function lengthOfLongestSubstring(s) {
    let longest = 0;
    if(s === "") return 0;
    
    return mainLoop();
    
    function mainLoop() {

        for(let cursor = 0; cursor < s.length; ) {
            const lookahead = findLongestSubstring(cursor); //{1,a}, {2,a}, {1,b}, 
            if(lookahead > longest) {
                longest = +lookahead;
            }
            // console.log('cursor='+cursor, lookahead)
            const nextStartPosition = s.indexOf(lookahead.char, cursor)+1;
            cursor = nextStartPosition;
            //console.log('cursor='+cursor, '(after)');
            if(cursor === -1 || longest >= s.length - cursor) {
                return longest;
            }
        }

        function findLongestSubstring(startIndex) {
            const charMap = {};
            let substringLength = 0;
            let currentChar;
            for(let j = startIndex; j < s.length; j++) {
                currentChar = s[j];
                if(currentChar in charMap) break;

                charMap[currentChar] = true;
                substringLength++;
            }
            const lookahead = new Number(substringLength);
            lookahead.char = currentChar;
            // console.log('jloop returning:', lookahead)
            return lookahead;
        }
    }
};