/* ******************************************************************

 * Huffman encoder, part of New Generation Entropy library

 * Copyright (c) Meta Platforms, Inc. and affiliates.

 *

 *  You can contact the author at :

 *  - FSE+HUF source repository : https://github.com/Cyan4973/FiniteStateEntropy

 *  - Public forum : https://groups.google.com/forum/#!forum/lz4c

 *

 * This source code is licensed under both the BSD-style license (found in the

 * LICENSE file in the root directory of this source tree) and the GPLv2 (found

 * in the COPYING file in the root directory of this source tree).

 * You may select, at your option, one of the above-listed licenses.

****************************************************************** */

/* **************************************************************

*  Compiler specifics

****************************************************************/

#ifdef _MSC_VER    /* Visual Studio */

#  pragma warning(disable : 4127)        /* disable: C4127: conditional expression is constant */

#endif

/* **************************************************************

*  Includes

****************************************************************/

#include "../common/zstd_deps.h"     /* ZSTD_memcpy, ZSTD_memset */

#include "../common/bmi2.h"

#include "../common/compiler.h"

#include "../common/bitstream.h"

#include "hist.h"

#define FSE_STATIC_LINKING_ONLY   /* FSE_optimalTableLog_internal */

#include "../common/fse.h"        /* header compression */

#include "../common/huf.h"

#include "../common/error_private.h"

#include "../common/bits.h"       /* ZSTD_highbit32 */

/* **************************************************************

*  Error Management

****************************************************************/

#define HUF_isError ERR_isError

#define HUF_STATIC_ASSERT(c) DEBUG_STATIC_ASSERT(c)   /* use only *after* variable declarations */

/* **************************************************************

*  Required declarations

****************************************************************/

typedef struct nodeElt_s {

    U32 count;

    U16 parent;

    BYTE byte;

    BYTE nbBits;

} nodeElt;

/* **************************************************************

*  Debug Traces

****************************************************************/

#if DEBUGLEVEL >= 2

static size_t showU32(const U32* arr, size_t size)

{

    size_t u;

    for (u=0; u<size; u++) {

        RAWLOG(6, " %u", arr[u]); (void)arr;

    }

    RAWLOG(6, " \n");

    return size;

}

static size_t HUF_getNbBits(HUF_CElt elt);

static size_t showCTableBits(const HUF_CElt* ctable, size_t size)

{

    size_t u;

    for (u=0; u<size; u++) {

        RAWLOG(6, " %zu", HUF_getNbBits(ctable[u])); (void)ctable;

    }

    RAWLOG(6, " \n");

    return size;

}

static size_t showHNodeSymbols(const nodeElt* hnode, size_t size)

{

    size_t u;

    for (u=0; u<size; u++) {

        RAWLOG(6, " %u", hnode[u].byte); (void)hnode;

    }

    RAWLOG(6, " \n");

    return size;

}

static size_t showHNodeBits(const nodeElt* hnode, size_t size)

{

    size_t u;

    for (u=0; u<size; u++) {

        RAWLOG(6, " %u", hnode[u].nbBits); (void)hnode;

    }

    RAWLOG(6, " \n");

    return size;

}

#endif

/* *******************************************************

*  HUF : Huffman block compression

*********************************************************/

#define HUF_WORKSPACE_MAX_ALIGNMENT 8

static void* HUF_alignUpWorkspace(void* workspace, size_t* workspaceSizePtr, size_t align)

{

    size_t const mask = align - 1;

    size_t const rem = (size_t)workspace & mask;

    size_t const add = (align - rem) & mask;

    BYTE* const aligned = (BYTE*)workspace + add;

    assert((align & (align - 1)) == 0); /* pow 2 */

    assert(align <= HUF_WORKSPACE_MAX_ALIGNMENT);

    if (*workspaceSizePtr >= add) {

        assert(add < align);

        assert(((size_t)aligned & mask) == 0);

        *workspaceSizePtr -= add;

        return aligned;

    } else {

        *workspaceSizePtr = 0;

        return NULL;

    }

}

/* HUF_compressWeights() :

 * Same as FSE_compress(), but dedicated to huff0's weights compression.

 * The use case needs much less stack memory.

 * Note : all elements within weightTable are supposed to be <= HUF_TABLELOG_MAX.

 */

#define MAX_FSE_TABLELOG_FOR_HUFF_HEADER 6

typedef struct {

    FSE_CTable CTable[FSE_CTABLE_SIZE_U32(MAX_FSE_TABLELOG_FOR_HUFF_HEADER, HUF_TABLELOG_MAX)];

    U32 scratchBuffer[FSE_BUILD_CTABLE_WORKSPACE_SIZE_U32(HUF_TABLELOG_MAX, MAX_FSE_TABLELOG_FOR_HUFF_HEADER)];

    unsigned count[HUF_TABLELOG_MAX+1];

    S16 norm[HUF_TABLELOG_MAX+1];

} HUF_CompressWeightsWksp;

static size_t

HUF_compressWeights(void* dst, size_t dstSize,

              const void* weightTable, size_t wtSize,

                    void* workspace, size_t workspaceSize)

{

    BYTE* const ostart = (BYTE*) dst;

    BYTE* op = ostart;

    BYTE* const oend = ostart + dstSize;

    unsigned maxSymbolValue = HUF_TABLELOG_MAX;

    U32 tableLog = MAX_FSE_TABLELOG_FOR_HUFF_HEADER;

    HUF_CompressWeightsWksp* wksp = (HUF_CompressWeightsWksp*)HUF_alignUpWorkspace(workspace, &workspaceSize, ZSTD_ALIGNOF(U32));

    if (workspaceSize < sizeof(HUF_CompressWeightsWksp)) return ERROR(GENERIC);

    /* init conditions */

    if (wtSize <= 1) return 0;  /* Not compressible */

    /* Scan input and build symbol stats */

    {   unsigned const maxCount = HIST_count_simple(wksp->count, &maxSymbolValue, weightTable, wtSize);   /* never fails */

        if (maxCount == wtSize) return 1;   /* only a single symbol in src : rle */

        if (maxCount == 1) return 0;        /* each symbol present maximum once => not compressible */

