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-rw-r--r--thirdparty/zstd/compress/zstd_opt.c186
1 files changed, 106 insertions, 80 deletions
diff --git a/thirdparty/zstd/compress/zstd_opt.c b/thirdparty/zstd/compress/zstd_opt.c
index 1b1ddad428..f02a760946 100644
--- a/thirdparty/zstd/compress/zstd_opt.c
+++ b/thirdparty/zstd/compress/zstd_opt.c
@@ -1,5 +1,5 @@
/*
- * Copyright (c) Przemyslaw Skibinski, Yann Collet, Facebook, Inc.
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
* All rights reserved.
*
* This source code is licensed under both the BSD-style license (found in the
@@ -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 */
/*-*************************************
@@ -26,27 +26,35 @@
#if 0 /* approximation at bit level (for tests) */
# define BITCOST_ACCURACY 0
# define BITCOST_MULTIPLIER (1 << BITCOST_ACCURACY)
-# define WEIGHT(stat, opt) ((void)opt, ZSTD_bitWeight(stat))
+# define WEIGHT(stat, opt) ((void)(opt), ZSTD_bitWeight(stat))
#elif 0 /* fractional bit accuracy (for tests) */
# define BITCOST_ACCURACY 8
# define BITCOST_MULTIPLIER (1 << BITCOST_ACCURACY)
-# define WEIGHT(stat,opt) ((void)opt, ZSTD_fracWeight(stat))
+# define WEIGHT(stat,opt) ((void)(opt), ZSTD_fracWeight(stat))
#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
+/* ZSTD_bitWeight() :
+ * provide estimated "cost" of a stat in full bits only */
MEM_STATIC U32 ZSTD_bitWeight(U32 stat)
{
return (ZSTD_highbit32(stat+1) * BITCOST_MULTIPLIER);
}
+/* ZSTD_fracWeight() :
+ * provide fractional-bit "cost" of a stat,
+ * using linear interpolation approximation */
MEM_STATIC U32 ZSTD_fracWeight(U32 rawStat)
{
U32 const stat = rawStat + 1;
U32 const hb = ZSTD_highbit32(stat);
U32 const BWeight = hb * BITCOST_MULTIPLIER;
+ /* Fweight was meant for "Fractional weight"
+ * but it's effectively a value between 1 and 2
+ * using fixed point arithmetic */
U32 const FWeight = (stat << BITCOST_ACCURACY) >> hb;
U32 const weight = BWeight + FWeight;
assert(hb + BITCOST_ACCURACY < 31);
@@ -57,7 +65,7 @@ MEM_STATIC U32 ZSTD_fracWeight(U32 rawStat)
/* debugging function,
* @return price in bytes as fractional value
* for debug messages only */
-MEM_STATIC double ZSTD_fCost(U32 price)
+MEM_STATIC double ZSTD_fCost(int price)
{
return (double)price / (BITCOST_MULTIPLIER*8);
}
@@ -88,20 +96,26 @@ 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);
+ DEBUGLOG(5, "ZSTD_downscaleStats (nbElts=%u, shift=%u)",
+ (unsigned)lastEltIndex+1, (unsigned)shift );
assert(shift < 30);
for (s=0; s<lastEltIndex+1; s++) {
- table[s] = 1 + (table[s] >> shift);
- sum += table[s];
+ unsigned const base = base1 ? 1 : (table[s]>0);
+ unsigned const newStat = base + (table[s] >> shift);
+ sum += newStat;
+ table[s] = newStat;
}
return sum;
}
/* ZSTD_scaleStats() :
- * reduce all elements in table is sum too large
+ * reduce all elt frequencies in table if sum too large
* return the resulting sum of elements */
static U32 ZSTD_scaleStats(unsigned* table, U32 lastEltIndex, U32 logTarget)
{
@@ -110,7 +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() :
@@ -129,18 +143,22 @@ ZSTD_rescaleFreqs(optState_t* const optPtr,
DEBUGLOG(5, "ZSTD_rescaleFreqs (srcSize=%u)", (unsigned)srcSize);
optPtr->priceType = zop_dynamic;
- if (optPtr->litLengthSum == 0) { /* first block : init */
- if (srcSize <= ZSTD_PREDEF_THRESHOLD) { /* heuristic */
