add xnet.CapturePrinter type

x/xnet/pcap-printer.go

@@ -0,0 +1,62 @@
+package xnet
+
+import (
+	"io"
+	"strconv"
+
+	"github.com/soypat/lneto/internet/pcap"
+)
+
+type CapturePrinterConfig struct {
+	NamespaceWidth int
+}
+
+// CapturePrinter prints internet packets using the [pcap.PacketBreakdown] and [pcap.Formatter] types.
+type CapturePrinter struct {
+	write      func(b []byte) (int, error)
+	frms       []pcap.Frame
+	cap        pcap.PacketBreakdown
+	pfmt       pcap.Formatter
+	fmtPcapBuf []byte
+	// minimum length of namespace on print.
+	namespaceminwidth int
+}
+
+func (stack *CapturePrinter) Configure(writer io.Writer, cfg CapturePrinterConfig) error {
+	stack.namespaceminwidth = cfg.NamespaceWidth
+	stack.write = writer.Write
+	return nil
+}
+
+// Formatter returns a pointer to the underlying pcap.Formatter type.
+// One can then configure the formatter's fields to affect printing.
+func (stack *CapturePrinter) Formatter() *pcap.Formatter {
+	return &stack.pfmt
+}
+
+func (stack *CapturePrinter) PrintPacket(prefix string, pkt []byte) {
+	fmtbuf := stack.fmtPcapBuf[:0]
+	var err error
+	stack.frms, err = stack.cap.CaptureEthernet(stack.frms[:0], pkt, 0)
+	if err == nil {
+		fmtbuf = append(fmtbuf, prefix...)
+		// Ensure minimum width of packet length display for less jitter in log viewline.
+		prevlen := len(prefix)
+		fmtbuf = strconv.AppendInt(fmtbuf, int64(len(pkt)), 10)
+		numLength := len(fmtbuf) - prevlen
+		appendSpaces := max(0, stack.namespaceminwidth-numLength) + 1 // add single space to separate actual format from packet length.
+		for range appendSpaces {
+			fmtbuf = append(fmtbuf, ' ')
+		}
+		fmtbuf, err = stack.pfmt.FormatFrames(fmtbuf, stack.frms, pkt)
+	}
+	fmtbuf = append(fmtbuf, '\n')
+	if err != nil {
+		fmtbuf = append(fmtbuf, "ERROR "...)
+		fmtbuf = append(fmtbuf, prefix...)
+		fmtbuf = append(fmtbuf, ": "...)
+		fmtbuf = append(fmtbuf, err.Error()...)
+	}
+	stack.write(fmtbuf)
+	stack.fmtPcapBuf = fmtbuf[:0] // Reuse buffer if allocated at larger size.
+}