140 lines
4.7 KiB
C++
140 lines
4.7 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <folly/Fingerprint.h>
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#include <folly/Portability.h>
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#include <folly/Utility.h>
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#include <folly/detail/FingerprintPolynomial.h>
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#include <utility>
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namespace folly {
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namespace detail {
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namespace {
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// The polynomials below were generated by a separate program that requires the
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// NTL (Number Theory Library) from http://www.shoup.net/ntl/
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//
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// Briefly: randomly generate a polynomial of degree D, test for
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// irreducibility, repeat until you find an irreducible polynomial
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// (roughly 1/D of all polynomials of degree D are irreducible, so
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// this will succeed in D/2 tries on average; D is small (64..128) so
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// this simple method works well)
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//
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// DO NOT REPLACE THE POLYNOMIALS USED, EVER, as that would change the value
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// of every single fingerprint in existence.
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template <size_t Deg>
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struct FingerprintTablePoly;
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template <>
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struct FingerprintTablePoly<63> {
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static constexpr uint64_t data[1] = {0xbf3736b51869e9b7};
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};
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template <>
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struct FingerprintTablePoly<95> {
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static constexpr uint64_t data[2] = {0x51555cb0aa8d39c3, 0xb679ec3700000000};
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};
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template <>
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struct FingerprintTablePoly<127> {
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static constexpr uint64_t data[2] = {0xc91bff9b8768b51b, 0x8c5d5853bd77b0d3};
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};
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template <typename D, size_t S0, size_t... I0>
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constexpr auto copy_table(D const (&table)[S0], std::index_sequence<I0...>) {
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using array = std::array<D, S0>;
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return array{{table[I0]...}};
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}
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template <typename D, size_t S0>
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constexpr auto copy_table(D const (&table)[S0]) {
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return copy_table(table, std::make_index_sequence<S0>{});
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}
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template <typename D, size_t S0, size_t S1, size_t... I0>
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constexpr auto copy_table(
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D const (&table)[S0][S1],
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std::index_sequence<I0...>) {
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using array = std::array<std::array<D, S1>, S0>;
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return array{{copy_table(table[I0])...}};
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}
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template <typename D, size_t S0, size_t S1>
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constexpr auto copy_table(D const (&table)[S0][S1]) {
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return copy_table(table, std::make_index_sequence<S0>{});
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}
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template <typename D, size_t S0, size_t S1, size_t S2, size_t... I0>
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constexpr auto copy_table(
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D const (&table)[S0][S1][S2],
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std::index_sequence<I0...>) {
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using array = std::array<std::array<std::array<D, S2>, S1>, S0>;
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return array{{copy_table(table[I0])...}};
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}
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template <typename D, size_t S0, size_t S1, size_t S2>
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constexpr auto copy_table(D const (&table)[S0][S1][S2]) {
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return copy_table(table, std::make_index_sequence<S0>{});
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}
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template <size_t Deg>
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constexpr poly_table<Deg> make_poly_table() {
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FingerprintPolynomial<Deg> poly(FingerprintTablePoly<Deg>::data);
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uint64_t table[8][256][poly_size(Deg)] = {};
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// table[i][q] is Q(X) * X^(k+8*i) mod P(X),
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// where k is the number of bits in the fingerprint (and deg(P)) and
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// Q(X) = q7*X^7 + q6*X^6 + ... + q1*X + q0 is a degree-7 polyonomial
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// whose coefficients are the bits of q.
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for (uint16_t x = 0; x < 256; x++) {
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FingerprintPolynomial<Deg> t;
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t.setHigh8Bits(uint8_t(x));
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for (auto& entry : table) {
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t.mulXkmod(8, poly);
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for (size_t j = 0; j < poly_size(Deg); ++j) {
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entry[x][j] = t.get(j);
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}
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}
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}
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return copy_table(table);
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}
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// private global variables marked constexpr to enforce that make_poly_table is
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// really invoked at constexpr time, which would not otherwise be guaranteed
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FOLLY_STORAGE_CONSTEXPR auto const poly_table_63 = make_poly_table<63>();
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FOLLY_STORAGE_CONSTEXPR auto const poly_table_95 = make_poly_table<95>();
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FOLLY_STORAGE_CONSTEXPR auto const poly_table_127 = make_poly_table<127>();
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} // namespace
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template <>
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const uint64_t FingerprintTable<64>::poly[poly_size(64)] = {
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FingerprintTablePoly<63>::data[0]};
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template <>
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const uint64_t FingerprintTable<96>::poly[poly_size(96)] = {
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FingerprintTablePoly<95>::data[0],
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FingerprintTablePoly<95>::data[1]};
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template <>
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const uint64_t FingerprintTable<128>::poly[poly_size(128)] = {
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FingerprintTablePoly<127>::data[0],
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FingerprintTablePoly<127>::data[1]};
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template <>
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const poly_table<64> FingerprintTable<64>::table = poly_table_63;
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template <>
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const poly_table<96> FingerprintTable<96>::table = poly_table_95;
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template <>
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const poly_table<128> FingerprintTable<128>::table = poly_table_127;
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} // namespace detail
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} // namespace folly
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