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(C++ DP Laplace noise) Avoid sampling with probability 0 for speed-up #424

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(C++ DP Laplace noise) Avoid sampling with probability 0 for speed-up #424

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14 changes: 4 additions & 10 deletions cc/algorithms/distributions.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -174,17 +174,11 @@ int64_t GeometricDistribution::Sample() { return Sample(1.0); }

const std::vector<double>& GeometricDistribution::GetProbs(double lambda) {
if (probs_.empty() || lambda != cached_lambda_) {
probs_.clear();
probs_.reserve(63);
probs_.resize(63);
for (int i = 0; i < 63; ++i) {
double c = lambda * (1LL << i);
if (c > 100) {
// If c is large, exp(c) is huge and 1 / (exp(c) + 1) is close to 0, no
// need to compute them.
break;
} else {
probs_.push_back(1.0 / (std::exp(c) + 1));
}
// If c is large, exp(c) is huge and 1 / (exp(c) + 1) is close to 0.
probs_[i] = c < 100 ? 1.0 / (std::exp(c) + 1) : 0.0;
}
cached_lambda_ = lambda;
}
Expand Down Expand Up @@ -235,7 +229,7 @@ int64_t GeometricDistribution::Sample(double scale) {
const std::vector<double>& probs = GetProbs(lambda);

int64_t result = 0;
for (int i = probs.size() - 1; i >= 0; --i) {
for (int i = 62; i >= 0; --i) {
if (GetUniformDouble() < probs[i]) {
result |= (1LL << i);
}
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