perf: add MarshalAppend APIs and increase buffer capacity

vmap/encoder_fast.go

@@ -6,18 +6,30 @@ import (
 
 // MarshalVmap marshals a VMAP to XML, producing output identical to encoding/xml.Marshal.
 func MarshalVmap(v *VMAP) ([]byte, error) {
-	buf := make([]byte, 0, 8192)
+	buf := make([]byte, 0, 256*1024)
 	buf = appendVMAP(buf, v)
 	return buf, nil
 }
 
+// MarshalVmapAppend appends the XML encoding of a VMAP to buf and returns the extended buffer.
+// This allows callers to manage buffer lifecycle (e.g. via sync.Pool) for reduced allocations.
+func MarshalVmapAppend(buf []byte, v *VMAP) ([]byte, error) {
+	return appendVMAP(buf, v), nil
+}
+
 // MarshalVast marshals a VAST to XML, producing output identical to encoding/xml.Marshal.
 func MarshalVast(v *VAST) ([]byte, error) {
-	buf := make([]byte, 0, 4096)
+	buf := make([]byte, 0, 128*1024)
 	buf = appendVAST(buf, v)
 	return buf, nil
 }
 
+// MarshalVastAppend appends the XML encoding of a VAST to buf and returns the extended buffer.
+// This allows callers to manage buffer lifecycle (e.g. via sync.Pool) for reduced allocations.
+func MarshalVastAppend(buf []byte, v *VAST) ([]byte, error) {
+	return appendVAST(buf, v), nil
+}
+
 // --- escape helpers ---
 
 // escText escapes text content, matching encoding/xml.EscapeText.

vmap/structure_test.go

@@ -572,6 +572,65 @@ func TestMarshalSpecialCharsFast(t *testing.T) {
 	is.Equal(string(expected), string(got))
 }
 
+// --- MarshalAppend Tests ---
+
+func TestMarshalVmapAppend(t *testing.T) {
+	is := is.New(t)
+	doc, err := os.ReadFile("sample-vmap/testVmap.xml")
+	is.NoErr(err)
+
+	var v VMAP
+	err = xml.Unmarshal(doc, &v)
+	is.NoErr(err)
+
+	expected, err := MarshalVmap(&v)
+	is.NoErr(err)
+
+	buf := make([]byte, 0, 1024)
+	got, err := MarshalVmapAppend(buf, &v)
+	is.NoErr(err)
+
+	is.Equal(string(expected), string(got))
+}
+
+func TestMarshalVastAppend(t *testing.T) {
+	is := is.New(t)
+	doc, err := os.ReadFile("sample-vmap/testVast.xml")
+	is.NoErr(err)
+
+	var v VAST
+	err = xml.Unmarshal(doc, &v)
+	is.NoErr(err)
+
+	expected, err := MarshalVast(&v)
+	is.NoErr(err)
+
+	buf := make([]byte, 0, 1024)
+	got, err := MarshalVastAppend(buf, &v)
+	is.NoErr(err)
+
+	is.Equal(string(expected), string(got))
+}
+
+func TestMarshalVmapAppendPreservesPrefix(t *testing.T) {
+	is := is.New(t)
+	doc, err := os.ReadFile("sample-vmap/testVmap.xml")
+	is.NoErr(err)
+
+	var v VMAP
+	err = xml.Unmarshal(doc, &v)
+	is.NoErr(err)
+
+	prefix := []byte("PREFIX")
+	buf := make([]byte, len(prefix), 1024)
+	copy(buf, prefix)
+
+	got, err := MarshalVmapAppend(buf, &v)
+	is.NoErr(err)
+
+	is.Equal(string(got[:len(prefix)]), "PREFIX")
+}
+
 // --- Fast Marshal Benchmarks ---
 
 func BenchmarkXMLMarshalVmap(b *testing.B) {