@stdlib/string/base/repeat/lib/polyfill.js

String manipulation functions.

/**
* @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';

// MAIN //

/**
* Repeats a string a specified number of times and returns the concatenated result.
*
* ## Method
*
* The algorithmic trick used in the implementation is to treat string concatenation the same as binary addition (i.e., any natural number (nonnegative integer) can be expressed as a sum of powers of two).
*
* For example,
*
* ```text
* n = 10 => 1010 => 2^3 + 2^0 + 2^1 + 2^0
* ```
*
* We can produce a 10-repeat string by "adding" the results of a 8-repeat string and a 2-repeat string.
*
* The implementation is then as follows:
*
* 1.  Let `s` be the string to be repeated and `o` be an output string.
*
* 2.  Initialize an output string `o`.
*
* 3.  Check the least significant bit to determine if the current `s` string should be "added" to the output "total".
*
*     -   if the bit is a one, add
*     -   otherwise, move on
*
* 4.  Double the string `s` by adding `s` to `s`.
*
* 5.  Right-shift the bits of `n`.
*
* 6.  Check if we have shifted off all bits.
*
*     -   if yes, done.
*     -   otherwise, move on
*
* 7.  Repeat 3-6.
*
* The result is that, as the string is repeated, we continually check to see if the doubled string is one which we want to add to our "total".
*
* The algorithm runs in `O(log_2(n))` compared to `O(n)`.
*
* @private
* @param {string} str - string to repeat
* @param {NonNegativeInteger} n - number of times to repeat the string
* @returns {string} repeated string
*
* @example
* var str = repeat( 'a', 5 );
* // returns 'aaaaa'
*
* @example
* var str = repeat( '', 100 );
* // returns ''
*
* @example
* var str = repeat( 'beep', 0 );
* // returns ''
*/
function repeat( str, n ) {
	var rpt;
	var cnt;
	if ( str.length === 0 || n === 0 ) {
		return '';
	}
	rpt = '';
	cnt = n;
	for ( ; ; ) {
		// If the count is odd, append the current concatenated string:
		if ( (cnt&1) === 1 ) {
			rpt += str;
		}
		// Right-shift the bits:
		cnt >>>= 1;
		if ( cnt === 0 ) {
			break;
		}
		// Double the string:
		str += str;
	}
	return rpt;
}


// EXPORTS //

module.exports = repeat;