9e5f4b9a99
This commit removes the uint64Slice type and performs sorting during filter construction and ZipMatchAny using sort.Slice. The benchmarks indicated that this speeds up BuildGCSFilter and ZipMatchAny by 10-12%, likely to the overhead of needing to resolve the sort.Interface methods. The benchmarks indicate that this improvement is not present for the smallest query size in our benchmarks, i.e. 100 elements, but the reduction is only about 3%. This would indicate the at these small values, the use of reflection is actually slightly slower than interface method resolution in total.
541 lines
15 KiB
Go
541 lines
15 KiB
Go
// Copyright (c) 2016-2017 The btcsuite developers
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// Copyright (c) 2016-2017 The Lightning Network Developers
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// Use of this source code is governed by an ISC
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// license that can be found in the LICENSE file.
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package gcs
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import (
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"bytes"
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"fmt"
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"io"
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"sort"
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"github.com/aead/siphash"
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"github.com/btcsuite/btcd/wire"
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"github.com/kkdai/bstream"
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)
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// Inspired by https://github.com/rasky/gcs
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var (
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// ErrNTooBig signifies that the filter can't handle N items.
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ErrNTooBig = fmt.Errorf("N is too big to fit in uint32")
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// ErrPTooBig signifies that the filter can't handle `1/2**P`
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// collision probability.
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ErrPTooBig = fmt.Errorf("P is too big to fit in uint32")
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)
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const (
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// KeySize is the size of the byte array required for key material for
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// the SipHash keyed hash function.
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KeySize = 16
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// varIntProtoVer is the protocol version to use for serializing N as a
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// VarInt.
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varIntProtoVer uint32 = 0
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)
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// fastReduction calculates a mapping that's more ore less equivalent to: x mod
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// N. However, instead of using a mod operation, which using a non-power of two
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// will lead to slowness on many processors due to unnecessary division, we
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// instead use a "multiply-and-shift" trick which eliminates all divisions,
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// described in:
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// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
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//
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// * v * N >> log_2(N)
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//
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// In our case, using 64-bit integers, log_2 is 64. As most processors don't
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// support 128-bit arithmetic natively, we'll be super portable and unfold the
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// operation into several operations with 64-bit arithmetic. As inputs, we the
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// number to reduce, and our modulus N divided into its high 32-bits and lower
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// 32-bits.
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func fastReduction(v, nHi, nLo uint64) uint64 {
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// First, we'll spit the item we need to reduce into its higher and
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// lower bits.
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vhi := v >> 32
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vlo := uint64(uint32(v))
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// Then, we distribute multiplication over each part.
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vnphi := vhi * nHi
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vnpmid := vhi * nLo
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npvmid := nHi * vlo
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vnplo := vlo * nLo
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// We calculate the carry bit.
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carry := (uint64(uint32(vnpmid)) + uint64(uint32(npvmid)) +
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(vnplo >> 32)) >> 32
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// Last, we add the high bits, the middle bits, and the carry.
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v = vnphi + (vnpmid >> 32) + (npvmid >> 32) + carry
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return v
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}
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// Filter describes an immutable filter that can be built from a set of data
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// elements, serialized, deserialized, and queried in a thread-safe manner. The
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// serialized form is compressed as a Golomb Coded Set (GCS), but does not
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// include N or P to allow the user to encode the metadata separately if
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// necessary. The hash function used is SipHash, a keyed function; the key used
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// in building the filter is required in order to match filter values and is
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// not included in the serialized form.
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type Filter struct {
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n uint32
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p uint8
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modulusNP uint64
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filterData []byte
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}
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// BuildGCSFilter builds a new GCS filter with the collision probability of
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// `1/(2**P)`, key `key`, and including every `[]byte` in `data` as a member of
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// the set.
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func BuildGCSFilter(P uint8, M uint64, key [KeySize]byte, data [][]byte) (*Filter, error) {
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// Some initial parameter checks: make sure we have data from which to
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// build the filter, and make sure our parameters will fit the hash
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// function we're using.
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if uint64(len(data)) >= (1 << 32) {
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return nil, ErrNTooBig
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}
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if P > 32 {
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return nil, ErrPTooBig
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}
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// Create the filter object and insert metadata.
