public class StringOp {

    public static String rovescia(String s){
        if (s== null || s.length()<= 1){
            return s;
        }
        else{
            return s.charAt(s.length() -1)+
                    rovescia(s.substring(0,s.length()-1));
        }

    }

    public static int BinDec(String v){
        if (v.equals("")) {
            return -1;
        }
        else {
            int right = v.charAt(v.length() - 1) - 48;
            if (v.length() == 1) {
                return right;
            }
            else {
                return (2 *
                        BinDec(v.substring(0, v.length() - 1))) +
                        right;
            }
        }
    }

    public static String BinSum(String v, String w){
        if (v.length()== 0 || w.length() == 0){
            return "";
        }
        else{
            return BinSum(v, w, 0);
        }
    }

    private static String BinSum(String v, String w, int rest){
        if (v.length()>0 && w.length()>0){
            char vl = v.charAt(v.length()-1);
            char wl = w.charAt(w.length()-1);
            int nl = vl - '0';
            int ml = wl - '0';
            String vr  = v.substring(0,v.length()-1);
            String wr  = w.substring(0,w.length()-1);
            if (rest+nl+ml == 3){
                return BinSum(vr,wr,1)+'1';
            }
            else if (rest+nl+ml == 2){
                return BinSum(vr,wr,1)+'0';
            }
            else if (rest+nl+ml == 1){
                return BinSum(vr,wr,0)+'1';
            }
            else{
                return BinSum(vr,wr,0)+'0';
            }

        }
        else if (v.length()==0 && w.length()>0 && rest ==1){
            return BinSum("1",w,0);
        }
        else if (v.length()>0 && w.length()==0 && rest ==1){
            return BinSum(v,"1",0);
        }
        else if (v.length()==0 && w.length()==0 && rest ==1){
            return "1";
        }
        else if (v.length()==0 && w.length()>0 && rest ==0){
            return w;
        }
        else if (v.length()>0 && w.length()==0 && rest ==0){
            return v;
        }
        else return "";
    }


// un altro modo per fare la somma
// poiche'
// x+s(y)= s(x+y)
// x+0 = x
// definiamo isZero(), binPred() e somma()
//
    public static boolean isZero (String v){
        if (v.length() == 0) {
            return true;
        }
        else if (v.charAt(0) =='1'){
                 return false;
        }
        else{
            return isZero(v.substring(1));
        }
    }

    public static String binPred (String v){
        if (isZero(v)){
            return v;
        }
        else if (v.charAt(v.length()-1)== '1'){
            return v.substring(0,v.length()-1)+'0';
        }
        else{
            return binPred(v.substring(0,v.length()-1))+'1';
        }
    }
    public static String succ(String v){
       if (v.length()==0){
           return "1";
       }
       else if (v.charAt(v.length()-1)== '0'){
            return v.substring(0,v.length()-1)+'1';
        }
        else{
            return succ(v.substring(0,v.length()-1))+'0';
        }

    }
    public static String somma(String v, String w){
        if (isZero(w)){
            return v;
        }
        else {
            return succ(somma(v, binPred(w)));
        }
    }
}