Add Jitter to backoff.

Amazon recently wrote about performance gains of using jitter in
concurrent systems. http://www.awsarchitectureblog.com/2015/03/backoff.html

Seemed simple enough to add and wont change any code that may already be
using Backoff.

README.md

@@ -25,6 +25,7 @@ b := &backoff.Backoff{
 	Min:    100 * time.Millisecond,
 	Max:    10 * time.Second,
 	Factor: 2,
+	Jitter: false,
 }
 
 fmt.Printf("%s\n", b.Duration())
@@ -72,6 +73,47 @@ for {
 
 ```
 
+**Exmaple using `Jitter`**
+
+Setting `Jitter` adds some randomization to the backoff durations.
+[See amazon's writeup of performance gains using jitter](http://www.awsarchitectureblog.com/2015/03/backoff.html).
+Seeding is not necessary but doing so gives repeatable results.
+
+```go
+import "math/rand"
+
+b := &backoff.Backoff{
+	//These are the defaults
+	Min:    100 * time.Millisecond,
+	Max:    10 * time.Second,
+	Factor: 2,
+	Jitter: true,
+}
+
+rand.Seed(42)
+
+fmt.Printf("%s\n", b.Duration())
+fmt.Printf("%s\n", b.Duration())
+fmt.Printf("%s\n", b.Duration())
+
+fmt.Printf("Reset!\n")
+b.Reset()
+
+fmt.Printf("%s\n", b.Duration())
+fmt.Printf("%s\n", b.Duration())
+fmt.Printf("%s\n", b.Duration())
+```
+
+```
+100ms
+106.600049ms
+281.228155ms
+Reset!
+100ms
+104.381845ms
+214.957989ms
+```
+
 #### Credits
 
 Ported from some JavaScript written by [@tj](https://github.com/tj)
@@ -97,4 +139,4 @@ MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
-SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

backoff.go

@@ -2,6 +2,7 @@ package backoff
 
 import (
 	"math"
+	"math/rand"
 	"time"
 )
 
@@ -12,6 +13,8 @@ import (
 type Backoff struct {
 	//Factor is the multiplying factor for each increment step
 	attempts, Factor float64
+	//Jitter eases contention by randomizing backoff steps
+	Jitter bool
 	//Min and Max are the minimum and maximum values of the counter
 	Min, Max time.Duration
 }
@@ -32,6 +35,9 @@ func (b *Backoff) Duration() time.Duration {
 	}
 	//calculate this duration
 	dur := float64(b.Min) * math.Pow(b.Factor, b.attempts)
+	if b.Jitter == true {
+		dur = rand.Float64()*(dur-float64(b.Min)) + float64(b.Min)
+	}
 	//cap!
 	if dur > float64(b.Max) {
 		return b.Max

backoff_test.go

@@ -50,6 +50,30 @@ func Test3(t *testing.T) {
 	equals(t, b.Duration(), 100*time.Nanosecond)
 }
 
+func TestJitter(t *testing.T) {
+	b := &Backoff{
+		Min:    100 * time.Millisecond,
+		Max:    10 * time.Second,
+		Factor: 2,
+		Jitter: true,
+	}
+
+	equals(t, b.Duration(), 100*time.Millisecond)
+	between(t, b.Duration(), 100*time.Millisecond, 200*time.Millisecond)
+	between(t, b.Duration(), 100*time.Millisecond, 400*time.Millisecond)
+	b.Reset()
+	equals(t, b.Duration(), 100*time.Millisecond)
+}
+
+func between(t *testing.T, actual, low, high time.Duration) {
+	if actual < low {
+		t.Fatalf("Got %s, Expecting >= %s", actual, low)
+	}
+	if actual > high {
+		t.Fatalf("Got %s, Expecting <= %s", actual, high)
+	}
+}
+
 func equals(t *testing.T, d1, d2 time.Duration) {
 	if d1 != d2 {
 		t.Fatalf("Got %s, Expecting %s", d1, d2)