package java.io;

public class PushbackReader extends Reader {
  private final Reader in;
  private char savedChar;
  private boolean hasSavedChar;

  public PushbackReader(Reader in, int bufferSize) {
    if (bufferSize > 1) {
      throw new IllegalArgumentException(bufferSize + " > 1");
    }
    this.in = in;
    this.hasSavedChar = false;
  }

  public PushbackReader(Reader in) {
    this(in, 1);
  }

  public int read(char[] b, int offset, int length) throws IOException {
    int count = 0;
    if (hasSavedChar && length > 0) {
      length--;
      b[offset++] = savedChar;
      hasSavedChar = false;
      count = 1;
    }
    if (length > 0) {
      int c = in.read(b, offset, length);
      if (c == -1) {
	if (count == 0) {
	  count = -1;
	}
      } else {
	count += c;
      }
    }

    return count;
  }

  public void unread(char[] b, int offset, int length) throws IOException {
    if (length != 1) {
      throw new IOException("Can only push back 1 char, not " + length);
    } else if (hasSavedChar) {
      throw new IOException("Already have a saved char");
    } else {
      hasSavedChar = true;
      savedChar = b[offset];
    }
  }

  public void unread(char[] b) throws IOException {
    unread(b, 0, b.length);
  }

  public void unread(int c) throws IOException {
    unread(new char[] { (char) c });
  }

  public void close() throws IOException {
    in.close();
  }
}