reservoir-sampling.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
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  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
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*/
 
# ifndef RESERVOIR_SAMPLING_H
# define RESERVOIR_SAMPLING_H
 
# include <chrono>
# include <cstdint>
# include <limits>
# include <memory>
# include <iterator>
# include <gsl/gsl_rng.h>
 
# include <ah-errors.H>
# include <tpl_array.H>
 
namespace Aleph
{
 
  template <typename T>
  class Reservoir_Sampler
  {
    using GslRngPtr = std::unique_ptr<gsl_rng, void(*)(gsl_rng *)>;
 
    size_t k_;             
    size_t n_seen_ = 0;    
    unsigned long seed_;   
    uint64_t rng_min_;     
    uint64_t rng_max_;     
    uint64_t rng_range_;   
    Array<T> reservoir_;   
    GslRngPtr rng_;        
  public:
    explicit Reservoir_Sampler(const size_t k,
                               const unsigned long seed = static_cast<unsigned long>(
                                 std::chrono::high_resolution_clock::now().time_since_epoch().count()))
      : k_(k), seed_(seed),
        rng_(gsl_rng_alloc(gsl_rng_mt19937), gsl_rng_free)
    {
      ah_runtime_error_unless(rng_ != nullptr) << "Reservoir_Sampler: GSL RNG allocation failed";
      gsl_rng_set(rng_.get(), seed_);
      rng_min_ = static_cast<uint64_t>(gsl_rng_min(rng_.get()));
      rng_max_ = static_cast<uint64_t>(gsl_rng_max(rng_.get()));
      rng_range_ = rng_max_ - rng_min_ + 1ULL;
      if (k_ > 0)
        reservoir_.reserve(k_);
    }
 
    void update(const T & val)
    {
      ah_overflow_error_if(n_seen_ == std::numeric_limits<size_t>::max())
        << "Reservoir_Sampler::update: stream length exceeded size_t capacity";
 
      if (k_ == 0)
        {
          ++n_seen_;
          return;
        }
 
      if (n_seen_ < k_)
        reservoir_.append(val);
      else
        {
          // Probability of replacement is k / (n_seen + 1)
          ah_overflow_error_if(n_seen_ >= std::numeric_limits<unsigned long>::max())
            << "Reservoir_Sampler::update: stream length exceeds unsigned long capacity";
 
          ah_runtime_error_unless(n_seen_ < rng_range_)
            << "Reservoir_Sampler::update: stream length exceeds RNG generator range (" << rng_range_ << ")";
 
          // Exclusive upper bound for the uniform random index in [0, n_seen_]
          const auto upper_bound = static_cast<unsigned long>(n_seen_ + 1);
          const size_t j = gsl_rng_uniform_int(rng_.get(), upper_bound);
          if (j < k_)
            reservoir_(j) = val;
        }
      ++n_seen_;
    }
 
    template <std::input_iterator Itor, std::sentinel_for<Itor> Sent>
    void update(Itor beg, Sent end)
    {
      while (beg != end)
        {
          update(*beg);
          ++beg;
        }
    }
 
    [[nodiscard]] const Array<T> & get_sample() const noexcept
    {
      return reservoir_;
    }
 
    [[nodiscard]] size_t total_seen() const noexcept { return n_seen_; }
 
    [[nodiscard]] size_t sample_size() const noexcept { return k_; }
 
    [[nodiscard]] uint64_t rng_range() const noexcept { return rng_range_; }
 
    void set_n_seen_for_testing(const size_t n)
    {
      const size_t expected = (n < k_) ? n : k_;
      ah_invalid_argument_if(reservoir_.size() != expected)
        << "set_n_seen_for_testing: inconsistent state: "
        << "n=" << n << ", k=" << k_
        << ", reservoir.size()=" << reservoir_.size()
        << " (expected " << expected << ")";
      n_seen_ = n;
    }
 
    void clear()
    {
      reservoir_.clear();
      n_seen_ = 0;
      gsl_rng_set(rng_.get(), seed_);
    }
  };
 
 
  template <std::input_iterator Itor, std::sentinel_for<Itor> Sent>
  [[nodiscard]] inline auto
  reservoir_sample(Itor beg, Sent end, size_t k,
                   unsigned long seed = static_cast<unsigned long>(
                     std::chrono::high_resolution_clock::now().time_since_epoch().count()))
  {
    using T = typename std::iterator_traits<Itor>::value_type;
    Reservoir_Sampler<T> sampler(k, seed);
    sampler.update(beg, end);
    return Array<T>(sampler.get_sample());
  }
 
} // namespace Aleph
 
# endif // RESERVOIR_SAMPLING_H