    }

    tableLog = FSE_optimalTableLog(tableLog, wtSize, maxSymbolValue);

    CHECK_F( FSE_normalizeCount(wksp->norm, tableLog, wksp->count, wtSize, maxSymbolValue, /* useLowProbCount */ 0) );

    /* Write table description header */

    {   CHECK_V_F(hSize, FSE_writeNCount(op, (size_t)(oend-op), wksp->norm, maxSymbolValue, tableLog) );

        op += hSize;

    }

    /* Compress */

    CHECK_F( FSE_buildCTable_wksp(wksp->CTable, wksp->norm, maxSymbolValue, tableLog, wksp->scratchBuffer, sizeof(wksp->scratchBuffer)) );

    {   CHECK_V_F(cSize, FSE_compress_usingCTable(op, (size_t)(oend - op), weightTable, wtSize, wksp->CTable) );

        if (cSize == 0) return 0;   /* not enough space for compressed data */

        op += cSize;

    }

    return (size_t)(op-ostart);

}

static size_t HUF_getNbBits(HUF_CElt elt)

{

    return elt & 0xFF;

}

static size_t HUF_getNbBitsFast(HUF_CElt elt)

{

    return elt;

}

static size_t HUF_getValue(HUF_CElt elt)

{

    return elt & ~(size_t)0xFF;

}

static size_t HUF_getValueFast(HUF_CElt elt)

{

    return elt;

}

static void HUF_setNbBits(HUF_CElt* elt, size_t nbBits)

{

    assert(nbBits <= HUF_TABLELOG_ABSOLUTEMAX);

    *elt = nbBits;

}

static void HUF_setValue(HUF_CElt* elt, size_t value)

{

    size_t const nbBits = HUF_getNbBits(*elt);

    if (nbBits > 0) {

        assert((value >> nbBits) == 0);

        *elt |= value << (sizeof(HUF_CElt) * 8 - nbBits);

    }

}

HUF_CTableHeader HUF_readCTableHeader(HUF_CElt const* ctable)

{

    HUF_CTableHeader header;

    ZSTD_memcpy(&header, ctable, sizeof(header));

    return header;

}

static void HUF_writeCTableHeader(HUF_CElt* ctable, U32 tableLog, U32 maxSymbolValue)

{

    HUF_CTableHeader header;

    HUF_STATIC_ASSERT(sizeof(ctable[0]) == sizeof(header));

    ZSTD_memset(&header, 0, sizeof(header));

    assert(tableLog < 256);

    header.tableLog = (BYTE)tableLog;

    assert(maxSymbolValue < 256);

    header.maxSymbolValue = (BYTE)maxSymbolValue;

    ZSTD_memcpy(ctable, &header, sizeof(header));

}

typedef struct {

    HUF_CompressWeightsWksp wksp;

    BYTE bitsToWeight[HUF_TABLELOG_MAX + 1];   /* precomputed conversion table */

    BYTE huffWeight[HUF_SYMBOLVALUE_MAX];

} HUF_WriteCTableWksp;

size_t HUF_writeCTable_wksp(void* dst, size_t maxDstSize,

                            const HUF_CElt* CTable, unsigned maxSymbolValue, unsigned huffLog,

                            void* workspace, size_t workspaceSize)

{

    HUF_CElt const* const ct = CTable + 1;

    BYTE* op = (BYTE*)dst;

    U32 n;

    HUF_WriteCTableWksp* wksp = (HUF_WriteCTableWksp*)HUF_alignUpWorkspace(workspace, &workspaceSize, ZSTD_ALIGNOF(U32));

    HUF_STATIC_ASSERT(HUF_CTABLE_WORKSPACE_SIZE >= sizeof(HUF_WriteCTableWksp));

    assert(HUF_readCTableHeader(CTable).maxSymbolValue == maxSymbolValue);

    assert(HUF_readCTableHeader(CTable).tableLog == huffLog);

    /* check conditions */

    if (workspaceSize < sizeof(HUF_WriteCTableWksp)) return ERROR(GENERIC);

    if (maxSymbolValue > HUF_SYMBOLVALUE_MAX) return ERROR(maxSymbolValue_tooLarge);

    /* convert to weight */

    wksp->bitsToWeight[0] = 0;

    for (n=1; n<huffLog+1; n++)

        wksp->bitsToWeight[n] = (BYTE)(huffLog + 1 - n);

    for (n=0; n<maxSymbolValue; n++)

        wksp->huffWeight[n] = wksp->bitsToWeight[HUF_getNbBits(ct[n])];

    /* attempt weights compression by FSE */

    if (maxDstSize < 1) return ERROR(dstSize_tooSmall);

    {   CHECK_V_F(hSize, HUF_compressWeights(op+1, maxDstSize-1, wksp->huffWeight, maxSymbolValue, &wksp->wksp, sizeof(wksp->wksp)) );

        if ((hSize>1) & (hSize < maxSymbolValue/2)) {   /* FSE compressed */

            op[0] = (BYTE)hSize;

            return hSize+1;

    }   }

    /* write raw values as 4-bits (max : 15) */

    if (maxSymbolValue > (256-128)) return ERROR(GENERIC);   /* should not happen : likely means source cannot be compressed */

    if (((maxSymbolValue+1)/2) + 1 > maxDstSize) return ERROR(dstSize_tooSmall);   /* not enough space within dst buffer */

    op[0] = (BYTE)(128 /*special case*/ + (maxSymbolValue-1));

    wksp->huffWeight[maxSymbolValue] = 0;   /* to be sure it doesn't cause msan issue in final combination */

    for (n=0; n<maxSymbolValue; n+=2)

        op[(n/2)+1] = (BYTE)((wksp->huffWeight[n] << 4) + wksp->huffWeight[n+1]);

    return ((maxSymbolValue+1)/2) + 1;

