xz: add support for check-ID None to Writer

So far we didn't support the check-ID None for Writer. To request it I
added a NoCheckSum boolean to the configuration, because CRC64 should
still be used as a default.

The commit is related to issue #27 - #27

writer.go

@@ -18,8 +18,10 @@ type WriterConfig struct {
 	DictCap    int
 	BufSize    int
 	BlockSize  int64
-	// checksum method: CRC32, CRC64 or SHA256
+	// checksum method: CRC32, CRC64 or SHA256 (default: CRC64)
 	CheckSum byte
+	// Forces NoChecksum (default: false)
+	NoCheckSum bool
 	// match algorithm
 	Matcher lzma.MatchAlgorithm
 }
@@ -41,6 +43,9 @@ func (c *WriterConfig) fill() {
 	if c.CheckSum == 0 {
 		c.CheckSum = CRC64
 	}
+	if c.NoCheckSum {
+		c.CheckSum = None
+	}
 }
 
 // Verify checks the configuration for errors. Zero values will be
@@ -284,7 +289,11 @@ func (c *WriterConfig) newBlockWriter(xz io.Writer, hash hash.Hash) (bw *blockWr
 	if err != nil {
 		return nil, err
 	}
-	bw.mw = io.MultiWriter(bw.w, bw.hash)
+	if bw.hash.Size() != 0 {
+		bw.mw = io.MultiWriter(bw.w, bw.hash)
+	} else {
+		bw.mw = bw.w
+	}
 	return bw, nil
 }
 

writer_test.go

@@ -136,3 +136,42 @@ func TestWriter2(t *testing.T) {
 		t.Fatal("decompressed data differs from original")
 	}
 }
+
+func TestWriterNoneCheck(t *testing.T) {
+	const txtlen = 1023
+	var buf bytes.Buffer
+	io.CopyN(&buf, randtxt.NewReader(rand.NewSource(41)), txtlen)
+	txt := buf.String()
+
+	buf.Reset()
+	w, err := WriterConfig{NoCheckSum: true}.NewWriter(&buf)
+	if err != nil {
+		t.Fatalf("NewWriter error %s", err)
+	}
+	n, err := io.WriteString(w, txt)
+	if err != nil {
+		t.Fatalf("WriteString error %s", err)
+	}
+	if n != len(txt) {
+		t.Fatalf("WriteString wrote %d bytes; want %d", n, len(txt))
+	}
+	if err = w.Close(); err != nil {
+		t.Fatalf("Close error %s", err)
+	}
+	t.Logf("buf.Len() %d", buf.Len())
+	r, err := NewReader(&buf)
+	if err != nil {
+		t.Fatalf("NewReader error %s", err)
+	}
+	var out bytes.Buffer
+	k, err := io.Copy(&out, r)
+	if err != nil {
+		t.Fatalf("Decompressing copy error %s after %d bytes", err, n)
+	}
+	if k != txtlen {
+		t.Fatalf("Decompression data length %d; want %d", k, txtlen)
+	}
+	if txt != out.String() {
+		t.Fatal("decompressed data differs from original")
+	}
+}