findSubsetBorders.js

const findSubsetBorders = (function(){

    function findSubsetBorders ( set, str ) {
        const
            max = set.length - 1,
            strl = str.length,
            low = findLowBorder( set, 0,   max, str, strl);

        return {
            start : low,
            end : findHighBorder(set, low, max, str, strl)
        }
    }

    function findLowBorder ( set, min, max, str, strl ) {
        var m;
        while ( m = bisect(min, max) ) {
            if ( compare(set[m], str, strl, false) >= 0 ) max = m;
            else min = m;
        }

        return compare(set[min], str, strl, true) === 0
            ? min
            : max;
    }

    function findHighBorder ( set, min, max, str, strl ) {
        var m;
        while ( m = bisect(min, max) ) {
            if ( compare(set[m], str, strl, true) <= 0 ) min = m;
            else max = m;
        }

        return compare(set[max], str, strl, true) === 0
            ? max
            : min;
    }

    function bisect ( min, max ) {
        var diff = max - min;
        return diff < 2 ? 0 : min + ( diff>>1 )
    }

    function compare ( a, b, bl, doSubstr ) {
        if ( doSubstr ) a = a.substring(0, bl);

        return a < b
                    ? -1
                    : a > b
                        ? 1
                        : 0;
    }

    return findSubsetBorders;
})();