hyperloglog.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 HYPERLOGLOG_H
# define HYPERLOGLOG_H
 
# include <cmath>
# include <algorithm>
# include <bit>
# include <cstdint>
 
# include <ah-errors.H>
# include <tpl_array.H>
# include <hash-fct.H>
 
namespace Aleph
{
  template <typename T>
  class HyperLogLog
  {
    uint8_t b_; 
    size_t m_; 
    double alpha_m_; 
    Array<uint8_t> registers_; 
    [[nodiscard]] static uint64_t hash64(const T & val) noexcept
    {
      static_assert(sizeof(size_t) >= 8,
        "HyperLogLog requires 64-bit size_t; murmur3hash is 32-bit on this platform");
      return static_cast<uint64_t>(murmur3hash(val, 0x12345678));
    }
 
    [[nodiscard]] static size_t compute_m_or_throw(const uint8_t b)
    {
      ah_domain_error_if(b < 4 or b > 16)
        << "HyperLogLog: b must be in range [4, 16] (got " << static_cast<int>(b) << ")";
      return static_cast<size_t>(1) << b;
    }
 
    [[nodiscard]] static double compute_alpha(const size_t m) noexcept
    {
      if (m == 16) return 0.673;
      if (m == 32) return 0.697;
      if (m == 64) return 0.709;
      return 0.7213 / (1.0 + 1.079 / static_cast<double>(m));
    }
 
  public:
    explicit HyperLogLog(const uint8_t b = 12)
      : b_(b), m_(compute_m_or_throw(b)),
        alpha_m_(compute_alpha(m_)),
        registers_(m_, static_cast<uint8_t>(0))
    {}
 
    void update(const T & val)
    {
      const uint64_t x = hash64(val);
      const size_t j = x >> (64 - b_); // upper b bits for index
      const uint64_t w = x << b_ | (static_cast<uint64_t>(1) << (b_ - 1)); // remaining bits
 
      // count leading zeros in the hash part not used for indexing
 
      if (const auto rho = static_cast<uint8_t>(std::countl_zero(w) + 1); rho > registers_[j])
        registers_(j) = rho;
    }
 
    [[nodiscard]] double estimate() const noexcept
    {
      double z = 0.0;
      size_t v = 0; // count of empty registers for Linear Counting
      for (size_t i = 0; i < m_; ++i)
        {
          // More efficient and stable than std::pow(2.0, -registers_[i])
          z += std::ldexp(1.0, -static_cast<int>(registers_[i]));
          if (registers_[i] == 0)
            ++v;
        }
 
      // Raw estimate
      double e = alpha_m_ * (static_cast<double>(m_) * static_cast<double>(m_)) / z;
 
      // Small range correction (Linear Counting)
      if (e <= 2.5 * static_cast<double>(m_))
        {
          if (v > 0)
            e = static_cast<double>(m_) * std::log(static_cast<double>(m_) / static_cast<double>(v));
        }
      // Large range correction: use log1p for numerical stability; saturate when ratio >= 1
      else
        {
          static const double two_to_64 = std::ldexp(1.0, 64);
          if (e > (1.0 / 30.0) * two_to_64)
            {
              const double ratio = e / two_to_64;
              if (ratio >= 1.0)
                return two_to_64; // saturate at hash-space limit
              e = -two_to_64 * std::log1p(-ratio);
            }
        }
 
      return e;
    }
 
    void merge(const HyperLogLog & other)
    {
      ah_domain_error_if(b_ != other.b_) << "HyperLogLog::merge: precision mismatch";
 
      for (size_t i = 0; i < m_; ++i)
        registers_(i) = std::max(registers_[i], other.registers_[i]);
    }
 
    void clear()
    {
      for (size_t i = 0; i < m_; ++i)
        registers_(i) = 0;
    }
 
    [[nodiscard]] size_t num_registers() const noexcept { return m_; }
  };
} // namespace Aleph
 
# endif // HYPERLOGLOG_H