missing map key match operator dispositions

evaluate.go

@@ -213,6 +213,27 @@ func getMatchExprValue(expression *grammar.MatchExpression, rvalue reflect.Kind)
 	}
 }
 
+// evaluateNotPresent is called after a pointerstructure.ErrNotFound is
+// encountered during evaluation.
+//
+// Returns true if the Selector Path's parent is a map as the missing key may
+// be handled by the MatchOperator's NotPresentDisposition method.
+//
+// Returns false if the Selector Path has a length of 1, or if the parent of
+// the Selector's Path is not a map, a pointerstructure.ErrrNotFound error is
+// returned.
+func evaluateNotPresent(ptr pointerstructure.Pointer, datum interface{}) bool {
+	if len(ptr.Parts) < 2 {
+		return false
+	}
+
+	// Pop the missing leaf part of the path
+	ptr.Parts = ptr.Parts[0 : len(ptr.Parts)-1]
+
+	val, _ := ptr.Get(datum)
+	return reflect.ValueOf(val).Kind() == reflect.Map
+}
+
 func evaluateMatchExpression(expression *grammar.MatchExpression, datum interface{}, opt ...Option) (bool, error) {
 	opts := getOpts(opt...)
 	ptr := pointerstructure.Pointer{
@@ -224,9 +245,16 @@ func evaluateMatchExpression(expression *grammar.MatchExpression, datum interfac
 	}
 	val, err := ptr.Get(datum)
 	if err != nil {
-		if errors.Is(err, pointerstructure.ErrNotFound) && opts.withUnknown != nil {
-			err = nil
-			val = *opts.withUnknown
+		if errors.Is(err, pointerstructure.ErrNotFound) {
+			// Prefer the withUnknown option if set, otherwise defer to NotPresent
+			// disposition
+			switch {
+			case opts.withUnknown != nil:
+				err = nil
+				val = *opts.withUnknown
+			case evaluateNotPresent(ptr, datum):
+				return expression.Operator.NotPresentDisposition(), nil
+			}
 		}
 
 		if err != nil {

evaluate_test.go

@@ -302,13 +302,30 @@ var evaluateTests map[string]expressionTest = map[string]expressionTest{
 			{expression: "Nested.MapOfStructs is empty or (Nested.SliceOfInts contains 7 and 9 in Nested.SliceOfInts)", result: true, benchQuick: true},
 			{expression: "Nested.SliceOfStructs.0.X == 1", result: true},
 			{expression: "Nested.SliceOfStructs.0.Y == 4", result: false},
-			{expression: "Nested.Map.notfound == 4", result: false, err: `error finding value in datum: /Nested/Map/notfound at part 2: couldn't find key "notfound"`},
 			{expression: "Map in Nested", result: false, err: "Cannot perform in/contains operations on type struct for selector: \"Nested\""},
 			{expression: `"foobar" in "/Nested/SliceOfInfs"`, result: true},
 			{expression: `"1" in "/Nested/SliceOfInfs"`, result: true},
 			{expression: `"2" in "/Nested/SliceOfInfs"`, result: false},
 			{expression: `"true" in "/Nested/SliceOfInfs"`, result: true},
 			{expression: `"/Nested/Map/email" matches "(foz|foo)@example.com"`, result: true},
+			// Missing key in map tests
+			{expression: "Nested.Map.notfound == 4", result: false},
+			{expression: "Nested.Map.notfound != 4", result: true},
+			{expression: "4 in Nested.Map.notfound", result: false},
+			{expression: "4 not in Nested.Map.notfound", result: true},
+			{expression: "Nested.Map.notfound is empty", result: true},
+			{expression: "Nested.Map.notfound is not empty", result: false},
+			{expression: `Nested.Map.notfound matches ".*"`, result: false},
+			{expression: `Nested.Map.notfound not matches ".*"`, result: true},
+			// Missing field in struct tests
+			{expression: "Nested.Notfound == 4", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: "Nested.Notfound != 4", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: "4 in Nested.Notfound", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: "4 not in Nested.Notfound", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: "Nested.Notfound is empty", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: "Nested.Notfound is not empty", result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: `Nested.Notfound matches ".*"`, result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
+			{expression: `Nested.Notfound not matches ".*"`, result: false, err: `error finding value in datum: /Nested/Notfound at part 1: couldn't find key: struct field with name "Notfound"`},
 		},
 	},
 }

grammar/ast.go

@@ -81,6 +81,42 @@ func (op MatchOperator) String() string {
 	}
 }
 
+// NotPresentDisposition is called during evaluation when Selector fails to
+// find a map key to determine the operator's behavior.
+func (op MatchOperator) NotPresentDisposition() bool {
+	// For a selector M["x"] against a map M that lacks an "x" key...
+	switch op {
+	case MatchEqual:
+		// ...M["x"] == <anything> is false. Nothing is equal to a missing key
+		return false
+	case MatchNotEqual:
+		// ...M["x"] != <anything> is true. Nothing is equal to a missing key
+		return true
+	case MatchIn:
+		// "a" in M["x"] is false. Missing keys contain no values
+		return false
+	case MatchNotIn:
+		// "a" not in M["x"] is true. Missing keys contain no values
+		return true
+	case MatchIsEmpty:
+		// M["x"] is empty is true. Missing keys contain no values
+		return true
+	case MatchIsNotEmpty:
+		// M["x"] is not empty is false. Missing keys contain no values
+		return false
+	case MatchMatches:
+		// M["x"] matches <anything> is false. Nothing matches a missing key
+		return false
+	case MatchNotMatches:
+		// M["x"] not matches <anything> is true. Nothing matches a missing key
+		return true
+	default:
+		// Should never be reached as every operator should explicitly define its
+		// behavior.
+		return false
+	}
+}
+
 type MatchValue struct {
 	Raw       string
 	Converted interface{}