}

size_t HUF_readCTable (HUF_CElt* CTable, unsigned* maxSymbolValuePtr, const void* src, size_t srcSize, unsigned* hasZeroWeights)

{

    BYTE huffWeight[HUF_SYMBOLVALUE_MAX + 1];   /* init not required, even though some static analyzer may complain */

    U32 rankVal[HUF_TABLELOG_ABSOLUTEMAX + 1];   /* large enough for values from 0 to 16 */

    U32 tableLog = 0;

    U32 nbSymbols = 0;

    HUF_CElt* const ct = CTable + 1;

    /* get symbol weights */

    CHECK_V_F(readSize, HUF_readStats(huffWeight, HUF_SYMBOLVALUE_MAX+1, rankVal, &nbSymbols, &tableLog, src, srcSize));

    *hasZeroWeights = (rankVal[0] > 0);

    /* check result */

    if (tableLog > HUF_TABLELOG_MAX) return ERROR(tableLog_tooLarge);

    if (nbSymbols > *maxSymbolValuePtr+1) return ERROR(maxSymbolValue_tooSmall);

    *maxSymbolValuePtr = nbSymbols - 1;

    HUF_writeCTableHeader(CTable, tableLog, *maxSymbolValuePtr);

    /* Prepare base value per rank */

    {   U32 n, nextRankStart = 0;

        for (n=1; n<=tableLog; n++) {

            U32 curr = nextRankStart;

            nextRankStart += (rankVal[n] << (n-1));

            rankVal[n] = curr;

    }   }

    /* fill nbBits */

    {   U32 n; for (n=0; n<nbSymbols; n++) {

            const U32 w = huffWeight[n];

            HUF_setNbBits(ct + n, (BYTE)(tableLog + 1 - w) & -(w != 0));

    }   }

    /* fill val */

    {   U16 nbPerRank[HUF_TABLELOG_MAX+2]  = {0};  /* support w=0=>n=tableLog+1 */

        U16 valPerRank[HUF_TABLELOG_MAX+2] = {0};

        { U32 n; for (n=0; n<nbSymbols; n++) nbPerRank[HUF_getNbBits(ct[n])]++; }

        /* determine stating value per rank */

        valPerRank[tableLog+1] = 0;   /* for w==0 */

        {   U16 min = 0;

            U32 n; for (n=tableLog; n>0; n--) {  /* start at n=tablelog <-> w=1 */

                valPerRank[n] = min;     /* get starting value within each rank */

                min += nbPerRank[n];

                min >>= 1;

        }   }

        /* assign value within rank, symbol order */

        { U32 n; for (n=0; n<nbSymbols; n++) HUF_setValue(ct + n, valPerRank[HUF_getNbBits(ct[n])]++); }

    }

    return readSize;

}

U32 HUF_getNbBitsFromCTable(HUF_CElt const* CTable, U32 symbolValue)

{

    const HUF_CElt* const ct = CTable + 1;

    assert(symbolValue <= HUF_SYMBOLVALUE_MAX);

    if (symbolValue > HUF_readCTableHeader(CTable).maxSymbolValue)

        return 0;

    return (U32)HUF_getNbBits(ct[symbolValue]);

}

/**

 * HUF_setMaxHeight():

 * Try to enforce @targetNbBits on the Huffman tree described in @huffNode.

 *

 * It attempts to convert all nodes with nbBits > @targetNbBits

 * to employ @targetNbBits instead. Then it adjusts the tree

 * so that it remains a valid canonical Huffman tree.

 *

 * @pre               The sum of the ranks of each symbol == 2^largestBits,

 *                    where largestBits == huffNode[lastNonNull].nbBits.

 * @post              The sum of the ranks of each symbol == 2^largestBits,

 *                    where largestBits is the return value (expected <= targetNbBits).

 *

 * @param huffNode    The Huffman tree modified in place to enforce targetNbBits.

 *                    It's presumed sorted, from most frequent to rarest symbol.

 * @param lastNonNull The symbol with the lowest count in the Huffman tree.

 * @param targetNbBits  The allowed number of bits, which the Huffman tree

 *                    may not respect. After this function the Huffman tree will

 *                    respect targetNbBits.

 * @return            The maximum number of bits of the Huffman tree after adjustment.

 */

static U32 HUF_setMaxHeight(nodeElt* huffNode, U32 lastNonNull, U32 targetNbBits)

{

    const U32 largestBits = huffNode[lastNonNull].nbBits;

    /* early exit : no elt > targetNbBits, so the tree is already valid. */

    if (largestBits <= targetNbBits) return largestBits;

    DEBUGLOG(5, "HUF_setMaxHeight (targetNbBits = %u)", targetNbBits);

    /* there are several too large elements (at least >= 2) */

    {   int totalCost = 0;

        const U32 baseCost = 1 << (largestBits - targetNbBits);

        int n = (int)lastNonNull;

        /* Adjust any ranks > targetNbBits to targetNbBits.

         * Compute totalCost, which is how far the sum of the ranks is

         * we are over 2^largestBits after adjust the offending ranks.

         */

        while (huffNode[n].nbBits > targetNbBits) {

            totalCost += baseCost - (1 << (largestBits - huffNode[n].nbBits));

            huffNode[n].nbBits = (BYTE)targetNbBits;

            n--;

        }

        /* n stops at huffNode[n].nbBits <= targetNbBits */

        assert(huffNode[n].nbBits <= targetNbBits);

        /* n end at index of smallest symbol using < targetNbBits */

        while (huffNode[n].nbBits == targetNbBits) --n;

        /* renorm totalCost from 2^largestBits to 2^targetNbBits

         * note : totalCost is necessarily a multiple of baseCost */

        assert(((U32)totalCost & (baseCost - 1)) == 0);

        totalCost >>= (largestBits - targetNbBits);

        assert(totalCost > 0);

        /* repay normalized cost */

        {   U32 const noSymbol = 0xF0F0F0F0;

            U32 rankLast[HUF_TABLELOG_MAX+2];

            /* Get pos of last (smallest = lowest cum. count) symbol per rank */

            ZSTD_memset(rankLast, 0xF0, sizeof(rankLast));

            {   U32 currentNbBits = targetNbBits;

                int pos;

                for (pos=n ; pos >= 0; pos--) {

                    if (huffNode[pos].nbBits >= currentNbBits) continue;

                    currentNbBits = huffNode[pos].nbBits;   /* < targetNbBits */

                    rankLast[targetNbBits-currentNbBits] = (U32)pos;

            }   }

            while (totalCost > 0) {

                /* Try to reduce the next power of 2 above totalCost because we

                 * gain back half the rank.

