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* @license Apache-2.0
*
* Copyright (c) 2025 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 numCodePoints = require( '@stdlib/string/num-code-points' );
var max = require( '@stdlib/math/base/special/fast/max' );
// 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 //
/**
* Slices a string based on Unicode code points.
*
* @param {string} str - input string
* @param {integer} start - the `ith` Unicode code point to start a slice (inclusive)
* @param {integer} end - the `jth` Unicode code point to end a slice (exclusive)
* @returns {string} output string
*
* @example
* var out = sliceCodePoints( 'Hello 👋 World', 1, 3 );
* // returns 'el'
*
* out = sliceCodePoints( '👋👋👋', 1, 2 );
* // returns '👋'
*/
function sliceCodePoints( str, start, end ) {
var totalCodePoints;
var codePoints;
var len;
var ch1;
var ch2;
var idx;
var ch;
var i;
len = str.length;
if ( len === 0 ) {
return '';
}
totalCodePoints = numCodePoints( str );
if ( start < 0 ) {
start = max( totalCodePoints + start, 0 );
}
if ( end < 0 ) {
end = max( totalCodePoints + end, 0 );
} else if ( end > totalCodePoints ) {
end = totalCodePoints;
}
if ( start >= totalCodePoints || end <= start ) {
return '';
}
codePoints = '';
idx = 0;
// Process the string one Unicode code unit at a time and handle UTF-16 surrogate pairs as a single Unicode code point...
for ( i = 0; i < len && idx < end; i++ ) {
ch1 = str[ i ];
ch = ch1;
// Check for a UTF-16 surrogate pair...
if ( i < len-1 && RE_UTF16_HIGH_SURROGATE.test( ch1 ) ) {
// 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:
ch += ch2;
i += 1; // bump the index to process the next code unit
}
}
// Note: `ch` may be a lone surrogate (e.g., a low surrogate without a preceding high surrogate or a high surrogate at the end of the input string).
if ( idx >= start && idx < end ) {
codePoints += ch;
}
idx += 1;
}
return codePoints;
}
// EXPORTS //
module.exports = sliceCodePoints;
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