1 files changed, 158 insertions, 0 deletions
diff --git a/network/protocol/sideband64k/decoder.go b/network/protocol/sideband64k/decoder.go
new file mode 100644
index 00000000..5b47ea87
--- /dev/null
+++ b/network/protocol/sideband64k/decoder.go
@@ -0,0 +1,158 @@
+package sideband64k
+
+import (
+	"fmt"
+	"io"
+
+	"codeberg.org/lindenii/furgit/network/protocol/pktline"
+)
+
+// ReadOptions controls sideband decoding behavior.
+type ReadOptions struct {
+	// ChompLF removes one trailing '\n' from FrameData payloads only.
+	ChompLF bool
+}
+
+// Decoder reads side-band-64k frames from an io.Reader.
+//
+// It preserves frame boundaries and supports one-frame lookahead via
+// PeekFrame.
+type Decoder struct {
+	dec     *pktline.Decoder
+	maxData int
+	opts    ReadOptions
+
+	peeked  bool
+	peek    Frame
+	peekErr error
+}
+
+// NewDecoder creates a decoder over r.
+func NewDecoder(r io.Reader, opts ReadOptions) *Decoder {
+	d := &Decoder{
+		dec:     pktline.NewDecoder(r, pktline.ReadOptions{}),
+		maxData: DataMax,
+		opts:    opts,
+	}
+	d.dec.SetMaxData(pktline.LargePacketDataMax)
+
+	return d
+}
+
+// SetMaxData sets maximum payload size accepted for one sideband data packet.
+//
+// Non-positive n resets to DataMax.
+func (d *Decoder) SetMaxData(n int) {
+	if n <= 0 {
+		d.maxData = DataMax
+
+		return
+	}
+
+	d.maxData = n
+}
+
+// ReadFrame reads one frame.
+func (d *Decoder) ReadFrame() (Frame, error) {
+	if d.peeked {
+		d.peeked = false
+
+		return cloneFrame(d.peek), d.peekErr
+	}
+
+	return d.readFrame()
+}
+
+// PeekFrame returns the next frame without consuming it.
+func (d *Decoder) PeekFrame() (Frame, error) {
+	if !d.peeked {
+		d.peek, d.peekErr = d.readFrame()
+		d.peeked = true
+	}
+
+	return cloneFrame(d.peek), d.peekErr
+}
+
+func (d *Decoder) readFrame() (Frame, error) {
+	f, err := d.dec.ReadFrame()
+	if err != nil {
+		return Frame{}, err
+	}
+
+	switch f.Type {
+	case pktline.PacketFlush:
+		return Frame{Type: FrameFlush}, nil
+	case pktline.PacketDelim:
+		return Frame{Type: FrameDelim}, nil
+	case pktline.PacketResponseEnd:
+		return Frame{Type: FrameResponseEnd}, nil
+	case pktline.PacketData:
+		if len(f.Payload) == 0 {
+			return Frame{}, &ProtocolError{Reason: "missing sideband designator"}
+		}
+
+		payload := f.Payload[1:]
+		if len(payload) > d.effectiveMaxData() {
+			return Frame{}, fmt.Errorf("%w: %d > %d", ErrTooLarge, len(payload), d.effectiveMaxData())
+		}
+
+		band := Band(f.Payload[0])
+		if !validBand(band) {
+			return Frame{}, &ProtocolError{Reason: fmt.Sprintf("%v: %d", ErrInvalidBand, band)}
+		}
+
+		payload = append([]byte(nil), payload...)
+		if d.opts.ChompLF && band == BandData && len(payload) > 0 && payload[len(payload)-1] == '\n' {
+			payload = payload[:len(payload)-1]
+		}
+
+		return Frame{
+			Type:    frameTypeForBand(band),
+			Payload: payload,
+		}, nil
+	default:
+		return Frame{}, &ProtocolError{Reason: "unknown pkt-line frame type"}
+	}
+}
+
+func (d *Decoder) effectiveMaxData() int {
+	return effectiveMaxData(d.maxData)
+}
+
+func cloneFrame(f Frame) Frame {
+	if f.Type == FrameFlush || f.Type == FrameDelim || f.Type == FrameResponseEnd {
+		return Frame{Type: f.Type}
+	}
+
+	out := Frame{Type: f.Type}
+	if f.Payload != nil {
+		out.Payload = append([]byte(nil), f.Payload...)
+	}
+
+	return out
+}
+
+func validBand(band Band) bool {
+	return band == BandData || band == BandProgress || band == BandError
+}
+
+func frameTypeForBand(band Band) FrameType {
+	switch band {
+	case BandData:
+		return FrameData
+	case BandProgress:
+		return FrameProgress
+	case BandError:
+		return FrameError
+	default:
+		panic("invalid sideband64k band")
+	}
+}
+
+func effectiveMaxData(n int) int {
+	if n <= 0 || n > DataMax {
+		return DataMax
+	}
+
+	return n
+}