time-to-botec

Benchmark sampling in different programming languages
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repeat.js (3745B)

      1 /**
      2 * @license Apache-2.0
      3 *
      4 * Copyright (c) 2018 The Stdlib Authors.
      5 *
      6 * Licensed under the Apache License, Version 2.0 (the "License");
      7 * you may not use this file except in compliance with the License.
      8 * You may obtain a copy of the License at
      9 *
     10 *    http://www.apache.org/licenses/LICENSE-2.0
     11 *
     12 * Unless required by applicable law or agreed to in writing, software
     13 * distributed under the License is distributed on an "AS IS" BASIS,
     14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     15 * See the License for the specific language governing permissions and
     16 * limitations under the License.
     17 */
     18 
     19 'use strict';
     20 
     21 // MODULES //
     22 
     23 var isNonNegativeInteger = require( '@stdlib/assert/is-nonnegative-integer' ).isPrimitive;
     24 var isString = require( '@stdlib/assert/is-string' ).isPrimitive;
     25 var FLOAT64_MAX_SAFE_INTEGER = require( '@stdlib/constants/float64/max-safe-integer' );
     26 var format = require( './../../format' );
     27 
     28 
     29 // MAIN //
     30 
     31 /**
     32 * Repeats a string a specified number of times and returns the concatenated result.
     33 *
     34 * ## Method
     35 *
     36 * 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).
     37 *
     38 * For example,
     39 *
     40 * ```text
     41 * n = 10 => 1010 => 2^3 + 2^0 + 2^1 + 2^0
     42 * ```
     43 *
     44 * We can produce a 10-repeat string by "adding" the results of a 8-repeat string and a 2-repeat string.
     45 *
     46 * The implementation is then as follows:
     47 *
     48 * 1.  Let `s` be the string to be repeated and `o` be an output string.
     49 *
     50 * 2.  Initialize an output string `o`.
     51 *
     52 * 3.  Check the least significant bit to determine if the current `s` string should be "added" to the output "total".
     53 *
     54 *     -   if the bit is a one, add
     55 *     -   otherwise, move on
     56 *
     57 * 4.  Double the string `s` by adding `s` to `s`.
     58 *
     59 * 5.  Right-shift the bits of `n`.
     60 *
     61 * 6.  Check if we have shifted off all bits.
     62 *
     63 *     -   if yes, done.
     64 *     -   otherwise, move on
     65 *
     66 * 7.  Repeat 3-6.
     67 *
     68 * 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".
     69 *
     70 * The algorithm runs in `O(log_2(n))` compared to `O(n)`.
     71 *
     72 *
     73 * @param {string} str - string to repeat
     74 * @param {NonNegativeInteger} n - number of times to repeat the string
     75 * @throws {TypeError} first argument must be a string
     76 * @throws {TypeError} second argument must be a nonnegative integer
     77 * @throws {RangeError} output string length must not exceed maximum allowed string length
     78 * @returns {string} repeated string
     79 *
     80 * @example
     81 * var str = repeat( 'a', 5 );
     82 * // returns 'aaaaa'
     83 *
     84 * @example
     85 * var str = repeat( '', 100 );
     86 * // returns ''
     87 *
     88 * @example
     89 * var str = repeat( 'beep', 0 );
     90 * // returns ''
     91 */
     92 function repeat( str, n ) {
     93 	var rpt;
     94 	var cnt;
     95 	if ( !isString( str ) ) {
     96 		throw new TypeError( format( 'invalid argument. First argument must be a string. Value: `%s`.', str ) );
     97 	}
     98 	if ( !isNonNegativeInteger( n ) ) {
     99 		throw new TypeError( format( 'invalid argument. Second argument must be a nonnegative integer. Value: `%s`.', n ) );
    100 	}
    101 	if ( str.length === 0 || n === 0 ) {
    102 		return '';
    103 	}
    104 	// Check that output string will not exceed the maximum string length:
    105 	if ( str.length * n > FLOAT64_MAX_SAFE_INTEGER ) {
    106 		throw new RangeError( format( 'invalid argument. Output string length exceeds maximum allowed string length. Value: `%u`.', str.length * n ) );
    107 	}
    108 	rpt = '';
    109 	cnt = n;
    110 	for ( ; ; ) {
    111 		// If the count is odd, append the current concatenated string:
    112 		if ( (cnt&1) === 1 ) {
    113 			rpt += str;
    114 		}
    115 		// Right-shift the bits:
    116 		cnt >>>= 1;
    117 		if ( cnt === 0 ) {
    118 			break;
    119 		}
    120 		// Double the string:
    121 		str += str;
    122 	}
    123 	return rpt;
    124 }
    125 
    126 
    127 // EXPORTS //
    128 
    129 module.exports = repeat;