                 */

                U32 nBitsToDecrease = ZSTD_highbit32((U32)totalCost) + 1;

                assert(nBitsToDecrease <= HUF_TABLELOG_MAX+1);

                for ( ; nBitsToDecrease > 1; nBitsToDecrease--) {

                    U32 const highPos = rankLast[nBitsToDecrease];

                    U32 const lowPos = rankLast[nBitsToDecrease-1];

                    if (highPos == noSymbol) continue;

                    /* Decrease highPos if no symbols of lowPos or if it is

                     * not cheaper to remove 2 lowPos than highPos.

                     */

                    if (lowPos == noSymbol) break;

                    {   U32 const highTotal = huffNode[highPos].count;

                        U32 const lowTotal = 2 * huffNode[lowPos].count;

                        if (highTotal <= lowTotal) break;

                }   }

                /* only triggered when no more rank 1 symbol left => find closest one (note : there is necessarily at least one !) */

                assert(rankLast[nBitsToDecrease] != noSymbol || nBitsToDecrease == 1);

                /* HUF_MAX_TABLELOG test just to please gcc 5+; but it should not be necessary */

                while ((nBitsToDecrease<=HUF_TABLELOG_MAX) && (rankLast[nBitsToDecrease] == noSymbol))

                    nBitsToDecrease++;

                assert(rankLast[nBitsToDecrease] != noSymbol);

                /* Increase the number of bits to gain back half the rank cost. */

                totalCost -= 1 << (nBitsToDecrease-1);

                huffNode[rankLast[nBitsToDecrease]].nbBits++;

                /* Fix up the new rank.

                 * If the new rank was empty, this symbol is now its smallest.

                 * Otherwise, this symbol will be the largest in the new rank so no adjustment.

                 */

                if (rankLast[nBitsToDecrease-1] == noSymbol)

                    rankLast[nBitsToDecrease-1] = rankLast[nBitsToDecrease];

                /* Fix up the old rank.

                 * If the symbol was at position 0, meaning it was the highest weight symbol in the tree,

                 * it must be the only symbol in its rank, so the old rank now has no symbols.

                 * Otherwise, since the Huffman nodes are sorted by count, the previous position is now

                 * the smallest node in the rank. If the previous position belongs to a different rank,

                 * then the rank is now empty.

                 */

                if (rankLast[nBitsToDecrease] == 0)    /* special case, reached largest symbol */

                    rankLast[nBitsToDecrease] = noSymbol;

                else {

                    rankLast[nBitsToDecrease]--;

                    if (huffNode[rankLast[nBitsToDecrease]].nbBits != targetNbBits-nBitsToDecrease)

                        rankLast[nBitsToDecrease] = noSymbol;   /* this rank is now empty */

                }

            }   /* while (totalCost > 0) */

            /* If we've removed too much weight, then we have to add it back.

             * To avoid overshooting again, we only adjust the smallest rank.

             * We take the largest nodes from the lowest rank 0 and move them

             * to rank 1. There's guaranteed to be enough rank 0 symbols because

             * TODO.

             */

            while (totalCost < 0) {  /* Sometimes, cost correction overshoot */

                /* special case : no rank 1 symbol (using targetNbBits-1);

                 * let's create one from largest rank 0 (using targetNbBits).

                 */

                if (rankLast[1] == noSymbol) {

                    while (huffNode[n].nbBits == targetNbBits) n--;

                    huffNode[n+1].nbBits--;

                    assert(n >= 0);

                    rankLast[1] = (U32)(n+1);

                    totalCost++;

                    continue;

                }

                huffNode[ rankLast[1] + 1 ].nbBits--;

                rankLast[1]++;

                totalCost ++;

            }

        }   /* repay normalized cost */

    }   /* there are several too large elements (at least >= 2) */

    return targetNbBits;

}

typedef struct {

    U16 base;

    U16 curr;

} rankPos;

typedef nodeElt huffNodeTable[2 * (HUF_SYMBOLVALUE_MAX + 1)];

/* Number of buckets available for HUF_sort() */

#define RANK_POSITION_TABLE_SIZE 192

typedef struct {

  huffNodeTable huffNodeTbl;

  rankPos rankPosition[RANK_POSITION_TABLE_SIZE];

} HUF_buildCTable_wksp_tables;

/* RANK_POSITION_DISTINCT_COUNT_CUTOFF == Cutoff point in HUF_sort() buckets for which we use log2 bucketing.

 * Strategy is to use as many buckets as possible for representing distinct

 * counts while using the remainder to represent all "large" counts.

 *

 * To satisfy this requirement for 192 buckets, we can do the following:

 * Let buckets 0-166 represent distinct counts of [0, 166]

 * Let buckets 166 to 192 represent all remaining counts up to RANK_POSITION_MAX_COUNT_LOG using log2 bucketing.

 */

#define RANK_POSITION_MAX_COUNT_LOG 32

#define RANK_POSITION_LOG_BUCKETS_BEGIN ((RANK_POSITION_TABLE_SIZE - 1) - RANK_POSITION_MAX_COUNT_LOG - 1 /* == 158 */)

#define RANK_POSITION_DISTINCT_COUNT_CUTOFF (RANK_POSITION_LOG_BUCKETS_BEGIN + ZSTD_highbit32(RANK_POSITION_LOG_BUCKETS_BEGIN) /* == 166 */)

/* Return the appropriate bucket index for a given count. See definition of

 * RANK_POSITION_DISTINCT_COUNT_CUTOFF for explanation of bucketing strategy.