- DEBUGLOG(5, "(srcSize <= ZSTD_PREDEF_THRESHOLD) => zop_predef");
+ if (optPtr->litLengthSum == 0) { /* no literals stats collected -> first block assumed -> init */
+
+ /* heuristic: use pre-defined stats for too small inputs */
+ if (srcSize <= ZSTD_PREDEF_THRESHOLD) {
+ DEBUGLOG(5, "srcSize <= %i : use predefined stats", ZSTD_PREDEF_THRESHOLD);
optPtr->priceType = zop_predef;
}
assert(optPtr->symbolCosts != NULL);
if (optPtr->symbolCosts->huf.repeatMode == HUF_repeat_valid) {
- /* huffman table presumed generated by dictionary */
+
+ /* huffman stats covering the full value set : table presumed generated by dictionary */
optPtr->priceType = zop_dynamic;
if (compressedLiterals) {
+ /* generate literals statistics from huffman table */
unsigned lit;
assert(optPtr->litFreq != NULL);
optPtr->litSum = 0;
@@ -188,13 +206,14 @@ ZSTD_rescaleFreqs(optState_t* const optPtr,
optPtr->offCodeSum += optPtr->offCodeFreq[of];
} }
- } else { /* 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] = {
@@ -224,10 +243,9 @@ ZSTD_rescaleFreqs(optState_t* const optPtr,
optPtr->offCodeSum = sum_u32(baseOFCfreqs, MaxOff+1);
}
-
}
- } else { /* new block : re-use previous statistics, scaled down */
+ } else { /* new block : scale down accumulated statistics */
if (compressedLiterals)
optPtr->litSum = ZSTD_scaleStats(optPtr->litFreq, MaxLit, 12);
@@ -255,11 +273,14 @@ static U32 ZSTD_rawLiteralsCost(const BYTE* const literals, U32 const litLength,
return (litLength*6) * BITCOST_MULTIPLIER; /* 6 bit per literal - no statistic used */
/* dynamic statistics */
- { U32 price = litLength * optPtr->litSumBasePrice;
+ { U32 price = optPtr->litSumBasePrice * litLength;
+ U32 const litPriceMax = optPtr->litSumBasePrice - BITCOST_MULTIPLIER;
U32 u;
+ assert(optPtr->litSumBasePrice >= BITCOST_MULTIPLIER);
for (u=0; u < litLength; u++) {
- assert(WEIGHT(optPtr->litFreq[literals[u]], optLevel) <= optPtr->litSumBasePrice); /* literal cost should never be negative */
- price -= WEIGHT(optPtr->litFreq[literals[u]], optLevel);
+ U32 litPrice = WEIGHT(optPtr->litFreq[literals[u]], optLevel);
+ if (UNLIKELY(litPrice > litPriceMax)) litPrice = litPriceMax;
+ price -= litPrice;
}
return price;
}
@@ -272,10 +293,11 @@ static U32 ZSTD_litLengthPrice(U32 const litLength, const optState_t* const optP
assert(litLength <= ZSTD_BLOCKSIZE_MAX);
if (optPtr->priceType == zop_predef)
return WEIGHT(litLength, optLevel);
- /* We can't compute the litLength price for sizes >= ZSTD_BLOCKSIZE_MAX
- * because it isn't representable in the zstd format. So instead just
- * call it 1 bit more than ZSTD_BLOCKSIZE_MAX - 1. In this case the block
- * would be all literals.
+
+ /* ZSTD_LLcode() can't compute litLength price for sizes >= ZSTD_BLOCKSIZE_MAX
+ * because it isn't representable in the zstd format.
+ * So instead just pretend it would cost 1 bit more than ZSTD_BLOCKSIZE_MAX - 1.
+ * In such a case, the block would be all literals.
*/
if (litLength == ZSTD_BLOCKSIZE_MAX)
return BITCOST_MULTIPLIER + ZSTD_litLengthPrice(ZSTD_BLOCKSIZE_MAX - 1, optPtr, optLevel);
@@ -289,24 +311,25 @@ static U32 ZSTD_litLengthPrice(U32 const litLength, const optState_t* const optP
}
/* ZSTD_getMatchPrice() :
- * Provides the cost of the match part (offset + matchLength) of a sequence
+ * Provides the cost of the match part (offset + matchLength) of a sequence.
