robin_growth_policy.h

Go to the documentation of this file.

/**
 * MIT License
 *
 * Copyright (c) 2017 Thibaut Goetghebuer-Planchon <tessil@gmx.com>
 *
 * 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_ROBIN_GROWTH_POLICY_H
#define TSL_ROBIN_GROWTH_POLICY_H
 
#include <algorithm>
#include <array>
#include <climits>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <ratio>
#include <stdexcept>
 
#ifdef TSL_DEBUG
#define tsl_rh_assert(expr) assert(expr)
#else
#define tsl_rh_assert(expr) (static_cast<void>(0))
#endif
 
/**
 * If exceptions are enabled, throw the exception passed in parameter, otherwise
 * call std::terminate.
 */
#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) ||                                         \
     (defined(_MSC_VER) && defined(_CPPUNWIND))) &&                                                \
    !defined(TSL_NO_EXCEPTIONS)
#define TSL_RH_THROW_OR_TERMINATE(ex, msg) throw ex(msg)
#else
#define TSL_RH_NO_EXCEPTIONS
#ifdef NDEBUG
#define TSL_RH_THROW_OR_TERMINATE(ex, msg) std::terminate()
#else
#include <iostream>
#define TSL_RH_THROW_OR_TERMINATE(ex, msg)                                                         \
  do {                                                                                             \
    std::cerr << msg << std::endl;                                                                 \
    std::terminate();                                                                              \
  } while (0)
#endif
#endif
 
#if defined(__GNUC__) || defined(__clang__)
#define TSL_RH_LIKELY(exp) (__builtin_expect(!!(exp), true))
#else
#define TSL_RH_LIKELY(exp) (exp)
#endif
 
#define TSL_RH_UNUSED(x) static_cast<void>(x)
 
namespace tsl {
  namespace rh {
 
    /**
     * 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 <std::size_t GrowthFactor> 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).
       *
       * If 0 is given, min_bucket_count_in_out must still be 0 after the policy
       * creation and bucket_for_hash must always return 0 in this case.
       */
      explicit power_of_two_growth_policy(std::size_t &min_bucket_count_in_out)
      {
        if (min_bucket_count_in_out > max_bucket_count()) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum size.");
        }
 
        if (min_bucket_count_in_out > 0) {
          min_bucket_count_in_out = round_up_to_power_of_two(min_bucket_count_in_out);
          m_mask                  = min_bucket_count_in_out - 1;
        }
        else {
          m_mask = 0;
        }
      }
 
      /**
       * Return the bucket [0, bucket_count()) to which the hash belongs.
       * If bucket_count() is 0, it must always return 0.
       */
      std::size_t bucket_for_hash(std::size_t hash) const noexcept { return hash & m_mask; }
 
      /**
       * Return the number of buckets that should be used on next growth.
       */
      std::size_t next_bucket_count() const
      {
        if ((m_mask + 1) > max_bucket_count() / GrowthFactor) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum 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<std::size_t>::max() / 2) + 1;
      }
 
      /**
       * Reset the growth policy as if it was created with a bucket count of 0.
       * After a clear, the policy must always return 0 when bucket_for_hash is
       * called.
       */
      void clear() noexcept { m_mask = 0; }
 
    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_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 useful if you want a slower
     * growth.
     */
    template <class GrowthFactor = std::ratio<3, 2>> class mod_growth_policy
    {
    public:
      explicit mod_growth_policy(std::size_t &min_bucket_count_in_out)
      {
        if (min_bucket_count_in_out > max_bucket_count()) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum size.");
        }
 
        if (min_bucket_count_in_out > 0) {
          m_mod = min_bucket_count_in_out;
        }
        else {
          m_mod = 1;
        }
      }
 
      std::size_t bucket_for_hash(std::size_t hash) const noexcept { return hash % m_mod; }
 
      std::size_t next_bucket_count() const
      {
        if (m_mod == max_bucket_count()) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum size.");
        }
 
        const double next_bucket_count =
            std::ceil(double(m_mod) * REHASH_SIZE_MULTIPLICATION_FACTOR);
        if (!std::isnormal(next_bucket_count)) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum 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; }
 
      void clear() noexcept { m_mod = 1; }
 
    private:
      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<std::size_t>::max() / REHASH_SIZE_MULTIPLICATION_FACTOR));
 
      static_assert(REHASH_SIZE_MULTIPLICATION_FACTOR >= 1.1, "Growth factor should be >= 1.1.");
 
      std::size_t m_mod;
    };
 
    namespace detail {
 
#if SIZE_MAX >= ULLONG_MAX
#define TSL_RH_NB_PRIMES 51
#elif SIZE_MAX >= ULONG_MAX
#define TSL_RH_NB_PRIMES 40
#else
#define TSL_RH_NB_PRIMES 23
#endif
 
      static constexpr const std::array<std::size_t, TSL_RH_NB_PRIMES> PRIMES = {{
          1u,
          5u,
          17u,
          29u,
          37u,
          53u,
          67u,
          79u,
          97u,
          131u,
          193u,
          257u,
          389u,
          521u,
          769u,
          1031u,
          1543u,
          2053u,
          3079u,
          6151u,
          12289u,
          24593u,
          49157u,
#if SIZE_MAX >= ULONG_MAX
          98317ul,
          196613ul,
          393241ul,
          786433ul,
          1572869ul,
          3145739ul,
          6291469ul,
          12582917ul,
          25165843ul,
          50331653ul,
          100663319ul,
          201326611ul,
          402653189ul,
          805306457ul,
          1610612741ul,
          3221225473ul,
          4294967291ul,
#endif
#if SIZE_MAX >= ULLONG_MAX
          6442450939ull,
          12884901893ull,
          25769803751ull,
          51539607551ull,
          103079215111ull,
          206158430209ull,
          412316860441ull,
          824633720831ull,
          1649267441651ull,
          3298534883309ull,
          6597069766657ull,
#endif
      }};
 
      template <unsigned int IPrime> 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<std::size_t (*)(std::size_t), TSL_RH_NB_PRIMES> 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>,
#if SIZE_MAX >= ULONG_MAX
              &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>,
              &mod<39>,
#endif
#if SIZE_MAX >= ULLONG_MAX
              &mod<40>, &mod<41>, &mod<42>, &mod<43>, &mod<44>, &mod<45>, &mod<46>, &mod<47>,
              &mod<48>, &mod<49>, &mod<50>,
#endif
          }};
 
    } // namespace detail
 
    /**
     * Grow the hash table by using prime numbers as bucket count. Slower than
     * tsl::rh::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 environment.
     */
    class prime_growth_policy
    {
    public:
      explicit 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()) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum size.");
        }
 
        m_iprime = static_cast<unsigned int>(std::distance(detail::PRIMES.begin(), it_prime));
        if (min_bucket_count_in_out > 0) {
          min_bucket_count_in_out = *it_prime;
        }
        else {
          min_bucket_count_in_out = 0;
        }
      }
 
      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()) {
          TSL_RH_THROW_OR_TERMINATE(std::length_error, "The hash table exceeds its maximum size.");
        }
 
        return detail::PRIMES[m_iprime + 1];
      }
 
      std::size_t max_bucket_count() const { return detail::PRIMES.back(); }
 
      void clear() noexcept { m_iprime = 0; }
 
    private:
      unsigned int m_iprime;
 
      static_assert(std::numeric_limits<decltype(m_iprime)>::max() >= detail::PRIMES.size(),
                    "The type of m_iprime is not big enough.");
    };
 
  } // namespace rh
} // namespace tsl
 
#endif