 */

static U32 HUF_getIndex(U32 const count) {

    return (count < RANK_POSITION_DISTINCT_COUNT_CUTOFF)

        ? count

        : ZSTD_highbit32(count) + RANK_POSITION_LOG_BUCKETS_BEGIN;

}

/* Helper swap function for HUF_quickSortPartition() */

static void HUF_swapNodes(nodeElt* a, nodeElt* b) {

  nodeElt tmp = *a;

  *a = *b;

  *b = tmp;

}

/* Returns 0 if the huffNode array is not sorted by descending count */

MEM_STATIC int HUF_isSorted(nodeElt huffNode[], U32 const maxSymbolValue1) {

    U32 i;

    for (i = 1; i < maxSymbolValue1; ++i) {

        if (huffNode[i].count > huffNode[i-1].count) {

            return 0;

        }

    }

    return 1;

}

/* Insertion sort by descending order */

HINT_INLINE void HUF_insertionSort(nodeElt huffNode[], int const low, int const high) {

    int i;

    int const size = high-low+1;

    huffNode += low;

    for (i = 1; i < size; ++i) {

        nodeElt const key = huffNode[i];

        int j = i - 1;

        while (j >= 0 && huffNode[j].count < key.count) {

            huffNode[j + 1] = huffNode[j];

            j--;

        }

        huffNode[j + 1] = key;

    }

}

/* Pivot helper function for quicksort. */

static int HUF_quickSortPartition(nodeElt arr[], int const low, int const high) {

    /* Simply select rightmost element as pivot. "Better" selectors like

     * median-of-three don't experimentally appear to have any benefit.

     */

    U32 const pivot = arr[high].count;

    int i = low - 1;

    int j = low;

    for ( ; j < high; j++) {

        if (arr[j].count > pivot) {

            i++;

            HUF_swapNodes(&arr[i], &arr[j]);

        }

    }

    HUF_swapNodes(&arr[i + 1], &arr[high]);

    return i + 1;

}

/* Classic quicksort by descending with partially iterative calls

 * to reduce worst case callstack size.

 */

static void HUF_simpleQuickSort(nodeElt arr[], int low, int high) {

    int const kInsertionSortThreshold = 8;

    if (high - low < kInsertionSortThreshold) {

        HUF_insertionSort(arr, low, high);

        return;

    }

    while (low < high) {

        int const idx = HUF_quickSortPartition(arr, low, high);

        if (idx - low < high - idx) {

            HUF_simpleQuickSort(arr, low, idx - 1);

            low = idx + 1;

        } else {

            HUF_simpleQuickSort(arr, idx + 1, high);

            high = idx - 1;

        }

    }

}

/**

 * HUF_sort():

 * Sorts the symbols [0, maxSymbolValue] by count[symbol] in decreasing order.

 * This is a typical bucket sorting strategy that uses either quicksort or insertion sort to sort each bucket.

 *

 * @param[out] huffNode       Sorted symbols by decreasing count. Only members `.count` and `.byte` are filled.

 *                            Must have (maxSymbolValue + 1) entries.

 * @param[in]  count          Histogram of the symbols.

 * @param[in]  maxSymbolValue Maximum symbol value.

 * @param      rankPosition   This is a scratch workspace. Must have RANK_POSITION_TABLE_SIZE entries.

 */

static void HUF_sort(nodeElt huffNode[], const unsigned count[], U32 const maxSymbolValue, rankPos rankPosition[]) {

    U32 n;

    U32 const maxSymbolValue1 = maxSymbolValue+1;

    /* Compute base and set curr to base.

     * For symbol s let lowerRank = HUF_getIndex(count[n]) and rank = lowerRank + 1.

     * See HUF_getIndex to see bucketing strategy.

     * We attribute each symbol to lowerRank's base value, because we want to know where

     * each rank begins in the output, so for rank R we want to count ranks R+1 and above.

     */

    ZSTD_memset(rankPosition, 0, sizeof(*rankPosition) * RANK_POSITION_TABLE_SIZE);

    for (n = 0; n < maxSymbolValue1; ++n) {

        U32 lowerRank = HUF_getIndex(count[n]);

        assert(lowerRank < RANK_POSITION_TABLE_SIZE - 1);

        rankPosition[lowerRank].base++;

    }

    assert(rankPosition[RANK_POSITION_TABLE_SIZE - 1].base == 0);

    /* Set up the rankPosition table */

    for (n = RANK_POSITION_TABLE_SIZE - 1; n > 0; --n) {

        rankPosition[n-1].base += rankPosition[n].base;

        rankPosition[n-1].curr = rankPosition[n-1].base;

    }

    /* Insert each symbol into their appropriate bucket, setting up rankPosition table. */

    for (n = 0; n < maxSymbolValue1; ++n) {

        U32 const c = count[n];

        U32 const r = HUF_getIndex(c) + 1;

        U32 const pos = rankPosition[r].curr++;

        assert(pos < maxSymbolValue1);

        huffNode[pos].count = c;

        huffNode[pos].byte  = (BYTE)n;

    }

    /* Sort each bucket. */

    for (n = RANK_POSITION_DISTINCT_COUNT_CUTOFF; n < RANK_POSITION_TABLE_SIZE - 1; ++n) {

        int const bucketSize = rankPosition[n].curr - rankPosition[n].base;

        U32 const bucketStartIdx = rankPosition[n].base;

        if (bucketSize > 1) {

            assert(bucketStartIdx < maxSymbolValue1);

            HUF_simpleQuickSort(huffNode + bucketStartIdx, 0, bucketSize-1);

        }

    }

    assert(HUF_isSorted(huffNode, maxSymbolValue1));

}

/** HUF_buildCTable_wksp() :

 *  Same as HUF_buildCTable(), but using externally allocated scratch buffer.

