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/*
 * Tencent is pleased to support the open source community by making Angel available.
 *
 * Copyright (C) 2017-2018 THL A29 Limited, a Tencent company. 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. You may obtain a copy of the License at
 *
 * https://opensource.org/licenses/Apache-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.tencent.angel.utils;

import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.util.List;

/**
 * Utility to present {@link java.nio.ByteBuffer} data as an {@link java.io.InputStream}. Copied
 * from avro.
 */
public class ByteBufferInputStream extends InputStream {
  private List buffers;
  private int current;

  public ByteBufferInputStream(List buffers) {
    this.buffers = buffers;
  }

  /**
   * @throws java.io.EOFException if EOF is reached.
   * @see java.io.InputStream#read()
   */
  @Override public int read() throws IOException {
    return getBuffer().get() & 0xff;
  }

  /**
   * @throws java.io.EOFException if EOF is reached before reading all the bytes.
   * @see java.io.InputStream#read(byte[], int, int)
   */
  @Override public int read(byte[] b, int off, int len) throws IOException {
    if (len == 0)
      return 0;
    ByteBuffer buffer = getBuffer();
    int remaining = buffer.remaining();
    if (len > remaining) {
      buffer.get(b, off, remaining);
      return remaining;
    } else {
      buffer.get(b, off, len);
      return len;
    }
  }

  /**
   * Read a buffer from the input without copying, if possible.
   *
   * @throws java.io.EOFException if EOF is reached before reading all the bytes.
   */
  public ByteBuffer readBuffer(int length) throws IOException {
    if (length == 0)
      return ByteBuffer.allocate(0);
    ByteBuffer buffer = getBuffer();
    if (buffer.remaining() == length) { // can return current as-is?
      current++;
      return buffer; // return w/o copying
    }
    // punt: allocate a new buffer & copy into it
    ByteBuffer result = ByteBuffer.allocate(length);
    int start = 0;
    while (start < length)
      start += read(result.array(), start, length - start);
    return result;
  }

  /**
   * Returns the next non-empty buffer.
   *
   * @throws java.io.EOFException if EOF is reached before reading all the bytes.
   */
  private ByteBuffer getBuffer() throws IOException {
    while (current < buffers.size()) {
      ByteBuffer buffer = buffers.get(current);
      if (buffer.hasRemaining())
        return buffer;
      current++;
    }
    throw new EOFException();
  }
}