def test_log_binomial_stirling(tol):
k = torch.arange(200.)
n_minus_k = k.unsqueeze(-1)
n = k + n_minus_k
# Test binomial coefficient choose(n, k).
expected = (n + 1).lgamma() - (k + 1).lgamma() - (n_minus_k + 1).lgamma()
actual = log_binomial(n, k, tol=tol)
assert (actual - expected).abs().max() < tol
def log_prob(self, value):
if self._validate_args:
self._validate_sample(value)
n = self.total_count
k = value
a = self.concentration1
b = self.concentration0
tol = self.approx_log_prob_tol
return (log_binomial(n, k, tol) + log_beta(k + a, n - k + b, tol) -
log_beta(a, b, tol))
def log_prob(self, value):
if self._validate_args:
self._validate_sample(value)
n = self.total_count
k = value
# k * log(p) + (n - k) * log(1 - p) = k * (log(p) - log(1 - p)) + n * log(1 - p)
# (case logit < 0) = k * logit - n * log1p(e^logit)
# (case logit > 0) = k * logit - n * (log(p) - log(1 - p)) + n * log(p)
# = k * logit - n * logit - n * log1p(e^-logit)
# (merge two cases) = k * logit - n * max(logit, 0) - n * log1p(e^-|logit|)
normalize_term = n * (_clamp_by_zero(self.logits) + self.logits.abs().neg().exp().log1p())
return (k * self.logits - normalize_term
+ log_binomial(n, k, tol=self.approx_log_prob_tol))
