diff options
Diffstat (limited to 'libgo/go/compress/lzw/writer.go')
| -rw-r--r-- | libgo/go/compress/lzw/writer.go | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/libgo/go/compress/lzw/writer.go b/libgo/go/compress/lzw/writer.go index 552bdc2ce17..cf06ea80c78 100644 --- a/libgo/go/compress/lzw/writer.go +++ b/libgo/go/compress/lzw/writer.go @@ -137,7 +137,19 @@ func (w *Writer) Write(p []byte) (n int, err error) { n = len(p) code := w.savedCode if code == invalidCode { - // The first code sent is always a literal code. + // This is the first write; send a clear code. + // https://www.w3.org/Graphics/GIF/spec-gif89a.txt Appendix F + // "Variable-Length-Code LZW Compression" says that "Encoders should + // output a Clear code as the first code of each image data stream". + // + // LZW compression isn't only used by GIF, but it's cheap to follow + // that directive unconditionally. + clear := uint32(1) << w.litWidth + if err := w.write(w, clear); err != nil { + return 0, err + } + // After the starting clear code, the next code sent (for non-empty + // input) is always a literal code. code, p = uint32(p[0]), p[1:] } loop: @@ -202,6 +214,12 @@ func (w *Writer) Close() error { if err := w.incHi(); err != nil && err != errOutOfCodes { return err } + } else { + // Write the starting clear code, as w.Write did not. + clear := uint32(1) << w.litWidth + if err := w.write(w, clear); err != nil { + return err + } } // Write the eof code. eof := uint32(1)<<w.litWidth + 1 |