Tape.java

Below is the syntax highlighted version of Tape.java from §7.4 Kleene Theorem.

public class Tape {
    private Stack<String> s1 = new Stack<String>();   // left part of tape
    private Stack<String> s2 = new Stack<String>();   // right part of tape
    private String current;                           // current symbol
    private final static String BLANK = "#";

    // allocate, initialize, and return a Tape whose contents are init
    public Tape() { this(BLANK); }
    public Tape(String init) {
        if (init == null || init.equals("")) init = BLANK;
        for (int i = init.length() - 1; i >= 0; i--) {
            current = init.substring(i, i+1);
            s2.push(current);
        }
        current = s2.pop();
    }
    
    // move tape head left one position
    public void left() {
        s2.push(current);
        if (s1.isEmpty()) s1.push(BLANK);  // increase size of tape if necessary
        current = s1.pop();       
    }
    
    // move tape head right one position
    public void right() {
        s1.push(current);
        if (s2.isEmpty()) s2.push(BLANK);  // increase size of tape if necessary
        current = s2.pop();
    }
    
    // set and get the symbol stored in the active cell
    public String read()             { return current;    }
    public void write(String symbol) { current = symbol;  }
    

    // return a reversed version of the string s
    public static String reverse(String s) {
        int N = s.length();
        if (N <= 1) return s;
        String left = s.substring(0, N/2);
        String right = s.substring(N/2, N);
        return reverse(right) + reverse(left);
    }
  
    // show representation of tape
    public String toString() {
        return BLANK + BLANK + reverse(s1.toString()) + "|" + current + "|" + s2 + BLANK + BLANK;
    }

}