fix: update implementation to preserve signed zeros

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---

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/dnansumkbn2/lib/ndarray.js

@@ -54,6 +54,7 @@ var abs = require( '@stdlib/math/base/special/abs' );
 function dnansumkbn2( N, x, strideX, offsetX ) {
 	var sum;
 	var ccs;
+	var flg;
 	var ix;
 	var cs;
 	var cc;
@@ -62,20 +63,50 @@ function dnansumkbn2( N, x, strideX, offsetX ) {
 	var c;
 	var i;
 
-	sum = 0.0;
 	if ( N <= 0 ) {
-		return sum;
+		return 0.0;
 	}
 	ix = offsetX;
 	if ( strideX === 0 ) {
 		if ( isnan( x[ ix ] ) ) {
-			return sum;
+			return 0.0;
 		}
 		return x[ ix ] * N;
 	}
+	// Find the first non-NaN element...
+	for ( i = 0; i < N; i++ ) {
+		v = x[ ix ];
+		if ( isnan( v ) === false ) {
+			break;
+		}
+		ix += strideX;
+	}
+	if ( i === N ) {
+		return NaN;
+	}
+	sum = v;
+	ix += strideX;
+	i += 1;
+
+	// In order to preserve the sign of zero which can be lost during compensated summation below, find the first non-zero element...
+	if ( sum === 0.0 ) {
+		for ( ; i < N; i++ ) {
+			v = x[ ix ];
+			if ( isnan( v ) === false ) {
+				if ( v !== 0.0 ) {
+					flg = true;
+					break;
+				}
+				sum += v;
+			}
+			ix += strideX;
+		}
+	} else {
+		flg = true;
+	}
 	ccs = 0.0; // second order correction term for lost low order bits
 	cs = 0.0; // first order correction term for lost low order bits
-	for ( i = 0; i < N; i++ ) {
+	for ( ; i < N; i++ ) {
 		v = x[ ix ];
 		if ( isnan( v ) === false ) {
 			t = sum + v;
@@ -96,7 +127,7 @@ function dnansumkbn2( N, x, strideX, offsetX ) {
 		}
 		ix += strideX;
 	}
-	return sum + cs + ccs;
+	return ( flg ) ? sum+cs+ccs : sum;
 }
 
 

lib/node_modules/@stdlib/blas/ext/base/dnansumkbn2/src/main.c

@@ -61,30 +61,62 @@ double API_SUFFIX(stdlib_strided_dnansumkbn2)( const CBLAS_INT N, const double *
 * @return         output value
 */
 double API_SUFFIX(stdlib_strided_dnansumkbn2_ndarray)( const CBLAS_INT N, const double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX ) {
-	double sum;
-	double ccs;
 	CBLAS_INT ix;
 	CBLAS_INT i;
+	double sum;
+	double ccs;
 	double cs;
 	double cc;
 	double v;
 	double t;
 	double c;
+	int flg;
 
-	sum = 0.0;
 	if ( N <= 0 ) {
-		return sum;
+		return 0.0;
 	}
 	ix = offsetX;
 	if ( strideX == 0 ) {
 		if ( stdlib_base_is_nan( X[ ix ] ) ) {
-			return sum;
+			return 0.0;
 		}
 		return X[ ix ] * N;
 	}
+	// Find the first non-NaN element...
+	for ( i = 0; i < N; i++ ) {
+		v = X[ ix ];
+		if ( !stdlib_base_is_nan( v ) ) {
+			break;
+		}
+		ix += strideX;
+	}
+	if ( i == N ) {
+		return 0.0;
+	}
+	sum = v;
+	ix += strideX;
+	flg = 0;
+	i += 1;
+
+	// In order to preserve the sign of zero which can be lost during compensated summation below, find the first non-zero element...
+	if ( sum == 0.0 ) {
+		for ( ; i < N; i++ ) {
+			v = X[ ix ];
+			if ( !stdlib_base_is_nan( v ) ) {
+				if ( v != 0.0 ) {
+					flg = 1;
+					break;
+				}
+				sum += v;
+			}
+			ix += strideX;
+		}
+	} else {
+		flg = 1;
+	}
 	ccs = 0.0; // second order correction term for lost lower order bits
 	cs = 0.0; // first order correction term for lost low order bits
-	for ( i = 0; i < N; i++ ) {
+	for ( ; i < N; i++ ) {
 		v = X[ ix ];
 		if ( !stdlib_base_is_nan( v ) ) {
 			t = sum + v;
@@ -105,5 +137,5 @@ double API_SUFFIX(stdlib_strided_dnansumkbn2_ndarray)( const CBLAS_INT N, const
 		}
 		ix += strideX;
 	}
-	return sum + cs + ccs;
+	return ( flg == 1 ) ? sum+cs+ccs : sum;
 }

lib/node_modules/@stdlib/blas/ext/base/dnansumkbn2/test/test.ndarray.js

@@ -22,6 +22,7 @@
 
 var tape = require( 'tape' );
 var isPositiveZero = require( '@stdlib/math/base/assert/is-positive-zero' );
+var isNegativeZero = require( '@stdlib/math/base/assert/is-negative-zero' );
 var Float64Array = require( '@stdlib/array/float64' );
 var dnansumkbn2 = require( './../lib/ndarray.js' );
 
@@ -78,14 +79,14 @@ tape( 'the function calculates the sum of strided array elements (ignoring NaN v
 	t.end();
 });
 
-tape( 'the function does not preserve the sign of zero', function test( t ) {
+tape( 'the function preserves the sign of zero', function test( t ) {
 	var x;
 	var v;
 
 	x = new Float64Array( [ -0.0, -0.0, -0.0 ] );
 
 	v = dnansumkbn2( x.length, x, 1, 0 );
-	t.strictEqual( isPositiveZero( v ), true, 'returns expected value' );
+	t.strictEqual( isNegativeZero( v ), true, 'returns expected value' );
 
 	x = new Float64Array( [ 0.0, -0.0, -0.0 ] );
 

lib/node_modules/@stdlib/blas/ext/base/dnansumkbn2/test/test.ndarray.native.js

@@ -23,6 +23,7 @@
 var resolve = require( 'path' ).resolve;
 var tape = require( 'tape' );
 var isPositiveZero = require( '@stdlib/math/base/assert/is-positive-zero' );
+var isNegativeZero = require( '@stdlib/math/base/assert/is-negative-zero' );
 var Float64Array = require( '@stdlib/array/float64' );
 var tryRequire = require( '@stdlib/utils/try-require' );
 
@@ -87,14 +88,14 @@ tape( 'the function calculates the sum of strided array elements (ignoring NaN v
 	t.end();
 });
 
-tape( 'the function does not preserve the sign of zero', opts, function test( t ) {
+tape( 'the function preserves the sign of zero', opts, function test( t ) {
 	var x;
 	var v;
 
 	x = new Float64Array( [ -0.0, -0.0, -0.0 ] );
 
 	v = dnansumkbn2( x.length, x, 1, 0 );
-	t.strictEqual( isPositiveZero( v ), true, 'returns expected value' );
+	t.strictEqual( isNegativeZero( v ), true, 'returns expected value' );
 
 	x = new Float64Array( [ 0.0, -0.0, -0.0 ] );