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Jing is a validator for RELAX NG and other schema languages. This project was taken from http://code.google.com/p/jing-trang and mavenized for inclusion in the Wicket Stuff HTML Validator. The code was taken from the 20091111 release.

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package com.thaiopensource.validate.auto;

import java.io.IOException;
import java.io.Reader;

/**
 * Rewindable implementation over a reader.
 * Modified from RewindableInputStream by replacing the input stream with a reader.
 * @author george
 */
public class RewindableReader extends Reader implements Rewindable {
  static class Block {
    Block next;
    final char[] buf;
    int used = 0;
    static final int MIN_SIZE = 1024;
    Block(int minSize) {
      buf = new char[Math.max(MIN_SIZE, minSize)];
    }

    Block() {
      this(0);
    }

    void append(char b) {
      buf[used++] = b;
    }

    void append(char[] b, int off, int len) {
      System.arraycopy(b, off, buf, used, len);
      used += len;
    }
  }

  private Block head;
  /**
   * If curBlockAvail > 0, then there are curBlockAvail chars available to be
   * returned starting at curBlockPos in curBlock.buf.
   */
  private int curBlockAvail;
  private Block curBlock;
  private int curBlockPos;
  private Block lastBlock;
  /**
   * true unless willNotRewind has been called
   */
  private boolean saving = true;
  private final Reader in;
  private boolean pretendClosed = false;
  /**
   * true if we have got an EOF from the underlying Reader
   */
  private boolean eof;

  public RewindableReader(Reader in) {
    if (in == null)
      throw new NullPointerException();
    this.in = in;
  }

  public void close() throws IOException {
    if (saving) {
      curBlockAvail = 0;
      curBlock = null;
      pretendClosed = true;
    }
    else {
      head = null;
      curBlock = null;
      lastBlock = null;
      saving = false;
      curBlockAvail = 0;
      in.close();
    }
  }

  public void rewind() {
    if (!saving)
      throw new IllegalStateException("rewind() after willNotRewind()");
    pretendClosed = false;
    if (head == null)
      return;
    curBlock = head;
    curBlockPos = 0;
    curBlockAvail = curBlock.used;
  }

  public boolean canRewind() {
    return saving;
  }

  public void willNotRewind() {
    saving = false;
    head = null;
    lastBlock = null;
    if (pretendClosed) {
      pretendClosed = false;
      try {
        in.close();
      }
      catch (IOException e) { }
    }
  }

  public int read() throws IOException {
    if (curBlockAvail > 0) {
      int c = curBlock.buf[curBlockPos++] & 0xFF;
      --curBlockAvail;
      if (curBlockAvail == 0) {
        curBlock = curBlock.next;
        if (curBlock != null) {
          curBlockPos = 0;
          curBlockAvail = curBlock.used;
        }
      }
      return c;
    }
    int c = in.read();
    if (saving && c != -1) {
      if (lastBlock == null)
        lastBlock = head = new Block();
      else if (lastBlock.used == lastBlock.buf.length)
        lastBlock = lastBlock.next = new Block();
      lastBlock.append((char)c);
    }
    return c;
  }

  public int read(char b[], int off, int len) throws IOException {
    if (curBlockAvail == 0 && !saving)
      return in.read(b, off, len);
    if (b == null)
      throw new NullPointerException();
    if (len < 0)
      throw new IndexOutOfBoundsException();
    int nRead = 0;
    if (curBlockAvail != 0) {
      for (;;) {
        if (len == 0)
          return nRead;
        b[off++] = curBlock.buf[curBlockPos++];
        --len;
        nRead++;
        --curBlockAvail;
        if (curBlockAvail == 0) {
          curBlock = curBlock.next;
          if (curBlock == null)
            break;
          curBlockAvail = curBlock.used;
          curBlockPos = 0;
        }
      }
    }
    if (len == 0)
      return nRead;
    if (eof)
      return nRead > 0 ? nRead : -1;
    try {
      int n = in.read(b, off, len);
      if (n < 0) {
        eof = true;
        return nRead > 0 ? nRead : -1;
      }
      nRead += n;
      if (saving) {
        if (lastBlock == null)
          lastBlock = head = new Block(n);
        else if (lastBlock.buf.length - lastBlock.used < n) {
          if (lastBlock.used != lastBlock.buf.length) {
            int free = lastBlock.buf.length - lastBlock.used;
            lastBlock.append(b, off, free);
            off += free;
            n -= free;
          }
          lastBlock = lastBlock.next = new Block(n);
        }
        lastBlock.append(b, off, n);
      }
    }
    catch (IOException e) {
      eof = true;
      if (nRead == 0)
        throw e;
    }
    return nRead;
  }
}