Add round function

docs/graphite.md

@@ -142,7 +142,7 @@ See also:
 | removeBelowValue(seriesList, n) seriesList                     |              | Stable     |
 | removeBetweenPercentile                                        |              | No         |
 | removeEmptySeries                                              |              | No         |
-| roundFunction                                                  |              | No         |
+| round                                                          |              | Stable     |
 | scale(seriesList, num) series                                  |              | Stable     |
 | scaleToSeconds(seriesList, seconds) seriesList                 |              | Stable     |
 | secondYAxis                                                    |              | No         |

expr/func_round.go

@@ -0,0 +1,96 @@
+package expr
+
+import (
+	"fmt"
+	"math"
+	"math/big"
+	"strconv"
+
+	"github.com/grafana/metrictank/api/models"
+	"github.com/grafana/metrictank/schema"
+)
+
+type FuncRound struct {
+	in        GraphiteFunc
+	precision int64
+}
+
+func NewRound() GraphiteFunc {
+	return &FuncRound{}
+}
+
+func (s *FuncRound) Signature() ([]Arg, []Arg) {
+	return []Arg{
+			ArgSeriesList{val: &s.in},
+			ArgInt{key: "precision", opt: true, validator: []Validator{IntPositive}, val: &s.precision},
+		}, []Arg{
+			ArgSeriesList{},
+		}
+}
+
+func (s *FuncRound) Context(context Context) Context {
+	return context
+}
+
+func (s *FuncRound) Exec(dataMap DataMap) ([]models.Series, error) {
+	series, err := s.in.Exec(dataMap)
+	if err != nil {
+		return nil, err
+	}
+
+	outputs := make([]models.Series, len(series))
+	precisionMult := float64(math.Pow10(int(s.precision)))
+	for i, serie := range series {
+		out := pointSlicePool.Get().([]schema.Point)
+
+		for _, v := range serie.Datapoints {
+			out = append(out, schema.Point{Val: roundToPrecision(v.Val, precisionMult, int(s.precision)), Ts: v.Ts})
+		}
+
+		s := models.Series{
+			Target:       fmt.Sprintf("round(%s,%d)", serie.Target, s.precision),
+			QueryPatt:    fmt.Sprintf("round(%s,%d)", serie.Target, s.precision),
+			Tags:         serie.CopyTagsWith("round", strconv.Itoa(int(s.precision))),
+			Datapoints:   out,
+			Interval:     serie.Interval,
+			Meta:         serie.Meta,
+			QueryMDP:     serie.QueryMDP,
+			QueryPNGroup: serie.QueryPNGroup,
+		}
+		outputs[i] = s
+	}
+	dataMap.Add(Req{}, outputs...)
+	return outputs, nil
+}
+
+func roundToPrecision(val float64, precisionMult float64, precision int) float64 {
+	if math.IsNaN(val) || math.IsInf(val, 0) {
+		return val
+	}
+
+	// Special case 1: Large negative precision, pow10 is too small to represent
+	if precisionMult == 0 {
+		return 0
+	}
+	// Special case 2: Large positive precision, pow10 is too large to represent
+	if math.IsInf(precisionMult, 0) {
+		return val
+	}
+
+	alignedVal := val * precisionMult
+	// Special case 3: underflow or overflow, need to do expensive "Big" version
+	if math.IsInf(alignedVal, 0) {
+		return roundToPrecisionSlow(val, precision)
+	}
+	return math.Round(alignedVal) / precisionMult
+}
+
+func roundToPrecisionSlow(val float64, precision int) float64 {
+	f := big.Rat{}
+	f.SetFloat64(val)
+	ret, err := strconv.ParseFloat(f.FloatString(precision), 64)
+	if err != nil {
+		return math.NaN()
+	}
+	return ret
+}