stronger kind tests and validation

internal/kind.go

@@ -7,11 +7,12 @@ const (
 	KindBuy
 	KindSell
 	KindSplit
+	sentinelKind
 )
 
-// String returns a human readable value
-func (d Kind) String() string {
-	switch d {
+// String returns a unique string value for Kind k
+func (k Kind) String() string {
+	switch k {
 	case KindBuy:
 		return "buy"
 	case KindSell:
@@ -23,8 +24,13 @@ func (d Kind) String() string {
 	}
 }
 
-// Is returns true when k equals o
+// Valid returns true if k is an accepted value for the Kind type
+func (k Kind) Valid() bool {
+	return k > 0 && k < sentinelKind
+}
+
+// Is returns true when k and o are valid and equal.
 func (k Kind) Is(o any) bool {
 	other, ok := o.(Kind)
-	return ok && k == other
+	return ok && k.Valid() && k == other
 }