* Must be combined with ZSTD_fullLiteralsCost() to get the full cost of a sequence.
- * @offcode : expects a scale where 0,1,2 are repcodes 1-3, and 3+ are real_offsets+2
+ * @offBase : sumtype, representing an offset or a repcode, and using numeric representation of ZSTD_storeSeq()
* @optLevel: when <2, favors small offset for decompression speed (improved cache efficiency)
*/
FORCE_INLINE_TEMPLATE U32
-ZSTD_getMatchPrice(U32 const offcode,
+ZSTD_getMatchPrice(U32 const offBase,
U32 const matchLength,
const optState_t* const optPtr,
int const optLevel)
{
U32 price;
- U32 const offCode = ZSTD_highbit32(STORED_TO_OFFBASE(offcode));
+ U32 const offCode = ZSTD_highbit32(offBase);
U32 const mlBase = matchLength - MINMATCH;
assert(matchLength >= MINMATCH);
- if (optPtr->priceType == zop_predef) /* fixed scheme, do not use statistics */
- return WEIGHT(mlBase, optLevel) + ((16 + offCode) * BITCOST_MULTIPLIER);
+ if (optPtr->priceType == zop_predef) /* fixed scheme, does not use statistics */
+ return WEIGHT(mlBase, optLevel)
+ + ((16 + offCode) * BITCOST_MULTIPLIER); /* emulated offset cost */
/* dynamic statistics */
price = (offCode * BITCOST_MULTIPLIER) + (optPtr->offCodeSumBasePrice - WEIGHT(optPtr->offCodeFreq[offCode], optLevel));
@@ -325,10 +348,10 @@ ZSTD_getMatchPrice(U32 const offcode,
}
/* ZSTD_updateStats() :
- * assumption : literals + litLengtn <= iend */
+ * assumption : literals + litLength <= iend */
static void ZSTD_updateStats(optState_t* const optPtr,
U32 litLength, const BYTE* literals,
- U32 offsetCode, U32 matchLength)
+ U32 offBase, U32 matchLength)
{
/* literals */
if (ZSTD_compressedLiterals(optPtr)) {
@@ -344,8 +367,8 @@ static void ZSTD_updateStats(optState_t* const optPtr,
optPtr->litLengthSum++;
}
- /* offset code : expected to follow storeSeq() numeric representation */
- { U32 const offCode = ZSTD_highbit32(STORED_TO_OFFBASE(offsetCode));
+ /* offset code : follows storeSeq() numeric representation */
+ { U32 const offCode = ZSTD_highbit32(offBase);
assert(offCode <= MaxOff);
optPtr->offCodeFreq[offCode]++;
optPtr->offCodeSum++;
@@ -552,16 +575,17 @@ void ZSTD_updateTree(ZSTD_matchState_t* ms, const BYTE* ip, const BYTE* iend) {
ZSTD_updateTree_internal(ms, ip, iend, ms->cParams.minMatch, ZSTD_noDict);
}
-FORCE_INLINE_TEMPLATE
-U32 ZSTD_insertBtAndGetAllMatches (
- ZSTD_match_t* matches, /* store result (found matches) in this table (presumed large enough) */
- ZSTD_matchState_t* ms,
- U32* nextToUpdate3,
- const BYTE* const ip, const BYTE* const iLimit, const ZSTD_dictMode_e dictMode,
- const U32 rep[ZSTD_REP_NUM],
- U32 const ll0, /* tells if associated literal length is 0 or not. This value must be 0 or 1 */
- const U32 lengthToBeat,
- U32 const mls /* template */)
+FORCE_INLINE_TEMPLATE U32
+ZSTD_insertBtAndGetAllMatches (
+ ZSTD_match_t* matches, /* store result (found matches) in this table (presumed large enough) */
+ ZSTD_matchState_t* ms,
+ U32* nextToUpdate3,
+ const BYTE* const ip, const BYTE* const iLimit,
+ const ZSTD_dictMode_e dictMode,
+ const U32 rep[ZSTD_REP_NUM],
+ const U32 ll0, /* tells if associated literal length is 0 or not. This value must be 0 or 1 */
