gcs: add package for building and using Golomb-coded set filters

LICENSE

@@ -1,6 +1,7 @@
 ISC License
 
-Copyright (c) 2013-2016 The btcsuite developers
+Copyright (c) 2013-2017 The btcsuite developers
+Copyright (c) 2016-2017 The Lightning Network Developers
 
 Permission to use, copy, modify, and distribute this software for any
 purpose with or without fee is hereby granted, provided that the above

gcs/README.md

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+base58
+==========
+
+[![Build Status](http://img.shields.io/travis/btcsuite/btcutil.svg)]
+(https://travis-ci.org/btcsuite/btcutil) [![ISC License]
+(http://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
+[![GoDoc](https://godoc.org/github.com/btcsuite/btcutil/gcs?status.png)]
+(http://godoc.org/github.com/btcsuite/btcutil/gcs)
+
+Package gcs provides an API for building and using a Golomb-coded set filter
+similar to that described [here].(http://giovanni.bajo.it/post/47119962313/golomb-coded-sets-smaller-than-bloom-filters)
+
+A comprehensive suite of tests is provided to ensure proper functionality.
+
+## Installation and Updating
+
+```bash
+$ go get -u github.com/btcsuite/btcutil/gcs
+```
+
+## License
+
+Package gcs is licensed under the [copyfree](http://copyfree.org) ISC
+License.

gcs/doc.go

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+// Copyright (c) 2016-2017 The btcsuite developers
+// Copyright (c) 2016-2017 The Lightning Network Developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+/*
+Package gcs provides an API for building and using a Golomb-coded set filter.
+
+Golomb-Coded Set
+
+A Golomb-coded set is a probabilistic data structure used similarly to a Bloom
+filter. A filter uses constant-size overhead plus on average n+2 bits per
+item added to the filter, where 2^-1 is the desired false positive (collision)
+probability.
+
+GCS use in Bitcoin
+
+GCS filters are a proposed mechanism for storing and transmitting per-block
+filters in Bitcoin. The usage is intended to be the inverse of Bloom filters:
+a full node would send an SPV node the GCS filter for a block, which the SPV
+node would check against its list of relevant items. The suggested collision
+probability for Bitcoin use is 2^-20.
+*/
+package gcs

