/***************************************************************************

  PUserFcn.cpp

  Author: Andreas Suter
  e-mail: andreas.suter@psi.ch

***************************************************************************/

/***************************************************************************
 *   Copyright (C) 2007-2025 by Andreas Suter                              *
 *   andreas.suter@psi.ch                                                  *
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   This program is distributed in the hope that it will be useful,       *
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
 *   GNU General Public License for more details.                          *
 *                                                                         *
 *   You should have received a copy of the GNU General Public License     *
 *   along with this program; if not, write to the                         *
 *   Free Software Foundation, Inc.,                                       *
 *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
 ***************************************************************************/

#include <cassert>

#include "PUserFcn.h"

ClassImp(PUserFcn)

//------------------------------------------------------
/**
 * \brief Default constructor for PUserFcn.
 *
 * Initializes a third-order polynomial user function. This implementation
 * requires no special initialization as it has no internal state - all
 * computation is done directly in the operator() method.
 *
 * \note This simple constructor serves as a template for more complex user
 * functions. Functions requiring initialization (e.g., loading lookup tables,
 * precomputing constants) should perform that work here or in SetGlobalPart().
 */
PUserFcn::PUserFcn()
{
}

//------------------------------------------------------
/**
 * \brief Destructor for PUserFcn.
 *
 * Cleans up any resources allocated by the polynomial function. Since this
 * implementation has no dynamically allocated resources, the destructor is
 * empty.
 *
 * \note User functions with allocated resources (lookup tables, buffers,
 * external library handles) must clean them up here to prevent memory leaks.
 * If using the global part interface, ensure proper coordination with
 * gGlobalUserFcn cleanup.
 */
PUserFcn::~PUserFcn()
{
}

//------------------------------------------------------
/**
 * \brief Evaluates the third-order polynomial at the given time.
 *
 * Computes a cubic polynomial of the form:
 *
 * \f[ P(t) = c_0 + c_1 t + c_2 t^2 + c_3 t^3 = \sum_{k=0}^{3} c_k t^k \f]
 *
 * where the coefficients \f$c_k\f$ are provided in the parameter vector.
 *
 * \section puserfcn_op_example Example MSR Configuration
 *
 * \code
 * FITPARAMETER
 * #  No Name       Value     Step      Pos_Error Boundaries
 *     1 c0         0.95      0.01      none
 *     2 c1         -0.001    0.0001    none
 *     3 c2         0.00001   0.000001  none
 *     4 c3         0.0       0.0000001 none      0  none   (fixed to zero for quadratic)
 *
 * THEORY
 * userFcn  libPUserFcn.so  PUserFcn  1 2 3 4
 * \endcode
 *
 * \section puserfcn_op_notes Implementation Notes
 *
 * - Uses direct polynomial evaluation (Horner's method could improve
 *   numerical stability for high-precision applications)
 * - Asserts exactly 4 parameters to catch MSR file configuration errors
 * - No special handling for negative time values
 *
 * \param t Independent variable (time in μs for μSR, or general x-axis
 *        value for non-μSR fits)
 * \param param Vector containing exactly 4 polynomial coefficients:
 *        - param[0]: \f$c_0\f$ - constant term (dimensionless)
 *        - param[1]: \f$c_1\f$ - linear coefficient (μs⁻¹)
 *        - param[2]: \f$c_2\f$ - quadratic coefficient (μs⁻²)
 *        - param[3]: \f$c_3\f$ - cubic coefficient (μs⁻³)
 *
 * \return The polynomial value \f$P(t)\f$ at the specified time
 *
 * \pre param.size() == 4 (enforced by assertion)
 *
 * \see PUserFcnBase::operator() for the virtual interface specification
 */
Double_t PUserFcn::operator()(Double_t t, const std::vector<Double_t> &param) const
{
  // expected parameters: c0, c1, c2, c3

  assert(param.size() == 4);

  return param[0] + param[1]*t + param[2]*t*t + param[3]*t*t*t;
}