+ const U32 lengthToBeat,
+ const U32 mls /* template */)
{
const ZSTD_compressionParameters* const cParams = &ms->cParams;
U32 const sufficient_len = MIN(cParams->targetLength, ZSTD_OPT_NUM -1);
@@ -644,7 +668,7 @@ U32 ZSTD_insertBtAndGetAllMatches (
DEBUGLOG(8, "found repCode %u (ll0:%u, offset:%u) of length %u",
repCode, ll0, repOffset, repLen);
bestLength = repLen;
- matches[mnum].off = STORE_REPCODE(repCode - ll0 + 1); /* expect value between 1 and 3 */
+ matches[mnum].off = REPCODE_TO_OFFBASE(repCode - ll0 + 1); /* expect value between 1 and 3 */
matches[mnum].len = (U32)repLen;
mnum++;
if ( (repLen > sufficient_len)
@@ -673,7 +697,7 @@ U32 ZSTD_insertBtAndGetAllMatches (
bestLength = mlen;
assert(curr > matchIndex3);
assert(mnum==0); /* no prior solution */
- matches[0].off = STORE_OFFSET(curr - matchIndex3);
+ matches[0].off = OFFSET_TO_OFFBASE(curr - matchIndex3);
matches[0].len = (U32)mlen;
mnum = 1;
if ( (mlen > sufficient_len) |
@@ -706,13 +730,13 @@ U32 ZSTD_insertBtAndGetAllMatches (
}
if (matchLength > bestLength) {
- DEBUGLOG(8, "found match of length %u at distance %u (offCode=%u)",
- (U32)matchLength, curr - matchIndex, STORE_OFFSET(curr - matchIndex));
+ DEBUGLOG(8, "found match of length %u at distance %u (offBase=%u)",
+ (U32)matchLength, curr - matchIndex, OFFSET_TO_OFFBASE(curr - matchIndex));
assert(matchEndIdx > matchIndex);
if (matchLength > matchEndIdx - matchIndex)
matchEndIdx = matchIndex + (U32)matchLength;
bestLength = matchLength;
- matches[mnum].off = STORE_OFFSET(curr - matchIndex);
+ matches[mnum].off = OFFSET_TO_OFFBASE(curr - matchIndex);
matches[mnum].len = (U32)matchLength;
mnum++;
if ( (matchLength > ZSTD_OPT_NUM)
@@ -754,12 +778,12 @@ U32 ZSTD_insertBtAndGetAllMatches (
if (matchLength > bestLength) {
matchIndex = dictMatchIndex + dmsIndexDelta;
- DEBUGLOG(8, "found dms match of length %u at distance %u (offCode=%u)",
- (U32)matchLength, curr - matchIndex, STORE_OFFSET(curr - matchIndex));
+ DEBUGLOG(8, "found dms match of length %u at distance %u (offBase=%u)",
+ (U32)matchLength, curr - matchIndex, OFFSET_TO_OFFBASE(curr - matchIndex));
if (matchLength > matchEndIdx - matchIndex)
matchEndIdx = matchIndex + (U32)matchLength;
bestLength = matchLength;
- matches[mnum].off = STORE_OFFSET(curr - matchIndex);
+ matches[mnum].off = OFFSET_TO_OFFBASE(curr - matchIndex);
matches[mnum].len = (U32)matchLength;
mnum++;
if ( (matchLength > ZSTD_OPT_NUM)
@@ -960,7 +984,7 @@ static void ZSTD_optLdm_maybeAddMatch(ZSTD_match_t* matches, U32* nbMatches,
const ZSTD_optLdm_t* optLdm, U32 currPosInBlock)
{
U32 const posDiff = currPosInBlock - optLdm->startPosInBlock;
- /* Note: ZSTD_match_t actually contains offCode and matchLength (before subtracting MINMATCH) */
+ /* Note: ZSTD_match_t actually contains offBase and matchLength (before subtracting MINMATCH) */
U32 const candidateMatchLength = optLdm->endPosInBlock - optLdm->startPosInBlock - posDiff;
/* Ensure that current block position is not outside of the match */
@@ -971,11 +995,11 @@ static void ZSTD_optLdm_maybeAddMatch(ZSTD_match_t* matches, U32* nbMatches,
}
if (*nbMatches == 0 || ((candidateMatchLength > matches[*nbMatches-1].len) && *nbMatches < ZSTD_OPT_NUM)) {