gcs/gcs.go

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+// Copyright (c) 2016-2017 The btcsuite developers
+// Copyright (c) 2016-2017 The Lightning Network Developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package gcs
+
+import (
+	"fmt"
+	"io"
+	"sort"
+
+	"github.com/aead/siphash"
+	"github.com/kkdai/bstream"
+)
+
+// Inspired by https://github.com/rasky/gcs
+
+var (
+	// ErrNTooBig signifies that the filter can't handle N items.
+	ErrNTooBig = fmt.Errorf("N is too big to fit in uint32")
+
+	// ErrPTooBig signifies that the filter can't handle `1/2**P`
+	// collision probability.
+	ErrPTooBig = fmt.Errorf("P is too big to fit in uint32")
+
+	// ErrNoData signifies that an empty slice was passed.
+	ErrNoData = fmt.Errorf("No data provided")
+)
+
+const (
+	//KeySize is the size of the byte array required for key material for
+	// the SipHash keyed hash function.
+	KeySize = 16
+)
+
+// gcsFilter describes an immutable filter that can be built from
+// a set of data elements, serialized, deserialized, and queried
+// in a thread-safe manner. The serialized form is compressed as
+// a Golomb Coded Set (GCS), but does not include N or P to allow
+// the user to encode the metadata separately if necessary. The
+// hash function used is SipHash, a keyed function; the key used
+// in building the filter is required in order to match filter
+// values and is not included in the serialized form.
+type gcsFilter struct {
+	n          uint32
+	p          uint8
+	modulusP   uint64
+	modulusNP  uint64
+	filterData []byte
+}
+
+// BuildGCSFilter builds a new GCS filter with the collision probability of
+// `1/(2**P)`, key `key`, and including every `[]byte` in `data` as a member of
+// the set.
+func BuildGCSFilter(P uint8, key [KeySize]byte,
+	data [][]byte) (*gcsFilter, error) {
+
+	// Some initial parameter checks: make sure we have data from which to
+	// build the filter, and make sure our parameters will fit the hash
+	// function we're using.
+	if len(data) == 0 {
+		return nil, ErrNoData
+	}
+	if len(data) > ((1 << 32) - 1) {
+		return nil, ErrNTooBig
+	}
+	if P > 32 {
+		return nil, ErrPTooBig
+	}
+
+	// Create the filter object and insert metadata.
+	f := gcsFilter{
+		n: uint32(len(data)),
+		p: P,
+	}
+	f.modulusP = uint64(1 << f.p)
+	f.modulusNP = uint64(f.n) * f.modulusP
+
+	// Build the filter.
+	var values uint64Slice
+	b := bstream.NewBStreamWriter(0)
+
+	// Insert the hash (modulo N*P) of each data element into a slice and
+	// sort the slice.
+	for _, d := range data {
+		v := siphash.Sum64(d, &key) % f.modulusNP
+		values = append(values, v)
+	}
+	sort.Sort(values)
+
+	// Write the sorted list of values into the filter bitstream,
+	// compressing it using Golomb coding.
+	var value, lastValue, remainder uint64
+	for _, v := range values {
+		// Calculate the difference between this value and the last,
+		// modulo P.
+		remainder = (v - lastValue) % f.modulusP
+		// Calculate the difference between this value and the last,
+		// divided by P.
+		value = (v - lastValue - remainder) / f.modulusP
+		lastValue = v
+		// Write the P multiple into the bitstream in unary; the
+		// average should be around 1 (2 bits - 0b10).
+		for value > 0 {
+			b.WriteBit(true)
+			value--
+		}
+		b.WriteBit(false)
+		// Write the remainder as a big-endian integer with enough bits
+		// to represent the appropriate collision probability.
+		b.WriteBits(remainder, int(f.p))
+	}
+
+	// Copy the bitstream into the filter object and return the object.
+	f.filterData = b.Bytes()
+	return &f, nil
+}
+
+// FromBytes deserializes a GCS filter from a known N, P, and serialized
+// filter as returned by Bytes().
+func FromBytes(N uint32, P uint8, d []byte) (*gcsFilter, error) {
+
+	// Basic sanity check.
+	if P > 32 {
+		return nil, ErrPTooBig
+	}
+
+	// Create the filter object and insert metadata.
+	f := &gcsFilter{
+		n: N,
+		p: P,
+	}
+	f.modulusP = uint64(1 << f.p)
+	f.modulusNP = uint64(f.n) * f.modulusP
+
+	// Copy the filter.
+	f.filterData = make([]byte, len(d))
+	copy(f.filterData, d)
+	return f, nil
+}
+
+// Bytes returns the serialized format of the GCS filter, which does not
+// include N or P (returned by separate methods) or the key used by SipHash.
+func (f *gcsFilter) Bytes() []byte {
+	filterData := make([]byte, len(f.filterData))
+	copy(filterData, f.filterData)
+	return filterData
+}
+
+// P returns the filter's collision probability as a negative power of 2 (that
+// is, a collision probability of `1/2**20` is represented as 20).
+func (f *gcsFilter) P() uint8 {
+	return f.p
+}
+
+// N returns the size of the data set used to build the filter.
+func (f *gcsFilter) N() uint32 {
+	return f.n
+}
+
+// Match checks whether a []byte value is likely (within collision
+// probability) to be a member of the set represented by the filter.
+func (f *gcsFilter) Match(key [KeySize]byte, data []byte) (bool, error) {
+
+	// Create a filter bitstream.
+	filterData := f.Bytes()
+	b := bstream.NewBStreamReader(filterData)
+
+	// Hash our search term with the same parameters as the filter.
+	term := siphash.Sum64(data, &key) % f.modulusNP
+
+	// Go through the search filter and look for the desired value.
+	var lastValue uint64
+	for lastValue < term {
+		// Read the difference between previous and new value
+		// from bitstream.
+		value, err := f.readFullUint64(b)
+		if err != nil {
+			if err == io.EOF {
+				return false, nil
+			}
+			return false, err
+		}
+		// Add the previous value to it.
+		value += lastValue
+		if value == term {
+			return true, nil
+		}
+		lastValue = value
+	}
+	return false, nil
+}
+
+// MatchAny returns checks whether any []byte value is likely (within
+// collision probability) to be a member of the set represented by the
+// filter faster than calling Match() for each value individually.
+func (f *gcsFilter) MatchAny(key [KeySize]byte, data [][]byte) (bool, error) {
+
+	// Basic sanity check.
+	if len(data) == 0 {
+		return false, ErrNoData
+	}
+
+	// Create a filter bitstream.
+	filterData := f.Bytes()
+	b := bstream.NewBStreamReader(filterData)
+
+	// Create an uncompressed filter of the search values.
+	var values uint64Slice
+	for _, d := range data {
+		v := siphash.Sum64(d, &key) % f.modulusNP
+		values = append(values, v)
+	}
+	sort.Sort(values)
+
+	// Zip down the filters, comparing values until we either run out of
+	// values to compare in one of the filters or we reach a matching value.
+	var lastValue1, lastValue2 uint64
+	lastValue2 = values[0]
+	i := 1
+	for lastValue1 != lastValue2 {
+		// Check which filter to advance to make sure we're comparing
+		// the right values.
+		switch {
+		case lastValue1 > lastValue2:
+			// Advance filter created from search terms or return
+			// false if we're at the end because nothing matched.
+			if i < len(values) {
+				lastValue2 = values[i]
+				i++
+			} else {
+				return false, nil
+			}
+		case lastValue2 > lastValue1:
+			// Advance filter we're searching or return false if
+			// we're at the end because nothing matched.
+			value, err := f.readFullUint64(b)
+			if err != nil {
+				if err == io.EOF {
+					return false, nil
+				}
+				return false, err
+			}
+			lastValue1 += value
+		}
+	}
+	// If we've made it this far, an element matched between filters so
+	// we return true.
+	return true, nil
+}
+
+// readFullUint64 reads a value represented by the sum of a unary multiple
+// of the filter's P modulus (`2**P`) and a big-endian P-bit remainder.
+func (f *gcsFilter) readFullUint64(b *bstream.BStream) (uint64, error) {
+	var v uint64
+
+	// Count the 1s until we reach a 0.
+	c, err := b.ReadBit()
+	if err != nil {
+		return 0, err
+	}
+	for c == true {
+		v++
+		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 = v*f.modulusP + remainder
+	return v, nil
+}