implemented fractional bit cost evaluation
for FSE symbols.
While it seems to work, the gains are negligible compared to rough maxNbBits evaluation.
There are even a few losses sometimes, that still need to be explained.
Furthermode, there are still cases where btlazy2 does a better job than btopt,
which seems rather strange too.
diff --git a/lib/common/fse.h b/lib/common/fse.h
index 556e6c5..6770785 100644
--- a/lib/common/fse.h
+++ b/lib/common/fse.h
@@ -577,10 +577,23 @@
MEM_STATIC U32 FSE_getMaxNbBits(const FSE_symbolCompressionTransform* symbolTT, U32 symbolValue)
{
- assert(symbolValue <= FSE_MAX_SYMBOL_VALUE);
return (symbolTT[symbolValue].deltaNbBits + ((1<<16)-1)) >> 16;
}
+/* FSE_bitCost_b256() :
+ * Approximate symbol cost,
+ * provide fractional value, using fixed-point format (8 bit) */
+MEM_STATIC U32 FSE_bitCost_b256(const FSE_symbolCompressionTransform* symbolTT, U32 tableLog, U32 symbolValue)
+{
+ U32 const minNbBits = symbolTT[symbolValue].deltaNbBits >> 16;
+ U32 const threshold = (minNbBits+1) << 16;
+ assert(symbolTT[symbolValue].deltaNbBits + (1<<tableLog) <= threshold);
+ U32 const deltaFromThreshold = threshold - (symbolTT[symbolValue].deltaNbBits + (1 << tableLog));
+ U32 const normalizedDeltaFromThreshold = (deltaFromThreshold << 8) >> tableLog; /* linear interpolation (very approximate) */
+ assert(normalizedDeltaFromThreshold <= 256);
+ return (minNbBits+1)*256 - normalizedDeltaFromThreshold;
+}
+
/* ====== Decompression ====== */
diff --git a/lib/compress/zstd_opt.c b/lib/compress/zstd_opt.c
index d800254..80edb1a 100644
--- a/lib/compress/zstd_opt.c
+++ b/lib/compress/zstd_opt.c
@@ -126,11 +126,13 @@
U32 const llCode = ZSTD_LLcode(litLength);
FSE_CState_t cstate;
FSE_initCState(&cstate, optPtr->symbolCosts->litlengthCTable);
- return (LL_bits[llCode] + FSE_getMaxNbBits(cstate.symbolTT, llCode)) << 8;
+ U32 const price = LL_bits[llCode]*256 + FSE_bitCost_b256(cstate.symbolTT, cstate.stateLog, llCode);
+ DEBUGLOG(8, "ZSTD_litLengthPrice: ll=%u, bitCost=%.2f", litLength, (double)price / 256);
+ return price;
}
if (optPtr->priceType == zop_predef) return ZSTD_highbit32((U32)litLength+1);
- /* literal Length */
+ /* dynamic statistics */
{ U32 const llCode = ZSTD_LLcode(litLength);
return (LL_bits[llCode] + optPtr->log2litLengthSum - ZSTD_highbit32(optPtr->litLengthFreq[llCode]+1)) << 8;
}
@@ -156,7 +158,9 @@
U32 const llCode = ZSTD_LLcode(litLength);
FSE_CState_t cstate;
FSE_initCState(&cstate, optPtr->symbolCosts->litlengthCTable);
- return ((int)(LL_bits[llCode] + FSE_getMaxNbBits(cstate.symbolTT, llCode)) - FSE_getMaxNbBits(cstate.symbolTT, 0)) * 256;
+ return (int)(LL_bits[llCode] * 256)
+ + FSE_bitCost_b256(cstate.symbolTT, cstate.stateLog, llCode)
+ - FSE_bitCost_b256(cstate.symbolTT, cstate.stateLog, 0);
}
if (optPtr->priceType >= zop_predef) return ZSTD_highbit32(litLength+1);
@@ -205,9 +209,8 @@
FSE_CState_t mlstate, offstate;
FSE_initCState(&mlstate, optPtr->symbolCosts->matchlengthCTable);
FSE_initCState(&offstate, optPtr->symbolCosts->offcodeCTable);
- return (FSE_getMaxNbBits(offstate.symbolTT, offCode) + offCode
- + FSE_getMaxNbBits(mlstate.symbolTT, mlCode) + ML_bits[mlCode])
- * 256;
+ return FSE_bitCost_b256(offstate.symbolTT, offstate.stateLog, offCode) + offCode*256
+ + FSE_bitCost_b256(mlstate.symbolTT, mlstate.stateLog, mlCode) + ML_bits[mlCode]*256;
}
if (optPtr->priceType == zop_predef) /* fixed scheme, do not use statistics */
diff --git a/lib/decompress/huf_decompress.c b/lib/decompress/huf_decompress.c
index 73f5c46..2c2d138 100644
--- a/lib/decompress/huf_decompress.c
+++ b/lib/decompress/huf_decompress.c
@@ -965,7 +965,7 @@
U32 HUF_selectDecoder (size_t dstSize, size_t cSrcSize)
{
assert(dstSize > 0);
- assert(dstSize <= 128 KB);
+ assert(dstSize <= 128*1024);
/* decoder timing evaluation */
{ U32 const Q = (cSrcSize >= dstSize) ? 15 : (U32)(cSrcSize * 16 / dstSize); /* Q < 16 */
U32 const D256 = (U32)(dstSize >> 8);
