Add a basic Holt-Winters algorithm to the query engine

CHANGELOG.md

@@ -15,6 +15,7 @@
 - [#6664](https://github.com/influxdata/influxdb/pull/6664): Adds monitoring statistic for on-disk shard size.
 - [#2926](https://github.com/influxdata/influxdb/issues/2926): Support bound parameters in the parser.
 - [#1310](https://github.com/influxdata/influxdb/issues/1310): Add https-private-key option to httpd config.
+- [#6621](https://github.com/influxdata/influxdb/pull/6621): Add Holt-Winter forecasting function.
 
 ### Bugfixes
 

influxql/ast.go

@@ -1604,6 +1604,29 @@ func (s *SelectStatement) validateAggregates(tr targetRequirement) error {
 				if err := s.validPercentileAggr(expr); err != nil {
 					return err
 				}
+			case "holt_winters", "holt_winters_with_fit":
+				if exp, got := 3, len(expr.Args); got != exp {
+					return fmt.Errorf("invalid number of arguments for %s, expected %d, got %d", expr.Name, exp, got)
+				}
+				// Validate that if they have grouping by time, they need a sub-call like min/max, etc.
+				groupByInterval, err := s.GroupByInterval()
+				if err != nil {
+					return fmt.Errorf("invalid group interval: %v", err)
+				}
+
+				if _, ok := expr.Args[0].(*Call); ok && groupByInterval == 0 {
+					return fmt.Errorf("%s aggregate requires a GROUP BY interval", expr.Name)
+				} else if !ok {
+					return fmt.Errorf("must use aggregate function with %s", expr.Name)
+				}
+				if arg, ok := expr.Args[1].(*IntegerLiteral); !ok {
+					return fmt.Errorf("expected integer argument as second arg in %s", expr.Name)
+				} else if arg.Val <= 0 {
+					return fmt.Errorf("second arg to %s must be greater than 0, got %d", expr.Name, arg.Val)
+				}
+				if _, ok := expr.Args[2].(*IntegerLiteral); !ok {
+					return fmt.Errorf("expected integer argument as third arg in %s", expr.Name)
+				}
 			default:
 				if err := s.validSelectWithAggregate(); err != nil {
 					return err

influxql/call_iterator.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math"
 	"sort"
+	"time"
 )
 
 /*
@@ -1058,3 +1059,23 @@ func newMovingAverageIterator(input Iterator, n int, opt IteratorOptions) (Itera
 		return nil, fmt.Errorf("unsupported moving average iterator type: %T", input)
 	}
 }
+
+// newHoltWintersIterator returns an iterator for operating on a elapsed() call.
+func newHoltWintersIterator(input Iterator, opt IteratorOptions, h, m int, includeFitData bool, interval time.Duration) (Iterator, error) {
+	switch input := input.(type) {
+	case FloatIterator:
+		createFn := func() (FloatPointAggregator, FloatPointEmitter) {
+			fn := NewFloatHoltWintersReducer(h, m, includeFitData, interval)
+			return fn, fn
+		}
+		return &floatReduceFloatIterator{input: newBufFloatIterator(input), opt: opt, create: createFn}, nil
+	case IntegerIterator:
+		createFn := func() (IntegerPointAggregator, FloatPointEmitter) {
+			fn := NewFloatHoltWintersReducer(h, m, includeFitData, interval)
+			return fn, fn
+		}
+		return &integerReduceFloatIterator{input: newBufIntegerIterator(input), opt: opt, create: createFn}, nil
+	default:
+		return nil, fmt.Errorf("unsupported elapsed iterator type: %T", input)
+	}
+}