 *  `workSpace` must be aligned on 4-bytes boundaries, and be at least as large as sizeof(HUF_buildCTable_wksp_tables).

 */

#define STARTNODE (HUF_SYMBOLVALUE_MAX+1)

/* HUF_buildTree():

 * Takes the huffNode array sorted by HUF_sort() and builds an unlimited-depth Huffman tree.

 *

 * @param huffNode        The array sorted by HUF_sort(). Builds the Huffman tree in this array.

 * @param maxSymbolValue  The maximum symbol value.

 * @return                The smallest node in the Huffman tree (by count).

 */

static int HUF_buildTree(nodeElt* huffNode, U32 maxSymbolValue)

{

    nodeElt* const huffNode0 = huffNode - 1;

    int nonNullRank;

    int lowS, lowN;

    int nodeNb = STARTNODE;

    int n, nodeRoot;

    DEBUGLOG(5, "HUF_buildTree (alphabet size = %u)", maxSymbolValue + 1);

    /* init for parents */

    nonNullRank = (int)maxSymbolValue;

    while(huffNode[nonNullRank].count == 0) nonNullRank--;

    lowS = nonNullRank; nodeRoot = nodeNb + lowS - 1; lowN = nodeNb;

    huffNode[nodeNb].count = huffNode[lowS].count + huffNode[lowS-1].count;

    huffNode[lowS].parent = huffNode[lowS-1].parent = (U16)nodeNb;

    nodeNb++; lowS-=2;

    for (n=nodeNb; n<=nodeRoot; n++) huffNode[n].count = (U32)(1U<<30);

    huffNode0[0].count = (U32)(1U<<31);  /* fake entry, strong barrier */

    /* create parents */

    while (nodeNb <= nodeRoot) {

        int const n1 = (huffNode[lowS].count < huffNode[lowN].count) ? lowS-- : lowN++;

        int const n2 = (huffNode[lowS].count < huffNode[lowN].count) ? lowS-- : lowN++;

        huffNode[nodeNb].count = huffNode[n1].count + huffNode[n2].count;

        huffNode[n1].parent = huffNode[n2].parent = (U16)nodeNb;

        nodeNb++;

    }

    /* distribute weights (unlimited tree height) */

    huffNode[nodeRoot].nbBits = 0;

    for (n=nodeRoot-1; n>=STARTNODE; n--)

        huffNode[n].nbBits = huffNode[ huffNode[n].parent ].nbBits + 1;

    for (n=0; n<=nonNullRank; n++)

        huffNode[n].nbBits = huffNode[ huffNode[n].parent ].nbBits + 1;

    DEBUGLOG(6, "Initial distribution of bits completed (%zu sorted symbols)", showHNodeBits(huffNode, maxSymbolValue+1));

    return nonNullRank;

}

/**

 * HUF_buildCTableFromTree():

 * Build the CTable given the Huffman tree in huffNode.

 *

 * @param[out] CTable         The output Huffman CTable.

 * @param      huffNode       The Huffman tree.

 * @param      nonNullRank    The last and smallest node in the Huffman tree.

 * @param      maxSymbolValue The maximum symbol value.

 * @param      maxNbBits      The exact maximum number of bits used in the Huffman tree.

 */

static void HUF_buildCTableFromTree(HUF_CElt* CTable, nodeElt const* huffNode, int nonNullRank, U32 maxSymbolValue, U32 maxNbBits)

{

    HUF_CElt* const ct = CTable + 1;

    /* fill result into ctable (val, nbBits) */

    int n;

    U16 nbPerRank[HUF_TABLELOG_MAX+1] = {0};

    U16 valPerRank[HUF_TABLELOG_MAX+1] = {0};

    int const alphabetSize = (int)(maxSymbolValue + 1);

    for (n=0; n<=nonNullRank; n++)

        nbPerRank[huffNode[n].nbBits]++;

    /* determine starting value per rank */

    {   U16 min = 0;

        for (n=(int)maxNbBits; n>0; n--) {

            valPerRank[n] = min;      /* get starting value within each rank */

            min += nbPerRank[n];

            min >>= 1;

    }   }

    for (n=0; n<alphabetSize; n++)

        HUF_setNbBits(ct + huffNode[n].byte, huffNode[n].nbBits);   /* push nbBits per symbol, symbol order */

    for (n=0; n<alphabetSize; n++)

        HUF_setValue(ct + n, valPerRank[HUF_getNbBits(ct[n])]++);   /* assign value within rank, symbol order */

    HUF_writeCTableHeader(CTable, maxNbBits, maxSymbolValue);

}

size_t

HUF_buildCTable_wksp(HUF_CElt* CTable, const unsigned* count, U32 maxSymbolValue, U32 maxNbBits,

                     void* workSpace, size_t wkspSize)

{

    HUF_buildCTable_wksp_tables* const wksp_tables =

        (HUF_buildCTable_wksp_tables*)HUF_alignUpWorkspace(workSpace, &wkspSize, ZSTD_ALIGNOF(U32));

    nodeElt* const huffNode0 = wksp_tables->huffNodeTbl;

    nodeElt* const huffNode = huffNode0+1;

    int nonNullRank;

    HUF_STATIC_ASSERT(HUF_CTABLE_WORKSPACE_SIZE == sizeof(HUF_buildCTable_wksp_tables));

    DEBUGLOG(5, "HUF_buildCTable_wksp (alphabet size = %u)", maxSymbolValue+1);

    /* safety checks */

    if (wkspSize < sizeof(HUF_buildCTable_wksp_tables))

        return ERROR(workSpace_tooSmall);

    if (maxNbBits == 0) maxNbBits = HUF_TABLELOG_DEFAULT;

    if (maxSymbolValue > HUF_SYMBOLVALUE_MAX)

        return ERROR(maxSymbolValue_tooLarge);

    ZSTD_memset(huffNode0, 0, sizeof(huffNodeTable));

