Multi_field_small_shared.h

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/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
 *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
 *    Author(s):       Hannah Schreiber, Clément Maria
 *
 *    Copyright (C) 2022 Inria
 *
 *    Modification(s):
 *      - YYYY/MM Author: Description of the modification
 */

 
#ifndef MATRIX_FIELD_MULTI_SMALL_SHARED_H_
#define MATRIX_FIELD_MULTI_SMALL_SHARED_H_
 
#include <utility>
#include <vector>
#include <climits>
#include <stdexcept>
#include <numeric>
 
#include <gudhi/Debug_utils.h>
 
namespace Gudhi {
namespace persistence_fields {

template <typename Unsigned_integer_type = unsigned int,
          class = std::enable_if_t<std::is_unsigned_v<Unsigned_integer_type> > >
class Shared_multi_field_element_with_small_characteristics
{
 public:
  using Element = Unsigned_integer_type; 
  using Characteristic = Element;        
  template <class T>
  using isInteger = std::enable_if_t<std::is_integral_v<T> >;

  Shared_multi_field_element_with_small_characteristics() : element_(0) {}

  template <typename Integer_type, class = isInteger<Integer_type> >
  Shared_multi_field_element_with_small_characteristics(Integer_type element) : element_(_get_value(element))
  {}

  static void initialize(unsigned int minimum, unsigned int maximum)
  {
    if (maximum < 2) throw std::invalid_argument("Characteristic must be strictly positive");
    if (minimum > maximum) throw std::invalid_argument("The given interval is not valid.");
    if (minimum == maximum && !_is_prime(minimum))
      throw std::invalid_argument("The given interval does not contain a prime number.");
 
    productOfAllCharacteristics_ = 1;
    primes_.clear();
    for (unsigned int i = minimum; i <= maximum; ++i) {
      if (_is_prime(i)) {
        primes_.push_back(i);
        productOfAllCharacteristics_ *= i;
      }
    }
 
    if (primes_.empty()) throw std::invalid_argument("The given interval does not contain a prime number.");
 
    partials_.resize(primes_.size());
    for (Characteristic i = 0; i < primes_.size(); ++i) {
      Characteristic p = primes_[i];
      Characteristic base = productOfAllCharacteristics_ / p;
      Characteristic exp = p - 1;
      partials_[i] = 1;
 
      while (exp > 0) {
        // If exp is odd, multiply with result
        if (exp & 1) partials_[i] = _multiply(partials_[i], base);
        // y must be even now
        exp = exp >> 1;  // y = y/2
        base = _multiply(base, base);
      }
    }
 
    // If I understood the paper well, multiplicativeID_ always equals to 1. But in Clement's code,
    // multiplicativeID_ is computed (see commented loop below). TODO: verify with Clement.
    //  for (unsigned int i = 0; i < partials_.size(); ++i){
    //    multiplicativeID_ = (multiplicativeID_ + partials_[i]) % productOfAllCharacteristics_;
    //  }
  }

  friend void operator+=(Shared_multi_field_element_with_small_characteristics& f1,
                         Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1.element_ = _add(f1.element_, f2.element_);
  }

  friend Shared_multi_field_element_with_small_characteristics operator+(
      Shared_multi_field_element_with_small_characteristics f1,
      Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1 += f2;
    return f1;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend void operator+=(Shared_multi_field_element_with_small_characteristics& f, const Integer_type& v)
  {
    f.element_ = _add(f.element_, _get_value(v));
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Shared_multi_field_element_with_small_characteristics operator+(
      Shared_multi_field_element_with_small_characteristics f,
      const Integer_type& v)
  {
    f += v;
    return f;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Integer_type operator+(const Integer_type& v, Shared_multi_field_element_with_small_characteristics f)
  {
    f += v;
    return f.element_;
  }

  friend void operator-=(Shared_multi_field_element_with_small_characteristics& f1,
                         Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1.element_ = _subtract(f1.element_, f2.element_);
  }