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f := Filter{
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n: uint32(len(data)),
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p: P,
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}
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// First we'll compute the value of m, which is the modulus we use
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// within our finite field. We want to compute: mScalar * 2^P. We use
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// math.Round in order to round the value up, rather than down.
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f.modulusNP = uint64(f.n) * M
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// Shortcut if the filter is empty.
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if f.n == 0 {
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return &f, nil
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}
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// Build the filter.
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values := make([]uint64, 0, len(data))
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b := bstream.NewBStreamWriter(0)
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// Insert the hash (fast-ranged over a space of N*P) of each data
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// element into a slice and sort the slice. This can be greatly
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// optimized with native 128-bit multiplication, but we're going to be
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// fully portable for now.
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//
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// First, we cache the high and low bits of modulusNP for the
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// multiplication of 2 64-bit integers into a 128-bit integer.
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nphi := f.modulusNP >> 32
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nplo := uint64(uint32(f.modulusNP))
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for _, d := range data {
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// For each datum, we assign the initial hash to a uint64.
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v := siphash.Sum64(d, &key)
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v = fastReduction(v, nphi, nplo)
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values = append(values, v)
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}
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sort.Slice(values, func(i, j int) bool { return values[i] < values[j] })
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// Write the sorted list of values into the filter bitstream,
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// compressing it using Golomb coding.
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var value, lastValue, remainder uint64
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for _, v := range values {
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// Calculate the difference between this value and the last,
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// modulo P.
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remainder = (v - lastValue) & ((uint64(1) << f.p) - 1)
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// Calculate the difference between this value and the last,
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// divided by P.
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value = (v - lastValue - remainder) >> f.p
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lastValue = v
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// Write the P multiple into the bitstream in unary; the
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// average should be around 1 (2 bits - 0b10).
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for value > 0 {
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b.WriteBit(true)
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value--
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}
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b.WriteBit(false)
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// Write the remainder as a big-endian integer with enough bits
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// to represent the appropriate collision probability.
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b.WriteBits(remainder, int(f.p))
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}
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// Copy the bitstream into the filter object and return the object.
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f.filterData = b.Bytes()
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return &f, nil
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}
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// FromBytes deserializes a GCS filter from a known N, P, and serialized filter
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// as returned by Bytes().
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func FromBytes(N uint32, P uint8, M uint64, d []byte) (*Filter, error) {
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// Basic sanity check.
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if P > 32 {
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return nil, ErrPTooBig
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}
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// Create the filter object and insert metadata.
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f := &Filter{
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n: N,
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p: P,
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}
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// First we'll compute the value of m, which is the modulus we use
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// within our finite field. We want to compute: mScalar * 2^P. We use
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// math.Round in order to round the value up, rather than down.
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f.modulusNP = uint64(f.n) * M
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// Copy the filter.
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f.filterData = make([]byte, len(d))
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copy(f.filterData, d)
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return f, nil
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}
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// FromNBytes deserializes a GCS filter from a known P, and serialized N and
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// filter as returned by NBytes().
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func FromNBytes(P uint8, M uint64, d []byte) (*Filter, error) {
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buffer := bytes.NewBuffer(d)
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N, err := wire.ReadVarInt(buffer, varIntProtoVer)
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if err != nil {
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return nil, err
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}
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if N >= (1 << 32) {
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return nil, ErrNTooBig
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}
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return FromBytes(uint32(N), P, M, buffer.Bytes())
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}
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// Bytes returns the serialized format of the GCS filter, which does not
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// include N or P (returned by separate methods) or the key used by SipHash.
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func (f *Filter) Bytes() ([]byte, error) {
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filterData := make([]byte, len(f.filterData))
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copy(filterData, f.filterData)
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return filterData, nil
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}
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// NBytes returns the serialized format of the GCS filter with N, which does
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// not include P (returned by a separate method) or the key used by SipHash.
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func (f *Filter) NBytes() ([]byte, error) {
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var buffer bytes.Buffer
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buffer.Grow(wire.VarIntSerializeSize(uint64(f.n)) + len(f.filterData))
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err := wire.WriteVarInt(&buffer, varIntProtoVer, uint64(f.n))
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if err != nil {
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return nil, err
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}
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_, err = buffer.Write(f.filterData)
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if err != nil {
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return nil, err
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}
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return buffer.Bytes(), nil
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}
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// PBytes returns the serialized format of the GCS filter with P, which does
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// not include N (returned by a separate method) or the key used by SipHash.
