xref: /kernel/linux/linux-5.10/lib/zstd/fse_compress.c

/*
 * FSE : Finite State Entropy encoder
 * Copyright (C) 2013-2015, Yann Collet.
 *
 * BSD 2-Clause License (http://www.opensource.org/licenses/bsd-license.php)
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *   * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * This program is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License version 2 as published by the
 * Free Software Foundation. This program is dual-licensed; you may select
 * either version 2 of the GNU General Public License ("GPL") or BSD license
 * ("BSD").
 *
 * You can contact the author at :
 * - Source repository : https://github.com/Cyan4973/FiniteStateEntropy
 */

/* **************************************************************
*  Compiler specifics
****************************************************************/
#define FORCE_INLINE static __always_inline

/* **************************************************************
*  Includes
****************************************************************/
#include "bitstream.h"
#include "fse.h"
#include <linux/compiler.h>
#include <linux/kernel.h>
#include <linux/math64.h>
#include <linux/string.h> /* memcpy, memset */

/* **************************************************************
*  Error Management
****************************************************************/
#define FSE_STATIC_ASSERT(c)                                   \
	{                                                      \
		enum { FSE_static_assert = 1 / (int)(!!(c)) }; \
	} /* use only *after* variable declarations */

/* **************************************************************
*  Templates
****************************************************************/
/*
  designed to be included
  for type-specific functions (template emulation in C)
  Objective is to write these functions only once, for improved maintenance
*/

/* safety checks */
#ifndef FSE_FUNCTION_EXTENSION
#error "FSE_FUNCTION_EXTENSION must be defined"
#endif
#ifndef FSE_FUNCTION_TYPE
#error "FSE_FUNCTION_TYPE must be defined"
#endif

/* Function names */
#define FSE_CAT(X, Y) X##Y
#define FSE_FUNCTION_NAME(X, Y) FSE_CAT(X, Y)
#define FSE_TYPE_NAME(X, Y) FSE_CAT(X, Y)

/* Function templates */

/* FSE_buildCTable_wksp() :
 * Same as FSE_buildCTable(), but using an externally allocated scratch buffer (`workSpace`).
 * wkspSize should be sized to handle worst case situation, which is `1<<max_tableLog * sizeof(FSE_FUNCTION_TYPE)`
 * workSpace must also be properly aligned with FSE_FUNCTION_TYPE requirements
 */
size_t FSE_buildCTable_wksp(FSE_CTable *ct, const short *normalizedCounter, unsigned maxSymbolValue, unsigned tableLog, void *workspace, size_t workspaceSize)
{
	U32 const tableSize = 1 << tableLog;
	U32 const tableMask = tableSize - 1;
	void *const ptr = ct;
	U16 *const tableU16 = ((U16 *)ptr) + 2;
	void *const FSCT = ((U32 *)ptr) + 1 /* header */ + (tableLog ? tableSize >> 1 : 1);
	FSE_symbolCompressionTransform *const symbolTT = (FSE_symbolCompressionTransform *)(FSCT);
	U32 const step = FSE_TABLESTEP(tableSize);
	U32 highThreshold = tableSize - 1;

	U32 *cumul;
	FSE_FUNCTION_TYPE *tableSymbol;
	size_t spaceUsed32 = 0;

	cumul = (U32 *)workspace + spaceUsed32;
	spaceUsed32 += FSE_MAX_SYMBOL_VALUE + 2;
	tableSymbol = (FSE_FUNCTION_TYPE *)((U32 *)workspace + spaceUsed32);
	spaceUsed32 += ALIGN(sizeof(FSE_FUNCTION_TYPE) * ((size_t)1 << tableLog), sizeof(U32)) >> 2;

	if ((spaceUsed32 << 2) > workspaceSize)
		return ERROR(tableLog_tooLarge);
	workspace = (U32 *)workspace + spaceUsed32;
	workspaceSize -= (spaceUsed32 << 2);

	/* CTable header */
	tableU16[-2] = (U16)tableLog;
	tableU16[-1] = (U16)maxSymbolValue;

	/* For explanations on how to distribute symbol values over the table :
	*  http://fastcompression.blogspot.fr/2014/02/fse-distributing-symbol-values.html */

