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+// Copyright 2017, OpenCensus 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.
+
+package trace
+
+import (
+ "time"
+)
+
+// samplePeriod is the minimum time between accepting spans in a single bucket.
+const samplePeriod = time.Second
+
+// defaultLatencies contains the default latency bucket bounds.
+// TODO: consider defaults, make configurable
+var defaultLatencies = [...]time.Duration{
+ 10 * time.Microsecond,
+ 100 * time.Microsecond,
+ time.Millisecond,
+ 10 * time.Millisecond,
+ 100 * time.Millisecond,
+ time.Second,
+ 10 * time.Second,
+ time.Minute,
+}
+
+// bucket is a container for a set of spans for a particular error code or latency range.
+type bucket struct {
+ nextTime time.Time // next time we can accept a span
+ buffer []*SpanData // circular buffer of spans
+ nextIndex int // location next SpanData should be placed in buffer
+ overflow bool // whether the circular buffer has wrapped around
+}
+
+func makeBucket(bufferSize int) bucket {
+ return bucket{
+ buffer: make([]*SpanData, bufferSize),
+ }
+}
+
+// add adds a span to the bucket, if nextTime has been reached.
+func (b *bucket) add(s *SpanData) {
+ if s.EndTime.Before(b.nextTime) {
+ return
+ }
+ if len(b.buffer) == 0 {
+ return
+ }
+ b.nextTime = s.EndTime.Add(samplePeriod)
+ b.buffer[b.nextIndex] = s
+ b.nextIndex++
+ if b.nextIndex == len(b.buffer) {
+ b.nextIndex = 0
+ b.overflow = true
+ }
+}
+
+// size returns the number of spans in the bucket.
+func (b *bucket) size() int {
+ if b.overflow {
+ return len(b.buffer)
+ }
+ return b.nextIndex
+}
+
+// span returns the ith span in the bucket.
+func (b *bucket) span(i int) *SpanData {
+ if !b.overflow {
+ return b.buffer[i]
+ }
+ if i < len(b.buffer)-b.nextIndex {
+ return b.buffer[b.nextIndex+i]
+ }
+ return b.buffer[b.nextIndex+i-len(b.buffer)]
+}
+
+// resize changes the size of the bucket to n, keeping up to n existing spans.
+func (b *bucket) resize(n int) {
+ cur := b.size()
+ newBuffer := make([]*SpanData, n)
+ if cur < n {
+ for i := 0; i < cur; i++ {
+ newBuffer[i] = b.span(i)
+ }
+ b.buffer = newBuffer
+ b.nextIndex = cur
+ b.overflow = false
+ return
+ }
+ for i := 0; i < n; i++ {
+ newBuffer[i] = b.span(i + cur - n)
+ }
+ b.buffer = newBuffer
+ b.nextIndex = 0
+ b.overflow = true
+}
+
+// latencyBucket returns the appropriate bucket number for a given latency.
+func latencyBucket(latency time.Duration) int {
+ i := 0
+ for i < len(defaultLatencies) && latency >= defaultLatencies[i] {
+ i++
+ }
+ return i
+}
+
+// latencyBucketBounds returns the lower and upper bounds for a latency bucket
+// number.
+//
+// The lower bound is inclusive, the upper bound is exclusive (except for the
+// last bucket.)
+func latencyBucketBounds(index int) (lower time.Duration, upper time.Duration) {
+ if index == 0 {
+ return 0, defaultLatencies[index]
+ }
+ if index == len(defaultLatencies) {
+ return defaultLatencies[index-1], 1<<63 - 1
+ }
+ return defaultLatencies[index-1], defaultLatencies[index]
+}