zstd_compress_sequences.c File Reference

#include "zstd_compress_sequences.h"

Include dependency graph for zstd_compress_sequences.c:

Go to the source code of this file.

Functions
static unsigned	ZSTD_getFSEMaxSymbolValue (FSE_CTable const *ctable)
static size_t	ZSTD_NCountCost (unsigned const *count, unsigned const max, size_t const nbSeq, unsigned const FSELog)
static size_t	ZSTD_entropyCost (unsigned const *count, unsigned const max, size_t const total)
size_t	ZSTD_fseBitCost (FSE_CTable const *ctable, unsigned const *count, unsigned const max)
size_t	ZSTD_crossEntropyCost (short const *norm, unsigned accuracyLog, unsigned const *count, unsigned const max)
symbolEncodingType_e	ZSTD_selectEncodingType (FSE_repeat *repeatMode, unsigned const *count, unsigned const max, size_t const mostFrequent, size_t nbSeq, unsigned const FSELog, FSE_CTable const *prevCTable, short const *defaultNorm, U32 defaultNormLog, ZSTD_defaultPolicy_e const isDefaultAllowed, ZSTD_strategy const strategy)
size_t	ZSTD_buildCTable (void *dst, size_t dstCapacity, FSE_CTable *nextCTable, U32 FSELog, symbolEncodingType_e type, unsigned *count, U32 max, const BYTE *codeTable, size_t nbSeq, const S16 *defaultNorm, U32 defaultNormLog, U32 defaultMax, const FSE_CTable *prevCTable, size_t prevCTableSize, void *entropyWorkspace, size_t entropyWorkspaceSize)
FORCE_INLINE_TEMPLATE size_t	ZSTD_encodeSequences_body (void *dst, size_t dstCapacity, FSE_CTable const *CTable_MatchLength, BYTE const *mlCodeTable, FSE_CTable const *CTable_OffsetBits, BYTE const *ofCodeTable, FSE_CTable const *CTable_LitLength, BYTE const *llCodeTable, seqDef const *sequences, size_t nbSeq, int longOffsets)
static size_t	ZSTD_encodeSequences_default (void *dst, size_t dstCapacity, FSE_CTable const *CTable_MatchLength, BYTE const *mlCodeTable, FSE_CTable const *CTable_OffsetBits, BYTE const *ofCodeTable, FSE_CTable const *CTable_LitLength, BYTE const *llCodeTable, seqDef const *sequences, size_t nbSeq, int longOffsets)
size_t	ZSTD_encodeSequences (void *dst, size_t dstCapacity, FSE_CTable const *CTable_MatchLength, BYTE const *mlCodeTable, FSE_CTable const *CTable_OffsetBits, BYTE const *ofCodeTable, FSE_CTable const *CTable_LitLength, BYTE const *llCodeTable, seqDef const *sequences, size_t nbSeq, int longOffsets, int bmi2)

Variables
static unsigned const	kInverseProbabilityLog256 [256]

Function Documentation

ZSTD_buildCTable()

size_t ZSTD_buildCTable	(	void *	dst,
		size_t	dstCapacity,
		FSE_CTable *	nextCTable,
		U32	FSELog,
		symbolEncodingType_e	type,
		unsigned *	count,
		U32	max,
		const BYTE *	codeTable,
		size_t	nbSeq,
		const S16 *	defaultNorm,
		U32	defaultNormLog,
		U32	defaultMax,
		const FSE_CTable *	prevCTable,
		size_t	prevCTableSize,
		void *	entropyWorkspace,
		size_t	entropyWorkspaceSize
	)

Definition at line 223 of file zstd_compress_sequences.c.

{
    BYTE* op = (BYTE*)dst;
    const BYTE* const oend = op + dstCapacity;
    DEBUGLOG(6, "ZSTD_buildCTable (dstCapacity=%u)", (unsigned)dstCapacity);
 
