add json persistence to allow store and restore

cardinality.go

@@ -1,6 +1,7 @@
 package probably
 
 import (
+	"encoding/json"
 	"math"
 )
 
@@ -104,3 +105,36 @@ func (h *HyperLogLog) Merge(from *HyperLogLog) {
 		}
 	}
 }
+
+func (h *HyperLogLog) MarshalJSON() ([]byte, error) {
+	m := map[string]interface{}{
+		"m":       h.m,
+		"k":       h.k,
+		"k_comp":  h.k_comp,
+		"alpha_m": h.alpha_m,
+		"bits":    h.bits,
+	}
+	return json.Marshal(m)
+}
+
+func (h *HyperLogLog) UnmarshalJSON(by []byte) error {
+
+	data := struct {
+		M      uint    `json:"m"`
+		K      float64 `json:"k"`
+		KComp  int     `json:"k_comp"`
+		AlphaM float64 `json:"alpha_m"`
+		Bits   []uint8 `json:"bits"`
+	}{}
+
+	err := json.Unmarshal(by, &data)
+	if err != nil {
+		return err
+	}
+	h.m = data.M
+	h.k = data.K
+	h.k_comp = data.KComp
+	h.alpha_m = data.AlphaM
+	h.bits = data.Bits
+	return nil
+}

cardinality_test.go

@@ -1,6 +1,7 @@
 package probably
 
 import (
+	"encoding/json"
 	"hash/crc32"
 	"testing"
 )
@@ -436,6 +437,37 @@ func TestCardinality(t *testing.T) {
 	}
 }
 
+func TestCardinalityJson(t *testing.T) {
+	hll := NewHyperLogLog(0.001)
+	for _, w := range words {
+		h := crc32.ChecksumIEEE([]byte(w))
+		hll.Add(h)
+	}
+	t.Logf("Word list is %v words, estimate is %v", len(words), hll.Count())
+	if hll.Count() != 2334 {
+		t.Fatalf("Expected estimate of 2,334, got %v", hll.Count())
+	}
+	by, err := json.Marshal(hll)
+	if err != nil {
+		t.Fatalf("Must not error on json serialization: %v", err)
+	}
+	var llJson HyperLogLog
+	err = json.Unmarshal(by, &llJson)
+	if err != nil {
+		t.Fatalf("Must not error on json deserialization: %v", err)
+	}
+	if llJson.Count() != 2334 {
+		t.Fatalf("2 Expected estimate of 2,334, got %v", llJson.Count())
+	}
+	for _, w := range words {
+		h := crc32.ChecksumIEEE([]byte(w))
+		llJson.Add(h)
+	}
+	if llJson.Count() != 2334 {
+		t.Fatalf("3 Expected estimate of 2,334, got %v", llJson.Count())
+	}
+}
+
 func TestCardinalityMerging(t *testing.T) {
 	counters := make([]*HyperLogLog, 10)
 	for i := 0; i < 10; i++ {