/**
* @license Apache-2.0
*
* Copyright (c) 2020 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

#include "stdlib/blas/ext/base/dnannsumkbn.h"
#include "stdlib/math/base/assert/is_nan.h"
#include <stdint.h>
#include <math.h>

/**
* Computes the sum of double-precision floating-point strided array elements, ignoring `NaN` values and using an improved Kahan–Babuška algorithm.
*
* ## Method
*
* -   This implementation uses an "improved Kahan–Babuška algorithm", as described by Neumaier (1974).
*
* ## References
*
* -   Neumaier, Arnold. 1974. "Rounding Error Analysis of Some Methods for Summing Finite Sums." _Zeitschrift Für Angewandte Mathematik Und Mechanik_ 54 (1): 39–51. doi:[10.1002/zamm.19740540106](https://doi.org/10.1002/zamm.19740540106).
*
* @param N       number of indexed elements
* @param X       input array
* @param stride  stride length
* @param n       pointer for storing the number of non-NaN elements
* @return        output value
*/
double stdlib_strided_dnannsumkbn( const int64_t N, const double *X, const int64_t stride, int64_t *n ) {
	double sum;
	int64_t ix;
	int64_t i;
	double v;
	double t;
	double c;

	sum = 0.0;
	*n = 0;
	if ( N <= 0 ) {
		return sum;
	}
	if ( N == 1 || stride == 0 ) {
		if ( stdlib_base_is_nan( X[ 0 ] ) ) {
			return sum;
		}
		*n += 1;
		return X[ 0 ];
	}
	if ( stride < 0 ) {
		ix = (1-N) * stride;
	} else {
		ix = 0;
	}
	c = 0.0;
	for ( i = 0; i < N; i++ ) {
		v = X[ ix ];
		if ( !stdlib_base_is_nan( v ) ) {
			t = sum + v;
			if ( fabs( sum ) >= fabs( v ) ) {
				c += (sum-t) + v;
			} else {
				c += (v-t) + sum;
			}
			sum = t;
			*n += 1;
		}
		ix += stride;
	}
	return sum + c;
}