    switch (type) {
    case set_rle:
        FORWARD_IF_ERROR(FSE_buildCTable_rle(nextCTable, (BYTE)max), "");
        RETURN_ERROR_IF(dstCapacity==0, dstSize_tooSmall, "not enough space");
        *op = codeTable[0];
        return 1;
    case set_repeat:
        memcpy(nextCTable, prevCTable, prevCTableSize);
        return 0;
    case set_basic:
        FORWARD_IF_ERROR(FSE_buildCTable_wksp(nextCTable, defaultNorm, defaultMax, defaultNormLog, entropyWorkspace, entropyWorkspaceSize), "");  /* note : could be pre-calculated */
        return 0;
    case set_compressed: {
        S16 norm[MaxSeq + 1];
        size_t nbSeq_1 = nbSeq;
        const U32 tableLog = FSE_optimalTableLog(FSELog, nbSeq, max);
        if (count[codeTable[nbSeq-1]] > 1) {
            count[codeTable[nbSeq-1]]--;
            nbSeq_1--;
        }
        assert(nbSeq_1 > 1);
        FORWARD_IF_ERROR(FSE_normalizeCount(norm, tableLog, count, nbSeq_1, max), "");
        {   size_t const NCountSize = FSE_writeNCount(op, oend - op, norm, max, tableLog);   /* overflow protected */
            FORWARD_IF_ERROR(NCountSize, "FSE_writeNCount failed");
            FORWARD_IF_ERROR(FSE_buildCTable_wksp(nextCTable, norm, max, tableLog, entropyWorkspace, entropyWorkspaceSize), "");
            return NCountSize;
        }
    }
    default: assert(0); RETURN_ERROR(GENERIC, "impossible to reach");
    }
}

Referenced by ZSTD_buildSuperBlockEntropy_sequences(), and ZSTD_compressSequences_internal().

ZSTD_crossEntropyCost()

size_t ZSTD_crossEntropyCost	(	short const *	norm,
		unsigned	accuracyLog,
		unsigned const *	count,
		unsigned const	max
	)

Returns the cost in bits of encoding the distribution in count using the table described by norm. The max symbol support by norm is assumed >= max. norm must be valid for every symbol with non-zero probability in count.

Definition at line 124 of file zstd_compress_sequences.c.

{
    unsigned const shift = 8 - accuracyLog;
    size_t cost = 0;
    unsigned s;
    assert(accuracyLog <= 8);
    for (s = 0; s <= max; ++s) {
        unsigned const normAcc = (norm[s] != -1) ? (unsigned)norm[s] : 1;
        unsigned const norm256 = normAcc << shift;
        assert(norm256 > 0);
        assert(norm256 < 256);
        cost += count[s] * kInverseProbabilityLog256[norm256];
    }
    return cost >> 8;
}

Referenced by ZSTD_estimateSubBlockSize_symbolType(), and ZSTD_selectEncodingType().

ZSTD_encodeSequences()

size_t ZSTD_encodeSequences	(	void *	dst,
		size_t	dstCapacity,
		FSE_CTable const *	CTable_MatchLength,
		BYTE const *	mlCodeTable,
		FSE_CTable const *	CTable_OffsetBits,
		BYTE const *	ofCodeTable,
		FSE_CTable const *	CTable_LitLength,
		BYTE const *	llCodeTable,
		seqDef const *	sequences,
		size_t	nbSeq,
		int	longOffsets,
		int	bmi2
	)

Definition at line 396 of file zstd_compress_sequences.c.

{
    DEBUGLOG(5, "ZSTD_encodeSequences: dstCapacity = %u", (unsigned)dstCapacity);
#if DYNAMIC_BMI2
    if (bmi2) {
        return ZSTD_encodeSequences_bmi2(dst, dstCapacity,
                                         CTable_MatchLength, mlCodeTable,
                                         CTable_OffsetBits, ofCodeTable,
                                         CTable_LitLength, llCodeTable,
                                         sequences, nbSeq, longOffsets);
    }
#endif
    (void)bmi2;
    return ZSTD_encodeSequences_default(dst, dstCapacity,
                                        CTable_MatchLength, mlCodeTable,
                                        CTable_OffsetBits, ofCodeTable,
                                        CTable_LitLength, llCodeTable,
                                        sequences, nbSeq, longOffsets);
}

Referenced by ZSTD_compressSequences_internal(), and ZSTD_compressSubBlock_sequences().

ZSTD_encodeSequences_body()

FORCE_INLINE_TEMPLATE size_t ZSTD_encodeSequences_body	(	void *	dst,
		size_t	dstCapacity,
		FSE_CTable const *	CTable_MatchLength,
		BYTE const *	mlCodeTable,
		FSE_CTable const *	CTable_OffsetBits,
		BYTE const *	ofCodeTable,
		FSE_CTable const *	CTable_LitLength,
		BYTE const *	llCodeTable,
		seqDef const *	sequences,
		size_t	nbSeq,
		int	longOffsets
	)

Definition at line 268 of file zstd_compress_sequences.c.

