Use ncount=1 when < 4096 symbols

lib/common/fse.h

@@ -137,10 +137,12 @@ FSE_PUBLIC_API unsigned FSE_optimalTableLog(unsigned maxTableLog, size_t srcSize
 /*! FSE_normalizeCount():
     normalize counts so that sum(count[]) == Power_of_2 (2^tableLog)
     'normalizedCounter' is a table of short, of minimum size (maxSymbolValue+1).
+    useLowProbCount is a bool param which is set to 1 to use count=-1 or set to 0 to
+    use count=1 instead, which speeds up FSE_readNCount() and FSE_buildDTable().
     @return : tableLog,
               or an errorCode, which can be tested using FSE_isError() */
 FSE_PUBLIC_API size_t FSE_normalizeCount(short* normalizedCounter, unsigned tableLog,
-                    const unsigned* count, size_t srcSize, unsigned maxSymbolValue);
+                    const unsigned* count, size_t srcSize, unsigned maxSymbolValue, unsigned useLowProbCount);
 
 /*! FSE_NCountWriteBound():
     Provides the maximum possible size of an FSE normalized table, given 'maxSymbolValue' and 'tableLog'.

lib/compress/fse_compress.c

@@ -341,13 +341,10 @@ unsigned FSE_optimalTableLog(unsigned maxTableLog, size_t srcSize, unsigned maxS
     return FSE_optimalTableLog_internal(maxTableLog, srcSize, maxSymbolValue, 2);
 }
 
-// TODO: Emit -1 based on # of symbols
-#define LOW_PROB 0
-
 /* 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)
+static size_t FSE_normalizeM2(short* norm, U32 tableLog, const unsigned* count, size_t total, U32 maxSymbolValue, short lowProbCount)
 {
     short const NOT_YET_ASSIGNED = -2;
     U32 s;
@@ -363,8 +360,8 @@ static size_t FSE_normalizeM2(short* norm, U32 tableLog, const unsigned* count,
             norm[s]=0;
             continue;
         }
-        if (LOW_PROB && count[s] <= lowThreshold) {
+        if (count[s] <= lowThreshold) {
-            norm[s] = -1;
+            norm[s] = lowProbCount;
             distributed++;
             total -= count[s];
             continue;
@@ -435,7 +432,7 @@ static size_t FSE_normalizeM2(short* norm, U32 tableLog, const unsigned* count,
 
 size_t FSE_normalizeCount (short* normalizedCounter, unsigned tableLog,
                            const unsigned* count, size_t total,
-                           unsigned maxSymbolValue)
+                           unsigned maxSymbolValue, unsigned useLowProbCount)
 {
     /* Sanity checks */
     if (tableLog==0) tableLog = FSE_DEFAULT_TABLELOG;
@@ -444,6 +441,7 @@ size_t FSE_normalizeCount (short* normalizedCounter, unsigned tableLog,
     if (tableLog < FSE_minTableLog(total, maxSymbolValue)) return ERROR(GENERIC);   /* Too small tableLog, compression potentially impossible */
 
     {   static U32 const rtbTable[] = {     0, 473195, 504333, 520860, 550000, 700000, 750000, 830000 };
+        short const lowProbCount = useLowProbCount ? -1 : 1;
         U64 const scale = 62 - tableLog;
         U64 const step = ((U64)1<<62) / total;   /* <== here, one division ! */
         U64 const vStep = 1ULL<<(scale-20);
@@ -456,8 +454,8 @@ size_t FSE_normalizeCount (short* normalizedCounter, unsigned 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 (LOW_PROB && count[s] <= lowThreshold) {
+            if (count[s] <= lowThreshold) {
-                normalizedCounter[s] = -1;
+                normalizedCounter[s] = lowProbCount;
                 stillToDistribute--;
             } else {
                 short proba = (short)((count[s]*step) >> scale);
@@ -471,7 +469,7 @@ size_t FSE_normalizeCount (short* normalizedCounter, unsigned tableLog,
         }   }
         if (-stillToDistribute >= (normalizedCounter[largest] >> 1)) {
             /* corner case, need another normalization method */
-            size_t const errorCode = FSE_normalizeM2(normalizedCounter, tableLog, count, total, maxSymbolValue);
+            size_t const errorCode = FSE_normalizeM2(normalizedCounter, tableLog, count, total, maxSymbolValue, lowProbCount);
             if (FSE_isError(errorCode)) return errorCode;
         }
         else normalizedCounter[largest] += (short)stillToDistribute;
@@ -643,7 +641,7 @@ size_t FSE_compress_wksp (void* dst, size_t dstSize, const void* src, size_t src
     }
 
