code: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   * 
   *      http://www.apache.org/licenses/LICENSE-2.0
   * 
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.io.filefilter;
  
  import java.io.File;
  import java.io.IOException;
  import java.io.RandomAccessFile;
  import java.io.Serializable;
  import java.util.Arrays;
  
  import org.apache.commons.io.IOUtils;
  
  /**
   * <p>
   * File filter for matching files containing a "magic number". A magic number 
   * is a unique series of bytes common to all files of a specific file format.
   * For instance, all Java class files begin with the bytes 
   * <code>0xCAFEBABE</code>. 
   * </p>
   * 
   * <code><pre>
   * File dir = new File(".");
   * MagicNumberFileFilter javaClassFileFilter =
   *     MagicNumberFileFilter(new byte[] {(byte) 0xCA, (byte) 0xFE, 
   *       (byte) 0xBA, (byte) 0xBE}); 
   * String[] javaClassFiles = dir.list(javaClassFileFilter);
   * for (String javaClassFile : javaClassFiles) {
   *     System.out.println(javaClassFile);
   * }
   * </pre></code>
   * 
   * <p>
   * Sometimes, such as in the case of TAR files, the
   * magic number will be offset by a certain number of bytes in the file. In the
   * case of TAR archive files, this offset is 257 bytes.
   * </p>
   * 
   * <code><pre>
   * File dir = new File(".");
   * MagicNumberFileFilter tarFileFilter = 
   *     MagicNumberFileFilter("ustar", 257); 
   * String[] tarFiles = dir.list(tarFileFilter);
   * for (String tarFile : tarFiles) {
   *     System.out.println(tarFile);
   * }
   * </pre></code>
   * @since Commons IO 2.0
   * @see FileFilterUtils#magicNumberFileFilter(byte[])
   * @see FileFilterUtils#magicNumberFileFilter(String)
   * @see FileFilterUtils#magicNumberFileFilter(byte[], long)
   * @see FileFilterUtils#magicNumberFileFilter(String, long)
   */
  public class MagicNumberFileFilter extends AbstractFileFilter implements
          Serializable {
      
      /**
       * The serialization version unique identifier.
       */
      private static final long serialVersionUID = -547733176983104172L;
  
      /**
       * The magic number to compare against the file's bytes at the provided 
       * offset.
       */
      private final byte[] magicNumbers;
      
      /**
       * The offset (in bytes) within the files that the magic number's bytes 
       * should appear.
       */
      private final long byteOffset;
      
      /**
       * <p>
       * Constructs a new MagicNumberFileFilter and associates it with the magic
       * number to test for in files. This constructor assumes a starting offset
       * of <code>0</code>.
       * </p>
       * 
       * <p>
       * It is important to note that <em>the array is not cloned</em> and that
       * any changes to the magic number array after construction will affect the
       * behavior of this file filter.
       * </p>
       * 
       * <code><pre>
       * MagicNumberFileFilter javaClassFileFilter =
       *     MagicNumberFileFilter(new byte[] {(byte) 0xCA, (byte) 0xFE, 
       *       (byte) 0xBA, (byte) 0xBE}); 
       * </pre></code>
       * 
       * @param magicNumber the magic number to look for in the file.
       * 
       * @throws IllegalArgumentException if <code>magicNumber</code> is 
       *         <code>null</code>, or contains no bytes.
       */
      public MagicNumberFileFilter(byte[] magicNumber) {
          this(magicNumber, 0);
      }
      
      /**
       * <p>
       * Constructs a new MagicNumberFileFilter and associates it with the magic
       * number to test for in files. This constructor assumes a starting offset
       * of <code>0</code>.
       * </p>
       * 
       * Example usage:
       * <pre>
       * {@code
       * MagicNumberFileFilter xmlFileFilter = 
       *     MagicNumberFileFilter("<?xml"); 
       * }
       * </pre>
       * 
       * @param magicNumber the magic number to look for in the file.
       *        The string is converted to bytes using the platform default charset.
       *
       * @throws IllegalArgumentException if <code>magicNumber</code> is 
       *         <code>null</code> or the empty String.
       */
      public MagicNumberFileFilter(String magicNumber) {
          this(magicNumber, 0);
      }
      