  friend Shared_multi_field_element_with_small_characteristics operator-(
      Shared_multi_field_element_with_small_characteristics f1,
      Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1 -= f2;
    return f1;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend void operator-=(Shared_multi_field_element_with_small_characteristics& f, const Integer_type& v)
  {
    f.element_ = _subtract(f.element_, _get_value(v));
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Shared_multi_field_element_with_small_characteristics operator-(
      Shared_multi_field_element_with_small_characteristics f,
      const Integer_type& v)
  {
    f -= v;
    return f;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Integer_type operator-(const Integer_type& v, const Shared_multi_field_element_with_small_characteristics& f)
  {
    return _subtract(_get_value(v), f.element_);
  }

  friend void operator*=(Shared_multi_field_element_with_small_characteristics& f1,
                         Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1.element_ = _multiply(f1.element_, f2.element_);
  }

  friend Shared_multi_field_element_with_small_characteristics operator*(
      Shared_multi_field_element_with_small_characteristics f1,
      Shared_multi_field_element_with_small_characteristics const& f2)
  {
    f1 *= f2;
    return f1;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend void operator*=(Shared_multi_field_element_with_small_characteristics& f, const Integer_type& v)
  {
    f.element_ = _multiply(f.element_, _get_value(v));
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Shared_multi_field_element_with_small_characteristics operator*(
      Shared_multi_field_element_with_small_characteristics f,
      const Integer_type& v)
  {
    f *= v;
    return f;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend Integer_type operator*(const Integer_type& v, Shared_multi_field_element_with_small_characteristics f)
  {
    f *= v;
    return f.element_;
  }

  friend bool operator==(const Shared_multi_field_element_with_small_characteristics& f1,
                         const Shared_multi_field_element_with_small_characteristics& f2)
  {
    return f1.element_ == f2.element_;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend bool operator==(const Integer_type& v, const Shared_multi_field_element_with_small_characteristics& f)
  {
    return _get_value(v) == f.element_;
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend bool operator==(const Shared_multi_field_element_with_small_characteristics& f, const Integer_type& v)
  {
    return _get_value(v) == f.element_;
  }

  friend bool operator!=(const Shared_multi_field_element_with_small_characteristics& f1,
                         const Shared_multi_field_element_with_small_characteristics& f2)
  {
    return !(f1 == f2);
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend bool operator!=(const Integer_type v, const Shared_multi_field_element_with_small_characteristics& f)
  {
    return !(v == f);
  }

  template <typename Integer_type, class = isInteger<Integer_type> >
  friend bool operator!=(const Shared_multi_field_element_with_small_characteristics& f, const Integer_type v)
  {
    return !(v == f);
  }
 
  // /**
  //  * @brief Assign operator.
  //  *
  //  * @tparam Integer_type A native integer type. Should be able to contain the characteristic if signed.
  //  */
  // template <typename Integer_type, class = isInteger<Integer_type> >
  // Shared_multi_field_element_with_small_characteristics& operator=(const Integer_type& value)
  // {
  //   element_ = _get_value(value);
  //   return *this;
  // }

  friend void swap(Shared_multi_field_element_with_small_characteristics& f1,
                   Shared_multi_field_element_with_small_characteristics& f2) noexcept
  {
    std::swap(f1.element_, f2.element_);
  }

  operator unsigned int() const { return element_; }

  Shared_multi_field_element_with_small_characteristics get_inverse() const
  {
    return get_partial_inverse(productOfAllCharacteristics_).first;
  }

  std::pair<Shared_multi_field_element_with_small_characteristics, Characteristic> get_partial_inverse(
      Characteristic productOfCharacteristics) const
  {
    GUDHI_CHECK(productOfCharacteristics >= 0 && productOfCharacteristics <= productOfAllCharacteristics_,
                std::invalid_argument(
                    "The given product is not the product of a subset of the current Multi-field characteristics."));
 
    Characteristic gcd = std::gcd(element_, productOfAllCharacteristics_);
 
    if (gcd == productOfCharacteristics)
      return {Shared_multi_field_element_with_small_characteristics(), multiplicativeID_};  // partial inverse is 0
 