	/* symbol start positions */
	{
		U32 u;
		cumul[0] = 0;
		for (u = 1; u <= maxSymbolValue + 1; u++) {
			if (normalizedCounter[u - 1] == -1) { /* Low proba symbol */
				cumul[u] = cumul[u - 1] + 1;
				tableSymbol[highThreshold--] = (FSE_FUNCTION_TYPE)(u - 1);
			} else {
				cumul[u] = cumul[u - 1] + normalizedCounter[u - 1];
			}
		}
		cumul[maxSymbolValue + 1] = tableSize + 1;
	}

	/* Spread symbols */
	{
		U32 position = 0;
		U32 symbol;
		for (symbol = 0; symbol <= maxSymbolValue; symbol++) {
			int nbOccurences;
			for (nbOccurences = 0; nbOccurences < normalizedCounter[symbol]; nbOccurences++) {
				tableSymbol[position] = (FSE_FUNCTION_TYPE)symbol;
				position = (position + step) & tableMask;
				while (position > highThreshold)
					position = (position + step) & tableMask; /* Low proba area */
			}
		}

		if (position != 0)
			return ERROR(GENERIC); /* Must have gone through all positions */
	}

	/* Build table */
	{
		U32 u;
		for (u = 0; u < tableSize; u++) {
			FSE_FUNCTION_TYPE s = tableSymbol[u];	/* note : static analyzer may not understand tableSymbol is properly initialized */
			tableU16[cumul[s]++] = (U16)(tableSize + u); /* TableU16 : sorted by symbol order; gives next state value */
		}
	}

	/* Build Symbol Transformation Table */
	{
		unsigned total = 0;
		unsigned s;
		for (s = 0; s <= maxSymbolValue; s++) {
			switch (normalizedCounter[s]) {
			case 0: break;

			case -1:
			case 1:
				symbolTT[s].deltaNbBits = (tableLog << 16) - (1 << tableLog);
				symbolTT[s].deltaFindState = total - 1;
				total++;
				break;
			default: {
				U32 const maxBitsOut = tableLog - BIT_highbit32(normalizedCounter[s] - 1);
				U32 const minStatePlus = normalizedCounter[s] << maxBitsOut;
				symbolTT[s].deltaNbBits = (maxBitsOut << 16) - minStatePlus;
				symbolTT[s].deltaFindState = total - normalizedCounter[s];
				total += normalizedCounter[s];
			}
			}
		}
	}

	return 0;
}

/*-**************************************************************
*  FSE NCount encoding-decoding
****************************************************************/
size_t FSE_NCountWriteBound(unsigned maxSymbolValue, unsigned tableLog)
{
	size_t const maxHeaderSize = (((maxSymbolValue + 1) * tableLog) >> 3) + 3;
	return maxSymbolValue ? maxHeaderSize : FSE_NCOUNTBOUND; /* maxSymbolValue==0 ? use default */
}

static size_t FSE_writeNCount_generic(void *header, size_t headerBufferSize, const short *normalizedCounter, unsigned maxSymbolValue, unsigned tableLog,
				      unsigned writeIsSafe)
{
	BYTE *const ostart = (BYTE *)header;
	BYTE *out = ostart;
	BYTE *const oend = ostart + headerBufferSize;
	int nbBits;
	const int tableSize = 1 << tableLog;
	int remaining;
	int threshold;
	U32 bitStream;
	int bitCount;
	unsigned charnum = 0;
	int previous0 = 0;

	bitStream = 0;
	bitCount = 0;
	/* Table Size */
	bitStream += (tableLog - FSE_MIN_TABLELOG) << bitCount;
	bitCount += 4;

	/* Init */
	remaining = tableSize + 1; /* +1 for extra accuracy */
	threshold = tableSize;
	nbBits = tableLog + 1;

