iamax.hh

// Copyright (c) 2017-2023, University of Tennessee. All rights reserved.
// SPDX-License-Identifier: BSD-3-Clause
// This program 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 BLAS_IAMAX_HH
#define BLAS_IAMAX_HH
 
#include "blas/util.hh"
 
#include <limits>
 
namespace blas {
 
// =============================================================================
 
template <typename T>
int64_t iamax(
    int64_t n,
    T const *x, int64_t incx )
{
    typedef real_type<T> real_t;
 
    // check arguments
    blas_error_if( n < 0 );      // standard BLAS returns, doesn't fail
    blas_error_if( incx <= 0 );  // standard BLAS returns, doesn't fail
 
    // todo: check NAN
    real_t result = -1;
    int64_t index = -1;
    if (incx == 1) {
        // unit stride
        for (int64_t i = 0; i < n; ++i) {
            real_t tmp = abs1( x[i] );
            if (tmp > result) {
                result = tmp;
                index = i;
            }
        }
    }
    else {
        // non-unit stride
        int64_t ix = 0;
        for (int64_t i = 0; i < n; ++i) {
            real_t tmp = abs1( x[ix] );
            if (tmp > result) {
                result = tmp;
                index = i;
            }
            ix += incx;
        }
    }
    return index;
}
 
}  // namespace blas
 
#endif        //  #ifndef BLAS_IAMAX_HH