From 922d2a0cbd1406c0b437c0a062a8c04129d7dda4 Mon Sep 17 00:00:00 2001
From: xengineering <me@xengineering.eu>
Date: Thu, 30 May 2024 20:00:44 +0200
Subject: software: communication: Implement unslip()

This implements to decode the Serial Line Internet Protocol (SLIP). It
splits the continuous byte stream into frames. Based on these frames the
rest of the data link layer and upper layers can be implemented.
---
 software/communication/data_link.go      | 63 ++++++++++++++++++++++++++++++++
 software/communication/data_link_test.go | 33 +++++++++++++++++
 software/communication/interface.go      | 10 +++--
 3 files changed, 103 insertions(+), 3 deletions(-)
 create mode 100644 software/communication/data_link.go
 create mode 100644 software/communication/data_link_test.go

diff --git a/software/communication/data_link.go b/software/communication/data_link.go
new file mode 100644
index 0000000..d6e752d
--- /dev/null
+++ b/software/communication/data_link.go
@@ -0,0 +1,63 @@
+package communication
+
+const (
+	SLIP_END         = 0xC0
+	SLIP_ESC         = 0xDB
+	SLIP_ESC_END     = 0xDC
+	SLIP_ESC_ESC     = 0xDD
+	SLIP_MAX_PAYLOAD = 1500
+)
+
+type dataLink struct {
+	rx chan []byte
+}
+
+func newDataLink() dataLink {
+	dl := dataLink{}
+
+	dl.rx = make(chan []byte)
+
+	return dl
+}
+
+func (dl *dataLink) start(source chan byte) {
+	go dl.receive(source)
+}
+
+func (dl *dataLink) receive(source chan byte) {
+	for {
+		frame := unslip(source)
+		dl.rx <- frame
+	}
+}
+
+func unslip(source chan byte) []byte {
+	escaped := false
+	buffer := make([]byte, 0)
+
+	for {
+		octet := <-source
+		if escaped {
+			switch octet {
+			case SLIP_ESC_END:
+				buffer = append(buffer, SLIP_END)
+			case SLIP_ESC_ESC:
+				buffer = append(buffer, SLIP_ESC)
+			}
+		} else {
+			switch octet {
+			case SLIP_END:
+				break
+			case SLIP_ESC:
+				escaped = true
+				continue
+			default:
+				buffer = append(buffer, octet)
+				continue
+			}
+			break
+		}
+	}
+
+	return buffer
+}
diff --git a/software/communication/data_link_test.go b/software/communication/data_link_test.go
new file mode 100644
index 0000000..91b941d
--- /dev/null
+++ b/software/communication/data_link_test.go
@@ -0,0 +1,33 @@
+package communication
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestUnslip(t *testing.T) {
+	input := []byte{
+		0xFF, 0x12, SLIP_END,
+	}
+
+	bytes := make(chan byte)
+	frames := make(chan []byte)
+
+	go func() {
+		for _, v := range input {
+			bytes <- v
+		}
+	}()
+
+	go func() {
+		for {
+			frames <- unslip(bytes)
+		}
+	}()
+
+	frame := <-frames
+	expected := []byte{0xFF, 0x12}
+	if !reflect.DeepEqual(frame, expected) {
+		t.Fatalf("Frame '%v' does not match expected '%v'\n", frame, expected)
+	}
+}
diff --git a/software/communication/interface.go b/software/communication/interface.go
index d124a48..df089f2 100644
--- a/software/communication/interface.go
+++ b/software/communication/interface.go
@@ -1,11 +1,12 @@
 package communication
 
 import (
-	"fmt"
+	"log"
 )
 
 type SerialInterface struct {
 	phy physical
+	dl  dataLink
 }
 
 func NewSerialInterface() (SerialInterface, error) {
@@ -17,16 +18,19 @@ func NewSerialInterface() (SerialInterface, error) {
 		return iface, err
 	}
 
+	iface.dl = newDataLink()
+
 	return iface, nil
 }
 
 func (i *SerialInterface) Start() {
 	i.phy.start()
+	i.dl.start(i.phy.rx)
 }
 
 func (i *SerialInterface) Cat() {
 	for {
-		data := <-i.phy.rx
-		fmt.Printf("%s", string(data))
+		data := <-i.dl.rx
+		log.Printf("RX: '%v'\n", data)
 	}
 }
-- 
cgit v1.2.3-70-g09d2