implement a WriteUintN function

internal/utils/byteoder_big_endian_test.go

@@ -73,6 +73,34 @@ var _ = Describe("Big Endian encoding / decoding", func() {
 		})
 	})
 
+	Context("WriteUintN", func() {
+		It("writes n bytes", func() {
+			expected := []byte{0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8}
+			m := map[uint8]uint64{
+				0: 0x0,
+				1: 0x01,
+				2: 0x0102,
+				3: 0x010203,
+				4: 0x01020304,
+				5: 0x0102030405,
+				6: 0x010203040506,
+				7: 0x01020304050607,
+				8: 0x0102030405060708,
+			}
+			for n, val := range m {
+				b := &bytes.Buffer{}
+				BigEndian.WriteUintN(b, n, val)
+				Expect(b.Bytes()).To(Equal(expected[:n]))
+			}
+		})
+
+		It("cuts off the higher order bytes", func() {
+			b := &bytes.Buffer{}
+			BigEndian.WriteUintN(b, 2, 0xdeadbeef)
+			Expect(b.Bytes()).To(Equal([]byte{0xbe, 0xef}))
+		})
+	})
+
 	Context("ReadUintN", func() {
 		It("reads n bytes", func() {
 			m := map[uint8]uint64{

internal/utils/byteorder.go

@@ -11,6 +11,7 @@ type ByteOrder interface {
 	ReadUint32(io.ByteReader) (uint32, error)
 	ReadUint16(io.ByteReader) (uint16, error)
 
+	WriteUintN(b *bytes.Buffer, length uint8, value uint64)
 	WriteUint32(*bytes.Buffer, uint32)
 	WriteUint16(*bytes.Buffer, uint16)
 }

internal/utils/byteorder_big_endian.go

@@ -57,6 +57,12 @@ func (bigEndian) ReadUint16(b io.ByteReader) (uint16, error) {
 	return uint16(b1) + uint16(b2)<<8, nil
 }
 
+func (bigEndian) WriteUintN(b *bytes.Buffer, length uint8, i uint64) {
+	for j := length; j > 0; j-- {
+		b.WriteByte(uint8(i >> (8 * (j - 1))))
+	}
+}
+
 // WriteUint32 writes a uint32
 func (bigEndian) WriteUint32(b *bytes.Buffer, i uint32) {
 	b.Write([]byte{uint8(i >> 24), uint8(i >> 16), uint8(i >> 8), uint8(i)})