GteInvSqrtEstimate.h

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// David Eberly, Geometric Tools, Redmond WA 98052
// Copyright (c) 1998-2017
// Distributed under the Boost Software License, Version 1.0.
// http://www.boost.org/LICENSE_1_0.txt
// http://www.geometrictools.com/License/Boost/LICENSE_1_0.txt
// File Version: 3.0.0 (2016/06/19)

#pragma once

#include <Mathematics/GteConstants.h>
#include <cmath>

// Minimax polynomial approximations to 1/sqrt(x).  The polynomial p(x) of
// degree D minimizes the quantity maximum{|1/sqrt(x) - p(x)| : x in [1,2]}
// over all polynomials of degree D.

namespace gte
{

template <typename Real>
class InvSqrtEstimate
{
public:
    // The input constraint is x in [1,2].  For example,
    //   float x; // in [1,2]
    //   float result = InvSqrtEstimate<float>::Degree<3>(x);
    template <int D>
    inline static Real Degree(Real x);

    // The input constraint is x > 0.  Range reduction is used to generate a
    // value y in [1,2], call Evaluate(y), and combine the output with the
    // proper exponent to obtain the approximation.  For example,
    //   float x;  // x > 0
    //   float result = InvSqrtEstimate<float>::DegreeRR<3>(x);
    template <int D>
    inline static Real DegreeRR(Real x);

private:
    // Metaprogramming and private implementation to allow specialization of
    // a template member function.
    template <int D> struct degree {};
    inline static Real Evaluate(degree<1>, Real t);
    inline static Real Evaluate(degree<2>, Real t);
    inline static Real Evaluate(degree<3>, Real t);
    inline static Real Evaluate(degree<4>, Real t);
    inline static Real Evaluate(degree<5>, Real t);
    inline static Real Evaluate(degree<6>, Real t);
    inline static Real Evaluate(degree<7>, Real t);
    inline static Real Evaluate(degree<8>, Real t);

    // Support for range reduction.
    inline static void Reduce(Real x, Real& adj, Real& y, int& p);
    inline static Real Combine(Real adj, Real y, int p);
};


template <typename Real>
template <int D>
inline Real InvSqrtEstimate<Real>::Degree(Real x)
{
    Real t = x - (Real)1;  // t in (0,1]
    return Evaluate(degree<D>(), t);
}

template <typename Real>
template <int D>
inline Real InvSqrtEstimate<Real>::DegreeRR(Real x)
{
    Real adj, y;
    int p;
    Reduce(x, adj, y, p);
    Real poly = Degree<D>(y);
    Real result = Combine(adj, poly, p);
    return result;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<1>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG1_C1;
    poly = (Real)GTE_C_INVSQRT_DEG1_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<2>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG2_C2;
    poly = (Real)GTE_C_INVSQRT_DEG2_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG2_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<3>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG3_C3;
    poly = (Real)GTE_C_INVSQRT_DEG3_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG3_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG3_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<4>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG4_C4;
    poly = (Real)GTE_C_INVSQRT_DEG4_C3 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG4_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG4_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG4_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<5>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG5_C5;
    poly = (Real)GTE_C_INVSQRT_DEG5_C4 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG5_C3 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG5_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG5_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG5_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<6>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG6_C6;
    poly = (Real)GTE_C_INVSQRT_DEG6_C5 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG6_C4 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG6_C3 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG6_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG6_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG6_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<7>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG7_C7;
    poly = (Real)GTE_C_INVSQRT_DEG7_C6 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C5 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C4 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C3 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG7_C0 + poly * t;
    return poly;
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Evaluate(degree<8>, Real t)
{
    Real poly;
    poly = (Real)GTE_C_INVSQRT_DEG8_C8;
    poly = (Real)GTE_C_INVSQRT_DEG8_C7 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C6 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C5 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C4 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C3 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C2 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C1 + poly * t;
    poly = (Real)GTE_C_INVSQRT_DEG8_C0 + poly * t;
    return poly;
}

template <typename Real>
inline void InvSqrtEstimate<Real>::Reduce(Real x, Real& adj, Real& y, int& p)
{
    y = frexp(x, &p);  // y in [1/2,1)
    y = ((Real)2)*y;  // y in [1,2)
    --p;
    adj = (1 & p)*(Real)GTE_C_INV_SQRT_2 + (1 & ~p)*(Real)1;
    p = -(p >> 1);
}

template <typename Real>
inline Real InvSqrtEstimate<Real>::Combine(Real adj, Real y, int p)
{
    return adj*ldexp(y, p);
}


}