blockfilter.cpp
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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 <crypto/siphash.h>
	#include <hash.h>
	#include <primitives/transaction.h>
	#include <script/script.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) {
	#ifdef __SIZEOF_INT128__
	return (static_cast<unsigned __int128>(x) *
	static_cast<unsigned __int128>(n)) >>
	64;
	#else
	// 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;
	#endif
	}

	uint64_t GCSFilter::HashToRange(const Element &element) const {
	uint64_t hash = CSipHasher(m_params.m_siphash_k0, m_params.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(const Params &params)
	: m_params(params), m_N(0), m_F(0), m_encoded{0} {}

	GCSFilter::GCSFilter(const Params &params, std::vector<uint8_t> encoded_filter)
	: m_params(params), 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_params.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_params.m_P);
	}
	if (!stream.empty()) {
	throw std::ios_base::failure("encoded_filter contains excess data");
	}
	}

	GCSFilter::GCSFilter(const Params &params, const ElementSet &elements)
	: m_params(params) {
	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_params.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_params.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_params.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());
	}

	static GCSFilter::ElementSet BasicFilterElements(const CBlock &block,
	const CBlockUndo &block_undo) {
	GCSFilter::ElementSet elements;

	for (const CTransactionRef &tx : block.vtx) {
	for (const CTxOut &txout : tx->vout) {
	const CScript &script = txout.scriptPubKey;
	if (script.empty() \|\| script[0] == OP_RETURN) {
	continue;
	}
	elements.emplace(script.begin(), script.end());
	}
	}

	for (const CTxUndo &tx_undo : block_undo.vtxundo) {
	for (const Coin &prevout : tx_undo.vprevout) {
	const CScript &script = prevout.GetTxOut().scriptPubKey;
	if (script.empty()) {
	continue;
	}
	elements.emplace(script.begin(), script.end());
	}
	}

	return elements;
	}

	BlockFilter::BlockFilter(BlockFilterType filter_type, const uint256 &block_hash,
	std::vector<uint8_t> filter)
	: m_filter_type(filter_type), m_block_hash(block_hash) {
	GCSFilter::Params params;
	if (!BuildParams(params)) {
	throw std::invalid_argument("unknown filter_type");
	}
	m_filter = GCSFilter(params, std::move(filter));
	}

	BlockFilter::BlockFilter(BlockFilterType filter_type, const CBlock &block,
	const CBlockUndo &block_undo)
	: m_filter_type(filter_type), m_block_hash(block.GetHash()) {
	GCSFilter::Params params;
	if (!BuildParams(params)) {
	throw std::invalid_argument("unknown filter_type");
	}
	m_filter = GCSFilter(params, BasicFilterElements(block, block_undo));
	}

	bool BlockFilter::BuildParams(GCSFilter::Params &params) const {
	switch (m_filter_type) {
	case BlockFilterType::BASIC:
	params.m_siphash_k0 = m_block_hash.GetUint64(0);
	params.m_siphash_k1 = m_block_hash.GetUint64(1);
	params.m_P = BASIC_FILTER_P;
	params.m_M = BASIC_FILTER_M;
	return true;
	case BlockFilterType::INVALID:
	return false;
	}

	return false;
	}

	uint256 BlockFilter::GetHash() const {
	const std::vector<uint8_t> &data = GetEncodedFilter();

	uint256 result;
	CHash256().Write(data.data(), data.size()).Finalize(result.begin());
	return result;
	}

	uint256 BlockFilter::ComputeHeader(const uint256 &prev_header) const {
	const uint256 &filter_hash = GetHash();

	uint256 result;
	CHash256()
	.Write(filter_hash.begin(), filter_hash.size())
	.Write(prev_header.begin(), prev_header.size())
	.Finalize(result.begin());
	return result;
	}

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