Add SeekableBuffer

Add a specialised bytes.Buffer variant with seeking capabilities. This allows writers without seeking support to serialize using NodeSerializerStreamed.

SBHPFv1/streamed_serializer.go

@@ -1,15 +1,16 @@
 package sbhpfv1
 
 import (
-	"bytes"
 	"io"
+
+	"git.zervo.org/FLUX/GoSBHPF/pkg/seekablebuffer"
 )
 
 // SerializeNodeStream serializes a node object into a SBHPF node.
-// It is a wrapper around SerializeNode that temporarily stores the serialized nodes in a memory buffer.
-// This is less efficient, but also works with writers that lack seeking support.
+// It is a wrapper around SerializeNode that temporarily stores the serialized nodes in a seekable memory buffer.
+// This is less efficient, but provides compatibility with writers that lack seeking support.
 func SerializeNodeStream(w io.Writer, node *Node) error {
-	buf := &bytes.Buffer{}
+	buf := &seekablebuffer.Buffer{}
 	if err := SerializeNode(buf, node); err != nil {
 		return err
 	}

pkg/seekablebuffer/seekablebuffer.go

@@ -0,0 +1,90 @@
+/*
+SeekableBuffer
+
+Copyright (c) 2023 aler9
+Copyright (c) 2025 Zervó Zadachin
+
+Implementation based on code from "bluenviron/mediacommon".
+See: https://github.com/bluenviron/mediacommon/blob/main/pkg/formats/fmp4/seekablebuffer/seekablebuffer.go
+
+Original code was licensed under the MIT License.
+See: https://github.com/bluenviron/mediacommon/blob/main/LICENSE
+*/
+
+package seekablebuffer
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+// Buffer is a bytes.Buffer with an additional Seek() method.
+type Buffer struct {
+	bytes.Buffer
+	pos int64
+}
+
+// Write implements io.Writer.
+func (b *Buffer) Write(p []byte) (int, error) {
+	var n int
+
+	if b.pos < int64(b.Len()) {
+		n = copy(b.Bytes()[b.pos:], p)
+		p = p[n:]
+	}
+
+	if len(p) > 0 {
+		// Buffer.Write should never return an error here.
+		nn, _ := b.Buffer.Write(p)
+		n += nn
+	}
+
+	b.pos += int64(n)
+	return n, nil
+}
+
+// Read (almost) implements io.Reader.
+func (b *Buffer) Read(_ []byte) (int, error) {
+	return 0, fmt.Errorf("read not implemented")
+}
+
+// Seek implements io.Seeker.
+func (b *Buffer) Seek(offset int64, whence int) (int64, error) {
+	var pos2 int64
+
+	switch whence {
+	case io.SeekStart:
+		pos2 = offset
+
+	case io.SeekCurrent:
+		pos2 = b.pos + offset
+
+	case io.SeekEnd:
+		pos2 = int64(b.Len()) + offset
+
+	default:
+		return 0, fmt.Errorf("invalid seek whence")
+	}
+
+	if pos2 < 0 {
+		return 0, fmt.Errorf("negative position")
+	}
+
+	b.pos = pos2
+
+	// Expand buffer if needed
+	diff := b.pos - int64(b.Len())
+	if diff > 0 {
+		// Buffer.Write should never return an error here.
+		b.Buffer.Write(make([]byte, diff))
+	}
+
+	return pos2, nil
+}
+
+// Reset resets the buffer state.
+func (b *Buffer) Reset() {
+	b.Buffer.Reset()
+	b.pos = 0
+}