/** * MIT License * * Copyright (c) 2017 Tessil * * Permission is hereby granted, free of charge, to any person obtaining a copy * of this software and associated documentation files (the "Software"), to deal * in the Software without restriction, including without limitation the rights * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell * copies of the Software, and to permit persons to whom the Software is * furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice shall be included in all * copies or substantial portions of the Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE * SOFTWARE. */ #ifndef TSL_ARRAY_GROWTH_POLICY_H #define TSL_ARRAY_GROWTH_POLICY_H #include #include #include #include #include #include #include #include #include namespace tsl { namespace ah { /** * Grow the hash table by a factor of GrowthFactor keeping the bucket count to a power of two. It allows * the table to use a mask operation instead of a modulo operation to map a hash to a bucket. * * GrowthFactor must be a power of two >= 2. */ template class power_of_two_growth_policy { public: /** * Called on the hash table creation and on rehash. The number of buckets for the table is passed in parameter. * This number is a minimum, the policy may update this value with a higher value if needed (but not lower). */ power_of_two_growth_policy(std::size_t& min_bucket_count_in_out) { if(min_bucket_count_in_out > max_bucket_count()) { throw std::length_error("The hash table exceeds its maxmimum size."); } static_assert(MIN_BUCKETS_SIZE > 0, "MIN_BUCKETS_SIZE must be > 0."); const std::size_t min_bucket_count = MIN_BUCKETS_SIZE; min_bucket_count_in_out = std::max(min_bucket_count, min_bucket_count_in_out); min_bucket_count_in_out = round_up_to_power_of_two(min_bucket_count_in_out); m_mask = min_bucket_count_in_out - 1; } /** * Return the bucket [0, bucket_count()) to which the hash belongs. */ std::size_t bucket_for_hash(std::size_t hash) const noexcept { return hash & m_mask; } /** * Return the bucket count to use when the bucket array grows on rehash. */ std::size_t next_bucket_count() const { if((m_mask + 1) > max_bucket_count() / GrowthFactor) { throw std::length_error("The hash table exceeds its maxmimum size."); } return (m_mask + 1) * GrowthFactor; } /** * Return the maximum number of buckets supported by the policy. */ std::size_t max_bucket_count() const { // Largest power of two. return (std::numeric_limits::max() / 2) + 1; } private: static std::size_t round_up_to_power_of_two(std::size_t value) { if(is_power_of_two(value)) { return value; } if(value == 0) { return 1; } --value; for(std::size_t i = 1; i < sizeof(std::size_t) * CHAR_BIT; i *= 2) { value |= value >> i; } return value + 1; } static constexpr bool is_power_of_two(std::size_t value) { return value != 0 && (value & (value - 1)) == 0; } protected: static const std::size_t MIN_BUCKETS_SIZE = 2; static_assert(is_power_of_two(GrowthFactor) && GrowthFactor >= 2, "GrowthFactor must be a power of two >= 2."); std::size_t m_mask; }; /** * Grow the hash table by GrowthFactor::num / GrowthFactor::den and use a modulo to map a hash * to a bucket. Slower but it can be usefull if you want a slower growth. */ template> class mod_growth_policy { public: mod_growth_policy(std::size_t& min_bucket_count_in_out) { if(min_bucket_count_in_out > max_bucket_count()) { throw std::length_error("The hash table exceeds its maxmimum size."); } static_assert(MIN_BUCKETS_SIZE > 0, "MIN_BUCKETS_SIZE must be > 0."); const std::size_t min_bucket_count = MIN_BUCKETS_SIZE; min_bucket_count_in_out = std::max(min_bucket_count, min_bucket_count_in_out); m_bucket_count = min_bucket_count_in_out; } std::size_t bucket_for_hash(std::size_t hash) const noexcept { return hash % m_bucket_count; } std::size_t next_bucket_count() const { if(m_bucket_count == max_bucket_count()) { throw