- U32 const candidateOffCode = STORE_OFFSET(optLdm->offset);
- DEBUGLOG(6, "ZSTD_optLdm_maybeAddMatch(): Adding ldm candidate match (offCode: %u matchLength %u) at block position=%u",
- candidateOffCode, candidateMatchLength, currPosInBlock);
+ U32 const candidateOffBase = OFFSET_TO_OFFBASE(optLdm->offset);
+ DEBUGLOG(6, "ZSTD_optLdm_maybeAddMatch(): Adding ldm candidate match (offBase: %u matchLength %u) at block position=%u",
+ candidateOffBase, candidateMatchLength, currPosInBlock);
matches[*nbMatches].len = candidateMatchLength;
- matches[*nbMatches].off = candidateOffCode;
+ matches[*nbMatches].off = candidateOffBase;
(*nbMatches)++;
}
}
@@ -1062,6 +1086,8 @@ ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
ZSTD_optimal_t lastSequence;
ZSTD_optLdm_t optLdm;
+ ZSTD_memset(&lastSequence, 0, sizeof(ZSTD_optimal_t));
+
optLdm.seqStore = ms->ldmSeqStore ? *ms->ldmSeqStore : kNullRawSeqStore;
optLdm.endPosInBlock = optLdm.startPosInBlock = optLdm.offset = 0;
ZSTD_opt_getNextMatchAndUpdateSeqStore(&optLdm, (U32)(ip-istart), (U32)(iend-ip));
@@ -1098,14 +1124,14 @@ ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
/* large match -> immediate encoding */
{ U32 const maxML = matches[nbMatches-1].len;
- U32 const maxOffcode = matches[nbMatches-1].off;
- DEBUGLOG(6, "found %u matches of maxLength=%u and maxOffCode=%u at cPos=%u => start new series",
- nbMatches, maxML, maxOffcode, (U32)(ip-prefixStart));
+ U32 const maxOffBase = matches[nbMatches-1].off;
+ DEBUGLOG(6, "found %u matches of maxLength=%u and maxOffBase=%u at cPos=%u => start new series",
+ nbMatches, maxML, maxOffBase, (U32)(ip-prefixStart));
if (maxML > sufficient_len) {
lastSequence.litlen = litlen;
lastSequence.mlen = maxML;
- lastSequence.off = maxOffcode;
+ lastSequence.off = maxOffBase;
DEBUGLOG(6, "large match (%u>%u), immediate encoding",
maxML, sufficient_len);
cur = 0;
@@ -1122,15 +1148,15 @@ ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
opt[pos].price = ZSTD_MAX_PRICE; /* mlen, litlen and price will be fixed during forward scanning */
}
for (matchNb = 0; matchNb < nbMatches; matchNb++) {
- U32 const offcode = matches[matchNb].off;
+ U32 const offBase = matches[matchNb].off;
U32 const end = matches[matchNb].len;
for ( ; pos <= end ; pos++ ) {
- U32 const matchPrice = ZSTD_getMatchPrice(offcode, pos, optStatePtr, optLevel);
+ U32 const matchPrice = ZSTD_getMatchPrice(offBase, pos, optStatePtr, optLevel);
U32 const sequencePrice = literalsPrice + matchPrice;
DEBUGLOG(7, "rPos:%u => set initial price : %.2f",
- pos, ZSTD_fCost(sequencePrice));
+ pos, ZSTD_fCost((int)sequencePrice));
opt[pos].mlen = pos;
- opt[pos].off = offcode;
+ opt[pos].off = offBase;
opt[pos].litlen = litlen;
opt[pos].price = (int)sequencePrice;
} }
@@ -1230,7 +1256,7 @@ ZSTD_compressBlock_opt_generic(ZSTD_matchState_t* ms,
U32 const startML = (matchNb>0) ? matches[matchNb-1].len+1 : minMatch;
U32 mlen;
- DEBUGLOG(7, "testing match %u => offCode=%4u, mlen=%2u, llen=%2u",
+ DEBUGLOG(7, "testing match %u => offBase=%4u, mlen=%2u, llen=%2u",
matchNb, matches[matchNb].off, lastML, litlen);
for (mlen = lastML; mlen >= startML; mlen--) { /* scan downward */
