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huff0: Speed up compression of short blocks #744

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Jan 20, 2023
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huff0: Speed up compression of short blocks #744

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90 changes: 55 additions & 35 deletions huff0/compress.go
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Original file line number Diff line number Diff line change
Expand Up @@ -484,53 +484,55 @@ func (s *Scratch) buildCTable() error {
// Different from reference implementation.
huffNode0 := s.nodes[0 : huffNodesLen+1]

for huffNode[nonNullRank].count == 0 {
for huffNode[nonNullRank].count() == 0 {
nonNullRank--
}

lowS := int16(nonNullRank)
nodeRoot := nodeNb + lowS - 1
lowN := nodeNb
huffNode[nodeNb].count = huffNode[lowS].count + huffNode[lowS-1].count
huffNode[lowS].parent, huffNode[lowS-1].parent = uint16(nodeNb), uint16(nodeNb)
huffNode[nodeNb].setCount(huffNode[lowS].count() + huffNode[lowS-1].count())
huffNode[lowS].setParent(nodeNb)
huffNode[lowS-1].setParent(nodeNb)
nodeNb++
lowS -= 2
for n := nodeNb; n <= nodeRoot; n++ {
huffNode[n].count = 1 << 30
huffNode[n].setCount(1 << 30)
}
// fake entry, strong barrier
huffNode0[0].count = 1 << 31
huffNode0[0].setCount(1 << 31)

// create parents
for nodeNb <= nodeRoot {
var n1, n2 int16
if huffNode0[lowS+1].count < huffNode0[lowN+1].count {
if huffNode0[lowS+1].count() < huffNode0[lowN+1].count() {
n1 = lowS
lowS--
} else {
n1 = lowN
lowN++
}
if huffNode0[lowS+1].count < huffNode0[lowN+1].count {
if huffNode0[lowS+1].count() < huffNode0[lowN+1].count() {
n2 = lowS
lowS--
} else {
n2 = lowN
lowN++
}

huffNode[nodeNb].count = huffNode0[n1+1].count + huffNode0[n2+1].count
huffNode0[n1+1].parent, huffNode0[n2+1].parent = uint16(nodeNb), uint16(nodeNb)
huffNode[nodeNb].setCount(huffNode0[n1+1].count() + huffNode0[n2+1].count())
huffNode0[n1+1].setParent(nodeNb)
huffNode0[n2+1].setParent(nodeNb)
nodeNb++
}

// distribute weights (unlimited tree height)
huffNode[nodeRoot].nbBits = 0
huffNode[nodeRoot].setNbBits(0)
for n := nodeRoot - 1; n >= startNode; n-- {
huffNode[n].nbBits = huffNode[huffNode[n].parent].nbBits + 1
huffNode[n].setNbBits(huffNode[huffNode[n].parent()].nbBits() + 1)
}
for n := uint16(0); n <= nonNullRank; n++ {
huffNode[n].nbBits = huffNode[huffNode[n].parent].nbBits + 1
huffNode[n].setNbBits(huffNode[huffNode[n].parent()].nbBits() + 1)
}
s.actualTableLog = s.setMaxHeight(int(nonNullRank))
maxNbBits := s.actualTableLog
Expand All @@ -542,7 +544,7 @@ func (s *Scratch) buildCTable() error {
var nbPerRank [tableLogMax + 1]uint16
var valPerRank [16]uint16
for _, v := range huffNode[:nonNullRank+1] {
nbPerRank[v.nbBits]++
nbPerRank[v.nbBits()]++
}
// determine stating value per rank
{
Expand All @@ -557,7 +559,7 @@ func (s *Scratch) buildCTable() error {

// push nbBits per symbol, symbol order
for _, v := range huffNode[:nonNullRank+1] {
s.cTable[v.symbol].nBits = v.nbBits
s.cTable[v.symbol()].nBits = v.nbBits()
}

// assign value within rank, symbol order
Expand Down Expand Up @@ -603,12 +605,12 @@ func (s *Scratch) huffSort() {
pos := rank[r].current
rank[r].current++
prev := nodes[(pos-1)&huffNodesMask]
for pos > rank[r].base && c > prev.count {
for pos > rank[r].base && c > prev.count() {
nodes[pos&huffNodesMask] = prev
pos--
prev = nodes[(pos-1)&huffNodesMask]
}
nodes[pos&huffNodesMask] = nodeElt{count: c, symbol: byte(n)}
nodes[pos&huffNodesMask] = makeNodeElt(c, byte(n))
}
}

Expand All @@ -617,7 +619,7 @@ func (s *Scratch) setMaxHeight(lastNonNull int) uint8 {
huffNode := s.nodes[1 : huffNodesLen+1]
//huffNode = huffNode[: huffNodesLen]

largestBits := huffNode[lastNonNull].nbBits
largestBits := huffNode[lastNonNull].nbBits()

