Cache.java

package com.osroyale.fs.cache;
 
import com.google.common.base.Preconditions;
 
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;

 
public final class Cache {

    public static final int INDEX_SIZE = 6;

    public static final int SECTOR_HEADER_SIZE = 8;

    public static final int SECTOR_SIZE = 520;

    private final ByteBuffer buffer = ByteBuffer.allocate(SECTOR_SIZE);

    private final SeekableByteChannel sectorChannel;

    private final SeekableByteChannel indexChannel;

    private final int id;

    protected Cache(SeekableByteChannel sectorChannel, SeekableByteChannel indexChannel, int id) {
        this.sectorChannel = sectorChannel;
        this.indexChannel = indexChannel;
        this.id = ++id;
    }

    public ByteBuffer get(final int indexId) throws IOException {
        final Index index = readIndex(indexId);
        if (!index.check()) {
            return null;
        }
 
        long position = (long) index.getId() * SECTOR_HEADER_SIZE;
 
        Preconditions.checkArgument(sectorChannel.size() >= position + SECTOR_SIZE);
 
        byte[] data = new byte[index.getLength()];
        int next = index.getId();
        int offset = 0;
 
        for (int chunk = 0; offset < index.getLength(); chunk++) {
            int read = Math.min(index.getLength() - offset, 512);
 
            Sector sector = readSector(next, data, offset, read);
            sector.check(id, indexId, chunk);
 
            next = sector.getNextIndexId();
            offset += read;
        }
 
        return ByteBuffer.wrap(data);
    }

    private Index readIndex(int indexId) throws IOException {
        long position = (long) indexId * INDEX_SIZE;
 
        buffer.clear().limit(INDEX_SIZE);
        indexChannel.position(position);
        indexChannel.read(buffer);
        buffer.flip();
 
        return Index.decode(buffer);
    }

    private Sector readSector(int sectorId, byte[] data, int offset, int length) throws IOException {
        long position = (long) sectorId * SECTOR_SIZE;
 
        buffer.clear().limit(length + SECTOR_HEADER_SIZE);
        sectorChannel.position(position);
        sectorChannel.read(buffer);
        buffer.flip();
 
        return Sector.decode(buffer, data, offset, length);
    }
 
}