Add a StatsBuffer circular buffer for ContainerStats.

storage/memory/stats_buffer.go

@@ -0,0 +1,56 @@
+package memory
+
+import (
+	"github.com/google/cadvisor/info"
+)
+
+// A circular buffer for ContainerStats.
+type StatsBuffer struct {
+	buffer []info.ContainerStats
+	size   int
+	index  int
+}
+
+// Returns a new thread-compatible StatsBuffer.
+func NewStatsBuffer(size int) *StatsBuffer {
+	return &StatsBuffer{
+		buffer: make([]info.ContainerStats, size),
+		size:   0,
+		index:  size - 1,
+	}
+}
+
+// Adds an element to the start of the buffer (removing one from the end if necessary).
+func (self *StatsBuffer) Add(item *info.ContainerStats) {
+	if self.size < len(self.buffer) {
+		self.size++
+	}
+	self.index = (self.index + 1) % len(self.buffer)
+	self.buffer[self.index] = *item
+}
+
+// Returns the first N elements in the buffer. If N > size of buffer, size of buffer elements are returned.
+func (self *StatsBuffer) FirstN(n int) []info.ContainerStats {
+	// Cap n at the number of elements we have.
+	if n > self.size {
+		n = self.size
+	}
+
+	// index points to the latest element, get n before that one (keeping in mind we may have gone through 0).
+	start := self.index - (n - 1)
+	if start < 0 {
+		start += len(self.buffer)
+	}
+
+	// Copy the elements.
+	res := make([]info.ContainerStats, n)
+	for i := 0; i < n; i++ {
+		index := (start + i) % len(self.buffer)
+		res[i] = self.buffer[index]
+	}
+	return res
+}
+
+func (self *StatsBuffer) Size() int {
+	return self.size
+}

storage/memory/stats_buffer_test.go

@@ -0,0 +1,63 @@
+package memory
+
+import (
+	"testing"
+
+	"github.com/google/cadvisor/info"
+)
+
+func createStats(id int32) *info.ContainerStats {
+	return &info.ContainerStats{
+		Cpu: info.CpuStats{
+			Load: id,
+		},
+	}
+}
+
+func expectSize(t *testing.T, sb *StatsBuffer, expectedSize int) {
+	if sb.Size() != expectedSize {
+		t.Errorf("Expected size %v, got %v", expectedSize, sb.Size())
+	}
+}
+
+func expectElements(t *testing.T, sb *StatsBuffer, expected []int32) {
+	res := sb.FirstN(sb.Size())
+	if len(res) != len(expected) {
+		t.Errorf("Expected elements %v, got %v", expected, res)
+		return
+	}
+	for i, el := range res {
+		if el.Cpu.Load != expected[i] {
+			t.Errorf("Expected elements %v, got %v", expected, res)
+		}
+	}
+}
+
+func TestAddAndFirstN(t *testing.T) {
+	sb := NewStatsBuffer(5)
+
+	// Add 1.
+	sb.Add(createStats(1))
+	expectSize(t, sb, 1)
+	expectElements(t, sb, []int32{1})
+
+	// Fill the buffer.
+	for i := 1; i <= 5; i++ {
+		expectSize(t, sb, i)
+		sb.Add(createStats(int32(i)))
+	}
+	expectSize(t, sb, 5)
+	expectElements(t, sb, []int32{1, 2, 3, 4, 5})
+
+	// Add more than is available in the buffer
+	sb.Add(createStats(6))
+	expectSize(t, sb, 5)
+	expectElements(t, sb, []int32{2, 3, 4, 5, 6})
+
+	// Replace all elements.
+	for i := 7; i <= 10; i++ {
+		sb.Add(createStats(int32(i)))
+	}
+	expectSize(t, sb, 5)
+	expectElements(t, sb, []int32{6, 7, 8, 9, 10})
+}