130 lines
3.7 KiB
JavaScript
130 lines
3.7 KiB
JavaScript
/**
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* @license Apache-2.0
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*
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* Copyright (c) 2018 The Stdlib Authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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'use strict';
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// MODULES //
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var isNonNegativeInteger = require( '@stdlib/assert/is-nonnegative-integer' ).isPrimitive;
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var isString = require( '@stdlib/assert/is-string' ).isPrimitive;
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var FLOAT64_MAX_SAFE_INTEGER = require( '@stdlib/constants/float64/max-safe-integer' );
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var format = require( './../../format' );
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// MAIN //
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/**
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* Repeats a string a specified number of times and returns the concatenated result.
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*
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* ## Method
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*
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* 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).
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*
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* For example,
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*
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* ```text
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* n = 10 => 1010 => 2^3 + 2^0 + 2^1 + 2^0
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* ```
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*
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* We can produce a 10-repeat string by "adding" the results of a 8-repeat string and a 2-repeat string.
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*
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* The implementation is then as follows:
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*
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* 1. Let `s` be the string to be repeated and `o` be an output string.
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*
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* 2. Initialize an output string `o`.
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*
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* 3. Check the least significant bit to determine if the current `s` string should be "added" to the output "total".
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*
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* - if the bit is a one, add
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* - otherwise, move on
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*
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* 4. Double the string `s` by adding `s` to `s`.
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*
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* 5. Right-shift the bits of `n`.
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*
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* 6. Check if we have shifted off all bits.
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*
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* - if yes, done.
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* - otherwise, move on
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*
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* 7. Repeat 3-6.
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*
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* 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".
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*
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* The algorithm runs in `O(log_2(n))` compared to `O(n)`.
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*
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*
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* @param {string} str - string to repeat
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* @param {NonNegativeInteger} n - number of times to repeat the string
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* @throws {TypeError} first argument must be a string
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* @throws {TypeError} second argument must be a nonnegative integer
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* @throws {RangeError} output string length must not exceed maximum allowed string length
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* @returns {string} repeated string
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*
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* @example
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* var str = repeat( 'a', 5 );
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* // returns 'aaaaa'
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*
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* @example
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* var str = repeat( '', 100 );
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* // returns ''
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*
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* @example
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* var str = repeat( 'beep', 0 );
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* // returns ''
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*/
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function repeat( str, n ) {
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var rpt;
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var cnt;
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if ( !isString( str ) ) {
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throw new TypeError( format( 'invalid argument. First argument must be a string. Value: `%s`.', str ) );
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}
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if ( !isNonNegativeInteger( n ) ) {
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throw new TypeError( format( 'invalid argument. Second argument must be a nonnegative integer. Value: `%s`.', n ) );
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}
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if ( str.length === 0 || n === 0 ) {
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return '';
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}
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// Check that output string will not exceed the maximum string length:
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if ( str.length * n > FLOAT64_MAX_SAFE_INTEGER ) {
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throw new RangeError( format( 'invalid argument. Output string length exceeds maximum allowed string length. Value: `%u`.', str.length * n ) );
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}
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rpt = '';
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cnt = n;
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for ( ; ; ) {
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// If the count is odd, append the current concatenated string:
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if ( (cnt&1) === 1 ) {
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rpt += str;
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}
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// Right-shift the bits:
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cnt >>>= 1;
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if ( cnt === 0 ) {
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break;
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}
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// Double the string:
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str += str;
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}
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return rpt;
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}
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// EXPORTS //
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module.exports = repeat;
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