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// Copyright 2023 gRPC authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "src/python/grpcio_observability/grpc_observability/sampler.h"
#include <cmath>
#include <cstdint>
#include "absl/strings/escaping.h"
namespace grpc_observability {
namespace {
// Converts a probability in [0, 1] to a threshold in [0, UINT64_MAX].
uint64_t CalculateThreshold(double probability) {
if (probability <= 0.0) return 0;
if (probability >= 1.0) return UINT64_MAX;
// We can't directly return probability * UINT64_MAX.
//
// UINT64_MAX is (2^64)-1, but as a double rounds up to 2^64.
// For probabilities >= 1-(2^-54), the product wraps to zero!
// Instead, calculate the high and low 32 bits separately.
const double product = UINT32_MAX * probability;
double hi_bits, lo_bits = ldexp(modf(product, &hi_bits), 32) + product;
return (static_cast<uint64_t>(hi_bits) << 32) +
static_cast<uint64_t>(lo_bits);
}
uint64_t CalculateThresholdFromBuffer(const std::string& trace_id) {
const std::string trace_id_bytes = absl::HexStringToBytes(trace_id);
const uint8_t* buf = reinterpret_cast<const uint8_t*>(trace_id_bytes.c_str());
uint64_t res = 0;
for (int i = 0; i < 8; ++i) {
res |= (static_cast<uint64_t>(buf[i]) << (i * 8));
}
return res;
}
} // namespace
ProbabilitySampler& ProbabilitySampler::Get() {
static ProbabilitySampler* sampler = new ProbabilitySampler;
return *sampler;
}
void ProbabilitySampler::SetThreshold(double probability) {
uint64_t threshold = CalculateThreshold(probability);
threshold_ = threshold;
}
bool ProbabilitySampler::ShouldSample(const std::string& trace_id) {
if (threshold_ == 0 || trace_id.length() < 32) return false;
// All Spans within the same Trace will get the same sampling decision, so
// full trees of Spans will be sampled.
return CalculateThresholdFromBuffer(trace_id) <= threshold_;
}
} // namespace grpc_observability
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