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func (f *Filter) PBytes() ([]byte, error) {
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filterData := make([]byte, len(f.filterData)+1)
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filterData[0] = f.p
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copy(filterData[1:], f.filterData)
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return filterData, nil
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}
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// NPBytes returns the serialized format of the GCS filter with N and P, which
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// does not include the key used by SipHash.
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func (f *Filter) NPBytes() ([]byte, error) {
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var buffer bytes.Buffer
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buffer.Grow(wire.VarIntSerializeSize(uint64(f.n)) + 1 + len(f.filterData))
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err := wire.WriteVarInt(&buffer, varIntProtoVer, uint64(f.n))
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if err != nil {
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return nil, err
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}
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err = buffer.WriteByte(f.p)
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if err != nil {
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return nil, err
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}
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_, err = buffer.Write(f.filterData)
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if err != nil {
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return nil, err
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}
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return buffer.Bytes(), nil
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}
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// P returns the filter's collision probability as a negative power of 2 (that
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// is, a collision probability of `1/2**20` is represented as 20).
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func (f *Filter) P() uint8 {
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return f.p
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}
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// N returns the size of the data set used to build the filter.
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func (f *Filter) N() uint32 {
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return f.n
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}
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// Match checks whether a []byte value is likely (within collision probability)
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// to be a member of the set represented by the filter.
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func (f *Filter) Match(key [KeySize]byte, data []byte) (bool, error) {
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// Create a filter bitstream.
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filterData, err := f.Bytes()
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if err != nil {
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return false, err
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}
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b := bstream.NewBStreamReader(filterData)
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// We take the high and low bits of modulusNP for the multiplication
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// of 2 64-bit integers into a 128-bit integer.
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nphi := f.modulusNP >> 32
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nplo := uint64(uint32(f.modulusNP))
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// Then we hash our search term with the same parameters as the filter.
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term := siphash.Sum64(data, &key)
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term = fastReduction(term, nphi, nplo)
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// Go through the search filter and look for the desired value.
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var value uint64
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for i := uint32(0); i < f.N(); i++ {
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// Read the difference between previous and new value from
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// bitstream.
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delta, err := f.readFullUint64(b)
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if err != nil {
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if err == io.EOF {
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return false, nil
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}
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return false, err
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}
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// Add the delta to the previous value.
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value += delta
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switch {
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// The current value matches our query term, success.
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case value == term:
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return true, nil
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// The current value is greater than our query term, thus no
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// future decoded value can match because the values
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// monotonically increase.
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case value > term:
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return false, nil
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}
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}
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// All values were decoded and none produced a successful match. This
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// indicates that the items in the filter were all smaller than our
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// target.
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return false, nil
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}
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// MatchAny returns checks whether any []byte value is likely (within collision
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// probability) to be a member of the set represented by the filter faster than
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// calling Match() for each value individually.
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func (f *Filter) MatchAny(key [KeySize]byte, data [][]byte) (bool, error) {
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// TODO(conner): add real heuristics to query optimization
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switch {
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case len(data) >= int(f.N()/2):
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return f.HashMatchAny(key, data)
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default:
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return f.ZipMatchAny(key, data)
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}
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}
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// ZipMatchAny returns checks whether any []byte value is likely (within
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// collision probability) to be a member of the set represented by the filter
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// faster than calling Match() for each value individually.
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//
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// NOTE: This method should outperform HashMatchAny when the number of query
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// entries is smaller than the number of filter entries.
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func (f *Filter) ZipMatchAny(key [KeySize]byte, data [][]byte) (bool, error) {
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// Basic anity check.
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if len(data) == 0 {
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return false, nil
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}
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// Create a filter bitstream.
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filterData, err := f.Bytes()
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if err != nil {
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return false, err
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}
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b := bstream.NewBStreamReader(filterData)
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// Create an uncompressed filter of the search values.
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values := make([]uint64, 0, len(data))
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// First, we cache the high and low bits of modulusNP for the
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// multiplication of 2 64-bit integers into a 128-bit integer.