{
    BIT_CStream_t blockStream;
    FSE_CState_t  stateMatchLength;
    FSE_CState_t  stateOffsetBits;
    FSE_CState_t  stateLitLength;
 
    RETURN_ERROR_IF(
        ERR_isError(BIT_initCStream(&blockStream, dst, dstCapacity)),
        dstSize_tooSmall, "not enough space remaining");
    DEBUGLOG(6, "available space for bitstream : %i  (dstCapacity=%u)",
                (int)(blockStream.endPtr - blockStream.startPtr),
                (unsigned)dstCapacity);
 
    /* first symbols */
    FSE_initCState2(&stateMatchLength, CTable_MatchLength, mlCodeTable[nbSeq-1]);
    FSE_initCState2(&stateOffsetBits,  CTable_OffsetBits,  ofCodeTable[nbSeq-1]);
    FSE_initCState2(&stateLitLength,   CTable_LitLength,   llCodeTable[nbSeq-1]);
    BIT_addBits(&blockStream, sequences[nbSeq-1].litLength, LL_bits[llCodeTable[nbSeq-1]]);
    if (MEM_32bits()) BIT_flushBits(&blockStream);
    BIT_addBits(&blockStream, sequences[nbSeq-1].matchLength, ML_bits[mlCodeTable[nbSeq-1]]);
    if (MEM_32bits()) BIT_flushBits(&blockStream);
    if (longOffsets) {
        U32 const ofBits = ofCodeTable[nbSeq-1];
        unsigned const extraBits = ofBits - MIN(ofBits, STREAM_ACCUMULATOR_MIN-1);
        if (extraBits) {
            BIT_addBits(&blockStream, sequences[nbSeq-1].offset, extraBits);
            BIT_flushBits(&blockStream);
        }
        BIT_addBits(&blockStream, sequences[nbSeq-1].offset >> extraBits,
                    ofBits - extraBits);
    } else {
        BIT_addBits(&blockStream, sequences[nbSeq-1].offset, ofCodeTable[nbSeq-1]);
    }
    BIT_flushBits(&blockStream);
 
    {   size_t n;
        for (n=nbSeq-2 ; n<nbSeq ; n--) {      /* intentional underflow */
            BYTE const llCode = llCodeTable[n];
            BYTE const ofCode = ofCodeTable[n];
            BYTE const mlCode = mlCodeTable[n];
            U32  const llBits = LL_bits[llCode];
            U32  const ofBits = ofCode;
            U32  const mlBits = ML_bits[mlCode];
            DEBUGLOG(6, "encoding: litlen:%2u - matchlen:%2u - offCode:%7u",
                        (unsigned)sequences[n].litLength,
                        (unsigned)sequences[n].matchLength + MINMATCH,
                        (unsigned)sequences[n].offset);
                                                                            /* 32b*/  /* 64b*/
                                                                            /* (7)*/  /* (7)*/
            FSE_encodeSymbol(&blockStream, &stateOffsetBits, ofCode);       /* 15 */  /* 15 */
            FSE_encodeSymbol(&blockStream, &stateMatchLength, mlCode);      /* 24 */  /* 24 */
            if (MEM_32bits()) BIT_flushBits(&blockStream);                  /* (7)*/
            FSE_encodeSymbol(&blockStream, &stateLitLength, llCode);        /* 16 */  /* 33 */
            if (MEM_32bits() || (ofBits+mlBits+llBits >= 64-7-(LLFSELog+MLFSELog+OffFSELog)))
                BIT_flushBits(&blockStream);                                /* (7)*/
            BIT_addBits(&blockStream, sequences[n].litLength, llBits);
            if (MEM_32bits() && ((llBits+mlBits)>24)) BIT_flushBits(&blockStream);
            BIT_addBits(&blockStream, sequences[n].matchLength, mlBits);
            if (MEM_32bits() || (ofBits+mlBits+llBits > 56)) BIT_flushBits(&blockStream);
            if (longOffsets) {
                unsigned const extraBits = ofBits - MIN(ofBits, STREAM_ACCUMULATOR_MIN-1);
                if (extraBits) {
                    BIT_addBits(&blockStream, sequences[n].offset, extraBits);
                    BIT_flushBits(&blockStream);                            /* (7)*/
                }
                BIT_addBits(&blockStream, sequences[n].offset >> extraBits,
                            ofBits - extraBits);                            /* 31 */
            } else {
                BIT_addBits(&blockStream, sequences[n].offset, ofBits);     /* 31 */
            }
            BIT_flushBits(&blockStream);                                    /* (7)*/
            DEBUGLOG(7, "remaining space : %i", (int)(blockStream.endPtr - blockStream.ptr));
    }   }
 
