tetrees/Plugins/Wwise/Source/AkAudio/Classes/GTE/Mathematics/IntpAkimaNonuniform1.h

// David Eberly, Geometric Tools, Redmond WA 98052
// Copyright (c) 1998-2020
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt
// https://www.geometrictools.com/License/Boost/LICENSE_1_0.txt
// Version: 4.0.2019.08.13

#pragma once

#include <Mathematics/IntpAkima1.h>

namespace WwiseGTE
{
    template <typename Real>
    class IntpAkimaNonuniform1 : public IntpAkima1<Real>
    {
    public:
        // Construction.  The interpolator is for arbitrarily spaced x-values.
        // The input arrays must have 'quantity' elements and the X[] array
        // must store increasing values:  X[i + 1] > X[i] for all i.
        IntpAkimaNonuniform1(int quantity, Real const* X, Real const* F)
            :
            IntpAkima1<Real>(quantity, F),
            mX(X)
        {
            LogAssert(X != nullptr, "Invalid input.");
            for (int j0 = 0, j1 = 1; j1 < quantity; ++j0, ++j1)
            {
                LogAssert(X[j1] > X[j0], "Invalid input.");
            }

            // Compute slopes.
            std::vector<Real> slope(quantity + 3);
            int i, ip1, ip2;
            for (i = 0, ip1 = 1, ip2 = 2; i < quantity - 1; ++i, ++ip1, ++ip2)
            {
                Real dx = X[ip1] - X[i];
                Real df = F[ip1] - F[i];
                slope[ip2] = df / dx;
            }

            slope[1] = (Real)2 * slope[2] - slope[3];
            slope[0] = (Real)2 * slope[1] - slope[2];
            slope[quantity + 1] = (Real)2 * slope[quantity] - slope[quantity - 1];
            slope[quantity + 2] = (Real)2 * slope[quantity + 1] - slope[quantity];

            // Construct derivatives.
            std::vector<Real> FDer(quantity);
            for (i = 0; i < quantity; ++i)
            {
                FDer[i] = this->ComputeDerivative(&slope[i]);
            }

            // Construct polynomials.
            for (i = 0, ip1 = 1; i < quantity - 1; ++i, ++ip1)
            {
                auto& poly = this->mPoly[i];

                Real F0 = F[i];
                Real F1 = F[ip1];
                Real FDer0 = FDer[i];
                Real FDer1 = FDer[ip1];
                Real df = F1 - F0;
                Real dx = X[ip1] - X[i];
                Real dx2 = dx * dx;
                Real dx3 = dx2 * dx;

                poly[0] = F0;
                poly[1] = FDer0;
                poly[2] = ((Real)3 * df - dx * (FDer1 + (Real)2 * FDer0)) / dx2;
                poly[3] = (dx * (FDer0 + FDer1) - (Real)2 * df) / dx3;
            }
        }

        virtual ~IntpAkimaNonuniform1() = default;

        // Member access.
        Real const* GetX() const
        {
            return mX;
        }

        virtual Real GetXMin() const override
        {
            return mX[0];
        }

        virtual Real GetXMax() const override
        {
            return mX[this->mQuantity - 1];
        }

    protected:
        virtual void Lookup(Real x, int& index, Real& dx) const override
        {
            // The caller has ensured that mXMin <= x <= mXMax.
            for (index = 0; index + 1 < this->mQuantity; ++index)
            {
                if (x < mX[index + 1])
                {
                    dx = x - mX[index];
                    return;
                }
            }

            --index;
            dx = x - mX[index];
        }

        Real const* mX;
    };
}