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* @license Apache-2.0
*
* Copyright (c) 2026 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.
*/
'use strict';
// MODULES //
var dlamch = require( '@stdlib/lapack/base/dlamch' );
var min = require( '@stdlib/math/base/special/min' );
// VARIABLES //
var SFMIN = dlamch( 'S' );
// MAIN //
/**
* Computes one dqd transform in ping-pong form.
*
* ## Notes
*
* - Z is a 1-D array of length >= 4*N0 storing interleaved q/e values.
* - PP is 0 for ping, 1 for pong.
*
* @private
* @param {integer} I0 - the first index
* @param {integer} N0 - the last index
* @param {Float64Array} Z - the QD array
* @param {integer} strideZ - stride length for `z`
* @param {NonNegativeInteger} offsetZ - starting index for `z`
* @param {boolean} PP - ping-pong flag (0 or 1)
* @param {Float64Array} out - output array containing `DMIN`, `DMIN1`, `DMIN2`, `DN`, `DNM1`, and `DNM2` respectively
* @param {integer} strideOut - stride length for `out`
* @param {NonNegativeInteger} offsetOut - starting index of `out`
* @returns {Float64Array} output array
*
* @example
* var Float64Array = require( '@stdlib/array/float64' );
*
* var out = new Float64Array( 6 );
* var Z = new Float64Array( [ 5, 0, 7, 0, 9, 0, 3, 0, 11, 0, 4, 0, 20, 0, 0, 0 ] );
*
* dlasq6( 0, 3, Z, 1, 0, 0, out, 1, 0 );
* // out => <Float64Array>[ 3.75, 3.75, 3.75, ~12.088, ~6.111, 3.75 ]
*/
function dlasq6( I0, N0, Z, strideZ, offsetZ, PP, out, strideOut, offsetOut ) {
var dmin1;
var dmin2;
var dnm1;
var dnm2;
var emin;
var dmin;
var temp;
var j4p2;
var idx;
var iz1;
var iz2;
var iz3;
var iz4;
var j4;
var dn;
var d;
// Quick return...
if ( ( N0 - I0 - 1 ) <= 0 ) {
return out;
}
idx = offsetZ;
j4 = offsetZ + ( strideZ*( ( 4*I0 ) + PP ) );
emin = Z[ j4 + ( 4*strideZ ) ];
d = Z[ j4 ];
dmin = d;
j4 = offsetZ + ( strideZ*( ( 4*I0 ) + 3 ) );
if ( PP === 0 ) {
for ( idx = ( 4*I0 ) + 3; idx <= ( 4*N0 ) - 9; idx += 4 ) {
iz1 = j4 + strideZ;
iz2 = j4 - strideZ;
iz3 = j4 - ( 2*strideZ );
Z[ iz3 ] = d + Z[ iz2 ];
if ( Z[ iz3 ] === 0 ) {
Z[ j4 ] = 0;
d = Z[ iz1 ];
dmin = d;
emin = 0;
} else if ( ( SFMIN*Z[ iz1 ] < Z[ iz3 ] ) &&
( SFMIN*Z[ iz3 ] < Z[ iz1 ] ) ) {
temp = Z[ iz1 ] / Z[ iz3 ];
Z[ j4 ] = Z[ iz2 ]*temp;
d *= temp;
} else {
Z[ j4 ] = Z[ iz1 ]*( Z[ iz2 ] / Z[ iz3 ] );
d = Z[ iz1 ]*( d / Z[ iz3 ] );
}
dmin = min( dmin, d );
emin = min( emin, Z[ j4 ] );
j4 += 4*strideZ;
}
} else {
for ( idx = ( 4*I0 ) + 3; idx <= ( 4*N0 ) - 9; idx += 4 ) {
iz1 = j4 + ( 2*strideZ );
iz2 = j4 - strideZ;
iz3 = j4 - ( 2*strideZ );
iz4 = j4 - ( 3*strideZ );
Z[ iz4 ] = d + Z[ j4 ];
if ( Z[ iz4 ] === 0 ) {
Z[ iz2 ] = 0;
d = Z[ iz1 ];
dmin = d;
emin = 0;
} else if ( ( SFMIN*Z[ iz1 ] < Z[ iz4 ] ) &&
( SFMIN*Z[ iz4 ] < Z[ iz1 ] ) ) {
temp = Z[ iz1 ] / Z[ iz4 ];
Z[ j4 - strideZ ] = Z[ j4 ]*temp;
d *= temp;
} else {
Z[ iz2 ] = Z[ iz1 ]*( Z[ j4 ] / Z[ iz4 ] );
d = Z[ iz1 ]*( d / Z[ iz4 ] );
}
dmin = min( dmin, d );
emin = min( emin, Z[ iz2 ] );
j4 += 4*strideZ;
}
}
// Unroll last two steps.
dnm2 = d;
dmin2 = dmin;
j4 = offsetZ + ( strideZ*( ( 4*N0 ) - 5 - PP ) );
j4p2 = j4 + ( strideZ*( ( 2*PP ) - 1 ) );
iz1 = j4 - ( 2*strideZ );
iz2 = j4p2 + ( 2*strideZ );
Z[ j4 - ( 2*strideZ ) ] = dnm2 + Z[ j4p2 ];
if ( Z[ iz1 ] === 0 ) {
Z[ j4 ] = 0;
dnm1 = Z[ iz2 ];
dmin = dnm1;
emin = 0;
} else if ( ( SFMIN*Z[ iz2 ] < Z[ iz1 ] ) &&
( SFMIN*Z[ iz1 ] < Z[ iz2 ] ) ) {
temp = Z[ iz2 ] / Z[ iz1 ];
Z[ j4 ] = Z[ j4p2 ]*temp;
dnm1 = dnm2*temp;
} else {
Z[ j4 ] = Z[ iz2 ]*( Z[ j4p2 ] / Z[ iz1 ] );
dnm1 = Z[ iz2 ]*( dnm2 / Z[ iz1 ] );
}
dmin = min( dmin, dnm1 );
dmin1 = dmin;
j4 += 4*strideZ;
j4p2 = j4 + ( strideZ*( ( 2*PP ) - 1 ) );
iz1 = j4 - ( 2*strideZ );
iz2 = j4p2 + ( 2*strideZ );
Z[ iz1 ] = dnm1 + Z[ j4p2 ];
if ( Z[ iz1 ] === 0 ) {
Z[ j4 ] = 0;
dn = Z[ iz2 ];
dmin = dn;
emin = 0;
} else if ( ( SFMIN*Z[ iz2 ] < Z[ iz1 ] ) &&
( SFMIN*Z[ iz1 ] < Z[ iz2 ] ) ) {
temp = Z[ iz2 ] / Z[ iz1 ];
Z[ j4 ] = Z[ j4p2 ]*temp;
dn = dnm1*temp;
} else {
Z[ j4 ] = Z[ iz2 ]*( Z[ j4p2 ] / Z[ iz1 ] );
dn = Z[ iz2 ]*( dnm1 / Z[ iz1 ] );
}
dmin = min( dmin, dn );
Z[ j4 + ( 2*strideZ ) ] = dn;
Z[ offsetZ + ( strideZ*( ( 4*( N0 ) ) + 3 - PP ) ) ] = emin;
idx = offsetOut;
out[ idx ] = dmin;
idx += strideOut;
out[ idx ] = dmin1;
idx += strideOut;
out[ idx ] = dmin2;
idx += strideOut;
out[ idx ] = dn;
idx += strideOut;
out[ idx ] = dnm1;
idx += strideOut;
out[ idx ] = dnm2;
return out;
}
// EXPORTS //
module.exports = dlasq6;
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