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Derive other indexes directly for binary fuse #54

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70 changes: 48 additions & 22 deletions binaryfusefilter.go
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Original file line number Diff line number Diff line change
Expand Up @@ -79,10 +79,6 @@ func buildBinaryFuse[T Unsigned](b *BinaryFuseBuilder, keys []uint64) (_ BinaryF
reverseOrder := reuseBuffer[uint64](&b.reverseOrder, int(size+1))
reverseOrder[size] = 1

// the array h0, h1, h2, h0, h1, h2
var h012 [6]uint32
// this could be used to compute the mod3
// tabmod3 := [5]uint8{0,1,2,0,1}
for {
iterations += 1
if iterations > MaxIterations {
Expand Down Expand Up @@ -194,6 +190,9 @@ func buildBinaryFuse[T Unsigned](b *BinaryFuseBuilder, keys []uint64) (_ BinaryF
}
}
stacksize := uint32(0)
segLen := filter.SegmentLength
// segLenToMinusSegLenX2 is used to change segLen to -2*segLen via XOR.
segLenToMinusSegLenX2 := segLen ^ (-(2 * segLen))
for Qsize > 0 {
Qsize--
index := alone[Qsize]
Expand All @@ -204,29 +203,63 @@ func buildBinaryFuse[T Unsigned](b *BinaryFuseBuilder, keys []uint64) (_ BinaryF
reverseOrder[stacksize] = hash
stacksize++

index1, index2, index3 := filter.getHashFromHash(hash)

h012[1] = index2
h012[2] = index3
h012[3] = index1
h012[4] = h012[1]
// Here, we could use filter.getHashFromHash(hash) to obtain the other
// two indexes. But we can manipulate the formulas to derive them more
// efficiently. We use bit tricks to avoid branching.

h01 := uint32(hash>>18) & filter.SegmentLengthMask
h02 := uint32(hash) & filter.SegmentLengthMask

// These variables are either 0 or all 1s.
is0 := -uint32((found - 1) >> 7) // all 1s if found==0 (relies on uint8 wrap)
is1 := -uint32(found & 1) // all 1s if found==1
is2 := -uint32(found >> 1) // all 1s if found==2

// First, adjust the segment index. other_index1 is:
// if found<2: index + segLen
// if found=2: index - segLen*2
other_index1 := index + (segLen ^ (segLenToMinusSegLenX2 & is2))
// other_index2 is:
// if found>0: index - segLen
// if found=0: index + 2*segLen
other_index2 := index - (segLen ^ (segLenToMinusSegLenX2 & is0))

// Now adjust the offset inside the segment.
// Three cases:
// 0: other_index1 ^= h01 other_index2 ^= h02
// 1: other_index1 ^= h01^h02 other_index2 ^= h01
// 2: other_index1 ^= h02 other_index2 ^= h01^h02
other_index1 ^= (h01 &^ is2) ^ (h02 &^ is0)
other_index2 ^= (h01 &^ is0) ^ (h02 &^ is1)

f1 := uint8(is0&1 | is1&2) // f1 = (found + 1) % 3
f2 := uint8(is0&2 | is2&1) // f2 = (found + 2) % 3

// Verification. Turn on for debugging.
if false {
index1, index2, index3 := filter.getHashFromHash(hash)
if other_index1 != []uint32{index1, index2, index3}[(found+1)%3] {
panic("incorrect other_index1")
}
if other_index2 != []uint32{index1, index2, index3}[(found+2)%3] {
panic("incorrect other_index2")
}
}

other_index1 := h012[found+1]
alone[Qsize] = other_index1
if (t2count[other_index1] >> 2) == 2 {
Qsize++
}
t2count[other_index1] -= 4
t2count[other_index1] ^= filter.mod3(found + 1) // could use this instead: tabmod3[found+1]
t2count[other_index1] ^= f1
t2hash[other_index1] ^= hash

other_index2 := h012[found+2]
alone[Qsize] = other_index2
if (t2count[other_index2] >> 2) == 2 {
Qsize++
}
t2count[other_index2] -= 4
t2count[other_index2] ^= filter.mod3(found + 2) // could use this instead: tabmod3[found+2]
t2count[other_index2] ^= f2
t2hash[other_index2] ^= hash
}
}
Expand Down Expand Up @@ -255,6 +288,7 @@ func buildBinaryFuse[T Unsigned](b *BinaryFuseBuilder, keys []uint64) (_ BinaryF
return filter, iterations, nil
}

var h012 [5]uint32
for i := int(size - 1); i >= 0; i-- {
// the hash of the key we insert next
hash := reverseOrder[i]
Expand Down Expand Up @@ -297,14 +331,6 @@ func (filter *BinaryFuse[T]) initializeParameters(b *BinaryFuseBuilder, size uin
filter.Fingerprints = reuseBuffer[T](&b.fingerprints, int(arrayLength))
}

func (filter *BinaryFuse[T]) mod3(x uint8) uint8 {
if x > 2 {
x -= 3
}

return x
}

func (filter *BinaryFuse[T]) getHashFromHash(hash uint64) (uint32, uint32, uint32) {
hi, _ := bits.Mul64(hash, uint64(filter.SegmentCountLength))
h0 := uint32(hi)
Expand Down
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