feat: add IsValid top level func

Author: adhocore

README.md

@@ -47,6 +47,14 @@ gron.IsDue(expr) // true|false, nil
 gron.IsDue(expr, time.Date(2021, time.April, 1, 1, 1, 0, 0, time.UTC)) // true|false, nil
 ```
 
+> Validity can be checked without instantiation:
+
+```go
+import "github.com/adhocore/gronx"
+
+gronx.IsValid("* * * * *") // true
+```
+
 ### Batch Due Check
 
 If you have multiple cron expressions to check due on same reference time use `BatchDue()`:

gronx.go

@@ -75,7 +75,9 @@ func New() *Gronx {
 // IsDue checks if cron expression is due for given reference time (or now).
 // It returns bool or error if any.
 func (g *Gronx) IsDue(expr string, ref ...time.Time) (bool, error) {
-	ref = append(ref, time.Now())
+	if len(ref) == 0 {
+		ref = append(ref, time.Now())
+	}
 	g.C.SetRef(ref[0])
 
 	segs, err := Segments(expr)
@@ -157,12 +159,16 @@ func (g *Gronx) SegmentsDue(segs []string) (bool, error) {
 	return true, nil
 }
 
+// IsValid checks if cron expression is valid.
+// It returns bool.
+func (g *Gronx) IsValid(expr string) bool { return IsValid(expr) }
+
 // checker for validity
 var checker = &SegmentChecker{ref: time.Now()}
 
 // IsValid checks if cron expression is valid.
 // It returns bool.
-func (g *Gronx) IsValid(expr string) bool {
+func IsValid(expr string) bool {
 	segs, err := Segments(expr)
 	if err != nil {
 		return false

gronx_test.go

@@ -60,6 +60,9 @@ func TestIsValid(t *testing.T) {
 		if !gron.IsValid("* 00 * * *") {
 			t.Errorf("expected true, got false")
 		}
+		if expr := "* * * * *"; IsValid(expr) != gron.IsValid(expr) {
+			t.Error("IsValid func and method must return same")
+		}
 	})
 
 	t.Run("is not valid", func(t *testing.T) {