/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch 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.elasticsearch.index.snapshots.blobstore;

import org.apache.lucene.util.IOUtils;

import java.io.IOException;
import java.io.InputStream;

/**
 *  A {@link SlicedInputStream} is a logical
 * concatenation one or more input streams. In contrast to the JDKs
 * {@link java.io.SequenceInputStream} this stream doesn't require the instantiation
 * of all logical sub-streams ahead of time. Instead, {@link #openSlice(long)} is called
 * if a new slice is required. Each slice is closed once it's been fully consumed or if
 * close is called before.
 */
public abstract class SlicedInputStream extends InputStream {
    private long slice = 0;
    private InputStream currentStream;
    private final long numSlices;
    private boolean initialized = false;

    /**
     * Creates a new SlicedInputStream
     * @param numSlices the number of slices to consume
     */
    protected SlicedInputStream(final long numSlices) {
        this.numSlices = numSlices;
    }

    private InputStream nextStream() throws IOException {
        assert initialized == false || currentStream != null;
        initialized = true;
        IOUtils.close(currentStream);
        if (slice < numSlices) {
            currentStream = openSlice(slice++);
        } else {
            currentStream = null;
        }
        return currentStream;
    }

    /**
     * Called for each logical slice given a zero based slice ordinal.
     */
    protected abstract InputStream openSlice(long slice) throws IOException;

    private InputStream currentStream() throws IOException {
        if (currentStream == null) {
            return initialized ? null : nextStream();
        }
        return currentStream;
    }

    @Override
    public final int read() throws IOException {
        InputStream stream = currentStream();
        if (stream == null) {
            return -1;
        }
        final int read = stream.read();
        if (read == -1) {
            nextStream();
            return read();
        }
        return read;
    }

    @Override
    public final int read(byte[] buffer, int offset, int length) throws IOException {
        final InputStream stream = currentStream();
        if (stream == null) {
            return -1;
        }
        final int read = stream.read(buffer, offset, length);
        if (read <= 0) {
            nextStream();
            return read(buffer, offset, length);
        }
        return read;
    }

    @Override
    public final void close() throws IOException {
        IOUtils.close(currentStream);
        initialized = true;
        currentStream = null;
    }

    @Override
    public final int available() throws IOException {
        InputStream stream = currentStream();
        return stream == null ? 0 : stream.available();
    }

}