fix: update implementation to preserve signed zeros

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---

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/dnannsumkbn2/lib/ndarray.js

@@ -58,6 +58,7 @@ var abs = require( '@stdlib/math/base/special/abs' );
 function dnannsumkbn2( N, x, strideX, offsetX, out, strideOut, offsetOut ) {
 	var sum;
 	var ccs;
+	var flg;
 	var cs;
 	var cc;
 	var ix;
@@ -68,28 +69,61 @@ function dnannsumkbn2( N, x, strideX, offsetX, out, strideOut, offsetOut ) {
 	var n;
 	var i;
 
-	sum = 0.0;
 	io = offsetOut;
 	if ( N <= 0 ) {
-		out[ io ] = sum;
+		out[ io ] = 0.0;
 		out[ io+strideOut ] = 0;
 		return out;
 	}
 	ix = offsetX;
 	if ( strideX === 0 ) {
 		if ( isnan( x[ ix ] ) ) {
-			out[ io ] = sum;
+			out[ io ] = 0.0;
 			out[ io+strideOut ] = 0;
 			return out;
 		}
 		out[ io ] = x[ ix ] * N;
 		out[ io+strideOut ] = N;
 		return out;
 	}
+	// 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 ) {
+		out[ io ] = 0.0;
+		out[ io+strideOut ] = 0;
+		return out;
+	}
+	n = 1;
+	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;
+				n += 1;
+			}
+			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
-	n = 0;
-	for ( i = 0; i < N; i++ ) {
+	for ( ; i < N; i++ ) {
 		v = x[ ix ];
 		if ( isnan( v ) === false ) {
 			t = sum + v;
@@ -111,7 +145,7 @@ function dnannsumkbn2( N, x, strideX, offsetX, out, strideOut, offsetOut ) {
 		}
 		ix += strideX;
 	}
-	out[ io ] = sum + cs + ccs;
+	out[ io ] = ( flg ) ? sum+cs+ccs : sum;
 	out[ io+strideOut ] = n;
 	return out;
 }

lib/node_modules/@stdlib/blas/ext/base/dnannsumkbn2/src/main.c

@@ -72,23 +72,57 @@ double API_SUFFIX(stdlib_strided_dnannsumkbn2_ndarray)( const CBLAS_INT N, const
 	double v;
 	double t;
 	double c;
+	int flg;
 
-	sum = 0.0;
 	*n = 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;
 		}
 		*n += N;
 		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;
+	}
+	*n += 1;
+	sum = v;
+	ix += strideX;
+	i += 1;
+	flg = 0;
+
+	// 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;
+				*n += 1;
+			}
+			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;
@@ -110,5 +144,5 @@ double API_SUFFIX(stdlib_strided_dnannsumkbn2_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/dnannsumkbn2/test/test.ndarray.js

@@ -22,6 +22,7 @@
 
 var tape = require( 'tape' );
 var Float64Array = require( '@stdlib/array/float64' );
+var isSameFloat64Array = require( '@stdlib/assert/is-same-float64array' );
 var dnannsumkbn2 = require( './../lib/ndarray.js' );
 
 
@@ -119,6 +120,24 @@ tape( 'the function calculates the sum of strided array elements (ignoring NaN v
 	t.end();
 });
 
+tape( 'the function preserves the sign of zero', function test( t ) {
+	var expected;
+	var out;
+	var x;
+	var v;
+
+	x = new Float64Array( [ -0.0, -0.0, -0.0, -0.0, -0.0 ] );
+
+	out = new Float64Array( 2 );
+	v = dnannsumkbn2( x.length, x, 1, 0, out, 1, 0 );
+
+	expected = new Float64Array( [ -0.0, 5.0 ] );
+	t.strictEqual( v, out, 'returns expected value' );
+	t.strictEqual( isSameFloat64Array( v, expected ), true, 'returns expected value' );
+
+	t.end();
+});
+
 tape( 'if provided an `N` parameter less than or equal to `0`, the function returns `0.0`', function test( t ) {
 	var expected;
 	var out;

lib/node_modules/@stdlib/blas/ext/base/dnannsumkbn2/test/test.ndarray.native.js

@@ -23,6 +23,7 @@
 var resolve = require( 'path' ).resolve;
 var tape = require( 'tape' );
 var Float64Array = require( '@stdlib/array/float64' );
+var isSameFloat64Array = require( '@stdlib/assert/is-same-float64array' );
 var tryRequire = require( '@stdlib/utils/try-require' );
 
 
@@ -128,6 +129,24 @@ tape( 'the function calculates the sum of strided array elements (ignoring NaN v
 	t.end();
 });
 
+tape( 'the function preserves the sign of zero', opts, function test( t ) {
+	var expected;
+	var out;
+	var x;
+	var v;
+
+	x = new Float64Array( [ -0.0, -0.0, -0.0, -0.0, -0.0 ] );
+
+	out = new Float64Array( 2 );
+	v = dnannsumkbn2( x.length, x, 1, 0, out, 1, 0 );
+
+	expected = new Float64Array( [ -0.0, 5.0 ] );
+	t.strictEqual( v, out, 'returns expected value' );
+	t.strictEqual( isSameFloat64Array( v, expected ), true, 'returns expected value' );
+
+	t.end();
+});
+
 tape( 'if provided an `N` parameter less than or equal to `0`, the function returns `0.0`', opts, function test( t ) {
 	var expected;
 	var out;