Performance refactor of running_output buffers

closes #914
closes #967

internal/buffer/buffer.go

@@ -0,0 +1,77 @@
+package buffer
+
+import (
+	"github.com/influxdata/telegraf"
+)
+
+// Buffer is an object for storing metrics in a circular buffer.
+type Buffer struct {
+	buf chan telegraf.Metric
+	// total dropped metrics
+	drops int
+	// total metrics added
+	total int
+}
+
+// NewBuffer returns a Buffer
+//   size is the maximum number of metrics that Buffer will cache. If Add is
+//   called when the buffer is full, then the oldest metric(s) will be dropped.
+func NewBuffer(size int) *Buffer {
+	return &Buffer{
+		buf: make(chan telegraf.Metric, size),
+	}
+}
+
+// IsEmpty returns true if Buffer is empty.
+func (b *Buffer) IsEmpty() bool {
+	return len(b.buf) == 0
+}
+
+// Len returns the current length of the buffer.
+func (b *Buffer) Len() int {
+	return len(b.buf)
+}
+
+// Drops returns the total number of dropped metrics that have occured in this
+// buffer since instantiation.
+func (b *Buffer) Drops() int {
+	return b.drops
+}
+
+// Total returns the total number of metrics that have been added to this buffer.
+func (b *Buffer) Total() int {
+	return b.total
+}
+
+// Add adds metrics to the buffer.
+func (b *Buffer) Add(metrics ...telegraf.Metric) {
+	for i, _ := range metrics {
+		b.total++
+		select {
+		case b.buf <- metrics[i]:
+		default:
+			b.drops++
+			<-b.buf
+			b.buf <- metrics[i]
+		}
+	}
+}
+
+// Batch returns a batch of metrics of size batchSize.
+// the batch will be of maximum length batchSize. It can be less than batchSize,
+// if the length of Buffer is less than batchSize.
+func (b *Buffer) Batch(batchSize int) []telegraf.Metric {
+	n := min(len(b.buf), batchSize)
+	out := make([]telegraf.Metric, n)
+	for i := 0; i < n; i++ {
+		out[i] = <-b.buf
+	}
+	return out
+}
+
+func min(a, b int) int {
+	if b < a {
+		return b
+	}
+	return a
+}