make OnlineStat marshal nicely to json

ostat.go

@@ -4,6 +4,7 @@
 package ostat
 
 import (
+	"encoding/json"
 	"fmt"
 	"math"
 )
@@ -128,3 +129,22 @@ func (os *OnlineStat) String() string {
 		},
 	)
 }
+
+func (os *OnlineStat) MarshalJSON() ([]byte, error) {
+	s := struct {
+		N        uint64  `json:"n"`
+		Min      float64 `json:"min"`
+		Max      float64 `json:"max"`
+		Mean     float64 `json:"mean"`
+		Variance float64 `json:"variance"`
+		StdDev   float64 `json:"std_dev"`
+	}{
+		N:        os.n,
+		Min:      os.Min,
+		Max:      os.Max,
+		Mean:     os.Mean(),
+		Variance: os.Variance(),
+		StdDev:   os.StdDev(),
+	}
+	return json.Marshal(s)
+}

ostat_test.go

@@ -1,6 +1,8 @@
 package ostat
 
 import (
+	"bytes"
+	"encoding/json"
 	"fmt"
 	"math"
 	"testing"
@@ -279,3 +281,19 @@ func TestString(t *testing.T) {
 		t.Errorf("failed to generate correct string")
 	}
 }
+
+
+func TestJson(t *testing.T) {
+	ps := NewPopulationStat()
+	for _, v := range []float64{1, 1, 2, 2} {
+		ps.Push(v)
+	}
+	expected := "{\"n\":4,\"min\":1,\"max\":2,\"mean\":1.5,\"variance\":0.25,\"std_dev\":0.5}\n"
+	buf := &bytes.Buffer{}
+	if err := json.NewEncoder(buf).Encode(ps); err != nil {
+		t.Fatalf("encoding problem:", err)
+	}
+	if got, want := expected, buf.String(); got != want {
+		t.Fatalf("wrong json encoding; got %q, want %q", got, want)
+	}
+}