Press n or j to go to the next uncovered block, b, p or k for the previous block.
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* @license Apache-2.0
*
* Copyright (c) 2023 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 RE_UTF16_SURROGATE_PAIR = require( '@stdlib/regexp/utf16-surrogate-pair' ).REGEXP;
// VARIABLES //
var RE_UTF16_LOW_SURROGATE = /[\uDC00-\uDFFF]/; // TODO: replace with stdlib pkg
var RE_UTF16_HIGH_SURROGATE = /[\uD800-\uDBFF]/; // TODO: replace with stdlib pkg
// MAIN //
/**
* Returns the first `n` Unicode code points of a string.
*
* @param {string} str - input string
* @param {NonNegativeInteger} n - number of Unicode code points to return
* @returns {string} output string
*
* @example
* var out = first( 'last man standing', 1 );
* // returns 'l'
*
* @example
* var out = first( 'presidential election', 1 );
* // returns 'p'
*
* @example
* var out = first( 'JavaScript', 1 );
* // returns 'J'
*
* @example
* var out = first( 'Hidden Treasures', 1 );
* // returns 'H'
*/
function first( str, n ) {
var len;
var out;
var ch1;
var ch2;
var cnt;
var i;
if ( str === '' || n === 0 ) {
return '';
}
if ( n === 1 ) {
str = str.substring( 0, 2 );
if ( RE_UTF16_SURROGATE_PAIR.test( str ) ) {
return str;
}
return str[ 0 ];
}
len = str.length;
out = '';
cnt = 0;
// Process the string one Unicode code unit at a time and count UTF-16 surrogate pairs as a single Unicode code point...
for ( i = 0; i < len; i++ ) {
ch1 = str[ i ];
out += ch1;
cnt += 1;
// Check for a high UTF-16 surrogate...
if ( RE_UTF16_HIGH_SURROGATE.test( ch1 ) ) {
// Check for an unpaired surrogate at the end of the input string...
if ( i === len-1 ) {
// We found an unpaired surrogate...
break;
}
// Check whether the high surrogate is paired with a low surrogate...
ch2 = str[ i+1 ];
if ( RE_UTF16_LOW_SURROGATE.test( ch2 ) ) {
// We found a surrogate pair:
out += ch2;
i += 1; // bump the index to process the next code unit
}
}
// Check whether we've found the desired number of code points...
if ( cnt === n ) {
break;
}
}
return out;
}
// EXPORTS //
module.exports = first;
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