ddidderr/zstd-rs
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improve compression ratio of small alphabets

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fix #3328

In situations where the alphabet size is very small,
the evaluation of literal costs from the Optimal Parser is initially incorrect.
It takes some time to converge, during which compression is less efficient.
This is especially important for small files,
because there will not be enough data to converge,
so most of the parsing is selected based on incorrect metrics.

After this patch, the scenario ##3328 gets fixed,
delivering the expected 29 bytes compressed size (smallest known compressed size).
This commit is contained in:
Yann Collet
2023-01-03 12:22:37 -08:00
parent 1c818e3a0a
commit 5434de01e2
5 changed files with 77 additions and 41 deletions
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lib/compress/zstd_opt.c
+10 -6
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@@ -16,7 +16,7 @@
#define ZSTD_LITFREQ_ADD 2 /* scaling factor for litFreq, so that frequencies adapt faster to new stats */
#define ZSTD_MAX_PRICE (1<<30)
#define ZSTD_PREDEF_THRESHOLD 1024 /* if srcSize < ZSTD_PREDEF_THRESHOLD, symbols' cost is assumed static, directly determined by pre-defined distributions */
#define ZSTD_PREDEF_THRESHOLD 8 /* if srcSize < ZSTD_PREDEF_THRESHOLD, symbols' cost is assumed static, directly determined by pre-defined distributions */
/*-*************************************
@@ -96,14 +96,18 @@ static U32 sum_u32(const unsigned table[], size_t nbElts)
return total;
}
static U32 ZSTD_downscaleStats(unsigned* table, U32 lastEltIndex, U32 shift)
typedef enum { base_0possible=0, base_1guaranteed=1 } base_directive_e;
static U32
ZSTD_downscaleStats(unsigned* table, U32 lastEltIndex, U32 shift, base_directive_e base1)
{
U32 s, sum=0;
DEBUGLOG(5, "ZSTD_downscaleStats (nbElts=%u, shift=%u)",
(unsigned)lastEltIndex+1, (unsigned)shift );
assert(shift < 30);
for (s=0; s<lastEltIndex+1; s++) {
unsigned newStat = 1 + (table[s] >> shift);
unsigned const base = base1 ? 1 : (table[s]>0);
unsigned const newStat = base + (table[s] >> shift);
sum += newStat;
table[s] = newStat;
}
@@ -120,7 +124,7 @@ static U32 ZSTD_scaleStats(unsigned* table, U32 lastEltIndex, U32 logTarget)
DEBUGLOG(5, "ZSTD_scaleStats (nbElts=%u, target=%u)", (unsigned)lastEltIndex+1, (unsigned)logTarget);
assert(logTarget < 30);
if (factor <= 1) return prevsum;
return ZSTD_downscaleStats(table, lastEltIndex, ZSTD_highbit32(factor));
return ZSTD_downscaleStats(table, lastEltIndex, ZSTD_highbit32(factor), base_1guaranteed);
}
/* ZSTD_rescaleFreqs() :
@@ -202,14 +206,14 @@ ZSTD_rescaleFreqs(optState_t* const optPtr,
optPtr->offCodeSum += optPtr->offCodeFreq[of];
} }
} else { /* huf.repeatMode != HUF_repeat_valid => presumed not a dictionary */
} else { /* first block, no dictionary */
assert(optPtr->litFreq != NULL);
if (compressedLiterals) {
/* base initial cost of literals on direct frequency within src */
unsigned lit = MaxLit;
HIST_count_simple(optPtr->litFreq, &lit, src, srcSize); /* use raw first block to init statistics */
optPtr->litSum = ZSTD_downscaleStats(optPtr->litFreq, MaxLit, 8);
optPtr->litSum = ZSTD_downscaleStats(optPtr->litFreq, MaxLit, 8, base_0possible);
}
{ unsigned const baseLLfreqs[MaxLL+1] = {