MagicNumberFileFilter

/*
 * 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
 *
 *      https://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.Serializable;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.Arrays;
import java.util.Objects;

import org.apache.commons.io.RandomAccessFileMode;
import org.apache.commons.io.RandomAccessFiles;

/**
 * <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}.
 * </p>
 * <h2>Using Classic IO</h2>
 * <pre>
 * File dir = FileUtils.current();
 * 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>
 *
 * <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>
 *
 * <pre>
 * File dir = FileUtils.current();
 * MagicNumberFileFilter tarFileFilter =
 *     MagicNumberFileFilter("ustar", 257);
 * String[] tarFiles = dir.list(tarFileFilter);
 * for (String tarFile : tarFiles) {
 *     System.out.println(tarFile);
 * }
 * </pre>
 * <h2>Using NIO</h2>
 * <pre>
 * final Path dir = PathUtils.current();
 * final AccumulatorPathVisitor visitor = AccumulatorPathVisitor.withLongCounters(MagicNumberFileFilter("ustar", 257));
 * //
 * // Walk one directory
 * Files.<strong>walkFileTree</strong>(dir, Collections.emptySet(), 1, visitor);
 * System.out.println(visitor.getPathCounters());
 * System.out.println(visitor.getFileList());
 * //
 * visitor.getPathCounters().reset();
 * //
 * // Walk directory tree
 * Files.<strong>walkFileTree</strong>(dir, visitor);
 * System.out.println(visitor.getPathCounters());
 * System.out.println(visitor.getDirList());
 * System.out.println(visitor.getFileList());
 * </pre>
 * <h2>Deprecating Serialization</h2>
 * <p>
 * <em>Serialization is deprecated and will be removed in 3.0.</em>
 * </p>
 *
 * <h2>Deprecating Serialization</h2>
 * <p>
 * <em>Serialization is deprecated and will be removed in 3.0.</em>
 * </p>
 *
 * @since 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}.
     * </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>
     *
     * <pre>
     * MagicNumberFileFilter javaClassFileFilter =
     *     MagicNumberFileFilter(new byte[] {(byte) 0xCA, (byte) 0xFE,
     *       (byte) 0xBA, (byte) 0xBE});
     * </pre>
     *
     * @param magicNumber the magic number to look for in the file.
     * @throws IllegalArgumentException if {@code magicNumber} is
     *         {@code null}, or contains no bytes.
     */
    public MagicNumberFileFilter(final byte[] 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>
     *
     * <pre>
     * MagicNumberFileFilter tarFileFilter =
     *     MagicNumberFileFilter(new byte[] {0x75, 0x73, 0x74, 0x61, 0x72}, 257);
     * </pre>
     *
     * <pre>
     * MagicNumberFileFilter javaClassFileFilter =
     *     MagicNumberFileFilter(new byte[] {0xCA, 0xFE, 0xBA, 0xBE}, 0);
     * </pre>
     *
     * @param magicNumbers 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}
     *         contains no bytes, or {@code offset}
     *         is a negative number.
     */
    public MagicNumberFileFilter(final byte[] magicNumbers, final long offset) {
        Objects.requireNonNull(magicNumbers, "magicNumbers");
        if (magicNumbers.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 = magicNumbers.clone();
        this.byteOffset = offset;
    }

    /**
     * <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}.
     * </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} is
     *         {@code null} or the empty String.
     */
    public MagicNumberFileFilter(final 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>
     *
     * <pre>
     * MagicNumberFileFilter tarFileFilter = MagicNumberFileFilter("ustar", 257);
     * </pre>
     * <p>
     * This method uses the virtual machine's {@linkplain Charset#defaultCharset() default charset}.
     * </p>
     *
     * @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} is
     *         the empty String, or {@code offset} is
     *         a negative number.
     */
    public MagicNumberFileFilter(final String magicNumber, final long offset) {
        this(magicNumber.getBytes(Charset.defaultCharset()), 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} if the file contains the filter's magic number
     *         at the specified offset, {@code false} otherwise.
     */
    @Override
    public boolean accept(final File file) {
        if (isFile(file) && file.canRead()) {
            try {
                return RandomAccessFileMode.READ_ONLY.apply(file.toPath(),
                        raf -> Arrays.equals(magicNumbers, RandomAccessFiles.read(raf, byteOffset, magicNumbers.length)));
            } catch (final IOException ignored) {
                // Do nothing, fall through and do not accept file
            }
        }
        return false;
    }

    /**
     * <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.
     * @param attributes the path's basic attributes (may be null).
     * @return {@code true} if the file contains the filter's magic number
     *         at the specified offset, {@code false} otherwise.
     * @since 2.9.0
     */
    @Override
    public FileVisitResult accept(final Path file, final BasicFileAttributes attributes) {
        if (file != null && Files.isRegularFile(file) && Files.isReadable(file)) {
            try {
                try (FileChannel fileChannel = FileChannel.open(file)) {
                    final ByteBuffer byteBuffer = ByteBuffer.allocate(this.magicNumbers.length);
                    fileChannel.position(byteOffset);
                    final int read = fileChannel.read(byteBuffer);
                    if (read != magicNumbers.length) {
                        return FileVisitResult.TERMINATE;
                    }
                    return toFileVisitResult(Arrays.equals(this.magicNumbers, byteBuffer.array()));
                }
            } catch (final IOException ignored) {
                // Do nothing, fall through and do not accept file
            }
        }
        return FileVisitResult.TERMINATE;
    }

    /**
     * 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() {
        final StringBuilder builder = new StringBuilder(super.toString());
        builder.append("(");
        // TODO perhaps use hex if value is not printable
        builder.append(new String(magicNumbers, Charset.defaultCharset()));
        builder.append(",");
        builder.append(this.byteOffset);
        builder.append(")");
        return builder.toString();
    }
}