ethernet: MTU semantic meaning corrections, add FCS logic (#28)

ethernet/crc_test.go

@@ -0,0 +1,100 @@
+package ethernet
+
+import (
+	"encoding/binary"
+	"testing"
+)
+
+func TestCRC32Search(t *testing.T) {
+	// Helper to create test data with CRC appended at a specific position.
+	makeDataWithCRC := func(payloadLen int) []byte {
+		data := make([]byte, payloadLen+4)
+		for i := range data[:payloadLen] {
+			data[i] = byte(i)
+		}
+		crc := CRC32(data[:payloadLen])
+		binary.LittleEndian.PutUint32(data[payloadLen:], crc)
+		return data
+	}
+
+	t.Run("finds CRC at end", func(t *testing.T) {
+		data := makeDataWithCRC(100)
+		off := CRC32Search(data, 0)
+		if off != 100 {
+			t.Errorf("expected offset 100, got %d", off)
+		}
+	})
+
+	t.Run("finds CRC with minOff before CRC", func(t *testing.T) {
+		data := makeDataWithCRC(100)
+		off := CRC32Search(data, 50)
+		if off != 100 {
+			t.Errorf("expected offset 100, got %d", off)
+		}
+	})
+
+	t.Run("finds CRC with minOff exactly at CRC", func(t *testing.T) {
+		data := makeDataWithCRC(100)
+		off := CRC32Search(data, 100)
+		if off != 100 {
+			t.Errorf("expected offset 100, got %d", off)
+		}
+	})
+
+	t.Run("returns -1 when minOff past CRC", func(t *testing.T) {
+		data := makeDataWithCRC(100)
+		off := CRC32Search(data, 101)
+		if off != -1 {
+			t.Errorf("expected -1, got %d", off)
+		}
+	})
+
+	t.Run("returns -1 when no valid CRC", func(t *testing.T) {
+		data := make([]byte, 100)
+		for i := range data {
+			data[i] = byte(i)
+		}
+		off := CRC32Search(data, 0)
+		if off != -1 {
+			t.Errorf("expected -1, got %d", off)
+		}
+	})
+
+	t.Run("returns -1 when data too short", func(t *testing.T) {
+		data := []byte{1, 2, 3}
+		off := CRC32Search(data, 0)
+		if off != -1 {
+			t.Errorf("expected -1, got %d", off)
+		}
+	})
+
+	t.Run("handles negative minOff", func(t *testing.T) {
+		data := makeDataWithCRC(20)
+		off := CRC32Search(data, -5)
+		if off != 20 {
+			t.Errorf("expected offset 20, got %d", off)
+		}
+	})
+
+	t.Run("finds CRC at position 0", func(t *testing.T) {
+		// Empty payload, just CRC
+		data := make([]byte, 4)
+		crc := CRC32(nil)
+		binary.LittleEndian.PutUint32(data, crc)
+		off := CRC32Search(data, 0)
+		if off != 0 {
+			t.Errorf("expected offset 0, got %d", off)
+		}
+	})
+
+	t.Run("finds first valid CRC when multiple could match", func(t *testing.T) {
+		// Create data where CRC is at position 50
+		data := makeDataWithCRC(50)
+		// Extend with more bytes (the search should still find first match)
+		data = append(data, make([]byte, 50)...)
+		off := CRC32Search(data, 0)
+		if off != 50 {
+			t.Errorf("expected offset 50 (first match), got %d", off)
+		}
+	})
+}