	while (remaining > 1) { /* stops at 1 */
		if (previous0) {
			unsigned start = charnum;
			while (!normalizedCounter[charnum])
				charnum++;
			while (charnum >= start + 24) {
				start += 24;
				bitStream += 0xFFFFU << bitCount;
				if ((!writeIsSafe) && (out > oend - 2))
					return ERROR(dstSize_tooSmall); /* Buffer overflow */
				out[0] = (BYTE)bitStream;
				out[1] = (BYTE)(bitStream >> 8);
				out += 2;
				bitStream >>= 16;
			}
			while (charnum >= start + 3) {
				start += 3;
				bitStream += 3 << bitCount;
				bitCount += 2;
			}
			bitStream += (charnum - start) << bitCount;
			bitCount += 2;
			if (bitCount > 16) {
				if ((!writeIsSafe) && (out > oend - 2))
					return ERROR(dstSize_tooSmall); /* Buffer overflow */
				out[0] = (BYTE)bitStream;
				out[1] = (BYTE)(bitStream >> 8);
				out += 2;
				bitStream >>= 16;
				bitCount -= 16;
			}
		}
		{
			int count = normalizedCounter[charnum++];
			int const max = (2 * threshold - 1) - remaining;
			remaining -= count < 0 ? -count : count;
			count++; /* +1 for extra accuracy */
			if (count >= threshold)
				count += max; /* [0..max[ [max..threshold[ (...) [threshold+max 2*threshold[ */
			bitStream += count << bitCount;
			bitCount += nbBits;
			bitCount -= (count < max);
			previous0 = (count == 1);
			if (remaining < 1)
				return ERROR(GENERIC);
			while (remaining < threshold)
				nbBits--, threshold >>= 1;
		}
		if (bitCount > 16) {
			if ((!writeIsSafe) && (out > oend - 2))
				return ERROR(dstSize_tooSmall); /* Buffer overflow */
			out[0] = (BYTE)bitStream;
			out[1] = (BYTE)(bitStream >> 8);
			out += 2;
			bitStream >>= 16;
			bitCount -= 16;
		}
	}

	/* flush remaining bitStream */
	if ((!writeIsSafe) && (out > oend - 2))
		return ERROR(dstSize_tooSmall); /* Buffer overflow */
	out[0] = (BYTE)bitStream;
	out[1] = (BYTE)(bitStream >> 8);
	out += (bitCount + 7) / 8;

	if (charnum > maxSymbolValue + 1)
		return ERROR(GENERIC);

	return (out - ostart);
}

size_t FSE_writeNCount(void *buffer, size_t bufferSize, const short *normalizedCounter, unsigned maxSymbolValue, unsigned tableLog)
{
	if (tableLog > FSE_MAX_TABLELOG)
		return ERROR(tableLog_tooLarge); /* Unsupported */
	if (tableLog < FSE_MIN_TABLELOG)
		return ERROR(GENERIC); /* Unsupported */

	if (bufferSize < FSE_NCountWriteBound(maxSymbolValue, tableLog))
		return FSE_writeNCount_generic(buffer, bufferSize, normalizedCounter, maxSymbolValue, tableLog, 0);

	return FSE_writeNCount_generic(buffer, bufferSize, normalizedCounter, maxSymbolValue, tableLog, 1);
}

/*-**************************************************************
*  Counting histogram
****************************************************************/
/*! FSE_count_simple
	This function counts byte values within `src`, and store the histogram into table `count`.
	It doesn't use any additional memory.
	But this function is unsafe : it doesn't check that all values within `src` can fit into `count`.
	For this reason, prefer using a table `count` with 256 elements.
	@return : count of most numerous element
*/
size_t FSE_count_simple(unsigned *count, unsigned *maxSymbolValuePtr, const void *src, size_t srcSize)
{
	const BYTE *ip = (const BYTE *)src;
	const BYTE *const end = ip + srcSize;
	unsigned maxSymbolValue = *maxSymbolValuePtr;
	unsigned max = 0;

	memset(count, 0, (maxSymbolValue + 1) * sizeof(*count));
	if (srcSize == 0) {
		*maxSymbolValuePtr = 0;
		return 0;
	}

	while (ip < end)
		count[*ip++]++;

	while (!count[maxSymbolValue])
		maxSymbolValue--;
	*maxSymbolValuePtr = maxSymbolValue;

	{
		U32 s;
		for (s = 0; s <= maxSymbolValue; s++)
			if (count[s] > max)
				max = count[s];
	}

	return (size_t)max;
}

/* FSE_count_parallel_wksp() :
 * Same as FSE_count_parallel(), but using an externally provided scratch buffer.
 * `workSpace` size must be a minimum of `1024 * sizeof(unsigned)`` */
static size_t FSE_count_parallel_wksp(unsigned *count, unsigned *maxSymbolValuePtr, const void *source, size_t sourceSize, unsigned checkMax,
				      unsigned *const workSpace)
{
	const BYTE *ip = (const BYTE *)source;
	const BYTE *const iend = ip + sourceSize;
	unsigned maxSymbolValue = *maxSymbolValuePtr;
	unsigned max = 0;
	U32 *const Counting1 = workSpace;
	U32 *const Counting2 = Counting1 + 256;
	U32 *const Counting3 = Counting2 + 256;
	U32 *const Counting4 = Counting3 + 256;

	memset(Counting1, 0, 4 * 256 * sizeof(unsigned));

