Provide custom marshalling for Point

Point has a non-standard marshalling, and is also
where the vast majority of CPU time is spent so
it is worth optimising.

web/api/v1/api.go

@@ -26,8 +26,9 @@ import (
 	"sort"
 	"strconv"
 	"time"
+	"unsafe"
 
-	"github.com/json-iterator/go"
+	jsoniter "github.com/json-iterator/go"
 	"github.com/prometheus/common/model"
 	"github.com/prometheus/common/route"
 	"github.com/prometheus/tsdb"
@@ -820,3 +821,41 @@ func parseDuration(s string) (time.Duration, error) {
 	}
 	return 0, fmt.Errorf("cannot parse %q to a valid duration", s)
 }
+
+func init() {
+	jsoniter.RegisterTypeEncoderFunc("promql.Point", marshalPointJSON, marshalPointJSONIsEmpty)
+}
+
+func marshalPointJSON(ptr unsafe.Pointer, stream *jsoniter.Stream) {
+	p := *((*promql.Point)(ptr))
+	stream.WriteArrayStart()
+	// Write out the timestamp as a float divided by 1000.
+	// This is ~3x faster than converting to a float.
+	t := p.T
+	if t < 0 {
+		stream.WriteRaw(`-`)
+		t = -t
+	}
+	stream.WriteInt64(t / 1000)
+	fraction := t % 1000
+	if fraction != 0 {
+		stream.WriteRaw(`.`)
+		if fraction < 100 {
+			stream.WriteRaw(`0`)
+		}
+		if fraction < 10 {
+			stream.WriteRaw(`0`)
+		}
+		stream.WriteInt64(fraction)
+	}
+	stream.WriteMore()
+	stream.WriteRaw(`"`)
+	stream.WriteFloat64(p.V)
+	stream.WriteRaw(`"`)
+	stream.WriteArrayEnd()
+
+}
+
+func marshalPointJSONIsEmpty(ptr unsafe.Pointer) bool {
+	return false
+}

web/api/v1/api_test.go

@@ -882,11 +882,11 @@ func TestRespond(t *testing.T) {
 		},
 		{
 			response: promql.Point{V: 20, T: 10},
-			expected: `{"status":"success","data":[0.01,"20"]}`,
+			expected: `{"status":"success","data":[0.010,"20"]}`,
 		},
 		{
 			response: promql.Point{V: 20, T: 100},
-			expected: `{"status":"success","data":[0.1,"20"]}`,
+			expected: `{"status":"success","data":[0.100,"20"]}`,
 		},
 		{
 			response: promql.Point{V: 20, T: 1001},
@@ -894,15 +894,15 @@ func TestRespond(t *testing.T) {
 		},
 		{
 			response: promql.Point{V: 20, T: 1010},
-			expected: `{"status":"success","data":[1.01,"20"]}`,
+			expected: `{"status":"success","data":[1.010,"20"]}`,
 		},
 		{
 			response: promql.Point{V: 20, T: 1100},
-			expected: `{"status":"success","data":[1.1,"20"]}`,
+			expected: `{"status":"success","data":[1.100,"20"]}`,
 		},
 		{
 			response: promql.Point{V: 20, T: 12345678123456555},
-			expected: `{"status":"success","data":[12345678123456.557,"20"]}`,
+			expected: `{"status":"success","data":[12345678123456.555,"20"]}`,
 		},
 		{
 			response: promql.Point{V: 20, T: -1},