    DEBUGLOG(6, "ZSTD_encodeSequences: flushing ML state with %u bits", stateMatchLength.stateLog);
    FSE_flushCState(&blockStream, &stateMatchLength);
    DEBUGLOG(6, "ZSTD_encodeSequences: flushing Off state with %u bits", stateOffsetBits.stateLog);
    FSE_flushCState(&blockStream, &stateOffsetBits);
    DEBUGLOG(6, "ZSTD_encodeSequences: flushing LL state with %u bits", stateLitLength.stateLog);
    FSE_flushCState(&blockStream, &stateLitLength);
 
    {   size_t const streamSize = BIT_closeCStream(&blockStream);
        RETURN_ERROR_IF(streamSize==0, dstSize_tooSmall, "not enough space");
        return streamSize;
    }
}

Referenced by ZSTD_encodeSequences_default().

ZSTD_encodeSequences_default()

static size_t ZSTD_encodeSequences_default ( void *  dst, size_t  dstCapacity, FSE_CTable const *  CTable_MatchLength, BYTE const *  mlCodeTable, FSE_CTable const *  CTable_OffsetBits, BYTE const *  ofCodeTable, FSE_CTable const *  CTable_LitLength, BYTE const *  llCodeTable, seqDef const *  sequences, size_t  nbSeq, int  longOffsets  )

static

Definition at line 362 of file zstd_compress_sequences.c.

{
    return ZSTD_encodeSequences_body(dst, dstCapacity,
                                    CTable_MatchLength, mlCodeTable,
                                    CTable_OffsetBits, ofCodeTable,
                                    CTable_LitLength, llCodeTable,
                                    sequences, nbSeq, longOffsets);
}

Referenced by ZSTD_encodeSequences().

ZSTD_entropyCost()

static size_t ZSTD_entropyCost ( unsigned const *  count, unsigned const  max, size_t const  total  )

static

Returns the cost in bits of encoding the distribution described by count using the entropy bound.

Definition at line 71 of file zstd_compress_sequences.c.

{
    unsigned cost = 0;
    unsigned s;
    for (s = 0; s <= max; ++s) {
        unsigned norm = (unsigned)((256 * count[s]) / total);
        if (count[s] != 0 && norm == 0)
            norm = 1;
        assert(count[s] < total);
        cost += count[s] * kInverseProbabilityLog256[norm];
    }
    return cost >> 8;
}

Referenced by ZSTD_selectEncodingType().

ZSTD_fseBitCost()

size_t ZSTD_fseBitCost	(	FSE_CTable const *	ctable,
		unsigned const *	count,
		unsigned const	max
	)

Returns the cost in bits of encoding the distribution in count using ctable. Returns an error if ctable cannot represent all the symbols in count.

Definition at line 89 of file zstd_compress_sequences.c.

{
    unsigned const kAccuracyLog = 8;
    size_t cost = 0;
    unsigned s;
    FSE_CState_t cstate;
    FSE_initCState(&cstate, ctable);
    if (ZSTD_getFSEMaxSymbolValue(ctable) < max) {
        DEBUGLOG(5, "Repeat FSE_CTable has maxSymbolValue %u < %u",
                    ZSTD_getFSEMaxSymbolValue(ctable), max);
        return ERROR(GENERIC);
    }
    for (s = 0; s <= max; ++s) {
        unsigned const tableLog = cstate.stateLog;
        unsigned const badCost = (tableLog + 1) << kAccuracyLog;
        unsigned const bitCost = FSE_bitCost(cstate.symbolTT, tableLog, s, kAccuracyLog);
        if (count[s] == 0)
            continue;
        if (bitCost >= badCost) {
            DEBUGLOG(5, "Repeat FSE_CTable has Prob[%u] == 0", s);
            return ERROR(GENERIC);
        }
        cost += (size_t)count[s] * bitCost;
    }
    return cost >> kAccuracyLog;
}

Referenced by ZSTD_estimateSubBlockSize_symbolType(), and ZSTD_selectEncodingType().