	/* safety checks */
	if (!sourceSize) {
		memset(count, 0, maxSymbolValue + 1);
		*maxSymbolValuePtr = 0;
		return 0;
	}
	if (!maxSymbolValue)
		maxSymbolValue = 255; /* 0 == default */

	/* by stripes of 16 bytes */
	{
		U32 cached = ZSTD_read32(ip);
		ip += 4;
		while (ip < iend - 15) {
			U32 c = cached;
			cached = ZSTD_read32(ip);
			ip += 4;
			Counting1[(BYTE)c]++;
			Counting2[(BYTE)(c >> 8)]++;
			Counting3[(BYTE)(c >> 16)]++;
			Counting4[c >> 24]++;
			c = cached;
			cached = ZSTD_read32(ip);
			ip += 4;
			Counting1[(BYTE)c]++;
			Counting2[(BYTE)(c >> 8)]++;
			Counting3[(BYTE)(c >> 16)]++;
			Counting4[c >> 24]++;
			c = cached;
			cached = ZSTD_read32(ip);
			ip += 4;
			Counting1[(BYTE)c]++;
			Counting2[(BYTE)(c >> 8)]++;
			Counting3[(BYTE)(c >> 16)]++;
			Counting4[c >> 24]++;
			c = cached;
			cached = ZSTD_read32(ip);
			ip += 4;
			Counting1[(BYTE)c]++;
			Counting2[(BYTE)(c >> 8)]++;
			Counting3[(BYTE)(c >> 16)]++;
			Counting4[c >> 24]++;
		}
		ip -= 4;
	}

	/* finish last symbols */
	while (ip < iend)
		Counting1[*ip++]++;

	if (checkMax) { /* verify stats will fit into destination table */
		U32 s;
		for (s = 255; s > maxSymbolValue; s--) {
			Counting1[s] += Counting2[s] + Counting3[s] + Counting4[s];
			if (Counting1[s])
				return ERROR(maxSymbolValue_tooSmall);
		}
	}

	{
		U32 s;
		for (s = 0; s <= maxSymbolValue; s++) {
			count[s] = Counting1[s] + Counting2[s] + Counting3[s] + Counting4[s];
			if (count[s] > max)
				max = count[s];
		}
	}

	while (!count[maxSymbolValue])
		maxSymbolValue--;
	*maxSymbolValuePtr = maxSymbolValue;
	return (size_t)max;
}

/* FSE_countFast_wksp() :
 * Same as FSE_countFast(), but using an externally provided scratch buffer.
 * `workSpace` size must be table of >= `1024` unsigned */
size_t FSE_countFast_wksp(unsigned *count, unsigned *maxSymbolValuePtr, const void *source, size_t sourceSize, unsigned *workSpace)
{
	if (sourceSize < 1500)
		return FSE_count_simple(count, maxSymbolValuePtr, source, sourceSize);
	return FSE_count_parallel_wksp(count, maxSymbolValuePtr, source, sourceSize, 0, workSpace);
}

/* FSE_count_wksp() :
 * Same as FSE_count(), but using an externally provided scratch buffer.
 * `workSpace` size must be table of >= `1024` unsigned */
size_t FSE_count_wksp(unsigned *count, unsigned *maxSymbolValuePtr, const void *source, size_t sourceSize, unsigned *workSpace)
{
	if (*maxSymbolValuePtr < 255)
		return FSE_count_parallel_wksp(count, maxSymbolValuePtr, source, sourceSize, 1, workSpace);
	*maxSymbolValuePtr = 255;
	return FSE_countFast_wksp(count, maxSymbolValuePtr, source, sourceSize, workSpace);
}