    Characteristic QT = productOfCharacteristics / gcd;
 
    const Element inv_qt = _get_inverse(element_, QT);
 
    auto res = get_partial_multiplicative_identity(QT);
    res *= inv_qt;
 
    return {res, QT};
  }

  static Shared_multi_field_element_with_small_characteristics get_additive_identity()
  {
    return Shared_multi_field_element_with_small_characteristics();
  }

  static Shared_multi_field_element_with_small_characteristics get_multiplicative_identity()
  {
    return Shared_multi_field_element_with_small_characteristics(multiplicativeID_);
  }

  static Shared_multi_field_element_with_small_characteristics get_partial_multiplicative_identity(
      const Characteristic& productOfCharacteristics)
  {
    GUDHI_CHECK(productOfCharacteristics >= 0 && productOfCharacteristics <= productOfAllCharacteristics_,
                std::invalid_argument(
                    "The given product is not the product of a subset of the current Multi-field characteristics."));
 
    if (productOfCharacteristics == 0 || productOfCharacteristics == productOfAllCharacteristics_) {
      return Shared_multi_field_element_with_small_characteristics(multiplicativeID_);
    }
    Shared_multi_field_element_with_small_characteristics mult;
    for (Characteristic idx = 0; idx < primes_.size(); ++idx) {
      if ((productOfCharacteristics % primes_[idx]) == 0) {
        mult += partials_[idx];
      }
    }
    return mult;
  }

  static Characteristic get_characteristic() { return productOfAllCharacteristics_; }

  Element get_value() const { return element_; }
 
 private:
  static constexpr bool _is_prime(const unsigned int p)
  {
    if (p <= 1) return false;
    if (p <= 3) return true;
    if (p % 2 == 0 || p % 3 == 0) return false;
 
    for (unsigned long i = 5; i * i <= p; i = i + 6)
      if (p % i == 0 || p % (i + 2) == 0) return false;
 
    return true;
  }
 
  static Element _multiply(Element a, Element b)
  {
    Element res = 0;
    Element temp_b = 0;
 
    if (b < a) std::swap(a, b);
 
    while (a != 0) {
      if (a & 1) {
        /* Add b to res, modulo m, without overflow */
        if (b >= productOfAllCharacteristics_ - res) res -= productOfAllCharacteristics_;
        res += b;
      }
      a >>= 1;
 
      /* Double b, modulo m */
      temp_b = b;
      if (b >= productOfAllCharacteristics_ - b) temp_b -= productOfAllCharacteristics_;
      b += temp_b;
    }
    return res;
  }
 
  static Element _add(Element element, Element v)
  {
    if (UINT_MAX - element < v) {
      // automatic unsigned integer overflow behaviour will make it work
      element += v;
      element -= productOfAllCharacteristics_;
      return element;
    }
 
    element += v;
    if (element >= productOfAllCharacteristics_) element -= productOfAllCharacteristics_;
 
    return element;
  }
 
  static Element _subtract(Element element, Element v)
  {
    if (element < v) {
      element += productOfAllCharacteristics_;
    }
    element -= v;
 
    return element;
  }
 
  static constexpr int _get_inverse(Element element, const Characteristic mod)
  {
    // to solve: Ax + My = 1
    int M = mod;
    int A = element;
    int y = 0, x = 1;
    // extended euclidean division
    while (A > 1) {
      int quotient = A / M;
      int temp = M;
 
      M = A % M, A = temp;
      temp = y;
 
      y = x - quotient * y;
      x = temp;
    }
 
    if (x < 0) x += mod;
 
    return x;
  }
 
  template <typename Integer_type, class = isInteger<Integer_type> >
  static constexpr Element _get_value(Integer_type e)
  {
    if constexpr (std::is_signed_v<Integer_type>) {
      if (e < -static_cast<Integer_type>(productOfAllCharacteristics_)) e = e % productOfAllCharacteristics_;
      if (e < 0) return e += productOfAllCharacteristics_;
      return e < static_cast<Integer_type>(productOfAllCharacteristics_) ? e : e % productOfAllCharacteristics_;
    } else {
      return e < productOfAllCharacteristics_ ? e : e % productOfAllCharacteristics_;
    }
  }
 
  Element element_;                                          
  static inline std::vector<Characteristic> primes_;         
  static inline Characteristic productOfAllCharacteristics_; 
  static inline std::vector<Characteristic> partials_;       
  static constexpr Element multiplicativeID_ = 1;            
};
 
}  // namespace persistence_fields
}  // namespace Gudhi
 
#endif  // MATRIX_FIELD_MULTI_SMALL_SHARED_H_