# Source code for gpytorch.utils.stochastic_lq

#!/usr/bin/env python3

import torch

from .lanczos import lanczos_tridiag

[docs]class StochasticLQ(object):
"""
Implements an approximate log determinant calculation for symmetric positive definite matrices
compute the trace of the inverse using the same probe vector the log determinant was computed
with. For more details, see Dong et al. 2017 (in submission).
"""

def __init__(self, max_iter=15, num_random_probes=10):
"""
The nature of stochastic Lanczos quadrature is that the calculation of tr(f(A)) is both inaccurate and
stochastic. An instance of StochasticLQ has two parameters that control these tradeoffs. Increasing either
parameter increases the running time of the algorithm.

Args:
- cls - Tensor constructor - to ensure correct type (default - default tensor)
- max_iter (scalar) - The number of Lanczos iterations to perform. Increasing this makes the estimate of
tr(f(A)) more accurate in expectation -- that is, the average value returned has lower error.
- num_random_probes (scalar) - The number of random probes to use in the stochastic trace estimation.
Increasing this makes the estimate of tr(f(A)) lower variance -- that is, the value
returned is more consistent.
"""
self.max_iter = max_iter
self.num_random_probes = num_random_probes

def lanczos_batch(self, matmul_closure, rhs_vectors):
return lanczos_tridiag(
matmul_closure,
self.max_iter,
init_vecs=rhs_vectors,
dtype=rhs_vectors.dtype,
device=rhs_vectors.device,
batch_shape=rhs_vectors.shape[-2:],
matrix_shape=torch.Size((rhs_vectors.size(-2), rhs_vectors.size(-2))),
)

[docs]    def evaluate(self, matrix_shape, eigenvalues, eigenvectors, funcs):
r"""
Computes tr(f(A)) for an arbitrary list of functions, where f(A) is equivalent to applying the function
elementwise to the eigenvalues of A, i.e., if A = V\LambdaV^{T}, then f(A) = Vf(\Lambda)V^{T}, where
f(\Lambda) is applied elementwise.
Note that calling this function with a list of functions to apply is significantly more efficient than
calling it multiple times with one function -- each additional function after the first requires negligible

Args:
- matrix_shape (torch.Size()) - size of underlying matrix (not including batch dimensions)
- eigenvalues (Tensor n_probes x ...batch_shape x k) - batches of eigenvalues from Lanczos tridiag mats
- eigenvectors (Tensor n_probes x ...batch_shape x k x k) - batches of eigenvectors from " " "
- funcs (list of closures) - A list of functions [f_1,...,f_k]. tr(f_i(A)) is computed for each function.
Each function in the closure should expect to take a torch vector of eigenvalues as input and apply
the function elementwise. For example, to compute logdet(A) = tr(log(A)), [lambda x: x.log()] would
be a reasonable value of funcs.

Returns:
- results (list of scalars) - The trace of each supplied function applied to the matrix, e.g.,
[tr(f_1(A)),tr(f_2(A)),...,tr(f_k(A))].
"""
batch_shape = torch.Size(eigenvalues.shape[1:-1])
results = [torch.zeros(batch_shape, dtype=eigenvalues.dtype, device=eigenvalues.device) for _ in funcs]
num_random_probes = eigenvalues.size(0)
for j in range(num_random_probes):
# These are (num_batch x k) and (num_batch x k x k)
eigenvalues_for_probe = eigenvalues[j]
eigenvectors_for_probe = eigenvectors[j]
for i, func in enumerate(funcs):
# First component of eigenvecs is (num_batch x k)
eigenvecs_first_component = eigenvectors_for_probe[..., 0, :]
func_eigenvalues = func(eigenvalues_for_probe)

dot_products = (eigenvecs_first_component.pow(2) * func_eigenvalues).sum(-1)
results[i] = results[i] + matrix_shape[-1] / float(num_random_probes) * dot_products

return results