simhash.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  version 2.0.0b
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  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 SIMHASH_H
# define SIMHASH_H
 
# include <algorithm>
# include <cmath>
# include <cstdint>
# include <bit>
 
# include <ah-errors.H>
# include <tpl_array.H>
# include <hash-fct.H>
 
namespace Aleph
{
  template <typename T>
  class SimHash
  {
  public:
    static constexpr size_t FINGERPRINT_SIZE = 64;
 
  private:
    Array<double> v_; 
    [[nodiscard]] static uint64_t hash64(const T & val) noexcept
    {
      if constexpr (sizeof(size_t) >= 8)
        return static_cast<uint64_t>(murmur3hash(val, 0x9e3779b9UL));
 
      const uint64_t lo =
        static_cast<uint64_t>(murmur3hash(val, 0x9e3779b9UL)) & 0xffffffffULL;
      const uint64_t hi =
        static_cast<uint64_t>(murmur3hash(val, 0x85ebca6bUL)) & 0xffffffffULL;
      return (hi << 32) | lo;
    }
 
  public:
    SimHash()
      : v_(FINGERPRINT_SIZE, 0.0)
    {}
 
    void update(const T & val, const double weight = 1.0)
    {
      const uint64_t hash = hash64(val);
      for (size_t i = 0; i < FINGERPRINT_SIZE; ++i)
        if ((hash >> i) & 1)
          v_(i) += weight;
        else
          v_(i) -= weight;
    }
 
    template <typename Itor>
    void update(Itor beg, const Itor & end)
    {
      while (beg != end)
        {
          update((*beg).first, (*beg).second);
          ++beg;
        }
    }
 
    [[nodiscard]] uint64_t get_fingerprint() const noexcept
    {
      uint64_t fingerprint = 0;
      for (size_t i = 0; i < FINGERPRINT_SIZE; ++i)
        if (v_(i) > 0)
          fingerprint |= (static_cast<uint64_t>(1) << i);
 
      return fingerprint;
    }
 
    [[nodiscard]] static double similarity(const uint64_t f1, const uint64_t f2) noexcept
    {
      const int distance = std::popcount(f1 ^ f2);
      return 1.0 - (static_cast<double>(distance) / static_cast<double>(FINGERPRINT_SIZE));
    }
 
    void clear()
    {
      for (size_t i = 0; i < FINGERPRINT_SIZE; ++i)
        v_(i) = 0.0;
    }
  };
} // namespace Aleph
 
# endif // SIMHASH_H