lascl.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_LASCL_HH
#define TLAPACK_LASCL_HH
 
#include "tlapack/base/constants.hpp"
#include "tlapack/base/utils.hpp"
 
namespace tlapack {
 
template <TLAPACK_UPLO uplo_t,
          TLAPACK_MATRIX matrix_t,
          TLAPACK_REAL a_type,
          TLAPACK_REAL b_type,
          enable_if_t<(
                          /* Requires: */
                          is_real<a_type> && is_real<b_type>),
                      int> = 0>
int lascl(uplo_t uplo, const b_type& b, const a_type& a, matrix_t& A)
{
    // data traits
    using idx_t = size_type<matrix_t>;
    using real_t = real_type<a_type, b_type>;
 
    // constants
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
 
    // constants
    const real_t small = safe_min<real_t>();
    const real_t big = safe_max<real_t>();
 
    // check arguments
    tlapack_check_false(
        (uplo != Uplo::General) && (uplo != Uplo::UpperHessenberg) &&
        (uplo != Uplo::LowerHessenberg) && (uplo != Uplo::Upper) &&
        (uplo != Uplo::Lower) && (uplo != Uplo::StrictUpper) &&
        (uplo != Uplo::StrictLower));
    tlapack_check_false((b == b_type(0)) || isnan(b));
    tlapack_check_false(isnan(a));
 
    // quick return
    if (m <= 0 || n <= 0) return 0;
 
    bool done = false;
    real_t a_ = a, b_ = b;
    while (!done) {
        real_t c, a1, b1 = b * small;
        if (b1 == b_) {
            // b is not finite:
            //  c is a correctly signed zero if a is finite,
            //  c is NaN otherwise.
            c = a_ / b_;
            done = true;
        }
        else {  // b is finite
            a1 = a_ / big;
            if (a1 == a_) {
                // a is either 0 or an infinity number:
                //  in both cases, c = a serves as the correct multiplication
                //  factor.
                c = a_;
                done = true;
            }
            else if ((abs(b1) > abs(a_)) && (a_ != real_t(0))) {
                // a is a non-zero finite number and abs(a/b) < small:
                //  Set c = small as the multiplication factor,
                //  Multiply b by the small factor.
                c = small;
                done = false;
                b_ = b1;
            }
            else if (abs(a1) > abs(b_)) {
                // abs(a/b) > big:
                //  Set c = big as the multiplication factor,
                //  Divide a by the big factor.
                c = big;
                done = false;
                a_ = a1;
            }
            else {
                // small <= abs(a/b) <= big:
                //  Set c = a/b as the multiplication factor.
                c = a_ / b_;
                done = true;
            }
        }
 
        if (uplo == Uplo::UpperHessenberg) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = 0; i < ((j < m) ? j + 2 : m); ++i)
                    A(i, j) *= c;
        }
        else if (uplo == Uplo::Upper) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = 0; i < ((j < m) ? j + 1 : m); ++i)
                    A(i, j) *= c;
        }
        else if (uplo == Uplo::StrictUpper) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = 0; i < ((j < m) ? j : m); ++i)
                    A(i, j) *= c;
        }
        else if (uplo == Uplo::LowerHessenberg) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = ((j > 1) ? j - 1 : 0); i < m; ++i)
                    A(i, j) *= c;
        }
        else if (uplo == Uplo::Lower) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = j; i < m; ++i)
                    A(i, j) *= c;
        }
        else if (uplo == Uplo::StrictLower) {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = j + 1; i < m; ++i)
                    A(i, j) *= c;
        }
        else  // if ( uplo == Uplo::General )
        {
            for (idx_t j = 0; j < n; ++j)
                for (idx_t i = 0; i < m; ++i)
                    A(i, j) *= c;
        }
    }
 
    return 0;
}
 
template <TLAPACK_MATRIX matrix_t,
          TLAPACK_REAL a_type,
          TLAPACK_REAL b_type,
          enable_if_t<(
                          /* Requires: */
                          is_real<a_type> && is_real<b_type>),
                      int> = 0>
int lascl(BandAccess accessType, const b_type& b, const a_type& a, matrix_t& A)
{
    // data traits
    using idx_t = size_type<matrix_t>;
    using real_t = real_type<a_type, b_type>;
 
    // constants
    const idx_t m = nrows(A);
    const idx_t n = ncols(A);
    const idx_t kl = accessType.lower_bandwidth;
    const idx_t ku = accessType.upper_bandwidth;
 
    // constants
    const real_t small = safe_min<real_t>();
    const real_t big = safe_max<real_t>();
 
    // check arguments
    tlapack_check_false((kl < 0) || (kl >= m) || (ku < 0) || (ku >= n));
    tlapack_check_false((b == b_type(0)) || isnan(b));
    tlapack_check_false(isnan(a));
 
    // quick return
    if (m <= 0 || n <= 0) return 0;
 
    bool done = false;
    real_t a_ = a, b_ = b;
    while (!done) {
        real_t c, a1, b1 = b * small;
        if (b1 == b_) {
            // b is not finite:
            //  c is a correctly signed zero if a is finite,
            //  c is NaN otherwise.
            c = a_ / b_;
            done = true;
        }
        else {  // b is finite
            a1 = a_ / big;
            if (a1 == a_) {
                // a is either 0 or an infinity number:
                //  in both cases, c = a serves as the correct multiplication
                //  factor.
                c = a_;
                done = true;
            }
            else if ((abs(b1) > abs(a_)) && (a_ != real_t(0))) {
                // a is a non-zero finite number and abs(a/b) < small:
                //  Set c = small as the multiplication factor,
                //  Multiply b by the small factor.
                c = small;
                done = false;
                b_ = b1;
            }
            else if (abs(a1) > abs(b_)) {
                // abs(a/b) > big:
                //  Set c = big as the multiplication factor,
                //  Divide a by the big factor.
                c = big;
                done = false;
                a_ = a1;
            }
            else {
                // small <= abs(a/b) <= big:
                //  Set c = a/b as the multiplication factor.
                c = a_ / b_;
                done = true;
            }
        }
 
        for (idx_t j = 0; j < n; ++j)
            for (idx_t i = ((j >= ku) ? (j - ku) : 0); i < min(m, j + kl + 1);
                 ++i)
                A(i, j) *= c;
    }
 
    return 0;
}
 
}  // namespace tlapack
 
#endif  // TLAPACK_LASCL_HH