// robert bradshaw 11/98


public class TuringTape {
  
  private int index;
  private char blankChar;
  private char[] data;
  
  public TuringTape()         { this(' '); }
  public TuringTape(String s) { this(s, 0); }
  public TuringTape(String s, int i) {
    blankChar=' ';
    data=s.toCharArray();
    index=i;
  }
  public TuringTape(char c) {
    blankChar=c;
    data=new char[25];
    for(int i=0; i<data.length; i++) data[i]=blankChar;
    index=data.length/2;
  }
  public TuringTape(String s, char c) {
    this(s);
    blankChar=c;
  }
  
  public char read() {
    return data[index];
  }
  
  public void write(char c) {
    data[index]=c;
  }
  
  public void move(char dir) {
    if (dir=='l' || dir=='L') index--;
    else if (dir=='r' || dir=='R') index++;
    //else throw something?
    if (index < 0) {
      index+=data.length;
      char[] newData=new char[2*data.length];
      for(int i=0; i<data.length; i++) {
        newData[i]=blankChar;
        newData[i+data.length]=data[i];
      }
      data=newData;
    }
    else if (index >= data.length) {
      char[] newData=new char[2*data.length];
      for(int i=0; i<data.length; i++) {
        newData[i]=data[i];
        newData[i+data.length]=blankChar;
      }
      data=newData;
    }
  }
  
  public String toString() {
    return toString(20);
  }
  public String toString(int sidelen) {
    return new String(getData(sidelen));
  }
  
  public char[] getData(int sidelen) { return getData(-sidelen, sidelen); }
  public char[] getData(int back, int fwd) {
    char[] dispChars=new char[-back+fwd+1];
    for(int i=back; i<fwd; i++) {
      if (0 <= index+i && index+i < data.length)
        dispChars[i-back]=data[index+i];
      else dispChars[i-back]=blankChar;
    }
    return dispChars;
  }
  
  
}