From bc29b4a8b4629a9ef0d335325612a02a6c07b4e8 Mon Sep 17 00:00:00 2001
From: xegineering <me@xegineering.eu>
Date: Wed, 2 Oct 2024 16:53:49 +0200
Subject: Add soundbox.toLinkLocal()

To communicate with soundbox devices it is required to convert the known
device MAC address (printed on the case) to an IPv6 link-local address.
---
 ipv6.go      | 19 +++++++++++++++++++
 ipv6_test.go | 19 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 ipv6.go
 create mode 100644 ipv6_test.go

diff --git a/ipv6.go b/ipv6.go
new file mode 100644
index 0000000..355e233
--- /dev/null
+++ b/ipv6.go
@@ -0,0 +1,19 @@
+package soundbox
+
+import (
+	"fmt"
+	"net"
+)
+
+// toLinkLocal converts a MAC address to the corresponding IPv6 link-local
+// address.
+func toLinkLocal(ha net.HardwareAddr) (net.IP, error) {
+	switch len(ha) {
+	case 6:
+		ip := net.IP{0xfe, 0x80, 0, 0, 0, 0, 0, 0,
+			ha[0] ^ 0b10, ha[1], ha[2], 0xff, 0xfe, ha[3], ha[4], ha[5]}
+		return ip, nil
+	default:
+		return nil, fmt.Errorf("Only IEEE 802 MAC-48 addresses supported")
+	}
+}
diff --git a/ipv6_test.go b/ipv6_test.go
new file mode 100644
index 0000000..d46408b
--- /dev/null
+++ b/ipv6_test.go
@@ -0,0 +1,19 @@
+package soundbox
+
+import (
+	"net"
+	"reflect"
+	"testing"
+)
+
+func TestToLinkLocal(t *testing.T) {
+	input := net.HardwareAddr{0xab, 0xcd, 0xef, 0x12, 0x34, 0x56}
+	expected := net.IP{0xfe, 0x80, 0, 0, 0, 0, 0, 0, 0xa9, 0xcd, 0xef, 0xff, 0xfe, 0x12, 0x34, 0x56}
+	computed, err := toLinkLocal(input)
+	if err != nil {
+		t.Errorf("Failed to call toLinkLocal(): %v", err)
+	}
+	if !reflect.DeepEqual(computed, expected) {
+		t.Errorf("Computed IPv6 link-local address %v did not match expectation %v", computed, expected)
+	}
+}
-- 
cgit v1.2.3-70-g09d2