/*-**************************************************************
*  FSE Compression Code
****************************************************************/
/*! FSE_sizeof_CTable() :
	FSE_CTable is a variable size structure which contains :
	`U16 tableLog;`
	`U16 maxSymbolValue;`
	`U16 nextStateNumber[1 << tableLog];`                         // This size is variable
	`FSE_symbolCompressionTransform symbolTT[maxSymbolValue+1];`  // This size is variable
Allocation is manual (C standard does not support variable-size structures).
*/
size_t FSE_sizeof_CTable(unsigned maxSymbolValue, unsigned tableLog)
{
	if (tableLog > FSE_MAX_TABLELOG)
		return ERROR(tableLog_tooLarge);
	return FSE_CTABLE_SIZE_U32(tableLog, maxSymbolValue) * sizeof(U32);
}

/* provides the minimum logSize to safely represent a distribution */
static unsigned FSE_minTableLog(size_t srcSize, unsigned maxSymbolValue)
{
	U32 minBitsSrc = BIT_highbit32((U32)(srcSize - 1)) + 1;
	U32 minBitsSymbols = BIT_highbit32(maxSymbolValue) + 2;
	U32 minBits = minBitsSrc < minBitsSymbols ? minBitsSrc : minBitsSymbols;
	return minBits;
}

unsigned FSE_optimalTableLog_internal(unsigned maxTableLog, size_t srcSize, unsigned maxSymbolValue, unsigned minus)
{
	U32 maxBitsSrc = BIT_highbit32((U32)(srcSize - 1)) - minus;
	U32 tableLog = maxTableLog;
	U32 minBits = FSE_minTableLog(srcSize, maxSymbolValue);
	if (tableLog == 0)
		tableLog = FSE_DEFAULT_TABLELOG;
	if (maxBitsSrc < tableLog)
		tableLog = maxBitsSrc; /* Accuracy can be reduced */
	if (minBits > tableLog)
		tableLog = minBits; /* Need a minimum to safely represent all symbol values */
	if (tableLog < FSE_MIN_TABLELOG)
		tableLog = FSE_MIN_TABLELOG;
	if (tableLog > FSE_MAX_TABLELOG)
		tableLog = FSE_MAX_TABLELOG;
	return tableLog;
}

unsigned FSE_optimalTableLog(unsigned maxTableLog, size_t srcSize, unsigned maxSymbolValue)
{
	return FSE_optimalTableLog_internal(maxTableLog, srcSize, maxSymbolValue, 2);
}

/* Secondary normalization method.
   To be used when primary method fails. */

static size_t FSE_normalizeM2(short *norm, U32 tableLog, const unsigned *count, size_t total, U32 maxSymbolValue)
{
	short const NOT_YET_ASSIGNED = -2;
	U32 s;
	U32 distributed = 0;
	U32 ToDistribute;

	/* Init */
	U32 const lowThreshold = (U32)(total >> tableLog);
	U32 lowOne = (U32)((total * 3) >> (tableLog + 1));

	for (s = 0; s <= maxSymbolValue; s++) {
		if (count[s] == 0) {
			norm[s] = 0;
			continue;
		}
		if (count[s] <= lowThreshold) {
			norm[s] = -1;
			distributed++;
			total -= count[s];
			continue;
		}
		if (count[s] <= lowOne) {
			norm[s] = 1;
			distributed++;
			total -= count[s];
			continue;
		}

		norm[s] = NOT_YET_ASSIGNED;
	}
	ToDistribute = (1 << tableLog) - distributed;

	if ((total / ToDistribute) > lowOne) {
		/* risk of rounding to zero */
		lowOne = (U32)((total * 3) / (ToDistribute * 2));
		for (s = 0; s <= maxSymbolValue; s++) {
			if ((norm[s] == NOT_YET_ASSIGNED) && (count[s] <= lowOne)) {
				norm[s] = 1;
				distributed++;
				total -= count[s];
				continue;
			}
		}
		ToDistribute = (1 << tableLog) - distributed;
	}

	if (distributed == maxSymbolValue + 1) {
		/* all values are pretty poor;
		   probably incompressible data (should have already been detected);
		   find max, then give all remaining points to max */
		U32 maxV = 0, maxC = 0;
		for (s = 0; s <= maxSymbolValue; s++)
			if (count[s] > maxC)
				maxV = s, maxC = count[s];
		norm[maxV] += (short)ToDistribute;
		return 0;
	}

	if (total == 0) {
		/* all of the symbols were low enough for the lowOne or lowThreshold */
		for (s = 0; ToDistribute > 0; s = (s + 1) % (maxSymbolValue + 1))
			if (norm[s] > 0)
				ToDistribute--, norm[s]++;
		return 0;
	}

