fix: preserve the sign of zero

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---

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/scusumkbn2/lib/ndarray.js

@@ -73,9 +73,23 @@ function scusumkbn2( N, sum, x, strideX, offsetX, y, strideY, offsetY ) {
 	ix = offsetX;
 	iy = offsetY;
 
+	// 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 = 0; i < N; i++ ) {
+			v = x[ ix ];
+			if ( v !== 0.0 ) {
+				break;
+			}
+			y[ iy ] = float64ToFloat32( sum + v );
+			ix += strideX;
+			iy += strideY;
+		}
+	} else {
+		i = 0;
+	}
 	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 ];
 		t = float64ToFloat32( sum+v );
 		if ( abs( sum ) >= abs( v ) ) {

lib/node_modules/@stdlib/blas/ext/base/scusumkbn2/src/main.c

@@ -59,10 +59,10 @@ void API_SUFFIX(stdlib_strided_scusumkbn2)( const CBLAS_INT N, const float sum,
 * @param N        number of indexed elements
 * @param sum      initial sum
 * @param X        input array
-* @param strideX  stride lengh for X
+* @param strideX  stride length for X
 * @param offsetX  starting index for X
 * @param Y        output array
-* @param strideY  stride lengt for Y
+* @param strideY  stride length for Y
 * @param offsetY  starting index for Y
 */
 void API_SUFFIX(stdlib_strided_scusumkbn2_ndarray)( const CBLAS_INT N, const float sum, const float *X, const CBLAS_INT strideX, const CBLAS_INT offsetX, float *Y, const CBLAS_INT strideY, const CBLAS_INT offsetY ) {
@@ -83,9 +83,24 @@ void API_SUFFIX(stdlib_strided_scusumkbn2_ndarray)( const CBLAS_INT N, const flo
 	ix = offsetX;
 	iy = offsetY;
 	s = sum;
+
+	// In order to preserve the sign of zero which can be lost during compensated summation below, find the first non-zero element...
+	if ( s == 0.0 ) {
+		for ( i = 0; i < N; i++ ) {
+			v = X[ ix ];
+			if ( v != 0.0f ) {
+				break;
+			}
+			Y[ iy ] = s + v;
+			ix += strideX;
+			iy += strideY;
+		}
+	} else {
+		i = 0;
+	}
 	ccs = 0.0f; // second order correction term for lost lower order bits
 	cs = 0.0f; // first order correction term for lost low order bits
-	for ( i = 0; i < N; i++ ) {
+	for ( ; i < N; i++ ) {
 		v = X[ ix ];
 		t = s + v;
 		if ( stdlib_base_absf( s ) >= stdlib_base_absf( v ) ) {

lib/node_modules/@stdlib/blas/ext/base/scusumkbn2/test/test.ndarray.js

@@ -21,8 +21,8 @@
 // MODULES //
 
 var tape = require( 'tape' );
-var floor = require( '@stdlib/math/base/special/floor' );
 var isnanf = require( '@stdlib/math/base/assert/is-nanf' );
+var isSameFloat32Array = require( '@stdlib/assert/is-same-float32array' );
 var Float32Array = require( '@stdlib/array/float32' );
 var scusumkbn2 = require( './../lib/ndarray.js' );
 
@@ -127,6 +127,26 @@ tape( 'the function calculates the cumulative sum', function test( t ) {
 	t.end();
 });
 
+tape( 'the function preserves the sign of zero', function test( t ) {
+	var expected;
+	var x;
+	var y;
+
+	x = new Float32Array( [ -0.0, -0.0, -0.0, 0.0, 1.0 ] );
+	y = new Float32Array( x.length );
+
+	scusumkbn2( x.length, -0.0, x, 1, 0, y, 1, 0 );
+	expected = new Float32Array([
+		-0.0,
+		-0.0,
+		-0.0,
+		0.0,
+		1.0
+	]);
+	t.strictEqual( isSameFloat32Array( y, expected ), true, 'returns expected value' );
+	t.end();
+});
+
 tape( 'the function returns a reference to the output array', function test( t ) {
 	var out;
 	var x;
@@ -292,7 +312,7 @@ tape( 'the function supports an `x` offset', function test( t ) {
 		0.0,
 		0.0
 	]);
-	N = floor( x.length / 2 );
+	N = 4;
 
 	scusumkbn2( N, 0.0, x, 2, 1, y, 1, 0 );
 
@@ -328,7 +348,7 @@ tape( 'the function supports a `y` offset', function test( t ) {
 		0.0,
 		0.0  // 3
 	]);
-	N = floor( x.length / 2 );
+	N = 4;
 
 	scusumkbn2( N, 0.0, x, 1, 0, y, 2, 1 );
 

lib/node_modules/@stdlib/blas/ext/base/scusumkbn2/test/test.ndarray.native.js

@@ -22,8 +22,8 @@
 
 var resolve = require( 'path' ).resolve;
 var tape = require( 'tape' );
-var floor = require( '@stdlib/math/base/special/floor' );
 var isnanf = require( '@stdlib/math/base/assert/is-nanf' );
+var isSameFloat32Array = require( '@stdlib/assert/is-same-float32array' );
 var Float32Array = require( '@stdlib/array/float32' );
 var tryRequire = require( '@stdlib/utils/try-require' );
 
@@ -136,6 +136,26 @@ tape( 'the function calculates the cumulative sum', opts, function test( t ) {
 	t.end();
 });
 
+tape( 'the function preserves the sign of zero', opts, function test( t ) {
+	var expected;
+	var x;
+	var y;
+
+	x = new Float32Array( [ -0.0, -0.0, -0.0, 0.0, 1.0 ] );
+	y = new Float32Array( x.length );
+
+	scusumkbn2( x.length, -0.0, x, 1, 0, y, 1, 0 );
+	expected = new Float32Array([
+		-0.0,
+		-0.0,
+		-0.0,
+		0.0,
+		1.0
+	]);
+	t.strictEqual( isSameFloat32Array( y, expected ), true, 'returns expected value' );
+	t.end();
+});
+
 tape( 'the function returns a reference to the output array', opts, function test( t ) {
 	var out;
 	var x;
@@ -301,7 +321,7 @@ tape( 'the function supports an `x` offset', opts, function test( t ) {
 		0.0,
 		0.0
 	]);
-	N = floor( x.length / 2 );
+	N = 4;
 
 	scusumkbn2( N, 0.0, x, 2, 1, y, 1, 0 );
 
@@ -337,7 +357,7 @@ tape( 'the function supports a `y` offset', opts, function test( t ) {
 		0.0,
 		0.0  // 3
 	]);
-	N = floor( x.length / 2 );
+	N = 4;
 
 	scusumkbn2( N, 0.0, x, 1, 0, y, 2, 1 );