compress/gzip, compress/zlib: add HuffmanOnly as compression levels.

This exposes HuffmanOnly in zlib and gzip packages, which is currently
unavailable.

Change-Id: If5d103bbc8b5fce2f5d740fd103a235c5d1ed7cd
Reviewed-on: https://go-review.googlesource.com/31186
Reviewed-by: Nigel Tao <[email protected]>
Reviewed-by: Joe Tsai <[email protected]>
Run-TryBot: Joe Tsai <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: klauspost

src/compress/gzip/gzip.go

@@ -19,6 +19,7 @@ const (
 	BestSpeed          = flate.BestSpeed
 	BestCompression    = flate.BestCompression
 	DefaultCompression = flate.DefaultCompression
+	HuffmanOnly        = flate.HuffmanOnly
 )
 
 // A Writer is an io.WriteCloser.
@@ -52,11 +53,11 @@ func NewWriter(w io.Writer) *Writer {
 // NewWriterLevel is like NewWriter but specifies the compression level instead
 // of assuming DefaultCompression.
 //
-// The compression level can be DefaultCompression, NoCompression, or any
-// integer value between BestSpeed and BestCompression inclusive. The error
-// returned will be nil if the level is valid.
+// The compression level can be DefaultCompression, NoCompression, HuffmanOnly
+// or any integer value between BestSpeed and BestCompression inclusive.
+// The error returned will be nil if the level is valid.
 func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
-	if level < DefaultCompression || level > BestCompression {
+	if level < HuffmanOnly || level > BestCompression {
 		return nil, fmt.Errorf("gzip: invalid compression level: %d", level)
 	}
 	z := new(Writer)

src/compress/zlib/writer.go

@@ -19,6 +19,7 @@ const (
 	BestSpeed          = flate.BestSpeed
 	BestCompression    = flate.BestCompression
 	DefaultCompression = flate.DefaultCompression
+	HuffmanOnly        = flate.HuffmanOnly
 )
 
 // A Writer takes data written to it and writes the compressed
@@ -47,9 +48,9 @@ func NewWriter(w io.Writer) *Writer {
 // NewWriterLevel is like NewWriter but specifies the compression level instead
 // of assuming DefaultCompression.
 //
-// The compression level can be DefaultCompression, NoCompression, or any
-// integer value between BestSpeed and BestCompression inclusive. The error
-// returned will be nil if the level is valid.
+// The compression level can be DefaultCompression, NoCompression, HuffmanOnly
+// or any integer value between BestSpeed and BestCompression inclusive.
+// The error returned will be nil if the level is valid.
 func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
 	return NewWriterLevelDict(w, level, nil)
 }
@@ -60,7 +61,7 @@ func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
 // The dictionary may be nil. If not, its contents should not be modified until
 // the Writer is closed.
 func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
-	if level < DefaultCompression || level > BestCompression {
+	if level < HuffmanOnly || level > BestCompression {
 		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
 	}
 	return &Writer{
@@ -99,7 +100,7 @@ func (z *Writer) writeHeader() (err error) {
 	// The next bit, FDICT, is set if a dictionary is given.
 	// The final five FCHECK bits form a mod-31 checksum.
 	switch z.level {
-	case 0, 1:
+	case -2, 0, 1:
 		z.scratch[1] = 0 << 6
 	case 2, 3, 4, 5:
 		z.scratch[1] = 1 << 6

src/compress/zlib/writer_test.go

@@ -147,6 +147,7 @@ func TestWriter(t *testing.T) {
 		tag := fmt.Sprintf("#%d", i)
 		testLevelDict(t, tag, b, DefaultCompression, "")
 		testLevelDict(t, tag, b, NoCompression, "")
+		testLevelDict(t, tag, b, HuffmanOnly, "")
 		for level := BestSpeed; level <= BestCompression; level++ {
 			testLevelDict(t, tag, b, level, "")
 		}
@@ -157,6 +158,7 @@ func TestWriterBig(t *testing.T) {
 	for i, fn := range filenames {
 		testFileLevelDict(t, fn, DefaultCompression, "")
 		testFileLevelDict(t, fn, NoCompression, "")
+		testFileLevelDict(t, fn, HuffmanOnly, "")
 		for level := BestSpeed; level <= BestCompression; level++ {
 			testFileLevelDict(t, fn, level, "")
 			if level >= 1 && testing.Short() && testenv.Builder() == "" {
@@ -174,6 +176,7 @@ func TestWriterDict(t *testing.T) {
 	for i, fn := range filenames {
 		testFileLevelDict(t, fn, DefaultCompression, dictionary)
 		testFileLevelDict(t, fn, NoCompression, dictionary)
+		testFileLevelDict(t, fn, HuffmanOnly, dictionary)
 		for level := BestSpeed; level <= BestCompression; level++ {
 			testFileLevelDict(t, fn, level, dictionary)
 			if level >= 1 && testing.Short() && testenv.Builder() == "" {
@@ -191,8 +194,10 @@ func TestWriterReset(t *testing.T) {
 	for _, fn := range filenames {
 		testFileLevelDictReset(t, fn, NoCompression, nil)
 		testFileLevelDictReset(t, fn, DefaultCompression, nil)
+		testFileLevelDictReset(t, fn, HuffmanOnly, nil)
 		testFileLevelDictReset(t, fn, NoCompression, []byte(dictionary))
 		testFileLevelDictReset(t, fn, DefaultCompression, []byte(dictionary))
+		testFileLevelDictReset(t, fn, HuffmanOnly, []byte(dictionary))
 		if testing.Short() {
 			break
 		}