	{
		U64 const vStepLog = 62 - tableLog;
		U64 const mid = (1ULL << (vStepLog - 1)) - 1;
		U64 const rStep = div_u64((((U64)1 << vStepLog) * ToDistribute) + mid, (U32)total); /* scale on remaining */
		U64 tmpTotal = mid;
		for (s = 0; s <= maxSymbolValue; s++) {
			if (norm[s] == NOT_YET_ASSIGNED) {
				U64 const end = tmpTotal + (count[s] * rStep);
				U32 const sStart = (U32)(tmpTotal >> vStepLog);
				U32 const sEnd = (U32)(end >> vStepLog);
				U32 const weight = sEnd - sStart;
				if (weight < 1)
					return ERROR(GENERIC);
				norm[s] = (short)weight;
				tmpTotal = end;
			}
		}
	}

	return 0;
}

size_t FSE_normalizeCount(short *normalizedCounter, unsigned tableLog, const unsigned *count, size_t total, unsigned maxSymbolValue)
{
	/* Sanity checks */
	if (tableLog == 0)
		tableLog = FSE_DEFAULT_TABLELOG;
	if (tableLog < FSE_MIN_TABLELOG)
		return ERROR(GENERIC); /* Unsupported size */
	if (tableLog > FSE_MAX_TABLELOG)
		return ERROR(tableLog_tooLarge); /* Unsupported size */
	if (tableLog < FSE_minTableLog(total, maxSymbolValue))
		return ERROR(GENERIC); /* Too small tableLog, compression potentially impossible */

	{
		U32 const rtbTable[] = {0, 473195, 504333, 520860, 550000, 700000, 750000, 830000};
		U64 const scale = 62 - tableLog;
		U64 const step = div_u64((U64)1 << 62, (U32)total); /* <== here, one division ! */
		U64 const vStep = 1ULL << (scale - 20);
		int stillToDistribute = 1 << tableLog;
		unsigned s;
		unsigned largest = 0;
		short largestP = 0;
		U32 lowThreshold = (U32)(total >> tableLog);

		for (s = 0; s <= maxSymbolValue; s++) {
			if (count[s] == total)
				return 0; /* rle special case */
			if (count[s] == 0) {
				normalizedCounter[s] = 0;
				continue;
			}
			if (count[s] <= lowThreshold) {
				normalizedCounter[s] = -1;
				stillToDistribute--;
			} else {
				short proba = (short)((count[s] * step) >> scale);
				if (proba < 8) {
					U64 restToBeat = vStep * rtbTable[proba];
					proba += (count[s] * step) - ((U64)proba << scale) > restToBeat;
				}
				if (proba > largestP)
					largestP = proba, largest = s;
				normalizedCounter[s] = proba;
				stillToDistribute -= proba;
			}
		}
		if (-stillToDistribute >= (normalizedCounter[largest] >> 1)) {
			/* corner case, need another normalization method */
			size_t const errorCode = FSE_normalizeM2(normalizedCounter, tableLog, count, total, maxSymbolValue);
			if (FSE_isError(errorCode))
				return errorCode;
		} else
			normalizedCounter[largest] += (short)stillToDistribute;
	}

	return tableLog;
}

/* fake FSE_CTable, for raw (uncompressed) input */
size_t FSE_buildCTable_raw(FSE_CTable *ct, unsigned nbBits)
{
	const unsigned tableSize = 1 << nbBits;
	const unsigned tableMask = tableSize - 1;
	const unsigned maxSymbolValue = tableMask;
	void *const ptr = ct;
	U16 *const tableU16 = ((U16 *)ptr) + 2;
	void *const FSCT = ((U32 *)ptr) + 1 /* header */ + (tableSize >> 1); /* assumption : tableLog >= 1 */
	FSE_symbolCompressionTransform *const symbolTT = (FSE_symbolCompressionTransform *)(FSCT);
	unsigned s;

	/* Sanity checks */
	if (nbBits < 1)
		return ERROR(GENERIC); /* min size */

	/* header */
	tableU16[-2] = (U16)nbBits;
	tableU16[-1] = (U16)maxSymbolValue;

	/* Build table */
	for (s = 0; s < tableSize; s++)
		tableU16[s] = (U16)(tableSize + s);