@@ -1296,7 +1322,7 @@ _shortestPath: /* cur, last_pos, best_mlen, best_off have to be set */
for (storePos=storeStart; storePos <= storeEnd; storePos++) {
U32 const llen = opt[storePos].litlen;
U32 const mlen = opt[storePos].mlen;
- U32 const offCode = opt[storePos].off;
+ U32 const offBase = opt[storePos].off;
U32 const advance = llen + mlen;
DEBUGLOG(6, "considering seq starting at %zi, llen=%u, mlen=%u",
anchor - istart, (unsigned)llen, (unsigned)mlen);
@@ -1308,8 +1334,8 @@ _shortestPath: /* cur, last_pos, best_mlen, best_off have to be set */
}
assert(anchor + llen <= iend);
- ZSTD_updateStats(optStatePtr, llen, anchor, offCode, mlen);
- ZSTD_storeSeq(seqStore, llen, anchor, iend, offCode, mlen);
+ ZSTD_updateStats(optStatePtr, llen, anchor, offBase, mlen);
+ ZSTD_storeSeq(seqStore, llen, anchor, iend, offBase, mlen);
anchor += advance;
ip = anchor;
} }
@@ -1349,7 +1375,7 @@ size_t ZSTD_compressBlock_btopt(
/* ZSTD_initStats_ultra():
* make a first compression pass, just to seed stats with more accurate starting values.
* only works on first block, with no dictionary and no ldm.
- * this function cannot error, hence its contract must be respected.
+ * this function cannot error out, its narrow contract must be respected.
*/
static void
ZSTD_initStats_ultra(ZSTD_matchState_t* ms,
@@ -1368,7 +1394,7 @@ ZSTD_initStats_ultra(ZSTD_matchState_t* ms,
ZSTD_compressBlock_opt2(ms, seqStore, tmpRep, src, srcSize, ZSTD_noDict); /* generate stats into ms->opt*/
- /* invalidate first scan from history */
+ /* invalidate first scan from history, only keep entropy stats */
ZSTD_resetSeqStore(seqStore);
ms->window.base -= srcSize;
ms->window.dictLimit += (U32)srcSize;
@@ -1392,20 +1418,20 @@ size_t ZSTD_compressBlock_btultra2(
U32 const curr = (U32)((const BYTE*)src - ms->window.base);
DEBUGLOG(5, "ZSTD_compressBlock_btultra2 (srcSize=%zu)", srcSize);
- /* 2-pass strategy:
+ /* 2-passes strategy:
* this strategy makes a first pass over first block to collect statistics
- * and seed next round's statistics with it.
- * After 1st pass, function forgets everything, and starts a new block.
+ * in order to seed next round's statistics with it.
+ * After 1st pass, function forgets history, and starts a new block.
* Consequently, this can only work if no data has been previously loaded in tables,
* aka, no dictionary, no prefix, no ldm preprocessing.
* The compression ratio gain is generally small (~0.5% on first block),
- * the cost is 2x cpu time on first block. */
+ ** the cost is 2x cpu time on first block. */
assert(srcSize <= ZSTD_BLOCKSIZE_MAX);
if ( (ms->opt.litLengthSum==0) /* first block */
&& (seqStore->sequences == seqStore->sequencesStart) /* no ldm */
&& (ms->window.dictLimit == ms->window.lowLimit) /* no dictionary */
- && (curr == ms->window.dictLimit) /* start of frame, nothing already loaded nor skipped */
- && (srcSize > ZSTD_PREDEF_THRESHOLD)
+ && (curr == ms->window.dictLimit) /* start of frame, nothing already loaded nor skipped */
+ && (srcSize > ZSTD_PREDEF_THRESHOLD) /* input large enough to not employ default stats */
) {
ZSTD_initStats_ultra(ms, seqStore, rep, src, srcSize);
}