1 files changed, 46 insertions, 13 deletions

diff --git a/src/pkg/compress/zlib/writer.go b/src/pkg/compress/zlib/writer.go
index 031586cd2..f1f9b2853 100644
--- a/src/pkg/compress/zlib/writer.go
+++ b/src/pkg/compress/zlib/writer.go
@@ -21,56 +21,80 @@ const (
 	DefaultCompression = flate.DefaultCompression
 )
 
-type writer struct {
+// A Writer takes data written to it and writes the compressed
+// form of that data to an underlying writer (see NewWriter).
+type Writer struct {
 	w          io.Writer
-	compressor io.WriteCloser
+	compressor *flate.Writer
 	digest     hash.Hash32
 	err        os.Error
 	scratch    [4]byte
 }
 
 // NewWriter calls NewWriterLevel with the default compression level.
-func NewWriter(w io.Writer) (io.WriteCloser, os.Error) {
+func NewWriter(w io.Writer) (*Writer, os.Error) {
 	return NewWriterLevel(w, DefaultCompression)
 }
 
-// NewWriterLevel creates a new io.WriteCloser that satisfies writes by compressing data written to w.
+// NewWriterLevel calls NewWriterDict with no dictionary.
+func NewWriterLevel(w io.Writer, level int) (*Writer, os.Error) {
+	return NewWriterDict(w, level, nil)
+}
+
+// NewWriterDict creates a new io.WriteCloser that satisfies writes by compressing data written to w.
 // It is the caller's responsibility to call Close on the WriteCloser when done.
 // level is the compression level, which can be DefaultCompression, NoCompression,
 // or any integer value between BestSpeed and BestCompression (inclusive).
-func NewWriterLevel(w io.Writer, level int) (io.WriteCloser, os.Error) {
-	z := new(writer)
+// dict is the preset dictionary to compress with, or nil to use no dictionary.
+func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, os.Error) {
+	z := new(Writer)
 	// ZLIB has a two-byte header (as documented in RFC 1950).
 	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
 	// The next four bits is the CM (compression method), which is 8 for deflate.
 	z.scratch[0] = 0x78
 	// The next two bits is the FLEVEL (compression level). The four values are:
 	// 0=fastest, 1=fast, 2=default, 3=best.
-	// The next bit, FDICT, is unused, in this implementation.
+	// The next bit, FDICT, is set if a dictionary is given.
 	// The final five FCHECK bits form a mod-31 checksum.
 	switch level {
 	case 0, 1:
-		z.scratch[1] = 0x01
+		z.scratch[1] = 0 << 6
 	case 2, 3, 4, 5:
-		z.scratch[1] = 0x5e
+		z.scratch[1] = 1 << 6
 	case 6, -1:
-		z.scratch[1] = 0x9c
+		z.scratch[1] = 2 << 6
 	case 7, 8, 9:
-		z.scratch[1] = 0xda
+		z.scratch[1] = 3 << 6
 	default:
 		return nil, os.NewError("level out of range")
 	}
+	if dict != nil {
+		z.scratch[1] |= 1 << 5
+	}
+	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
 	_, err := w.Write(z.scratch[0:2])
 	if err != nil {
 		return nil, err
 	}
+	if dict != nil {
+		// The next four bytes are the Adler-32 checksum of the dictionary.
+		checksum := adler32.Checksum(dict)
+		z.scratch[0] = uint8(checksum >> 24)
+		z.scratch[1] = uint8(checksum >> 16)
+		z.scratch[2] = uint8(checksum >> 8)
+		z.scratch[3] = uint8(checksum >> 0)
+		_, err = w.Write(z.scratch[0:4])
+		if err != nil {
+			return nil, err
+		}
+	}
 	z.w = w
 	z.compressor = flate.NewWriter(w, level)
 	z.digest = adler32.New()
 	return z, nil
 }
 
-func (z *writer) Write(p []byte) (n int, err os.Error) {
+func (z *Writer) Write(p []byte) (n int, err os.Error) {
 	if z.err != nil {
 		return 0, z.err
 	}
@@ -86,8 +110,17 @@ func (z *writer) Write(p []byte) (n int, err os.Error) {
 	return
 }
 
+// Flush flushes the underlying compressor.
+func (z *Writer) Flush() os.Error {
+	if z.err != nil {
+		return z.err
+	}
+	z.err = z.compressor.Flush()
+	return z.err
+}
+
 // Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
-func (z *writer) Close() os.Error {
+func (z *Writer) Close() os.Error {
 	if z.err != nil {
 		return z.err
 	}