OpenSubtitlesHasher.java

package org.lodder.subtools.multisubdownloader.subtitleproviders.opensubtitles;

import java.io.*;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.LongBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;

/**
 * Hash code is based on Media Player Classic. In natural language it calculates: size + 64bit
 * checksum of the first and last 64k (even if they overlap because the file is smaller than
 * 128k).
 */
public class OpenSubtitlesHasher {

    /**
     * Size of the chunks that will be hashed in bytes (64 KB)
     */
    private static final int HASH_CHUNK_SIZE = 64 * 1024;


    public static String computeHash(File file) throws IOException {
        long size = file.length();
        long chunkSizeForFile = Math.min(HASH_CHUNK_SIZE, size);

        FileInputStream fiStream = new FileInputStream(file);
        FileChannel fileChannel = fiStream.getChannel();

        try {
            long head = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, 0, chunkSizeForFile));
            long tail = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, Math.max(size - HASH_CHUNK_SIZE, 0), chunkSizeForFile));

            return String.format("%016x", size + head + tail);
        } finally {
        	fileChannel.close();
            fiStream.close();
        }
    }


    public static String computeHash(InputStream stream, long length) throws IOException {

        int chunkSizeForFile = (int) Math.min(HASH_CHUNK_SIZE, length);

        // buffer that will contain the head and the tail chunk, chunks will overlap if length is smaller than two chunks
        byte[] chunkBytes = new byte[(int) Math.min(2 * HASH_CHUNK_SIZE, length)];

        DataInputStream in = new DataInputStream(stream);

        // first chunk
        in.readFully(chunkBytes, 0, chunkSizeForFile);

        long position = chunkSizeForFile;
        long tailChunkPosition = length - chunkSizeForFile;

        // seek to position of the tail chunk, or not at all if length is smaller than two chunks
        while (position < tailChunkPosition && (position += in.skip(tailChunkPosition - position)) >= 0) ;

        // second chunk, or the rest of the data if length is smaller than two chunks
        in.readFully(chunkBytes, chunkSizeForFile, chunkBytes.length - chunkSizeForFile);

        long head = computeHashForChunk(ByteBuffer.wrap(chunkBytes, 0, chunkSizeForFile));
        long tail = computeHashForChunk(ByteBuffer.wrap(chunkBytes, chunkBytes.length - chunkSizeForFile, chunkSizeForFile));

        return String.format("%016x", length + head + tail);
    }


    private static long computeHashForChunk(ByteBuffer buffer) {

        LongBuffer longBuffer = buffer.order(ByteOrder.LITTLE_ENDIAN).asLongBuffer();
        long hash = 0;

        while (longBuffer.hasRemaining()) {
            hash += longBuffer.get();
        }

        return hash;
    }

}