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compress/lzw: output a Clear code first, per GIF spec

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The TestStartsWithClearCode test is new, but if it existed beforehand,
the want strings would be "\x81" and "Hi\x81" without a starting "\x80".

Fixes #26108
Fixes #33748
Updates makeworld-the-better-one/didder#7
Updates nothings/stb#1222

Change-Id: I35ac0ed862ba6ee921ba9aee257bc19828abaa82
Reviewed-on: https://go-review.googlesource.com/c/go/+/354710
Trust: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
This commit is contained in:
Nigel Tao 2021-10-09 16:52:19 +11:00
parent d887d3be5f
commit 9c1dbdf60e
2 changed files with 47 additions and 1 deletions
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20
src/compress/lzw/writer.go
View File

@ -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

28
src/compress/lzw/writer_test.go
View File

@ -168,6 +168,34 @@ func TestSmallLitWidth(t *testing.T) {
}
}
func TestStartsWithClearCode(t *testing.T) {
// A literal width of 7 bits means that the code width starts at 8 bits,
// which makes it easier to visually inspect the output (provided that the
// output is short so codes don't get longer). Each byte is a code:
// - ASCII bytes are literal codes,
// - 0x80 is the clear code,
// - 0x81 is the end code.
// - 0x82 and above are copy codes (unused in this test case).
for _, empty := range []bool{false, true} {
var buf bytes.Buffer
w := NewWriter(&buf, LSB, 7)
if !empty {
w.Write([]byte("Hi"))
}
w.Close()
got := buf.String()
want := "\x80\x81"
if !empty {
want = "\x80Hi\x81"
}
if got != want {
t.Errorf("empty=%t: got %q, want %q", empty, got, want)
}
}
}
func BenchmarkEncoder(b *testing.B) {
buf, err := os.ReadFile("../testdata/e.txt")
if err != nil {