132 lines
4.0 KiB
JavaScript
132 lines
4.0 KiB
JavaScript
/**
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* @license Apache-2.0
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*
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* Copyright (c) 2020 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 isBoolean = require( '@stdlib/assert/is-boolean' ).isPrimitive;
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var isString = require( '@stdlib/assert/is-string' ).isPrimitive;
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var isNonNegativeInteger = require( '@stdlib/assert/is-nonnegative-integer' ).isPrimitive;
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var format = require( './../../format' );
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// VARIABLES //
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// Factors for converting individual surrogates
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var Ox10000 = 0x10000|0; // 65536
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var Ox400 = 0x400|0; // 1024
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// Range for a high surrogate
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var OxD800 = 0xD800|0; // 55296
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var OxDBFF = 0xDBFF|0; // 56319
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// Range for a low surrogate
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var OxDC00 = 0xDC00|0; // 56320
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var OxDFFF = 0xDFFF|0; // 57343
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// MAIN //
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/**
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* Returns a Unicode code point from a string at a specified position.
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*
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* ## Notes
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*
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* - UTF-16 encoding uses one 16-bit unit for non-surrogates (U+0000 to U+D7FF and U+E000 to U+FFFF).
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* - UTF-16 encoding uses two 16-bit units (surrogate pairs) for U+10000 to U+10FFFF and encodes U+10000-U+10FFFF by subtracting 0x10000 from the code point, expressing the result as a 20-bit binary, and splitting the 20 bits of 0x0-0xFFFFF as upper and lower 10-bits. The respective 10-bits are stored in two 16-bit words: a high and a low surrogate.
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*
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*
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* @param {string} str - input string
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* @param {NonNegativeInteger} idx - position
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* @param {boolean} [backward=false] - backward iteration for low surrogates
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* @throws {TypeError} first argument must be a string
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* @throws {TypeError} second argument must be a number having a nonnegative integer
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* @throws {TypeError} third argument must be a boolean
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* @throws {RangeError} position must be a valid index in string
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* @returns {NonNegativeInteger} code point
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*
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* @example
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* var out = codePointAt( 'last man standing', 4 );
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* // returns 32
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*
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* @example
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* var out = codePointAt( 'presidential election', 8, true );
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* // returns 116
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*
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* @example
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* var out = codePointAt( 'अनुच्छेद', 2 );
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* // returns 2369
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*
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* @example
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* var out = codePointAt( '🌷', 1, true );
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* // returns 127799
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*/
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function codePointAt( str, idx, backward ) {
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var code;
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var FLG;
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var low;
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var hi;
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if ( !isString( str ) ) {
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throw new TypeError( format( 'invalid argument. Must provide a string. Value: `%s`.', str ) );
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}
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if ( !isNonNegativeInteger( idx ) ) {
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throw new TypeError( format( 'invalid argument. Must provide a valid position (i.e., a nonnegative integer). Value: `%s`.', idx ) );
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}
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if ( idx >= str.length ) {
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throw new RangeError( format( 'invalid argument. Must provide a valid position (i.e., within string bounds). Value: `%u`.', idx ) );
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}
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if ( arguments.length > 2 ) {
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if ( !isBoolean( backward ) ) {
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throw new TypeError( format( 'invalid argument. Third argument must be a boolean. Value: `%s`.', backward ) );
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}
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FLG = backward;
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} else {
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FLG = false;
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}
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code = str.charCodeAt( idx );
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// High surrogate
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if ( code >= OxD800 && code <= OxDBFF && idx < str.length - 1 ) {
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hi = code;
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low = str.charCodeAt( idx+1 );
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if ( OxDC00 <= low && low <= OxDFFF ) {
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return ( ( hi - OxD800 ) * Ox400 ) + ( low - OxDC00 ) + Ox10000;
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}
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return hi;
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}
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// Low surrogate - support only if backward iteration is desired
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if ( FLG ) {
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if ( code >= OxDC00 && code <= OxDFFF && idx >= 1 ) {
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hi = str.charCodeAt( idx-1 );
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low = code;
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if ( OxD800 <= hi && hi <= OxDBFF ) {
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return ( ( hi - OxD800 ) * Ox400 ) + ( low - OxDC00 ) + Ox10000;
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}
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return low;
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}
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}
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return code;
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}
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// EXPORTS //
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module.exports = codePointAt;
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