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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 isPositiveZero = require( '@stdlib/math/base/assert/is-positive-zero' );
var gcopy = require( '@stdlib/blas/base/gcopy' ).ndarray;
var floor = require( '@stdlib/math/base/special/floor' );
// FUNCTIONS //
/**
* Sifts a value down the heap to restore the min-heap.
*
* @private
* @param {NonNegativeInteger} root - logical root index to sift down from
* @param {PositiveInteger} size - heap size
* @param {number} value - value to sift down
* @param {Object} arr - heap storage array
* @param {Collection} arr.data - heap storage array data
* @param {Array<Function>} arr.accessors - heap storage array element accessors
* @param {integer} stride - stride length
* @param {NonNegativeInteger} offset - starting offset
*/
function siftDown( root, size, value, arr, stride, offset ) {
var ridx;
var cidx;
var abuf;
var aget;
var aset;
var j;
abuf = arr.data;
aget = arr.accessors[ 0 ];
aset = arr.accessors[ 1 ];
ridx = offset + (root*stride);
j = ( root * 2 ) + 1;
while ( j < size ) {
cidx = offset + (j*stride);
// Find smaller child.
if ( j + 1 < size && aget( abuf, offset + ((j+1)*stride) ) < aget( abuf, cidx ) ) { // eslint-disable-line max-len
j += 1;
cidx = offset + (j*stride);
}
// Stop if element is already smaller than child.
if ( aget( abuf, cidx ) >= value ) {
break;
}
// Move child up.
aset( abuf, ridx, aget( abuf, cidx ) );
root = j;
ridx = cidx;
j = ( root * 2 ) + 1;
}
aset( abuf, ridx, value );
}
// MAIN //
/**
* Computes the k maximum values of a strided array.
*
* @private
* @param {PositiveInteger} N - number of indexed elements
* @param {PositiveInteger} k - number of maximum values to return
* @param {Object} x - input array object
* @param {Collection} x.data - input array data
* @param {Array<Function>} x.accessors - array element accessors
* @param {integer} strideX - stride length for `x`
* @param {NonNegativeInteger} offsetX - starting index for `x`
* @param {Object} out - output array object
* @param {Collection} out.data - output array data
* @param {Array<Function>} out.accessors - array element accessors
* @param {integer} strideOut - stride length for `out`
* @param {NonNegativeInteger} offsetOut - starting index for `out`
* @returns {Object} output array object
*
* @example
* var toAccessorArray = require( '@stdlib/array/base/to-accessor-array' );
* var arraylike2object = require( '@stdlib/array/base/arraylike2object' );
*
* var x = [ 1.0, 2.0, 3.0, 4.0, 5.0 ];
* var out = [ 0.0, 0.0, 0.0 ];
*
* maxk( 5, 3, arraylike2object( toAccessorArray( x ) ), 1, 0, arraylike2object( toAccessorArray( out ) ), 1, 0 );
* // out => [ 3.0, 4.0, 5.0 ]
*/
function maxk( N, k, x, strideX, offsetX, out, strideOut, offsetOut ) {
var xbuf;
var obuf;
var xget;
var oget;
var oset;
var min;
var ix;
var i;
var v;
xbuf = x.data;
obuf = out.data;
xget = x.accessors[ 0 ];
oget = out.accessors[ 0 ];
oset = out.accessors[ 1 ];
// Copy first k elements from input to output
gcopy( k, xbuf, strideX, offsetX, obuf, strideOut, offsetOut );
// Build min-heap from first k elements in output
for ( i = floor(k/2)-1; i >= 0; i-- ) {
siftDown( i, k, oget( obuf, (offsetOut + i)*strideOut ), out, strideOut, offsetOut ); // eslint-disable-line max-len
}
// Cache minimum value
min = oget( obuf, offsetOut );
// Scan remaining elements and update heap if larger than current minimum
ix = offsetX + ( k * strideX );
for ( i = k; i < N; i++ ) {
v = xget( xbuf, ix );
// If element is larger than min, replace root and sift down
if ( v > min || (v === min && isPositiveZero( v )) ) {
oset( obuf, offsetOut, v );
siftDown( 0, k, v, out, strideOut, offsetOut );
min = oget( obuf, offsetOut );
}
ix += strideX;
}
return out;
}
// EXPORTS //
module.exports = maxk;
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