
return log_erfc(neg_inv_sqrt_2 * x) - log_2
[docs]
def log_erfc(x: Tensor) -> Tensor:
"""Computes the logarithm of the complementary error function in a numerically
stable manner. The GitHub issue https://github.com/pytorch/pytorch/issues/31945
tracks progress toward moving this feature into PyTorch in C++.
Args:
x: An input tensor with dtype torch.float32 or torch.float64.
Returns:
A tensor of values of the same type and shape as x containing log(erfc(x)).
"""
if not (x.dtype == torch.float32 or x.dtype == torch.float64):
raise TypeError(
f"log_erfc only supports torch.float32 and torch.float64 "
f"dtypes, but received {x.dtype = }."
)
is_pos = x > 0
x_pos = x.masked_fill(~is_pos, 0)
x_neg = x.masked_fill(is_pos, 0)
return torch.where(
is_pos,
torch.log(torch.special.erfcx(x_pos)) - x_pos.square(),
torch.log(torch.special.erfc(x_neg)),
)
[docs]
def log_erfcx(x: Tensor) -> Tensor:
"""Computes the logarithm of the complementary scaled error function in a
numerically stable manner. The GitHub issue tracks progress toward moving this
feature into PyTorch in C++: https://github.com/pytorch/pytorch/issues/31945.
Args:
x: An input tensor with dtype torch.float32 or torch.float64.
Returns:
A tensor of values of the same type and shape as x containing log(erfcx(x)).
"""
is_pos = x > 0
x_pos = x.masked_fill(~is_pos, 0)
x_neg = x.masked_fill(is_pos, 0)
return torch.where(
is_pos,
torch.special.erfcx(x_pos).log(),
torch.special.erfc(x_neg).log() + x.square(),
)
[docs]
def standard_normal_log_hazard(x: Tensor) -> Tensor:
"""Computes the logarithm of the hazard function of the standard normal
distribution, i.e. `log(phi(x) / Phi(-x))`.
Args:
x: A tensor of any shape, with either float32 or float64 dtypes.
Returns:
A Tensor of the same shape `x`, containing the values of the logarithm of the
hazard function evaluated at `x`.
"""
# NOTE: using _inv_sqrt_2 instead of _neg_inv_sqrt_2 means we are computing Phi(-x).
a, b = get_constants_like((_log_two_inv_sqrt_2pi, _inv_sqrt_2), x)
return a - log_erfcx(b * x)
[docs]
def log_prob_normal_in(a: Tensor, b: Tensor) -> Tensor:
r"""Computes the probability that a standard normal random variable takes a value
in \[a, b\], i.e. log(Phi(b) - Phi(a)), where Phi is the standard normal CDF.
Returns accurate values and permits numerically stable backward passes for inputs
in [-1e100, 1e100] for double precision and [-1e20, 1e20] for single precision.
In contrast, a naive approach is not numerically accurate beyond [-10, 10].
Args:
a: Tensor of lower integration bounds of the Gaussian probability measure.
b: Tensor of upper integration bounds of the Gaussian probability measure.
Returns:
Tensor of the log probabilities.
"""
if not (a < b).all():
raise ValueError("Received input tensors a, b for which not all a < b.")
# if abs(b) > abs(a), we use Phi(b) - Phi(a) = Phi(-a) - Phi(-b), since the
# right tail converges to 0 from below, leading to digit cancellation issues, while
# the left tail of log_ndtr is well behaved and results in large negative numbers
rev_cond = b.abs() > a.abs() # condition for reversal of inputs
if rev_cond.any():
c = torch.where(rev_cond, -b, a)
b = torch.where(rev_cond, -a, b)
a = c # after we updated b, can assign c to a
return logdiffexp(log_a=log_ndtr(a), log_b=log_ndtr(b))
[docs]
def swap_along_dim_(
values: Tensor,
i: Union[int, LongTensor],
j: Union[int, LongTensor],
dim: int,
buffer: Optional[Tensor] = None,
) -> Tensor:
r"""Swaps Tensor slices in-place along dimension `dim`.
When passed as Tensors, `i` (and `j`) should be `dim`-dimensional tensors
with the same shape as `values.shape[:dim]`. The xception to this rule occurs
when `dim=0`, in which case `i` (and `j`) should be (at most) one-dimensional
when passed as a Tensor.
Args:
values: Tensor whose values are to be swapped.
i: Indices for slices along dimension `dim`.
j: Indices for slices along dimension `dim`.
dim: The dimension of `values` along which to swap slices.
buffer: Optional buffer used internally to store copied values.
Returns:
The original `values` tensor.
"""
dim = values.ndim + dim if dim < 0 else dim
if dim and (
(isinstance(i, Tensor) and i.ndim) or (isinstance(j, Tensor) and j.ndim)
):
# Handle n-dimensional batches of heterogeneous swaps via linear indexing
if isinstance(i, Tensor) and i.shape != values.shape[:dim]:
raise ValueError("Batch shapes of `i` and `values` do not match.")
if isinstance(j, Tensor) and j.shape != values.shape[:dim]:
raise ValueError("Batch shapes of `j` and `values` do not match.")
pidx = build_positional_indices(
shape=values.shape[: dim + 1], dim=-2, device=values.device
)
swap_along_dim_(
values.view(-1, *values.shape[dim + 1 :]),
i=(pidx + i).view(-1),
j=(pidx + j).view(-1),
dim=0,
buffer=buffer,
)
else:
# Base cases: homogeneous swaps and 1-dimenensional heterogeneous swaps
if isinstance(i, Tensor) and i.ndim > 1:
raise ValueError("Tensor `i` must be at most 1-dimensional when `dim=0`.")
if isinstance(j, Tensor) and j.ndim > 1:
raise ValueError("Tensor `j` must be at most 1-dimensional when `dim=0`.")
if dim:
ctx = tuple(slice(None) for _ in range(dim))
i = ctx + (i,)
j = ctx + (j,)
if buffer is None:
buffer = values[i].clone()
else:
buffer.copy_(values[i])
values[i] = values[j]
values[j] = buffer
return values