     tableLog = FSE_optimalTableLog(tableLog, srcSize, maxSymbolValue);
-    CHECK_F( FSE_normalizeCount(norm, tableLog, count, srcSize, maxSymbolValue) );
+    CHECK_F( FSE_normalizeCount(norm, tableLog, count, srcSize, maxSymbolValue, /* useLowProbCount */ srcSize >= 2048) );
 
     /* Write table description header */
     {   CHECK_V_F(nc_err, FSE_writeNCount(op, oend-op, norm, maxSymbolValue, tableLog) );

lib/compress/huf_compress.c

@@ -85,7 +85,7 @@ static size_t HUF_compressWeights (void* dst, size_t dstSize, const void* weight
     }
 
     tableLog = FSE_optimalTableLog(tableLog, wtSize, maxSymbolValue);
-    CHECK_F( FSE_normalizeCount(norm, tableLog, count, wtSize, maxSymbolValue) );
+    CHECK_F( FSE_normalizeCount(norm, tableLog, count, wtSize, maxSymbolValue, /* useLowProbCount */ 0) );
 
     /* Write table description header */
     {   CHECK_V_F(hSize, FSE_writeNCount(op, (size_t)(oend-op), norm, maxSymbolValue, tableLog) );

lib/compress/zstd_compress_sequences.c

@@ -50,6 +50,19 @@ static unsigned ZSTD_getFSEMaxSymbolValue(FSE_CTable const* ctable) {
   return maxSymbolValue;
 }
 
+/**
+ * Returns true if we should use ncount=-1 else we should
+ * use ncount=1 for low probability symbols instead.
+ */
+static unsigned ZSTD_useLowProbCount(size_t const nbSeq)
+{
+    /* Heuristic: This should cover most blocks <= 16K and
+     * start to fade out after 16K to about 32K depending on
+     * comprssibility.
+     */
+    return nbSeq >= 2048;
+}
+
 /**
  * Returns the cost in bytes of encoding the normalized count header.
  * Returns an error if any of the helper functions return an error.
@@ -60,7 +73,7 @@ static size_t ZSTD_NCountCost(unsigned const* count, unsigned const max,
     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), "");
+    FORWARD_IF_ERROR(FSE_normalizeCount(norm, tableLog, count, nbSeq, max, ZSTD_useLowProbCount(nbSeq)), "");
     return FSE_writeNCount(wksp, sizeof(wksp), norm, max, tableLog);
 }
 
@@ -253,7 +266,7 @@ ZSTD_buildCTable(void* dst, size_t dstCapacity,
             nbSeq_1--;
         }
         assert(nbSeq_1 > 1);
-        FORWARD_IF_ERROR(FSE_normalizeCount(norm, tableLog, count, nbSeq_1, max), "");
+        FORWARD_IF_ERROR(FSE_normalizeCount(norm, tableLog, count, nbSeq_1, max, ZSTD_useLowProbCount(nbSeq_1)), "");
         {   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), "");

lib/decompress/zstd_decompress_block.c

@@ -376,14 +376,14 @@ void ZSTD_buildFSETable_body(ZSTD_seqSymbol* dt,
 
     U16* symbolNext = (U16*)wksp;
     BYTE* spread = (BYTE*)(symbolNext + MaxSeq + 1);
+    U32 highThreshold = tableSize - 1;
 
-    assert(wkspSize >= ZSTD_BUILD_FSE_TABLE_WKSP_SIZE);
-    (void)wkspSize;
 
