Opentracing support

vendor/github.com/openzipkin/zipkin-go-opentracing/sample.go

@@ -0,0 +1,99 @@
+package zipkintracer
+
+import (
+	"math"
+	"math/rand"
+	"sync"
+	"time"
+)
+
+// Sampler functions return if a Zipkin span should be sampled, based on its
+// traceID.
+type Sampler func(id uint64) bool
+
+func neverSample(_ uint64) bool { return false }
+
+func alwaysSample(_ uint64) bool { return true }
+
+// ModuloSampler provides a typical OpenTracing type Sampler.
+func ModuloSampler(mod uint64) Sampler {
+	if mod < 2 {
+		return alwaysSample
+	}
+	return func(id uint64) bool {
+		return (id % mod) == 0
+	}
+}
+
+// NewBoundarySampler is appropriate for high-traffic instrumentation who
+// provision random trace ids, and make the sampling decision only once.
+// It defends against nodes in the cluster selecting exactly the same ids.
+func NewBoundarySampler(rate float64, salt int64) Sampler {
+	if rate <= 0 {
+		return neverSample
+	}
+	if rate >= 1.0 {
+		return alwaysSample
+	}
+	var (
+		boundary = int64(rate * 10000)
+		usalt    = uint64(salt)
+	)
+	return func(id uint64) bool {
+		return int64(math.Abs(float64(id^usalt)))%10000 < boundary
+	}
+}
+
+// NewCountingSampler is appropriate for low-traffic instrumentation or
+// those who do not provision random trace ids. It is not appropriate for
+// collectors as the sampling decision isn't idempotent (consistent based
+// on trace id).
+func NewCountingSampler(rate float64) Sampler {
+	if rate <= 0 {
+		return neverSample
+	}
+	if rate >= 1.0 {
+		return alwaysSample
+	}
+	var (
+		i         = 0
+		outOf100  = int(rate*100 + math.Copysign(0.5, rate*100)) // for rounding float to int conversion instead of truncation
+		decisions = randomBitSet(100, outOf100, rand.New(rand.NewSource(time.Now().UnixNano())))
+		mtx       = &sync.Mutex{}
+	)
+
+	return func(_ uint64) bool {
+		mtx.Lock()
+		defer mtx.Unlock()
+		result := decisions[i]
+		i++
+		if i == 100 {
+			i = 0
+		}
+		return result
+	}
+}
+
+/**
+ * Reservoir sampling algorithm borrowed from Stack Overflow.
+ *
+ * http://stackoverflow.com/questions/12817946/generate-a-random-bitset-with-n-1s
+ */
+func randomBitSet(size int, cardinality int, rnd *rand.Rand) []bool {
+	result := make([]bool, size)
+	chosen := make([]int, cardinality)
+	var i int
+	for i = 0; i < cardinality; i++ {
+		chosen[i] = i
+		result[i] = true
+	}
+	for ; i < size; i++ {
+		j := rnd.Intn(i + 1)
+		if j < cardinality {
+			result[chosen[j]] = false
+			result[i] = true
+			chosen[j] = i
+		}
+	}
+	return result
+}