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/*
 * Copyright 2018 Proofpoint, Inc.
 *
 * Licensed 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 com.proofpoint.http.client;

import com.google.errorprone.annotations.concurrent.GuardedBy;

import javax.annotation.concurrent.ThreadSafe;
import java.io.InputStream;
import java.util.Iterator;
import java.util.List;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkState;
import static java.lang.Math.min;
import static java.lang.System.arraycopy;
import static java.util.Objects.checkFromToIndex;
import static java.util.Objects.requireNonNull;

@ThreadSafe
public class GatheringByteArrayInputStream
        extends InputStream
{
    @GuardedBy("this")
    private final Iterator buffers;
    @GuardedBy("this")
    private final byte[] singleByte = new byte[1];
    @GuardedBy("this")
    private byte[] currentBuffer = new byte[0];
    @GuardedBy("this")
    private int currentBufferPosition;
    @GuardedBy("this")
    private long remainingBytes;

    public GatheringByteArrayInputStream(List buffers, long totalBytes)
    {
        checkArgument(totalBytes >= 0, "totalBytes should equal to or greater than 0");

        this.buffers = requireNonNull(buffers, "buffers is null").iterator();
        this.remainingBytes = totalBytes;
    }

    @Override
    public synchronized int read(byte[] buffer)
    {
        return read(buffer, 0, buffer.length);
    }

    @Override
    public synchronized int read()
    {
        int bytes = read(singleByte);
        if (bytes == -1) {
            return -1;
        }
        return singleByte[0] & 0xFF;
    }

    @Override
    public synchronized long skip(long n)
    {
        if (n < 0) {
            return 0;
        }

        long totalSkippedBytes = min(n, remainingBytes);

        n = totalSkippedBytes;
        while (n > 0) {
            if (currentBufferPosition >= currentBuffer.length) {
                advanceCurrentBuffer();
            }
            int skippedBytes = (int) min(n, currentBuffer.length - currentBufferPosition);
            n -= skippedBytes;
            currentBufferPosition += skippedBytes;
        }
        remainingBytes -= totalSkippedBytes;
        return totalSkippedBytes;
    }

    @Override
    public synchronized int read(byte[] buffer, int offset, int length)
    {
        requireNonNull(buffer, "buffer is null");
        checkFromToIndex(offset, offset + length, buffer.length);

        if (remainingBytes == 0) {
            return -1;
        }

        int totalReadBytes = (int) min(length, remainingBytes);

        length = totalReadBytes;
        while (length > 0) {
            if (currentBufferPosition >= currentBuffer.length) {
                advanceCurrentBuffer();
            }
            int readBytes = min(length, currentBuffer.length - currentBufferPosition);
            arraycopy(currentBuffer, currentBufferPosition, buffer, offset, readBytes);
            offset += readBytes;
            length -= readBytes;
            currentBufferPosition += readBytes;
        }
        remainingBytes -= totalReadBytes;
        return totalReadBytes;
    }

    @Override
    public void close() {}

    private synchronized void advanceCurrentBuffer()
    {
        checkState(currentBufferPosition >= currentBuffer.length, "there is still un-read space in currentBuffer");
        checkState(buffers.hasNext(), "buffers should have more data when remainingBytes is greater than 0");

        currentBuffer = buffers.next();
        currentBufferPosition = 0;
    }
}