lassq.hpp

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//
// Copyright (c) 2025, University of Colorado Denver. All rights reserved.
//
// This file is part of <T>LAPACK.
// <T>LAPACK is free software: you can redistribute it and/or modify it under
// the terms of the BSD 3-Clause license. See the accompanying LICENSE file.
 
#ifndef TLAPACK_LASSQ_HH
#define TLAPACK_LASSQ_HH
 
#include "tlapack/base/constants.hpp"
#include "tlapack/base/utils.hpp"
 
namespace tlapack {
 
template <class abs_f, TLAPACK_VECTOR vector_t>
void lassq(const vector_t& x,
           real_type<type_t<vector_t>>& scale,
           real_type<type_t<vector_t>>& sumsq,
           abs_f absF)
{
    using real_t = real_type<type_t<vector_t>>;
    using idx_t = size_type<vector_t>;
 
    // constants
    const idx_t n = size(x);
 
    // constants
    const real_t zero(0);
    const real_t one(1);
    const real_t tsml = blue_min<real_t>();
    const real_t tbig = blue_max<real_t>();
    const real_t ssml = blue_scalingMin<real_t>();
    const real_t sbig = blue_scalingMax<real_t>();
 
    // quick return
    if (isnan(scale) || isnan(sumsq)) return;
 
    if (sumsq == zero) scale = one;
    if (scale == zero) {
        scale = one;
        sumsq = zero;
    }
 
    // quick return
    if (n <= 0) return;
 
    //  Compute the sum of squares in 3 accumulators:
    //     abig -- sums of squares scaled down to avoid overflow
    //     asml -- sums of squares scaled up to avoid underflow
    //     amed -- sums of squares that do not require scaling
    //  The thresholds and multipliers are
    //     tbig -- values bigger than this are scaled down by sbig
    //     tsml -- values smaller than this are scaled up by ssml
 
    real_t asml = zero;
    real_t amed = zero;
    real_t abig = zero;
 
    for (idx_t i = 0; i < n; ++i) {
        real_t ax = absF(x[i]);
        if (ax > tbig)
            abig += (ax * sbig) * (ax * sbig);
        else if (ax < tsml) {
            if (abig == zero) asml += (ax * ssml) * (ax * ssml);
        }
        else
            amed += ax * ax;
    }
 
    // Put the existing sum of squares into one of the accumulators
    if (sumsq > zero) {
        real_t ax = scale * sqrt(sumsq);
        if (ax > tbig) {
            if (scale > one) {
                scale *= sbig;
                abig += scale * (scale * sumsq);
            }
            else {
                // sumsq > tbig^2 => (sbig * (sbig * sumsq)) is representable
                abig += scale * (scale * (sbig * (sbig * sumsq)));
            }
        }
        else if (ax < tsml) {
            if (abig == zero) {
                if (scale < one) {
                    scale *= ssml;
                    asml += scale * (scale * sumsq);
                }
                else {
                    // sumsq < tsml^2 => (ssml * (ssml * sumsq)) is
                    // representable
                    asml += scale * (scale * (ssml * (ssml * sumsq)));
                }
            }
        }
        else {
            amed += scale * (scale * sumsq);
        }
    }
 
    // Combine abig and amed or amed and asml if
    // more than one accumulator was used.
 
    if (abig > zero) {
        // Combine abig and amed if abig > 0
        if (amed > zero || isnan(amed)) abig += (amed * sbig) * sbig;
        scale = one / sbig;
        sumsq = abig;
    }
    else if (asml > zero) {
        // Combine amed and asml if asml > 0
        if (amed > zero || isnan(amed)) {
            amed = sqrt(amed);
            asml = sqrt(asml) / ssml;
 
            real_t ymin, ymax;
            if (asml > amed) {
                ymin = amed;
                ymax = asml;
            }
            else {
                ymin = asml;
                ymax = amed;
            }
 
            scale = one;
            sumsq = (ymax * ymax) * (one + (ymin / ymax) * (ymin / ymax));
        }
        else {
            scale = one / ssml;
            sumsq = asml;
        }
    }
    else {
        // Otherwise all values are mid-range or zero
        scale = one;
        sumsq = amed;
    }
}
 
template <TLAPACK_VECTOR vector_t>
void lassq(const vector_t& x,
           real_type<type_t<vector_t>>& scale,
           real_type<type_t<vector_t>>& sumsq)
{
    using T = type_t<vector_t>;
    return lassq(x, scale, sumsq,
                 // Lambda function that returns the absolute value using abs :
                 [](const T& x) { return abs(x); });
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_LASSQ_HH