Merge pull request #918 from prometheus/time-split

model: reduce allocations in Time.UnmarshalJSON

Author: roidelapluie

model/time.go

@@ -123,44 +123,38 @@ func (t Time) MarshalJSON() ([]byte, error) {
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
 func (t *Time) UnmarshalJSON(b []byte) error {
-	p := strings.Split(string(b), ".")
-	switch len(p) {
-	case 1:
-		v, err := strconv.ParseInt(p[0], 10, 64)
+	base, frac, found := strings.Cut(string(b), ".")
+	if !found {
+		v, err := strconv.ParseInt(base, 10, 64)
 		if err != nil {
 			return err
 		}
 		*t = Time(v * second)
-
-	case 2:
-		v, err := strconv.ParseInt(p[0], 10, 64)
+	} else {
+		v, err := strconv.ParseInt(base, 10, 64)
 		if err != nil {
 			return err
 		}
-		v *= second
 
-		prec := dotPrecision - len(p[1])
+		prec := dotPrecision - len(frac)
 		if prec < 0 {
-			p[1] = p[1][:dotPrecision]
-		} else if prec > 0 {
-			p[1] += strings.Repeat("0", prec)
+			frac = frac[:dotPrecision]
 		}
-
-		va, err := strconv.ParseInt(p[1], 10, 32)
+		va, err := strconv.ParseInt(frac, 10, 32)
 		if err != nil {
 			return err
 		}
-
-		// If the value was something like -0.1 the negative is lost in the
-		// parsing because of the leading zero, this ensures that we capture it.
-		if len(p[0]) > 0 && p[0][0] == '-' && v+va > 0 {
-			*t = Time(v+va) * -1
-		} else {
-			*t = Time(v + va)
+		switch prec {
+		case 1:
+			va *= 10
+		case 2:
+			va *= 100
 		}
 
-	default:
-		return fmt.Errorf("invalid time %q", string(b))
+		if len(base) > 0 && base[0] == '-' {
+			va = -va
+		}
+		*t = Time(v*second + va)
 	}
 	return nil
 }

model/time_test.go

@@ -432,6 +432,11 @@ func TestTimeJSON(t *testing.T) {
 	}{
 		{Time(1), `0.001`},
 		{Time(-1), `-0.001`},
+		{Time(1001), `1.001`},
+		{Time(-1001), `-1.001`},
+		{Time(123000), `123`},
+		{Time(123100), `123.1`},
+		{Time(123010), `123.01`},
 	}
 
 	for i, test := range tests {
@@ -460,3 +465,17 @@ func BenchmarkParseDuration(b *testing.B) {
 		require.NoError(b, err)
 	}
 }
+
+func BenchmarkUnmarshalTime(b *testing.B) {
+	cases := []string{"1780924784", "1780924784.01", "1780924784.001"}
+
+	for _, c := range cases {
+		b.Run(c, func(b *testing.B) {
+			var t Time
+			data := []byte(c)
+			for b.Loop() {
+				_ = t.UnmarshalJSON(data)
+			}
+		})
+	}
+}