function assert(a,b){
    if (a !== b){
        throw "Assertion fail! " + a + " != " + b;
    }
}

function getLastRevision(revs, branch){
    var ind = branch.lastIndexOf(".");
    var base = branch.slice(0, ind-1) + branch.slice(ind+1);

    function getTail(rev){
        if(rev.indexOf(base) === 0){
            return parseInt(rev.slice(base.length + 1), 10);
        }
    }

    var max = Number.MIN_VALUE;
    for (var i=0; i < revs.length; i++){
        var tail = getTail(revs[i]);
        if(tail && tail > max){
            max = tail;
        }
    }

    return base + "." + max;
}

revs = ["1.18", "1.19","1.20","1.21","2.50",
        "1.18.2.1", "1.18.2.2","1.18.2.3","1.18.2.6", "1.19.2.6",
        "1.18.2.1.3.1", "1.18.2.1.3.2","1.18.2.1.3.3","1.18.2.1.3.61","1.18.3.1.3.6"]

assert(getLastRevision(revs, "1.18.0.2"), "1.18.2.6")
assert(getLastRevision(revs, "1.18.2.1.0.3"), "1.18.2.1.3.61")