ZSTD_getFSEMaxSymbolValue()

static unsigned ZSTD_getFSEMaxSymbolValue ( FSE_CTable const *  ctable )

static

Definition at line 46 of file zstd_compress_sequences.c.

                                                                    {
  void const* ptr = ctable;
  U16 const* u16ptr = (U16 const*)ptr;
  U32 const maxSymbolValue = MEM_read16(u16ptr + 1);
  return maxSymbolValue;
}

Referenced by ZSTD_fseBitCost().

ZSTD_NCountCost()

static size_t ZSTD_NCountCost ( unsigned const *  count, unsigned const  max, size_t const  nbSeq, unsigned const  FSELog  )

static

Returns the cost in bytes of encoding the normalized count header. Returns an error if any of the helper functions return an error.

Definition at line 57 of file zstd_compress_sequences.c.

{
    BYTE wksp[FSE_NCOUNTBOUND];
    S16 norm[MaxSeq + 1];
    const U32 tableLog = FSE_optimalTableLog(FSELog, nbSeq, max);
    FORWARD_IF_ERROR(FSE_normalizeCount(norm, tableLog, count, nbSeq, max), "");
    return FSE_writeNCount(wksp, sizeof(wksp), norm, max, tableLog);
}

Referenced by ZSTD_selectEncodingType().

ZSTD_selectEncodingType()

symbolEncodingType_e ZSTD_selectEncodingType	(	FSE_repeat *	repeatMode,
		unsigned const *	count,
		unsigned const	max,
		size_t const	mostFrequent,
		size_t	nbSeq,
		unsigned const	FSELog,
		FSE_CTable const *	prevCTable,
		short const *	defaultNorm,
		U32	defaultNormLog,
		ZSTD_defaultPolicy_e const	isDefaultAllowed,
		ZSTD_strategy const	strategy
	)

Definition at line 142 of file zstd_compress_sequences.c.

{
    ZSTD_STATIC_ASSERT(ZSTD_defaultDisallowed == 0 && ZSTD_defaultAllowed != 0);
    if (mostFrequent == nbSeq) {
        *repeatMode = FSE_repeat_none;
        if (isDefaultAllowed && nbSeq <= 2) {
            /* Prefer set_basic over set_rle when there are 2 or less symbols,
             * since RLE uses 1 byte, but set_basic uses 5-6 bits per symbol.
             * If basic encoding isn't possible, always choose RLE.
             */
            DEBUGLOG(5, "Selected set_basic");
            return set_basic;
        }
        DEBUGLOG(5, "Selected set_rle");
        return set_rle;
    }
    if (strategy < ZSTD_lazy) {
        if (isDefaultAllowed) {
            size_t const staticFse_nbSeq_max = 1000;
            size_t const mult = 10 - strategy;
            size_t const baseLog = 3;
            size_t const dynamicFse_nbSeq_min = (((size_t)1 << defaultNormLog) * mult) >> baseLog;  /* 28-36 for offset, 56-72 for lengths */
            assert(defaultNormLog >= 5 && defaultNormLog <= 6);  /* xx_DEFAULTNORMLOG */
            assert(mult <= 9 && mult >= 7);
            if ( (*repeatMode == FSE_repeat_valid)
              && (nbSeq < staticFse_nbSeq_max) ) {
                DEBUGLOG(5, "Selected set_repeat");
                return set_repeat;
            }
            if ( (nbSeq < dynamicFse_nbSeq_min)
              || (mostFrequent < (nbSeq >> (defaultNormLog-1))) ) {
                DEBUGLOG(5, "Selected set_basic");
                /* The format allows default tables to be repeated, but it isn't useful.
                 * When using simple heuristics to select encoding type, we don't want
                 * to confuse these tables with dictionaries. When running more careful
                 * analysis, we don't need to waste time checking both repeating tables
                 * and default tables.
                 */
                *repeatMode = FSE_repeat_none;
                return set_basic;
            }
        }
    } else {
        size_t const basicCost = isDefaultAllowed ? ZSTD_crossEntropyCost(defaultNorm, defaultNormLog, count, max) : ERROR(GENERIC);
        size_t const repeatCost = *repeatMode != FSE_repeat_none ? ZSTD_fseBitCost(prevCTable, count, max) : ERROR(GENERIC);
        size_t const NCountCost = ZSTD_NCountCost(count, max, nbSeq, FSELog);
        size_t const compressedCost = (NCountCost << 3) + ZSTD_entropyCost(count, max, nbSeq);
 
