Adding "if" function to tick

CHANGELOG.md

@@ -65,6 +65,7 @@ stream
     Also allows for grouping by measurement.
     The breaking change is that the group ID format has changed to allow for the measurement name.
 - [#759](https://github.com/influxdata/kapacitor/pull/759): Add mechanism for token based subscription auth.
+- [#745](https://github.com/influxdata/kapacitor/pull/745): Add if function for tick script, for example: `if("value" > 6, 1, 2)`.
 
 ### Bugfixes
 

tick/stateful/functions.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"reflect"
 	"strconv"
 	"time"
 
@@ -87,6 +88,9 @@ func init() {
 
 	// Humanize functions
 	statelessFuncs["humanBytes"] = &humanBytes{}
+
+	// Conditionals
+	statelessFuncs["if"] = &ifFunc{}
 }
 
 // Return set of built-in Funcs
@@ -624,3 +628,33 @@ func (*humanBytes) Call(args ...interface{}) (v interface{}, err error) {
 	}
 	return
 }
+
+type ifFunc struct {
+}
+
+func (*ifFunc) Reset() {
+
+}
+
+func (*ifFunc) Call(args ...interface{}) (interface{}, error) {
+	if len(args) != 3 {
+		return nil, errors.New("if expects exactly three arguments")
+	}
+
+	var condition bool
+	var isBool bool
+
+	if condition, isBool = args[0].(bool); !isBool {
+		return nil, fmt.Errorf("first argument to if must be a condition with type of bool - got %T", args[0])
+	}
+
+	if reflect.TypeOf(args[1]) != reflect.TypeOf(args[2]) {
+		return nil, fmt.Errorf("Multiple return types isn't supported - second argument is %T and third argument is %T", args[1], args[2])
+	}
+
+	if condition {
+		return args[1], nil
+	}
+
+	return args[2], nil
+}

tick/stateful/functions_test.go

@@ -385,6 +385,7 @@ func Test_String(t *testing.T) {
 		t.Errorf("unexpected error from call to string() got %s exp %s", got, expErr)
 	}
 }
+
 func Test_Duration(t *testing.T) {
 	f := &duration{}
 
@@ -485,3 +486,63 @@ func Test_Duration(t *testing.T) {
 		}
 	}
 }
+
+func Test_If(t *testing.T) {
+	f := &ifFunc{}
+
+	testCases := []struct {
+		args []interface{}
+		exp  interface{}
+		err  error
+	}{
+		// Error cases
+		{
+			args: []interface{}{true},
+			err:  errors.New("if expects exactly three arguments"),
+		},
+		{
+			args: []interface{}{true, 6},
+			err:  errors.New("if expects exactly three arguments"),
+		},
+		{
+			args: []interface{}{12, 6, false},
+			err:  errors.New("first argument to if must be a condition with type of bool - got int"),
+		},
+
+		// Simple if
+		{
+			args: []interface{}{true, 1, 2},
+			exp:  1,
+		},
+		{
+			args: []interface{}{false, 1, 2},
+			exp:  2,
+		},
+
+		// multiple types
+		{
+			args: []interface{}{false, 1, "1"},
+			err:  errors.New("Multiple return types isn't supported - second argument is int and third argument is string"),
+		},
+	}
+
+	for _, tc := range testCases {
+		result, err := f.Call(tc.args...)
+		if tc.err != nil {
+			if err == nil {
+				t.Errorf("expected error from if(%v)\ngot: nil\nexp: exp %s", tc.args, tc.err)
+			} else if got, exp := err.Error(), tc.err.Error(); got != exp {
+				t.Errorf("unexpected error from if(%v)\ngot: %s\nexp: %s", tc.args, got, exp)
+			}
+			continue
+		} else if err != nil {
+			t.Errorf("unexpected error from if(%v) %s", tc.args, err)
+			continue
+		}
+
+		if result != tc.exp {
+			t.Errorf("unexpected result from if(%v)\ngot: %+v\nexp: %+v", tc.args, result, tc.exp)
+		}
+
+	}
+}