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// Copyright (c) 2018 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#include <blockfilter.h>
#include <hash.h>
#include <streams.h>
/// SerType used to serialize parameters in GCS filter encoding.
static constexpr int GCS_SER_TYPE = SER_NETWORK;
/// Protocol version used to serialize parameters in GCS filter encoding.
static constexpr int GCS_SER_VERSION = 0;
template <typename OStream>
static void GolombRiceEncode(BitStreamWriter<OStream>& bitwriter, uint8_t P, uint64_t x)
{
// Write quotient as unary-encoded: q 1's followed by one 0.
uint64_t q = x >> P;
while (q > 0) {
int nbits = q <= 64 ? static_cast<int>(q) : 64;
bitwriter.Write(~0ULL, nbits);
q -= nbits;
}
bitwriter.Write(0, 1);
// Write the remainder in P bits. Since the remainder is just the bottom
// P bits of x, there is no need to mask first.
bitwriter.Write(x, P);
}
template <typename IStream>
static uint64_t GolombRiceDecode(BitStreamReader<IStream>& bitreader, uint8_t P)
{
// Read unary-encoded quotient: q 1's followed by one 0.
uint64_t q = 0;
while (bitreader.Read(1) == 1) {
++q;
}
uint64_t r = bitreader.Read(P);
return (q << P) + r;
}
// Map a value x that is uniformly distributed in the range [0, 2^64) to a
// value uniformly distributed in [0, n) by returning the upper 64 bits of
// x * n.
//
// See: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
static uint64_t MapIntoRange(uint64_t x, uint64_t n)
{
// To perform the calculation on 64-bit numbers without losing the
// result to overflow, split the numbers into the most significant and
// least significant 32 bits and perform multiplication piece-wise.
//
// See: https://stackoverflow.com/a/26855440
uint64_t x_hi = x >> 32;
uint64_t x_lo = x & 0xFFFFFFFF;
uint64_t n_hi = n >> 32;
uint64_t n_lo = n & 0xFFFFFFFF;
uint64_t ac = x_hi * n_hi;
uint64_t ad = x_hi * n_lo;
uint64_t bc = x_lo * n_hi;
uint64_t bd = x_lo * n_lo;
uint64_t mid34 = (bd >> 32) + (bc & 0xFFFFFFFF) + (ad & 0xFFFFFFFF);
uint64_t upper64 = ac + (bc >> 32) + (ad >> 32) + (mid34 >> 32);
return upper64;
}
uint64_t GCSFilter::HashToRange(const Element& element) const
{
uint64_t hash = CSipHasher(m_siphash_k0, m_siphash_k1)
.Write(element.data(), element.size())
.Finalize();
return MapIntoRange(hash, m_F);
}
std::vector<uint64_t> GCSFilter::BuildHashedSet(const ElementSet& elements) const
{
std::vector<uint64_t> hashed_elements;
hashed_elements.reserve(elements.size());
for (const Element& element : elements) {
hashed_elements.push_back(HashToRange(element));
}
std::sort(hashed_elements.begin(), hashed_elements.end());
return hashed_elements;
}
GCSFilter::GCSFilter(uint64_t siphash_k0, uint64_t siphash_k1, uint8_t P, uint32_t M)
: m_siphash_k0(siphash_k0), m_siphash_k1(siphash_k1), m_P(P), m_M(M), m_N(0), m_F(0)
{}
GCSFilter::GCSFilter(uint64_t siphash_k0, uint64_t siphash_k1, uint8_t P, uint32_t M,
std::vector<unsigned char> encoded_filter)
: GCSFilter(siphash_k0, siphash_k1, P, M)
{
m_encoded = std::move(encoded_filter);
VectorReader stream(GCS_SER_TYPE, GCS_SER_VERSION, m_encoded, 0);
uint64_t N = ReadCompactSize(stream);
m_N = static_cast<uint32_t>(N);
if (m_N != N) {
throw std::ios_base::failure("N must be <2^32");
}
m_F = static_cast<uint64_t>(m_N) * static_cast<uint64_t>(m_M);
// Verify that the encoded filter contains exactly N elements. If it has too much or too little
// data, a std::ios_base::failure exception will be raised.
BitStreamReader<VectorReader> bitreader(stream);
for (uint64_t i = 0; i < m_N; ++i) {
GolombRiceDecode(bitreader, m_P);
}
if (!stream.empty()) {
throw std::ios_base::failure("encoded_filter contains excess data");
}
}
GCSFilter::GCSFilter(uint64_t siphash_k0, uint64_t siphash_k1, uint8_t P, uint32_t M,
const ElementSet& elements)
: GCSFilter(siphash_k0, siphash_k1, P, M)
{
size_t N = elements.size();
m_N = static_cast<uint32_t>(N);
if (m_N != N) {
throw std::invalid_argument("N must be <2^32");
}
m_F = static_cast<uint64_t>(m_N) * static_cast<uint64_t>(m_M);
CVectorWriter stream(GCS_SER_TYPE, GCS_SER_VERSION, m_encoded, 0);
WriteCompactSize(stream, m_N);
if (elements.empty()) {
return;
}
BitStreamWriter<CVectorWriter> bitwriter(stream);
uint64_t last_value = 0;
for (uint64_t value : BuildHashedSet(elements)) {
uint64_t delta = value - last_value;
GolombRiceEncode(bitwriter, m_P, delta);
last_value = value;
}
bitwriter.Flush();
}
bool GCSFilter::MatchInternal(const uint64_t* element_hashes, size_t size) const
{
VectorReader stream(GCS_SER_TYPE, GCS_SER_VERSION, m_encoded, 0);
// Seek forward by size of N
uint64_t N = ReadCompactSize(stream);
assert(N == m_N);
BitStreamReader<VectorReader> bitreader(stream);
uint64_t value = 0;
size_t hashes_index = 0;
for (uint32_t i = 0; i < m_N; ++i) {
uint64_t delta = GolombRiceDecode(bitreader, m_P);
value += delta;
while (true) {
if (hashes_index == size) {
return false;
} else if (element_hashes[hashes_index] == value) {
return true;
} else if (element_hashes[hashes_index] > value) {
break;
}
hashes_index++;
}
}
return false;
}
bool GCSFilter::Match(const Element& element) const
{
uint64_t query = HashToRange(element);
return MatchInternal(&query, 1);
}
bool GCSFilter::MatchAny(const ElementSet& elements) const
{
const std::vector<uint64_t> queries = BuildHashedSet(elements);
return MatchInternal(queries.data(), queries.size());
}
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