/*
 *  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.tools.ant.types.selectors.modifiedselector;


import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.security.DigestInputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.util.Locale;

import org.apache.tools.ant.BuildException;


/**
 * Computes a 'hashvalue' for the content of file using
 * java.security.MessageDigest.
 * Use of this algorithm doesn't require any additional nested &lt;param&gt;s.
 * Supported &lt;param&gt;s are:
 * <table>
 * <caption>Digest algorithm parameters</caption>
 * <tr>
 *   <th>name</th><th>values</th><th>description</th><th>required</th>
 * </tr>
 * <tr>
 *   <td>algorithm.algorithm</td>
 *   <td>MD5 | SHA (default provider)</td>
 *   <td>name of the algorithm the provider should use</td>
 *   <td>no, defaults to MD5</td>
 * </tr>
 * <tr>
 *   <td> algorithm.provider</td>
 *   <td></td>
 *   <td>name of the provider to use</td>
 *   <td>no, defaults to <i>null</i></td>
 * </tr>
 * </table>
 *
 * @version 2004-07-08
 * @since  Ant 1.6
 */
public class DigestAlgorithm implements Algorithm {

    private static final int BYTE_MASK = 0xFF;
    private static final int BUFFER_SIZE = 8192;

    // -----  member variables  -----


    /**
     * MessageDigest algorithm to be used.
     */
    private String algorithm = "MD5";

    /**
     * MessageDigest Algorithm provider
     */
    private String provider = null;

    /**
     * Message Digest instance
     */
    private MessageDigest messageDigest = null;

    /**
     * Size of the read buffer to use.
     */
    private int readBufferSize = BUFFER_SIZE;


    // -----  Algorithm-Configuration  -----


    /**
     * Specifies the algorithm to be used to compute the checksum.
     * Defaults to "MD5". Other popular algorithms like "SHA" may be used as well.
     * @param algorithm the digest algorithm to use
     */
    public void setAlgorithm(String algorithm) {
        this.algorithm = algorithm != null
            ? algorithm.toUpperCase(Locale.ENGLISH) : null;
    }


    /**
     * Sets the MessageDigest algorithm provider to be used
     * to calculate the checksum.
     * @param provider provider to use
     */
    public void setProvider(String provider) {
        this.provider = provider;
    }


    /** Initialize the security message digest. */
    public void initMessageDigest() {
        if (messageDigest != null) {
            return;
        }

        if (provider != null && !provider.isEmpty() && !"null".equals(provider)) {
            try {
                messageDigest = MessageDigest.getInstance(algorithm, provider);
            } catch (NoSuchAlgorithmException | NoSuchProviderException e) {
                throw new BuildException(e);
            }
        } else {
            try {
                messageDigest = MessageDigest.getInstance(algorithm);
            } catch (NoSuchAlgorithmException noalgo) {
                throw new BuildException(noalgo);
            }
        }
    }


    // -----  Logic  -----


    /**
     * This algorithm supports only MD5 and SHA.
     * @return <i>true</i> if all is ok, otherwise <i>false</i>.
     */
    @Override
    public boolean isValid() {
        return "SHA".equals(algorithm) || "MD5".equals(algorithm);
    }

    /**
     * Computes a value for a file content with the specified digest algorithm.
     * @param file    File object for which the value should be evaluated.
     * @return        The value for that file
     */
    // implementation adapted from ...taskdefs.Checksum, thanks to Magesh for hint
    @Override
    public String getValue(File file) {
        if (!file.canRead()) {
            return null;
        }
        initMessageDigest();
        byte[] buf = new byte[readBufferSize];
        messageDigest.reset();
        try (DigestInputStream dis = new DigestInputStream(Files.newInputStream(file.toPath()),
                messageDigest)) {
            // read the whole stream
            while (dis.read(buf, 0, readBufferSize) != -1) {
            }
            StringBuilder checksumSb = new StringBuilder();
            for (byte digestByte : messageDigest.digest()) {
                checksumSb.append(String.format("%02x", BYTE_MASK & digestByte));
            }
            return checksumSb.toString();
        } catch (IOException ignored) {
            return null;
        }
    }

    /**
     * Override Object.toString().
     * @return some information about this algorithm.
     */
    @Override
    public String toString() {
        return String.format("<DigestAlgorithm:algorithm=%s;provider=%s>",
            algorithm, provider);
    }
}
