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slightly improved weight calculation

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translating into a tiny compression ratio improvement
This commit is contained in:
Yann Collet
2018-05-17 11:19:44 -07:00
parent 18fc3d3cd5
commit a243020d37
3 changed files with 16 additions and 14 deletions
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lib/compress/zstd_opt.c
+7 -7
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@@ -25,29 +25,29 @@
# define BITCOST_ACCURACY 0
# define BITCOST_MULTIPLIER (1 << BITCOST_ACCURACY)
# define WEIGHT(stat) ((void)opt, ZSTD_bitWeight(stat))
#elif 0 /* fractional bit accuracy */
#elif 1 /* fractional bit accuracy */
# define BITCOST_ACCURACY 8
# define BITCOST_MULTIPLIER (1 << BITCOST_ACCURACY)
# define WEIGHT(stat,opt) ((void)opt, ZSTD_fracWeight(stat))
#else /* opt==approx, ultra==accurate */
#else /* opt==approx, ultra==accurate */
# define BITCOST_ACCURACY 8
# define BITCOST_MULTIPLIER (1 << BITCOST_ACCURACY)
# define WEIGHT(stat,opt) (opt ? ZSTD_fracWeight(stat) : ZSTD_bitWeight(stat) )
# define WEIGHT(stat,opt) (opt ? ZSTD_fracWeight(stat) : ZSTD_bitWeight(stat))
#endif
MEM_STATIC U32 ZSTD_bitWeight(U32 stat)
{
return (ZSTD_highbit32((stat)+1) * BITCOST_MULTIPLIER);
return (ZSTD_highbit32(stat+1) * BITCOST_MULTIPLIER);
}
MEM_STATIC U32 ZSTD_fracWeight(U32 stat)
MEM_STATIC U32 ZSTD_fracWeight(U32 rawStat)
{
U32 const hb = stat ? ZSTD_highbit32(stat) : 0;
U32 const stat = rawStat + 1;
U32 const hb = ZSTD_highbit32(stat);
U32 const BWeight = hb * BITCOST_MULTIPLIER;
U32 const FWeight = (stat << BITCOST_ACCURACY) >> hb;
U32 const weight = BWeight + FWeight;
assert(hb + BITCOST_ACCURACY < 31);
DEBUGLOG(2, "stat=%u, hb=%u, weight=%u", stat, hb, weight)
return weight;
}