	/* Build Symbol Transformation Table */
	{
		const U32 deltaNbBits = (nbBits << 16) - (1 << nbBits);
		for (s = 0; s <= maxSymbolValue; s++) {
			symbolTT[s].deltaNbBits = deltaNbBits;
			symbolTT[s].deltaFindState = s - 1;
		}
	}

	return 0;
}

/* fake FSE_CTable, for rle input (always same symbol) */
size_t FSE_buildCTable_rle(FSE_CTable *ct, BYTE symbolValue)
{
	void *ptr = ct;
	U16 *tableU16 = ((U16 *)ptr) + 2;
	void *FSCTptr = (U32 *)ptr + 2;
	FSE_symbolCompressionTransform *symbolTT = (FSE_symbolCompressionTransform *)FSCTptr;

	/* header */
	tableU16[-2] = (U16)0;
	tableU16[-1] = (U16)symbolValue;

	/* Build table */
	tableU16[0] = 0;
	tableU16[1] = 0; /* just in case */

	/* Build Symbol Transformation Table */
	symbolTT[symbolValue].deltaNbBits = 0;
	symbolTT[symbolValue].deltaFindState = 0;

	return 0;
}

static size_t FSE_compress_usingCTable_generic(void *dst, size_t dstSize, const void *src, size_t srcSize, const FSE_CTable *ct, const unsigned fast)
{
	const BYTE *const istart = (const BYTE *)src;
	const BYTE *const iend = istart + srcSize;
	const BYTE *ip = iend;

	BIT_CStream_t bitC;
	FSE_CState_t CState1, CState2;

	/* init */
	if (srcSize <= 2)
		return 0;
	{
		size_t const initError = BIT_initCStream(&bitC, dst, dstSize);
		if (FSE_isError(initError))
			return 0; /* not enough space available to write a bitstream */
	}

#define FSE_FLUSHBITS(s) (fast ? BIT_flushBitsFast(s) : BIT_flushBits(s))

	if (srcSize & 1) {
		FSE_initCState2(&CState1, ct, *--ip);
		FSE_initCState2(&CState2, ct, *--ip);
		FSE_encodeSymbol(&bitC, &CState1, *--ip);
		FSE_FLUSHBITS(&bitC);
	} else {
		FSE_initCState2(&CState2, ct, *--ip);
		FSE_initCState2(&CState1, ct, *--ip);
	}

	/* join to mod 4 */
	srcSize -= 2;
	if ((sizeof(bitC.bitContainer) * 8 > FSE_MAX_TABLELOG * 4 + 7) && (srcSize & 2)) { /* test bit 2 */
		FSE_encodeSymbol(&bitC, &CState2, *--ip);
		FSE_encodeSymbol(&bitC, &CState1, *--ip);
		FSE_FLUSHBITS(&bitC);
	}

	/* 2 or 4 encoding per loop */
	while (ip > istart) {

		FSE_encodeSymbol(&bitC, &CState2, *--ip);

		if (sizeof(bitC.bitContainer) * 8 < FSE_MAX_TABLELOG * 2 + 7) /* this test must be static */
			FSE_FLUSHBITS(&bitC);

		FSE_encodeSymbol(&bitC, &CState1, *--ip);

		if (sizeof(bitC.bitContainer) * 8 > FSE_MAX_TABLELOG * 4 + 7) { /* this test must be static */
			FSE_encodeSymbol(&bitC, &CState2, *--ip);
			FSE_encodeSymbol(&bitC, &CState1, *--ip);
		}

		FSE_FLUSHBITS(&bitC);
	}

	FSE_flushCState(&bitC, &CState2);
	FSE_flushCState(&bitC, &CState1);
	return BIT_closeCStream(&bitC);
}

size_t FSE_compress_usingCTable(void *dst, size_t dstSize, const void *src, size_t srcSize, const FSE_CTable *ct)
{
	unsigned const fast = (dstSize >= FSE_BLOCKBOUND(srcSize));

	if (fast)
		return FSE_compress_usingCTable_generic(dst, dstSize, src, srcSize, ct, 1);
	else
		return FSE_compress_usingCTable_generic(dst, dstSize, src, srcSize, ct, 0);
}

size_t FSE_compressBound(size_t size) { return FSE_COMPRESSBOUND(size); }