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nphi := f.modulusNP >> 32
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nplo := uint64(uint32(f.modulusNP))
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for _, d := range data {
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// For each datum, we assign the initial hash to a uint64.
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v := siphash.Sum64(d, &key)
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// We'll then reduce the value down to the range of our
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// modulus.
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v = fastReduction(v, nphi, nplo)
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values = append(values, v)
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}
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sort.Slice(values, func(i, j int) bool { return values[i] < values[j] })
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querySize := len(values)
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// Zip down the filters, comparing values until we either run out of
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// values to compare in one of the filters or we reach a matching
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// value.
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var (
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value uint64
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queryIndex int
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)
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out:
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for i := uint32(0); i < f.N(); i++ {
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// Advance filter we're searching or return false if we're at
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// the end because nothing matched.
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delta, err := f.readFullUint64(b)
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if err != nil {
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if err == io.EOF {
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return false, nil
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}
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return false, err
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}
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value += delta
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for {
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switch {
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// All query items have been exhausted and we haven't
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// had a match, therefore there are no matches.
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case queryIndex == querySize:
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return false, nil
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// The current item in the query matches the decoded
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// value, success.
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case values[queryIndex] == value:
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return true, nil
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// The current item in the query is greater than the
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// current decoded value, continue to decode the next
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// delta and try again.
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case values[queryIndex] > value:
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continue out
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}
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queryIndex++
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}
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}
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// All items in the filter were decoded and none produced a successful
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// match.
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return false, nil
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}
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// HashMatchAny returns checks whether any []byte value is likely (within
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// collision probability) to be a member of the set represented by the filter
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// faster than calling Match() for each value individually.
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//
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// NOTE: This method should outperform MatchAny if the number of query entries
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// approaches the number of filter entries, len(data) >= f.N().
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func (f *Filter) HashMatchAny(key [KeySize]byte, data [][]byte) (bool, error) {
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// Basic sanity check.
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if len(data) == 0 {
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return false, nil
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}
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|
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// Create a filter bitstream.
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filterData, err := f.Bytes()
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if err != nil {
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return false, err
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}
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b := bstream.NewBStreamReader(filterData)
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var (
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values = make(map[uint32]struct{}, f.N())
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lastValue uint64
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)
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// First, decompress the filter and construct an index of the keys
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// contained within the filter. Index construction terminates after all
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// values have been read from the bitstream.
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for {
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// Read the next diff value from the filter, add it to the
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// last value, and set the new value in the index.
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value, err := f.readFullUint64(b)
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if err == nil {
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lastValue += value
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values[uint32(lastValue)] = struct{}{}
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continue
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} else if err == io.EOF {
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break
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}
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return false, err
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}
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|
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// We cache the high and low bits of modulusNP for the multiplication of
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// 2 64-bit integers into a 128-bit integer.
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nphi := f.modulusNP >> 32
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nplo := uint64(uint32(f.modulusNP))
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// Finally, run through the provided data items, querying the index to
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// determine if the filter contains any elements of interest.
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for _, d := range data {
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// For each datum, we assign the initial hash to
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// a uint64.
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v := siphash.Sum64(d, &key)
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// We'll then reduce the value down to the range
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// of our modulus.
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v = fastReduction(v, nphi, nplo)
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if _, ok := values[uint32(v)]; !ok {
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continue
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}
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return true, nil
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}
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return false, nil
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}
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|
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// readFullUint64 reads a value represented by the sum of a unary multiple of
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// the filter's P modulus (`2**P`) and a big-endian P-bit remainder.
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func (f *Filter) readFullUint64(b *bstream.BStream) (uint64, error) {
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var quotient uint64
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// Count the 1s until we reach a 0.
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c, err := b.ReadBit()
|
|
if err != nil {
|
|
return 0, err
|
|
}
|
|
for c {
|
|
quotient++
|
|
c, err = b.ReadBit()
|
|
if err != nil {
|
|
return 0, err
|
|
}
|
|
}
|
|
|
|
// Read P bits.
|
|
remainder, err := b.ReadBits(int(f.p))
|
|
if err != nil {
|
|
return 0, err
|
|
}
|
|
|
|
// Add the multiple and the remainder.
|
|
v := (quotient << f.p) + remainder
|
|
return v, nil
|
|
}
|