var TEST = function () {  
    
    /**
	 * i = index
	 * ┌ len=1 => (i  , i  ) ┐ // origin, odd  
	 * └ len=2 => (i  , i+1) ┘ // go forward, even
	 * ┌ len=3 => (i-1, i+1) ┐ // go backward, odd
	 * └ len=4 => (i-1, i+2) ┘ // go forward, even
	 * ┌ len=5 => (i-2, i+2) ┐ // go backward, odd
	 * └ len=6 => (i-2, i+3) ┘ // go forward, even
	 */
     
	let _strLen = 0;
	
	const _getMatchLen = (str, si, ei) => {
		while (si >= 0 && ei < _strLen && str[si] === str[ei]) {
			si--;
			ei++;
		}
		return ei - si - 1; // (ei-1) - (si+1) + 1;
	}
	
    return {
        exec: function (str) {
            if (!str) { return ''; }
			
            _strLen = str.length;
			
			let start = NaN;
			let len = 0;
			for (let i = 0; i < _strLen; ++i) {
				if (!str[Math.ceil(len / 2)]) {
					break;
				}
				let matchLen = Math.max(_getMatchLen(str, i, i), _getMatchLen(str, i, i + 1));
				if (matchLen > len) {
					len = matchLen;
					start = i - Math.ceil(matchLen / 2) + 1;
				}
			}
            return str.substr(start, len);
        }
    };
}();