@stdlib/array/base/cartesian-power/lib/main.js

Arrays.

/**
* @license Apache-2.0
*
* Copyright (c) 2022 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 resolveGetter = require( './../../../base/resolve-getter' );
var pow = require( '@stdlib/math/base/special/pow' );


// MAIN //

/**
* Returns the Cartesian power.
*
* ## Notes
*
* -   The Cartesian power is an n-fold Cartesian product involving a single array. The main insight of this implementation is that the n-fold Cartesian product can be represented as an n-dimensional array stored in row-major order. As such, we can
*
*     -   Compute the total number of tuples, which is simply the size of the provided array (set) raised to the specified power `n`. For n-dimensional arrays, this is the equivalent of computing the product of array dimensions to determine the total number of elements.
*     -   Initialize an array for storing indices for indexing into the provided array. For n-dimensional arrays, the index array is equivalent to an array of subscripts for indexing into each dimension.
*     -   For the outermost loop, treat the loop index as a linear index into an n-dimensional array and resolve the corresponding subscripts.
*     -   Continue iterating until all tuples have been generated.
*
* @param {Collection} x - input array
* @param {NonNegativeInteger} n - power
* @returns {Array<Array>} list of ordered tuples comprising the Cartesian product
*
* @example
* var x = [ 1, 2 ];
*
* var out = cartesianPower( x, 2 );
* // returns [ [ 1, 1 ], [ 1, 2 ], [ 2, 1 ], [ 2, 2 ] ]
*/
function cartesianPower( x, n ) {
	var get;
	var out;
	var tmp;
	var idx;
	var len;
	var N;
	var s;
	var i;
	var j;
	var k;

	N = x.length;
	if ( N <= 0 || n <= 0 ) {
		return [];
	}
	// Resolve an array element accessor:
	get = resolveGetter( x );

	// Compute the total number of ordered tuples:
	len = pow( N, n );

	// Initialize a list of indices for indexing into the array (equivalent to ndarray subscripts):
	idx = [];
	for ( i = 0; i < n; i++ ) {
		idx.push( 0 );
	}
	// Compute the n-fold Cartesian product...
	out = [];
	for ( i = 0; i < len; i++ ) {
		// Resolve a linear index to array indices (logic is equivalent to what is found in ndarray/base/ind2sub for an ndarray stored in row-major order; see https://github.com/stdlib-js/stdlib/blob/215ca5355f3404f15996fd0ced58a98e46f22be6/lib/node_modules/%40stdlib/ndarray/base/ind2sub/lib/assign.js)...
		k = i;
		for ( j = n-1; j >= 0; j-- ) {
			s = k % N;
			k -= s;
			k /= N;
			idx[ j ] = s;
		}
		// Generate the next ordered tuple...
		tmp = [];
		for ( j = 0; j < n; j++ ) {
			tmp.push( get( x, idx[ j ] ) );
		}
		out.push( tmp );
	}
	return out;
}


// EXPORTS //

module.exports = cartesianPower;