work.suroh.tk/node_modules/engine.io-parser/lib/utf8.js

/*! https://mths.be/utf8js v2.1.2 by @mathias */

var stringFromCharCode = String.fromCharCode;

// Taken from https://mths.be/punycode
function ucs2decode(string) {
	var output = [];
	var counter = 0;
	var length = string.length;
	var value;
	var extra;
	while (counter < length) {
		value = string.charCodeAt(counter++);
		if (value >= 0xD800 && value <= 0xDBFF && counter < length) {
			// high surrogate, and there is a next character
			extra = string.charCodeAt(counter++);
			if ((extra & 0xFC00) == 0xDC00) { // low surrogate
				output.push(((value & 0x3FF) << 10) + (extra & 0x3FF) + 0x10000);
			} else {
				// unmatched surrogate; only append this code unit, in case the next
				// code unit is the high surrogate of a surrogate pair
				output.push(value);
				counter--;
			}
		} else {
			output.push(value);
		}
	}
	return output;
}

// Taken from https://mths.be/punycode
function ucs2encode(array) {
	var length = array.length;
	var index = -1;
	var value;
	var output = '';
	while (++index < length) {
		value = array[index];
		if (value > 0xFFFF) {
			value -= 0x10000;
			output += stringFromCharCode(value >>> 10 & 0x3FF | 0xD800);
			value = 0xDC00 | value & 0x3FF;
		}
		output += stringFromCharCode(value);
	}
	return output;
}

function checkScalarValue(codePoint, strict) {
	if (codePoint >= 0xD800 && codePoint <= 0xDFFF) {
		if (strict) {
			throw Error(
				'Lone surrogate U+' + codePoint.toString(16).toUpperCase() +
				' is not a scalar value'
			);
		}
		return false;
	}
	return true;
}
/*--------------------------------------------------------------------------*/

function createByte(codePoint, shift) {
	return stringFromCharCode(((codePoint >> shift) & 0x3F) | 0x80);
}

function encodeCodePoint(codePoint, strict) {
	if ((codePoint & 0xFFFFFF80) == 0) { // 1-byte sequence
		return stringFromCharCode(codePoint);
	}
	var symbol = '';
	if ((codePoint & 0xFFFFF800) == 0) { // 2-byte sequence
		symbol = stringFromCharCode(((codePoint >> 6) & 0x1F) | 0xC0);
	}
	else if ((codePoint & 0xFFFF0000) == 0) { // 3-byte sequence
		if (!checkScalarValue(codePoint, strict)) {
			codePoint = 0xFFFD;
		}
		symbol = stringFromCharCode(((codePoint >> 12) & 0x0F) | 0xE0);
		symbol += createByte(codePoint, 6);
	}
	else if ((codePoint & 0xFFE00000) == 0) { // 4-byte sequence
		symbol = stringFromCharCode(((codePoint >> 18) & 0x07) | 0xF0);
		symbol += createByte(codePoint, 12);
		symbol += createByte(codePoint, 6);
	}
	symbol += stringFromCharCode((codePoint & 0x3F) | 0x80);
	return symbol;
}

function utf8encode(string, opts) {
	opts = opts || {};
	var strict = false !== opts.strict;

	var codePoints = ucs2decode(string);
	var length = codePoints.length;
	var index = -1;
	var codePoint;
	var byteString = '';
	while (++index < length) {
		codePoint = codePoints[index];
		byteString += encodeCodePoint(codePoint, strict);
	}
	return byteString;
}

/*--------------------------------------------------------------------------*/

function readContinuationByte() {
	if (byteIndex >= byteCount) {
		throw Error('Invalid byte index');
	}

	var continuationByte = byteArray[byteIndex] & 0xFF;
	byteIndex++;

	if ((continuationByte & 0xC0) == 0x80) {
		return continuationByte & 0x3F;
	}

	// If we end up here, it’s not a continuation byte
	throw Error('Invalid continuation byte');
}

function decodeSymbol(strict) {
	var byte1;
	var byte2;
	var byte3;
	var byte4;
	var codePoint;

	if (byteIndex > byteCount) {
		throw Error('Invalid byte index');
	}

	if (byteIndex == byteCount) {
		return false;
	}

	// Read first byte
	byte1 = byteArray[byteIndex] & 0xFF;
	byteIndex++;

	// 1-byte sequence (no continuation bytes)
	if ((byte1 & 0x80) == 0) {
		return byte1;
	}

	// 2-byte sequence
	if ((byte1 & 0xE0) == 0xC0) {
		byte2 = readContinuationByte();
		codePoint = ((byte1 & 0x1F) << 6) | byte2;
		if (codePoint >= 0x80) {
			return codePoint;
		} else {
			throw Error('Invalid continuation byte');
		}
	}

	// 3-byte sequence (may include unpaired surrogates)
	if ((byte1 & 0xF0) == 0xE0) {
		byte2 = readContinuationByte();
		byte3 = readContinuationByte();
		codePoint = ((byte1 & 0x0F) << 12) | (byte2 << 6) | byte3;
		if (codePoint >= 0x0800) {
			return checkScalarValue(codePoint, strict) ? codePoint : 0xFFFD;
		} else {
			throw Error('Invalid continuation byte');
		}
	}

	// 4-byte sequence
	if ((byte1 & 0xF8) == 0xF0) {
		byte2 = readContinuationByte();
		byte3 = readContinuationByte();
		byte4 = readContinuationByte();
		codePoint = ((byte1 & 0x07) << 0x12) | (byte2 << 0x0C) |
			(byte3 << 0x06) | byte4;
		if (codePoint >= 0x010000 && codePoint <= 0x10FFFF) {
			return codePoint;
		}
	}

	throw Error('Invalid UTF-8 detected');
}

var byteArray;
var byteCount;
var byteIndex;
function utf8decode(byteString, opts) {
	opts = opts || {};
	var strict = false !== opts.strict;

	byteArray = ucs2decode(byteString);
	byteCount = byteArray.length;
	byteIndex = 0;
	var codePoints = [];
	var tmp;
	while ((tmp = decodeSymbol(strict)) !== false) {
		codePoints.push(tmp);
	}
	return ucs2encode(codePoints);
}

module.exports = {
	version: '2.1.2',
	encode: utf8encode,
	decode: utf8decode
};