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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): Clément Maria
*
* Copyright (C) 2014 Inria
*
* Modification(s):
* - YYYY/MM Author: Description of the modification
*/
#ifndef PERSISTENT_COHOMOLOGY_FIELD_ZP_H_
#define PERSISTENT_COHOMOLOGY_FIELD_ZP_H_
#include <utility>
#include <vector>
namespace Gudhi {
namespace persistent_cohomology {
/** \brief Structure representing the coefficient field \f$\mathbb{Z}/p\mathbb{Z}\f$
*
* \implements CoefficientField
* \ingroup persistent_cohomology
*/
class Field_Zp {
public:
typedef int Element;
Field_Zp()
: Prime(0),
inverse_() {
}
void init(int charac) {
assert(charac > 0); // division by zero + non negative values
Prime = charac;
inverse_.clear();
inverse_.reserve(charac);
inverse_.push_back(0);
for (int i = 1; i < Prime; ++i) {
int inv = 1;
while (((inv * i) % Prime) != 1)
++inv;
inverse_.push_back(inv);
}
}
/** Set x <- x + w * y*/
Element plus_times_equal(const Element& x, const Element& y, const Element& w) {
assert(Prime > 0); // division by zero + non negative values
Element result = (x + w * y) % Prime;
if (result < 0)
result += Prime;
return result;
}
// operator= defined on Element
/** Returns y * w */
Element times(const Element& y, const Element& w) {
return plus_times_equal(0, y, (Element)w);
}
Element plus_equal(const Element& x, const Element& y) {
return plus_times_equal(x, y, (Element)1);
}
/** \brief Returns the additive idendity \f$0_{\Bbbk}\f$ of the field.*/
Element additive_identity() const {
return 0;
}
/** \brief Returns the multiplicative identity \f$1_{\Bbbk}\f$ of the field.*/
Element multiplicative_identity(Element = 0) const {
return 1;
}
/** Returns the inverse in the field. Modifies P. ??? */
std::pair<Element, Element> inverse(Element x, Element P) {
return std::pair<Element, Element>(inverse_[x], P);
} // <------ return the product of field characteristic for which x is invertible
/** Returns -x * y.*/
Element times_minus(Element x, Element y) {
assert(Prime > 0); // division by zero + non negative values
Element out = (-x * y) % Prime;
return (out < 0) ? out + Prime : out;
}
/** \brief Returns the characteristic \f$p\f$ of the field.*/
int characteristic() const {
return Prime;
}
private:
int Prime;
/** Property map Element -> Element, which associate to an element its inverse in the field.*/
std::vector<Element> inverse_;
};
} // namespace persistent_cohomology
} // namespace Gudhi
#endif // PERSISTENT_COHOMOLOGY_FIELD_ZP_H_
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