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switchable bit-approximation / fractional-bit accuracy modes

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also : makes it possible to select nb of fractional bits.
This commit is contained in:
Yann Collet
2018-05-09 10:48:09 -07:00
parent ba2ad9b6b9
commit c0da0f5e9e
2 changed files with 37 additions and 23 deletions
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lib/common/fse.h
+11 -7
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@@ -582,16 +582,20 @@ MEM_STATIC U32 FSE_getMaxNbBits(const FSE_symbolCompressionTransform* symbolTT,
/* 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)
* provide fractional value, using fixed-point format (accuracyLog fractional bits) */
MEM_STATIC U32 FSE_bitCost(const FSE_symbolCompressionTransform* symbolTT, U32 tableLog, U32 symbolValue, U32 accuracyLog)
{
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;
assert(tableLog < 16);
U32 const tableSize = 1 << tableLog;
assert(symbolTT[symbolValue].deltaNbBits + tableSize <= threshold);
U32 const deltaFromThreshold = threshold - (symbolTT[symbolValue].deltaNbBits + tableSize);
assert(accuracyLog < 31-tableLog); /* ensure enough room for renormalization double shift */
U32 const normalizedDeltaFromThreshold = (deltaFromThreshold << accuracyLog) >> tableLog; /* linear interpolation (very approximate) */
U32 const bitMultiplier = 1 << accuracyLog;
assert(normalizedDeltaFromThreshold <= bitMultiplier);
return (minNbBits+1)*bitMultiplier - normalizedDeltaFromThreshold;
}