minhash.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 MINHASH_H
# define MINHASH_H
 
# include <algorithm>
# include <iterator>
# include <limits>
# include <cstdint>
 
# include <ah-errors.H>
# include <tpl_array.H>
# include <hash-fct.H>
 
namespace Aleph
{
  template <typename T>
  class MinHash
  {
    size_t k_; 
    Array<uint64_t> signature_; 
    Array<uint32_t> seeds_; 
  public:
    explicit MinHash(size_t k = 128)
      : k_(k)
    {
      ah_domain_error_if(k == 0) << "MinHash: k must be > 0";
      
      signature_.reserve(k);
      for (size_t i = 0; i < k; ++i)
        signature_.append(std::numeric_limits<uint64_t>::max());
      seeds_.reserve(k_);
 
      // Use dft_hash_fct to generate independent seeds
      for (size_t i = 0; i < k_; ++i)
        seeds_.append(static_cast<uint32_t>(dft_hash_fct(i, 0xdeadbeef)));
    }
 
    void update(const T & val)
    {
      for (size_t i = 0; i < k_; ++i)
        if (const auto h = static_cast<uint64_t>(murmur3hash(val, seeds_[i])); h < signature_[i])
          signature_(i) = h;
    }
 
    template <std::input_iterator Itor>
    void update(Itor beg, const Itor & end)
    {
      while (beg != end)
        {
          update(*beg);
          ++beg;
        }
    }
 
    [[nodiscard]] double similarity(const MinHash & other) const
    {
      ah_domain_error_if(k_ != other.k_)
        << "MinHash::similarity: signature size mismatch";
 
      size_t matches = 0;
      for (size_t i = 0; i < k_; ++i)
        if (signature_[i] == other.signature_[i])
          ++matches;
 
      return static_cast<double>(matches) / static_cast<double>(k_);
    }
 
    [[nodiscard]] const Array<uint64_t> &get_signature() const noexcept
    {
      return signature_;
    }
 
    void clear()
    {
      constexpr uint64_t inf = std::numeric_limits<uint64_t>::max();
      for (size_t i = 0; i < k_; ++i)
        signature_(i) = inf;
    }
 
    MinHash & merge(const MinHash & other)
    {
      ah_domain_error_if(k_ != other.k_)
        << "MinHash::merge: signature size mismatch ("
        << k_ << " vs " << other.k_ << ")";
      for (size_t i = 0; i < k_; ++i)
        signature_(i) = std::min(signature_(i), other.signature_(i));
      return *this;
    }
 
    [[nodiscard]] size_t size() const noexcept { return k_; }
  };
} // namespace Aleph
 
# endif // MINHASH_H