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/*
 * Copyright 2010-2016 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "license" file accompanying this file. This file 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 com.amazonaws.services.s3.internal.crypto;

import com.amazonaws.internal.SdkInputStream;

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

/**
 * Reads only a specific range of bytes from the underlying input stream.
 */
public class AdjustedRangeInputStream extends SdkInputStream {
    private InputStream decryptedContents;
    private long virtualAvailable;
    private boolean closed;

    /**
     * Creates a new DecryptedContentsInputStream object.
     *
     * @param objectContents The input stream containing the object contents
     *            retrieved from S3
     * @param rangeBeginning The position of the left-most byte desired by the
     *            user
     * @param rangeEnd The position of the right-most byte desired by the user
     * @throws IOException If there are errors skipping to the left-most byte
     *             desired by the user.
     */
    public AdjustedRangeInputStream(InputStream objectContents, long rangeBeginning, long rangeEnd)
            throws IOException {
        this.decryptedContents = objectContents;
        this.closed = false;
        initializeForRead(rangeBeginning, rangeEnd);
    }

    /**
     * Skip to the start location of the range of bytes desired by the user.
     */
    private void initializeForRead(long rangeBeginning, long rangeEnd) throws IOException {
        // To get to the left-most byte desired by a user, we must skip over the
        // 16 bytes of the
        // preliminary cipher block, and then possibly skip a few more bytes
        // into the next block
        // to where the the left-most byte is located.
        int numBytesToSkip;
        if (rangeBeginning < JceEncryptionConstants.SYMMETRIC_CIPHER_BLOCK_SIZE) {
            numBytesToSkip = (int) rangeBeginning;
        } else {
            int offsetIntoBlock = (int) (rangeBeginning % JceEncryptionConstants.SYMMETRIC_CIPHER_BLOCK_SIZE);
            numBytesToSkip = JceEncryptionConstants.SYMMETRIC_CIPHER_BLOCK_SIZE + offsetIntoBlock;
        }
        if (numBytesToSkip != 0) {
            // Skip to the left-most desired byte. The read() method is used
            // instead of the skip() method
            // since the skip() method will not block if the underlying input
            // stream is waiting for more input.
            while (numBytesToSkip > 0) {
                this.decryptedContents.read();
                numBytesToSkip--;
            }
        }
        // The number of bytes the user may read is equal to the number of the
        // bytes in the range.
        // Note that the range includes the endpoints.
        this.virtualAvailable = (rangeEnd - rangeBeginning) + 1;
    }

    /*
     * (non-Javadoc)
     * @see java.io.InputStream#read()
     */
    @Override
    public int read() throws IOException {
        abortIfNeeded();
        int result = 0;
        // If there are no more available bytes, mark that we are at the end of
        // the stream.
        if (this.virtualAvailable <= 0) {
            result = -1;
        } else {
            // Otherwise, read a byte.
            result = this.decryptedContents.read();
        }

        // If we have not reached the end of the stream, decrement the number of
        // available bytes.
        if (result != -1) {
            this.virtualAvailable--;
        } else {
            // If we are at the end of the stream, close it.
            close();
            this.virtualAvailable = 0;
        }
        return result;
    }

    /*
     * (non-Javadoc)
     * @see java.io.InputStream#read(byte[], int, int)
     */
    @Override
    public int read(byte[] buffer, int offset, int length) throws IOException {
        abortIfNeeded();
        int numBytesRead;
        // If no more bytes are available, do not read any bytes into the buffer
        if (this.virtualAvailable <= 0) {
            numBytesRead = -1;
        } else {
            // If the desired read length is greater than the number of
            // available bytes,
            // shorten the read length to the number of available bytes.
            if (length > this.virtualAvailable) {
                // If the number of available bytes is greater than the maximum
                // value of a 32 bit int, then
                // read as many bytes as an int can.
                length = (this.virtualAvailable < Integer.MAX_VALUE) ? (int) this.virtualAvailable
                        : Integer.MAX_VALUE;
            }
            // Read bytes into the buffer.
            numBytesRead = this.decryptedContents.read(buffer, offset, length);
        }
        // If we were able to read bytes, decrement the number of bytes
        // available to be read.
        if (numBytesRead != -1) {
            this.virtualAvailable -= numBytesRead;
        } else {
            // If we've reached the end of the stream, close it
            close();
            this.virtualAvailable = 0;
        }
        return numBytesRead;
    }

    /*
     * (non-Javadoc)
     * @see java.io.InputStream#available()
     */
    @Override
    public int available() throws IOException {
        abortIfNeeded();
        int available = this.decryptedContents.available();
        if (available < this.virtualAvailable) {
            return available;
        } else {
            // Limit the number of bytes available to the number
            // of bytes remaining in the range.
            return (int) this.virtualAvailable;
        }
    }

    /*
     * (non-Javadoc)
     * @see java.io.InputStream#close()
     */
    @Override
    public void close() throws IOException {
        // If not already closed, then close the input stream.
        if (!this.closed) {
            this.closed = true;
            this.decryptedContents.close();
        }
        abortIfNeeded();
    }

    @Override
    protected InputStream getWrappedInputStream() {
        return decryptedContents;
    }
}