        if (isDefaultAllowed) {
            assert(!ZSTD_isError(basicCost));
            assert(!(*repeatMode == FSE_repeat_valid && ZSTD_isError(repeatCost)));
        }
        assert(!ZSTD_isError(NCountCost));
        assert(compressedCost < ERROR(maxCode));
        DEBUGLOG(5, "Estimated bit costs: basic=%u\trepeat=%u\tcompressed=%u",
                    (unsigned)basicCost, (unsigned)repeatCost, (unsigned)compressedCost);
        if (basicCost <= repeatCost && basicCost <= compressedCost) {
            DEBUGLOG(5, "Selected set_basic");
            assert(isDefaultAllowed);
            *repeatMode = FSE_repeat_none;
            return set_basic;
        }
        if (repeatCost <= compressedCost) {
            DEBUGLOG(5, "Selected set_repeat");
            assert(!ZSTD_isError(repeatCost));
            return set_repeat;
        }
        assert(compressedCost < basicCost && compressedCost < repeatCost);
    }
    DEBUGLOG(5, "Selected set_compressed");
    *repeatMode = FSE_repeat_check;
    return set_compressed;
}

Referenced by ZSTD_buildSuperBlockEntropy_sequences(), and ZSTD_compressSequences_internal().

Variable Documentation

kInverseProbabilityLog256

unsigned const kInverseProbabilityLog256[256]

static

Initial value:

= {
    0,    2048, 1792, 1642, 1536, 1453, 1386, 1329, 1280, 1236, 1197, 1162,
    1130, 1100, 1073, 1047, 1024, 1001, 980,  960,  941,  923,  906,  889,
    874,  859,  844,  830,  817,  804,  791,  779,  768,  756,  745,  734,
    724,  714,  704,  694,  685,  676,  667,  658,  650,  642,  633,  626,
    618,  610,  603,  595,  588,  581,  574,  567,  561,  554,  548,  542,
    535,  529,  523,  517,  512,  506,  500,  495,  489,  484,  478,  473,
    468,  463,  458,  453,  448,  443,  438,  434,  429,  424,  420,  415,
    411,  407,  402,  398,  394,  390,  386,  382,  377,  373,  370,  366,
    362,  358,  354,  350,  347,  343,  339,  336,  332,  329,  325,  322,
    318,  315,  311,  308,  305,  302,  298,  295,  292,  289,  286,  282,
    279,  276,  273,  270,  267,  264,  261,  258,  256,  253,  250,  247,
    244,  241,  239,  236,  233,  230,  228,  225,  222,  220,  217,  215,
    212,  209,  207,  204,  202,  199,  197,  194,  192,  190,  187,  185,
    182,  180,  178,  175,  173,  171,  168,  166,  164,  162,  159,  157,
    155,  153,  151,  149,  146,  144,  142,  140,  138,  136,  134,  132,
    130,  128,  126,  123,  121,  119,  117,  115,  114,  112,  110,  108,
    106,  104,  102,  100,  98,   96,   94,   93,   91,   89,   87,   85,
    83,   82,   80,   78,   76,   74,   73,   71,   69,   67,   66,   64,
    62,   61,   59,   57,   55,   54,   52,   50,   49,   47,   46,   44,
    42,   41,   39,   37,   36,   34,   33,   31,   30,   28,   26,   25,
    23,   22,   20,   19,   17,   16,   14,   13,   11,   10,   8,    7,
    5,    4,    2,    1,
}

-log2(x / 256) lookup table for x in [0, 256). If x == 0: Return 0 Else: Return floor(-log2(x / 256) * 256)

Definition at line 21 of file zstd_compress_sequences.c.

Referenced by ZSTD_crossEntropyCost(), and ZSTD_entropyCost().