      /**
       * <p>
       * Constructs a new MagicNumberFileFilter and associates it with the magic
       * number to test for in files and the byte offset location in the file to
       * to look for that magic number.
       * </p>
       * 
       * <code><pre>
       * MagicNumberFileFilter tarFileFilter = 
       *     MagicNumberFileFilter("ustar", 257); 
       * </pre></code>
       * 
       * @param magicNumber the magic number to look for in the file. 
       *        The string is converted to bytes using the platform default charset.
       * @param offset the byte offset in the file to start comparing bytes.
       * 
       * @throws IllegalArgumentException if <code>magicNumber</code> is 
       *         <code>null</code> or the empty String, or <code>offset</code> is 
       *         a negative number.
       */
      public MagicNumberFileFilter(String magicNumber, long offset) {
          if (magicNumber == null) {
              throw new IllegalArgumentException("The magic number cannot be null");
          }
          if (magicNumber.length() == 0) {
              throw new IllegalArgumentException("The magic number must contain at least one byte");
          }
          if (offset < 0) {
              throw new IllegalArgumentException("The offset cannot be negative");
          }
          
          this.magicNumbers = magicNumber.getBytes(); // uses the platform default charset
          this.byteOffset = offset;
      }
      
      /**
       * <p>
       * Constructs a new MagicNumberFileFilter and associates it with the magic
       * number to test for in files and the byte offset location in the file to
       * to look for that magic number.
       * </p>
       * 
       * <p>
       * It is important to note that <em>the array is not cloned</em> and that
       * any changes to the magic number array after construction will affect the
       * behavior of this file filter.
       * </p>
       * 
       * <code><pre>
       * MagicNumberFileFilter tarFileFilter =
       *     MagicNumberFileFilter(new byte[] {0x75, 0x73, 0x74, 0x61, 0x72}, 257); 
       * </pre></code>
       * 
       * <code><pre>
       * MagicNumberFileFilter javaClassFileFilter =
       *     MagicNumberFileFilter(new byte[] {0xCA, 0xFE, 0xBA, 0xBE}, 0); 
       * </pre></code>
       * 
       * @param magicNumber the magic number to look for in the file.
       * @param offset the byte offset in the file to start comparing bytes.
       * 
       * @throws IllegalArgumentException if <code>magicNumber</code> is 
       *         <code>null</code>, or contains no bytes, or <code>offset</code> 
       *         is a negative number.
       */
      public MagicNumberFileFilter(byte[] magicNumber, long offset) {
          if (magicNumber == null) {
              throw new IllegalArgumentException("The magic number cannot be null");
          }
          if (magicNumber.length == 0) {
              throw new IllegalArgumentException("The magic number must contain at least one byte");
          }
          if (offset < 0) {
              throw new IllegalArgumentException("The offset cannot be negative");
          }
          
          this.magicNumbers = new byte[magicNumber.length];
          System.arraycopy(magicNumber, 0, this.magicNumbers, 0, magicNumber.length);
          this.byteOffset = offset;
      }
      
      /**
       * <p>
       * Accepts the provided file if the file contains the file filter's magic
       * number at the specified offset.
       * </p>
       * 
       * <p>
       * If any {@link IOException}s occur while reading the file, the file will
       * be rejected.
       * </p>
       * 
       * @param file the file to accept or reject.
       * 
       * @return <code>true</code> if the file contains the filter's magic number 
       *         at the specified offset, <code>false</code> otherwise.
       */
      @Override
      public boolean accept(File file) {
          if (file != null && file.isFile() && file.canRead()) {
              RandomAccessFile randomAccessFile = null;
              try {
                  byte[] fileBytes = new byte[this.magicNumbers.length]; 
                  randomAccessFile = new RandomAccessFile(file, "r");
                  randomAccessFile.seek(byteOffset);
                  int read = randomAccessFile.read(fileBytes);
                  if (read != magicNumbers.length) {
                      return false;
                  }
                  return Arrays.equals(this.magicNumbers, fileBytes);
              } catch (IOException ioe) {
                  // Do nothing, fall through and do not accept file
              } finally {
                  IOUtils.closeQuietly(randomAccessFile);
              }
          }
          
          return false;
      }
  
      /**
       * Returns a String representation of the file filter, which includes the 
       * magic number bytes and byte offset.
       * 
       * @return a String representation of the file filter.
       */
      @Override
      public String toString() {
          StringBuilder builder = new StringBuilder(super.toString());
          builder.append("(");
          builder.append(new String(magicNumbers));// TODO perhaps use hex if value is not printable
          builder.append(",");
          builder.append(this.byteOffset);
          builder.append(")");
          return builder.toString();
      }
  }
  