fix: append scalars when Set targets array index beyond length

mcdc_supplement_test.go

@@ -338,16 +338,13 @@ func TestDeleteSupplementalEdgeCases(t *testing.T) {
 // MCDC SYS-REQ-009: N/A
 func TestSetSupplementalArrayInsertionCoverage(t *testing.T) {
 	t.Run("append into existing top level array path", func(t *testing.T) {
-		// When setting an index beyond the current array length for a
-		// primitive (non-object) array, the code overwrites rather than
-		// appends because createInsertComponent with object=true wraps
-		// the value. This is the actual parser behavior.
+		// Setting an index beyond the current array length appends the value.
 		got, err := Set([]byte(`{"top":[1]}`), []byte(`2`), "top", "[1]")
 		if err != nil {
 			t.Fatalf("Set array append returned error: %v", err)
 		}
-		if string(got) != `{"top":[2]}` {
-			t.Fatalf("Set array append result = %s, want %s", string(got), `{"top":[2]}`)
+		if string(got) != `{"top":[1,2]}` {
+			t.Fatalf("Set array append result = %s, want %s", string(got), `{"top":[1,2]}`)
 		}
 	})
 

parser.go

@@ -883,9 +883,12 @@ func Set(data []byte, setValue []byte, keys ...string) (value []byte, err error)
 		depthOffset := endOffset
 		if depth != 0 {
 			// if subpath is a non-empty object, add to it
-			// or if subpath is a non-empty array, add to it
-			if (data[startOffset] == '{' && data[startOffset+1+nextToken(data[startOffset+1:])] != '}') ||
-				(data[startOffset] == '[' && data[startOffset+1+nextToken(data[startOffset+1:])] == '{') && keys[depth:][0][0] == 91 {
+			// or if subpath is a non-empty array and next key is an index, append to it
+			nextInside := data[startOffset+1+nextToken(data[startOffset+1:])]
+			nextKeyIsIndex := len(keys[depth:]) > 0 && len(keys[depth:][0]) > 0 && keys[depth:][0][0] == '['
+			canExtendObject := data[startOffset] == '{' && nextInside != '}'
+			canExtendArray := data[startOffset] == '[' && nextInside != ']' && nextKeyIsIndex
+			if canExtendObject || canExtendArray {
 				depthOffset--
 				startOffset = depthOffset
 				// otherwise, over-write it with a new object

set_spec_test.go

@@ -52,3 +52,26 @@ func TestFuzzSetHarnessCoverage(t *testing.T) {
 		t.Fatalf("expected FuzzSet failure path to return 0, got %d", got)
 	}
 }
+
+
+// Regression for issue #267: Set with successive array indexes must append scalars.
+func TestSetAppendsScalarToExistingArrayByIndex(t *testing.T) {
+	value := []byte(`{}`)
+	var err error
+	value, err = Set(value, []byte(`1`), "test", "[0]")
+	if err != nil {
+		t.Fatalf("Set [0]: %v", err)
+	}
+	value, err = Set(value, []byte(`2`), "test", "[1]")
+	if err != nil {
+		t.Fatalf("Set [1]: %v", err)
+	}
+	value, err = Set(value, []byte(`3`), "test", "[2]")
+	if err != nil {
+		t.Fatalf("Set [2]: %v", err)
+	}
+	expected := `{"test":[1,2,3]}`
+	if string(value) != expected {
+		t.Fatalf("Set result mismatch: expected %s, got %s", expected, string(value))
+	}
+}