    /* sort, decreasing order */

    HUF_sort(huffNode, count, maxSymbolValue, wksp_tables->rankPosition);

    DEBUGLOG(6, "sorted symbols completed (%zu symbols)", showHNodeSymbols(huffNode, maxSymbolValue+1));

    /* build tree */

    nonNullRank = HUF_buildTree(huffNode, maxSymbolValue);

    /* determine and enforce maxTableLog */

    maxNbBits = HUF_setMaxHeight(huffNode, (U32)nonNullRank, maxNbBits);

    if (maxNbBits > HUF_TABLELOG_MAX) return ERROR(GENERIC);   /* check fit into table */

    HUF_buildCTableFromTree(CTable, huffNode, nonNullRank, maxSymbolValue, maxNbBits);

    return maxNbBits;

}

size_t HUF_estimateCompressedSize(const HUF_CElt* CTable, const unsigned* count, unsigned maxSymbolValue)

{

    HUF_CElt const* ct = CTable + 1;

    size_t nbBits = 0;

    int s;

    for (s = 0; s <= (int)maxSymbolValue; ++s) {

        nbBits += HUF_getNbBits(ct[s]) * count[s];

    }

    return nbBits >> 3;

}

int HUF_validateCTable(const HUF_CElt* CTable, const unsigned* count, unsigned maxSymbolValue) {

    HUF_CTableHeader header = HUF_readCTableHeader(CTable);

    HUF_CElt const* ct = CTable + 1;

    int bad = 0;

    int s;

    assert(header.tableLog <= HUF_TABLELOG_ABSOLUTEMAX);

    if (header.maxSymbolValue < maxSymbolValue)

        return 0;

    for (s = 0; s <= (int)maxSymbolValue; ++s) {

        bad |= (count[s] != 0) & (HUF_getNbBits(ct[s]) == 0);

    }

    return !bad;

}

size_t HUF_compressBound(size_t size) { return HUF_COMPRESSBOUND(size); }

/** HUF_CStream_t:

 * Huffman uses its own BIT_CStream_t implementation.

 * There are three major differences from BIT_CStream_t:

 *   1. HUF_addBits() takes a HUF_CElt (size_t) which is

 *      the pair (nbBits, value) in the format:

 *      format:

 *        - Bits [0, 4)            = nbBits

 *        - Bits [4, 64 - nbBits)  = 0

 *        - Bits [64 - nbBits, 64) = value

 *   2. The bitContainer is built from the upper bits and

 *      right shifted. E.g. to add a new value of N bits

 *      you right shift the bitContainer by N, then or in

 *      the new value into the N upper bits.

 *   3. The bitstream has two bit containers. You can add

 *      bits to the second container and merge them into

 *      the first container.

 */

#define HUF_BITS_IN_CONTAINER (sizeof(size_t) * 8)

typedef struct {

    size_t bitContainer[2];

    size_t bitPos[2];

    BYTE* startPtr;

    BYTE* ptr;

    BYTE* endPtr;

} HUF_CStream_t;

/**! HUF_initCStream():

 * Initializes the bitstream.

 * @returns 0 or an error code.

 */

static size_t HUF_initCStream(HUF_CStream_t* bitC,

                                  void* startPtr, size_t dstCapacity)

{

    ZSTD_memset(bitC, 0, sizeof(*bitC));

    bitC->startPtr = (BYTE*)startPtr;

    bitC->ptr = bitC->startPtr;

    bitC->endPtr = bitC->startPtr + dstCapacity - sizeof(bitC->bitContainer[0]);

    if (dstCapacity <= sizeof(bitC->bitContainer[0])) return ERROR(dstSize_tooSmall);

    return 0;

}

/*! HUF_addBits():

 * Adds the symbol stored in HUF_CElt elt to the bitstream.

 *

 * @param elt   The element we're adding. This is a (nbBits, value) pair.

 *              See the HUF_CStream_t docs for the format.

 * @param idx   Insert into the bitstream at this idx.

 * @param kFast This is a template parameter. If the bitstream is guaranteed

 *              to have at least 4 unused bits after this call it may be 1,

 *              otherwise it must be 0. HUF_addBits() is faster when fast is set.

 */

FORCE_INLINE_TEMPLATE void HUF_addBits(HUF_CStream_t* bitC, HUF_CElt elt, int idx, int kFast)

{

    assert(idx <= 1);

    assert(HUF_getNbBits(elt) <= HUF_TABLELOG_ABSOLUTEMAX);

    /* This is efficient on x86-64 with BMI2 because shrx

     * only reads the low 6 bits of the register. The compiler

     * knows this and elides the mask. When fast is set,

     * every operation can use the same value loaded from elt.

     */

    bitC->bitContainer[idx] >>= HUF_getNbBits(elt);

    bitC->bitContainer[idx] |= kFast ? HUF_getValueFast(elt) : HUF_getValue(elt);

    /* We only read the low 8 bits of bitC->bitPos[idx] so it

     * doesn't matter that the high bits have noise from the value.

     */

    bitC->bitPos[idx] += HUF_getNbBitsFast(elt);

    assert((bitC->bitPos[idx] & 0xFF) <= HUF_BITS_IN_CONTAINER);

    /* The last 4-bits of elt are dirty if fast is set,

     * so we must not be overwriting bits that have already been

     * inserted into the bit container.

     */

#if DEBUGLEVEL >= 1

    {

        size_t const nbBits = HUF_getNbBits(elt);

        size_t const dirtyBits = nbBits == 0 ? 0 : ZSTD_highbit32((U32)nbBits) + 1;

        (void)dirtyBits;

        /* Middle bits are 0. */

        assert(((elt >> dirtyBits) << (dirtyBits + nbBits)) == 0);

        /* We didn't overwrite any bits in the bit container. */

        assert(!kFast || (bitC->bitPos[idx] & 0xFF) <= HUF_BITS_IN_CONTAINER);

        (void)dirtyBits;

    }

#endif

}

FORCE_INLINE_TEMPLATE void HUF_zeroIndex1(HUF_CStream_t* bitC)

{

    bitC->bitContainer[1] = 0;

    bitC->bitPos[1] = 0;

}

/*! HUF_mergeIndex1() :

 * Merges the bit container @ index 1 into the bit container @ index 0

 * and zeros the bit container @ index 1.

