blockfilter: Implement GCSFilter constructors.

1 files changed, 151 insertions, 0 deletions

diff --git a/src/blockfilter.cpp b/src/blockfilter.cpp
new file mode 100644
index 0000000000..e4c95ccfbd
--- /dev/null
+++ b/src/blockfilter.cpp
@@ -0,0 +1,151 @@
+// 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();
+}