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compress/lzw: clarify code invariants

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This follows on from https://go-review.googlesource.com/c/go/+/191358
which was submitted as a comment-only change.

Benchmarks don't show any significant change:

compress/lzw
name            old speed      new speed      delta
Decoder/1e4-56  92.8MB/s ± 1%  92.7MB/s ± 1%   ~     (p=1.000 n=5+5)
Decoder/1e5-56   100MB/s ± 1%   100MB/s ± 1%   ~     (p=0.746 n=5+5)
Decoder/1e6-56   101MB/s ± 1%   101MB/s ± 1%   ~     (p=0.381 n=5+5)

image/gif
name                old speed      new speed      delta
Decode-56           63.2MB/s ± 1%  63.2MB/s ± 1%   ~     (p=0.690 n=5+5)

Change-Id: Ic36b5410cb06ca258da32e40da1f1ff6c44cff86
Reviewed-on: https://go-review.googlesource.com/c/go/+/194938
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Nigel Tao 2019-09-12 17:56:57 +10:00
parent 8cc57c0ccc
commit 4e215554aa
1 changed files with 4 additions and 1 deletions
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5
src/compress/lzw/reader.go
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@ -199,6 +199,9 @@ loop:
} }
d.last, d.hi = code, d.hi+1 d.last, d.hi = code, d.hi+1
if d.hi >= d.overflow { if d.hi >= d.overflow {
if d.hi > d.overflow {
panic("unreachable")
}
if d.width == maxWidth { if d.width == maxWidth {
d.last = decoderInvalidCode d.last = decoderInvalidCode
// Undo the d.hi++ a few lines above, so that (1) we maintain // Undo the d.hi++ a few lines above, so that (1) we maintain
@ -207,7 +210,7 @@ loop:
d.hi-- d.hi--
} else { } else {
d.width++ d.width++
d.overflow <<= 1 d.overflow = 1 << d.width
} }
} }
if d.o >= flushBuffer { if d.o >= flushBuffer {