 */

FORCE_INLINE_TEMPLATE void HUF_mergeIndex1(HUF_CStream_t* bitC)

{

    assert((bitC->bitPos[1] & 0xFF) < HUF_BITS_IN_CONTAINER);

    bitC->bitContainer[0] >>= (bitC->bitPos[1] & 0xFF);

    bitC->bitContainer[0] |= bitC->bitContainer[1];

    bitC->bitPos[0] += bitC->bitPos[1];

    assert((bitC->bitPos[0] & 0xFF) <= HUF_BITS_IN_CONTAINER);

}

/*! HUF_flushBits() :

* Flushes the bits in the bit container @ index 0.

*

* @post bitPos will be < 8.

* @param kFast If kFast is set then we must know a-priori that

*              the bit container will not overflow.

*/

FORCE_INLINE_TEMPLATE void HUF_flushBits(HUF_CStream_t* bitC, int kFast)

{

    /* The upper bits of bitPos are noisy, so we must mask by 0xFF. */

    size_t const nbBits = bitC->bitPos[0] & 0xFF;

    size_t const nbBytes = nbBits >> 3;

    /* The top nbBits bits of bitContainer are the ones we need. */

    size_t const bitContainer = bitC->bitContainer[0] >> (HUF_BITS_IN_CONTAINER - nbBits);

    /* Mask bitPos to account for the bytes we consumed. */

    bitC->bitPos[0] &= 7;

    assert(nbBits > 0);

    assert(nbBits <= sizeof(bitC->bitContainer[0]) * 8);

    assert(bitC->ptr <= bitC->endPtr);

    MEM_writeLEST(bitC->ptr, bitContainer);

    bitC->ptr += nbBytes;

    assert(!kFast || bitC->ptr <= bitC->endPtr);

    if (!kFast && bitC->ptr > bitC->endPtr) bitC->ptr = bitC->endPtr;

    /* bitContainer doesn't need to be modified because the leftover

     * bits are already the top bitPos bits. And we don't care about

     * noise in the lower values.

     */

}

/*! HUF_endMark()

 * @returns The Huffman stream end mark: A 1-bit value = 1.

 */

static HUF_CElt HUF_endMark(void)

{

    HUF_CElt endMark;

    HUF_setNbBits(&endMark, 1);

    HUF_setValue(&endMark, 1);

    return endMark;

}

/*! HUF_closeCStream() :

 *  @return Size of CStream, in bytes,

 *          or 0 if it could not fit into dstBuffer */

static size_t HUF_closeCStream(HUF_CStream_t* bitC)

{

    HUF_addBits(bitC, HUF_endMark(), /* idx */ 0, /* kFast */ 0);

    HUF_flushBits(bitC, /* kFast */ 0);

    {

        size_t const nbBits = bitC->bitPos[0] & 0xFF;

        if (bitC->ptr >= bitC->endPtr) return 0; /* overflow detected */

        return (size_t)(bitC->ptr - bitC->startPtr) + (nbBits > 0);

    }

}

FORCE_INLINE_TEMPLATE void

HUF_encodeSymbol(HUF_CStream_t* bitCPtr, U32 symbol, const HUF_CElt* CTable, int idx, int fast)

{

    HUF_addBits(bitCPtr, CTable[symbol], idx, fast);

}

FORCE_INLINE_TEMPLATE void

HUF_compress1X_usingCTable_internal_body_loop(HUF_CStream_t* bitC,

                                   const BYTE* ip, size_t srcSize,

                                   const HUF_CElt* ct,

                                   int kUnroll, int kFastFlush, int kLastFast)

{

    /* Join to kUnroll */

    int n = (int)srcSize;

    int rem = n % kUnroll;

    if (rem > 0) {

        for (; rem > 0; --rem) {

            HUF_encodeSymbol(bitC, ip[--n], ct, 0, /* fast */ 0);

        }

        HUF_flushBits(bitC, kFastFlush);

    }

    assert(n % kUnroll == 0);

    /* Join to 2 * kUnroll */

    if (n % (2 * kUnroll)) {

        int u;

        for (u = 1; u < kUnroll; ++u) {

            HUF_encodeSymbol(bitC, ip[n - u], ct, 0, 1);

        }

        HUF_encodeSymbol(bitC, ip[n - kUnroll], ct, 0, kLastFast);

        HUF_flushBits(bitC, kFastFlush);

        n -= kUnroll;

    }

    assert(n % (2 * kUnroll) == 0);

    for (; n>0; n-= 2 * kUnroll) {

        /* Encode kUnroll symbols into the bitstream @ index 0. */

        int u;

        for (u = 1; u < kUnroll; ++u) {

            HUF_encodeSymbol(bitC, ip[n - u], ct, /* idx */ 0, /* fast */ 1);

        }

        HUF_encodeSymbol(bitC, ip[n - kUnroll], ct, /* idx */ 0, /* fast */ kLastFast);

        HUF_flushBits(bitC, kFastFlush);

        /* Encode kUnroll symbols into the bitstream @ index 1.

         * This allows us to start filling the bit container

         * without any data dependencies.

         */

        HUF_zeroIndex1(bitC);

        for (u = 1; u < kUnroll; ++u) {

            HUF_encodeSymbol(bitC, ip[n - kUnroll - u], ct, /* idx */ 1, /* fast */ 1);

        }

        HUF_encodeSymbol(bitC, ip[n - kUnroll - kUnroll], ct, /* idx */ 1, /* fast */ kLastFast);

        /* Merge bitstream @ index 1 into the bitstream @ index 0 */

        HUF_mergeIndex1(bitC);

        HUF_flushBits(bitC, kFastFlush);

    }

    assert(n == 0);

}