     /* Sanity Checks */
     assert(maxSymbolValue <= MaxSeq);
     assert(tableLog <= MaxFSELog);
-    U32 highThreshold = tableSize - 1;
+    assert(wkspSize >= ZSTD_BUILD_FSE_TABLE_WKSP_SIZE);
+    (void)wkspSize;
     /* Init, lay down lowprob symbols */
     {   ZSTD_seqSymbol_header DTableH;
         DTableH.tableLog = tableLog;

lib/dictBuilder/zdict.c

@@ -786,7 +786,7 @@ static size_t ZDICT_analyzeEntropy(void*  dstBuffer, size_t maxDstSize,
     /* note : the result of this phase should be used to better appreciate the impact on statistics */
 
     total=0; for (u=0; u<=offcodeMax; u++) total+=offcodeCount[u];
-    errorCode = FSE_normalizeCount(offcodeNCount, Offlog, offcodeCount, total, offcodeMax);
+    errorCode = FSE_normalizeCount(offcodeNCount, Offlog, offcodeCount, total, offcodeMax, /* useLowProbCount */ 1);
     if (FSE_isError(errorCode)) {
         eSize = errorCode;
         DISPLAYLEVEL(1, "FSE_normalizeCount error with offcodeCount \n");
@@ -795,7 +795,7 @@ static size_t ZDICT_analyzeEntropy(void*  dstBuffer, size_t maxDstSize,
     Offlog = (U32)errorCode;
 
     total=0; for (u=0; u<=MaxML; u++) total+=matchLengthCount[u];
-    errorCode = FSE_normalizeCount(matchLengthNCount, mlLog, matchLengthCount, total, MaxML);
+    errorCode = FSE_normalizeCount(matchLengthNCount, mlLog, matchLengthCount, total, MaxML, /* useLowProbCount */ 1);
     if (FSE_isError(errorCode)) {
         eSize = errorCode;
         DISPLAYLEVEL(1, "FSE_normalizeCount error with matchLengthCount \n");
@@ -804,7 +804,7 @@ static size_t ZDICT_analyzeEntropy(void*  dstBuffer, size_t maxDstSize,
     mlLog = (U32)errorCode;
 
     total=0; for (u=0; u<=MaxLL; u++) total+=litLengthCount[u];
-    errorCode = FSE_normalizeCount(litLengthNCount, llLog, litLengthCount, total, MaxLL);
+    errorCode = FSE_normalizeCount(litLengthNCount, llLog, litLengthCount, total, MaxLL, /* useLowProbCount */ 1);
     if (FSE_isError(errorCode)) {
         eSize = errorCode;
         DISPLAYLEVEL(1, "FSE_normalizeCount error with litLengthCount \n");

tests/fuzzer.c

@@ -1573,11 +1573,11 @@ static int basicUnitTests(U32 const seed, double compressibility)
             const void* const contentStart = (const char*)dict + flatdictSize;
             size_t const target_nodict_cSize[22+1] = { 3840, 3770, 3870, 3830, 3770,
                                                        3770, 3770, 3770, 3750, 3750,
-                                                       3740, 3670, 3670, 3660, 3660,
+                                                       3742, 3670, 3670, 3660, 3660,
                                                        3660, 3660, 3660, 3660, 3660,
                                                        3660, 3660, 3660 };
             size_t const target_wdict_cSize[22+1] =  { 2830, 2890, 2890, 2820, 2940,
-                                                       2950, 2950, 2920, 2900, 2890,
+                                                       2950, 2950, 2921, 2900, 2891,
                                                        2910, 2910, 2910, 2770, 2760,
                                                        2750, 2750, 2750, 2750, 2750,
                                                        2750, 2750, 2750 };
@@ -2744,7 +2744,7 @@ static int basicUnitTests(U32 const seed, double compressibility)
         /* Calling FSE_normalizeCount() on a uniform distribution should not
          * cause a division by zero.
          */
-        FSE_normalizeCount(norm, tableLog, count, nbSeq, maxSymbolValue);
+        FSE_normalizeCount(norm, tableLog, count, nbSeq, maxSymbolValue, /* useLowProbCount */ 1);
     }
     DISPLAYLEVEL(3, "OK \n");
 #ifdef ZSTD_MULTITHREAD