// early exit : no elt > maxNbBits
if largestBits <= maxNbBits {
Expand All @@ -627,14 +629,14 @@ func (s *Scratch) setMaxHeight(lastNonNull int) uint8 {
baseCost := int(1) << (largestBits - maxNbBits)
n := uint32(lastNonNull)

for huffNode[n].nbBits > maxNbBits {
totalCost += baseCost - (1 << (largestBits - huffNode[n].nbBits))
huffNode[n].nbBits = maxNbBits
for huffNode[n].nbBits() > maxNbBits {
totalCost += baseCost - (1 << (largestBits - huffNode[n].nbBits()))
huffNode[n].setNbBits(maxNbBits)
n--
}
// n stops at huffNode[n].nbBits <= maxNbBits

for huffNode[n].nbBits == maxNbBits {
for huffNode[n].nbBits() == maxNbBits {
n--
}
// n end at index of smallest symbol using < maxNbBits
Expand All @@ -655,10 +657,10 @@ func (s *Scratch) setMaxHeight(lastNonNull int) uint8 {
{
currentNbBits := maxNbBits
for pos := int(n); pos >= 0; pos-- {
if huffNode[pos].nbBits >= currentNbBits {
if huffNode[pos].nbBits() >= currentNbBits {
continue
}
currentNbBits = huffNode[pos].nbBits // < maxNbBits
currentNbBits = huffNode[pos].nbBits() // < maxNbBits
rankLast[maxNbBits-currentNbBits] = uint32(pos)
}
}
Expand All @@ -675,8 +677,8 @@ func (s *Scratch) setMaxHeight(lastNonNull int) uint8 {
if lowPos == noSymbol {
break
}
highTotal := huffNode[highPos].count
lowTotal := 2 * huffNode[lowPos].count
highTotal := huffNode[highPos].count()
lowTotal := 2 * huffNode[lowPos].count()
if highTotal <= lowTotal {
break
}
Expand All @@ -692,39 +694,57 @@ func (s *Scratch) setMaxHeight(lastNonNull int) uint8 {
// this rank is no longer empty
rankLast[nBitsToDecrease-1] = rankLast[nBitsToDecrease]
}
huffNode[rankLast[nBitsToDecrease]].nbBits++
huffNode[rankLast[nBitsToDecrease]].setNbBits(1 +
huffNode[rankLast[nBitsToDecrease]].nbBits())
if rankLast[nBitsToDecrease] == 0 {
/* special case, reached largest symbol */
rankLast[nBitsToDecrease] = noSymbol
} else {
rankLast[nBitsToDecrease]--
if huffNode[rankLast[nBitsToDecrease]].nbBits != maxNbBits-nBitsToDecrease {
if huffNode[rankLast[nBitsToDecrease]].nbBits() != maxNbBits-nBitsToDecrease {
rankLast[nBitsToDecrease] = noSymbol /* this rank is now empty */
}
}
}

for totalCost < 0 { /* Sometimes, cost correction overshoot */
if rankLast[1] == noSymbol { /* special case : no rank 1 symbol (using maxNbBits-1); let's create one from largest rank 0 (using maxNbBits) */
for huffNode[n].nbBits == maxNbBits {
for huffNode[n].nbBits() == maxNbBits {
n--
}
huffNode[n+1].nbBits--
huffNode[n+1].setNbBits(huffNode[n+1].nbBits() - 1)
rankLast[1] = n + 1
totalCost++
continue
}
huffNode[rankLast[1]+1].nbBits--
huffNode[rankLast[1]+1].setNbBits(huffNode[rankLast[1]+1].nbBits() - 1)
rankLast[1]++
totalCost++
}
}
return maxNbBits
}

type nodeElt struct {
count uint32
parent uint16
symbol byte
nbBits uint8
// A nodeElt is the fields
//
// count uint32
// parent uint16
// symbol byte
// nbBits uint8
//
// in some order, all squashed into an integer so that the compiler
// always loads and stores entire nodeElts instead of separate fields.
type nodeElt uint64

func makeNodeElt(count uint32, symbol byte) nodeElt {
return nodeElt(count) | nodeElt(symbol)<<48
}

func (e *nodeElt) count() uint32 { return uint32(*e) }
func (e *nodeElt) parent() uint16 { return uint16(*e >> 32) }
func (e *nodeElt) symbol() byte { return byte(*e >> 48) }
func (e *nodeElt) nbBits() uint8 { return uint8(*e >> 56) }

func (e *nodeElt) setCount(c uint32) { *e = (*e)&0xffffffff00000000 | nodeElt(c) }
func (e *nodeElt) setParent(p int16) { *e = (*e)&0xffff0000ffffffff | nodeElt(uint16(p))<<32 }
func (e *nodeElt) setNbBits(n uint8) { *e = (*e)&0x00ffffffffffffff | nodeElt(n)<<56 }