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/*
 * This code provides Chebyshev approximation utilities.
 */

#ifndef LBCRYPTO_INC_MATH_CHEBYSHEV_H
#define LBCRYPTO_INC_MATH_CHEBYSHEV_H

#include <cstdint>
#include <functional>
#include <vector>

/**
 * @namespace lbcrypto
 * The namespace of lbcrypto
 */
namespace lbcrypto {

/**
 * Method for calculating Chebyshev coefficients for an input function
 * over the range [a,b]. These coefficents are used in EValChebyshevSeries
 * to approximate the function func as
 *   func(x) ~ coeffs[0]/2 + sum_{i=1}^{degree} coeffs[i] * T_{i}(x),
 * where T_{i}(x) are Chebyshev polynomials of the first kind. (Note that
 * the 1st coeffiicent is divided by two.)
 * @param func is the function to be approximated
 * @param a - lower bound of argument for which the coefficients were found
 * @param b - upper bound of argument for which the coefficients were found
 * @param degree Desired degree of approximation
 * @return the coefficients of the Chebyshev approximation.
 */
std::vector<double> EvalChebyshevCoefficients(std::function<double(double)> f, double a, double b, uint32_t d);

/**
 * A cleartext version of CryptoContext<...>::EvalChebyshevFunction(...).
 * It evaluates an approximation of func via Chebyshev polynomials of a
 * bounded degree, over a specified interval [a,b].
 *
 * @param func is the function to be approximated
 * @param ptxt is a vector of plaintext inputs
 * @param a is the bottom of the interval over chichfunc is approximated
 * @param b is the top of the interval over chichfunc is approximated
 * @param degree is the desired degree of approximation
 * @return Evaluation of the approximated function over the plaintexts.
 */
std::vector<double> EvalChebyshevFunctionPtxt(std::function<double(double)> f, const std::vector<double>& ptxt,
                                              double a, double b, size_t d);

}  // namespace lbcrypto

#endif
