ioss/vtkioss/hopscotch_growth_policy.h

/**
 * MIT License
 *
 * Copyright (c) 2018 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_HOPSCOTCH_GROWTH_POLICY_H
#define TSL_HOPSCOTCH_GROWTH_POLICY_H

#include <algorithm>
#include <array>
#include <climits>
#include <cmath>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <ratio>
#include <stdexcept>

/**
 * Only activate tsl_hh_assert if TSL_DEBUG is defined.
 * This way we avoid the performance hit when NDEBUG is not defined with assert
 * as tsl_hh_assert is used a lot (people usually compile with "-O3" and not
 * "-O3 -DNDEBUG").
 */
#ifdef TSL_DEBUG
#define tsl_hh_assert(expr) assert(expr)
#else
#define tsl_hh_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_HH_THROW_OR_TERMINATE(ex, msg) throw ex(msg)
#else
#define TSL_HH_NO_EXCEPTIONS
#ifdef NDEBUG
#define TSL_HH_THROW_OR_TERMINATE(ex, msg) std::terminate()
#else
#include <iostream>
#define TSL_HH_THROW_OR_TERMINATE(ex, msg) \
  do {                                     \
    std::cerr << msg << std::endl;         \
    std::terminate();                      \
  } while (0)
#endif
#endif

namespace tsl {
namespace hh {

/**
 * 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_HH_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 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) {
      TSL_HH_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;
  }

 private:
  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_HH_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_HH_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_HH_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_HH_NB_PRIMES 51
#elif SIZE_MAX >= ULONG_MAX
#define TSL_HH_NB_PRIMES 40
#else
#define TSL_HH_NB_PRIMES 23
#endif

static constexpr const std::array<std::size_t, TSL_HH_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_HH_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::hh::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_HH_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_HH_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 hh
}  // namespace tsl

#endif