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// Copyright 2017, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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//     http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package trace
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import (
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	"encoding/binary"
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)
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const defaultSamplingProbability = 1e-4
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// Sampler decides whether a trace should be sampled and exported.
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type Sampler func(SamplingParameters) SamplingDecision
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// SamplingParameters contains the values passed to a Sampler.
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type SamplingParameters struct {
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	ParentContext   SpanContext
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	TraceID         TraceID
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	SpanID          SpanID
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	Name            string
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	HasRemoteParent bool
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}
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// SamplingDecision is the value returned by a Sampler.
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type SamplingDecision struct {
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	Sample bool
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}
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// ProbabilitySampler returns a Sampler that samples a given fraction of traces.
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//
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// It also samples spans whose parents are sampled.
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func ProbabilitySampler(fraction float64) Sampler {
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	if !(fraction >= 0) {
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		fraction = 0
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	} else if fraction >= 1 {
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		return AlwaysSample()
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	}
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	traceIDUpperBound := uint64(fraction * (1 << 63))
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	return Sampler(func(p SamplingParameters) SamplingDecision {
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		if p.ParentContext.IsSampled() {
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			return SamplingDecision{Sample: true}
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		}
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		x := binary.BigEndian.Uint64(p.TraceID[0:8]) >> 1
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		return SamplingDecision{Sample: x < traceIDUpperBound}
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	})
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}
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// AlwaysSample returns a Sampler that samples every trace.
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// Be careful about using this sampler in a production application with
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// significant traffic: a new trace will be started and exported for every
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// request.
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func AlwaysSample() Sampler {
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	return func(p SamplingParameters) SamplingDecision {
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		return SamplingDecision{Sample: true}
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	}
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}
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// NeverSample returns a Sampler that samples no traces.
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func NeverSample() Sampler {
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	return func(p SamplingParameters) SamplingDecision {
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		return SamplingDecision{Sample: false}
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	}
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}
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