std::length_error("The hash table exceeds its maxmimum size."); } const double next_bucket_count = std::ceil(double(m_bucket_count) * REHASH_SIZE_MULTIPLICATION_FACTOR); if(!std::isnormal(next_bucket_count)) { throw std::length_error("The hash table exceeds its maxmimum size."); } if(next_bucket_count > double(max_bucket_count())) { return max_bucket_count(); } else { return std::size_t(next_bucket_count); } } std::size_t max_bucket_count() const { return MAX_BUCKET_COUNT; } private: static const std::size_t MIN_BUCKETS_SIZE = 2; static constexpr double REHASH_SIZE_MULTIPLICATION_FACTOR = 1.0 * GrowthFactor::num / GrowthFactor::den; static const std::size_t MAX_BUCKET_COUNT = std::size_t(double( std::numeric_limits::max() / REHASH_SIZE_MULTIPLICATION_FACTOR )); static_assert(REHASH_SIZE_MULTIPLICATION_FACTOR >= 1.1, "Growth factor should be >= 1.1."); std::size_t m_bucket_count; }; namespace detail { static constexpr const std::array PRIMES = {{ 5ul, 17ul, 29ul, 37ul, 53ul, 67ul, 79ul, 97ul, 131ul, 193ul, 257ul, 389ul, 521ul, 769ul, 1031ul, 1543ul, 2053ul, 3079ul, 6151ul, 12289ul, 24593ul, 49157ul, 98317ul, 196613ul, 393241ul, 786433ul, 1572869ul, 3145739ul, 6291469ul, 12582917ul, 25165843ul, 50331653ul, 100663319ul, 201326611ul, 402653189ul, 805306457ul, 1610612741ul, 3221225473ul, 4294967291ul }}; template static constexpr std::size_t mod(std::size_t hash) { return hash % PRIMES[IPrime]; } // MOD_PRIME[iprime](hash) returns hash % PRIMES[iprime]. This table allows for faster modulo as the // compiler can optimize the modulo code better with a constant known at the compilation. static constexpr const std::array MOD_PRIME = {{ &mod<0>, &mod<1>, &mod<2>, &mod<3>, &mod<4>, &mod<5>, &mod<6>, &mod<7>, &mod<8>, &mod<9>, &mod<10>, &mod<11>, &mod<12>, &mod<13>, &mod<14>, &mod<15>, &mod<16>, &mod<17>, &mod<18>, &mod<19>, &mod<20>, &mod<21>, &mod<22>, &mod<23>, &mod<24>, &mod<25>, &mod<26>, &mod<27>, &mod<28>, &mod<29>, &mod<30>, &mod<31>, &mod<32>, &mod<33>, &mod<34>, &mod<35>, &mod<36>, &mod<37> , &mod<38> }}; } /** * Grow the hash table by using prime numbers as bucket count. Slower than tsl::ah::power_of_two_growth_policy in * general but will probably distribute the values around better in the buckets with a poor hash function. * * To allow the compiler to optimize the modulo operation, a lookup table is used with constant primes numbers. * * With a switch the code would look like: * \code * switch(iprime) { // iprime is the current prime of the hash table * case 0: hash % 5ul; * break; * case 1: hash % 17ul; * break; * case 2: hash % 29ul; * break; * ... * } * \endcode * * Due to the constant variable in the modulo the compiler is able to optimize the operation * by a series of multiplications, substractions and shifts. * * The 'hash % 5' could become something like 'hash - (hash * 0xCCCCCCCD) >> 34) * 5' in a 64 bits environement. */ class prime_growth_policy { public: prime_growth_policy(std::size_t& min_bucket_count_in_out) { auto it_prime = std::lower_bound(detail::PRIMES.begin(), detail::PRIMES.end(), min_bucket_count_in_out); if(it_prime == detail::PRIMES.end()) { throw std::length_error("The hash table exceeds its maxmimum size."); } m_iprime = static_cast(std::distance(detail::PRIMES.begin(), it_prime)); min_bucket_count_in_out = *it_prime; } std::size_t bucket_for_hash(std::size_t hash) const noexcept { return detail::MOD_PRIME[m_iprime](hash); } std::size_t next_bucket_count() const { if(m_iprime + 1 >= detail::PRIMES.size()) { throw std::length_error("The hash table exceeds its maxmimum size."); } return detail::PRIMES[m_iprime + 1]; } std::size_t max_bucket_count() const { return detail::PRIMES.back(); } private: unsigned int m_iprime; static_assert(std::numeric_limits::max() >= detail::PRIMES.size(), "The type of m_iprime is not big enough."); }; } } #endif