BitStream.java

import java.nio.ByteBuffer;

public class BitStream {

  private final byte[] bytes;
  private final int numOfBits;
  private int pos;

  public BitStream(ByteBuffer buf) {
    bytes = new byte[buf.remaining()];
    int p = buf.position();
    buf.get(bytes);
    buf.position(p);
    this.numOfBits = bytes.length * 8;
    this.pos = 0;
  }

  public int getBit() {
    if (pos >= numOfBits)
      throw new IllegalStateException();

    int i = pos / 8;
    int a = pos % 8 + 1;
    ++pos;
    return (bytes[i] >> (8 - a)) & 0x1;
  }

  public int getBits(int bits) {
    if (bits > 32)
      throw new IllegalArgumentException();
    if (bits + pos > numOfBits)
      throw new IllegalArgumentException();
    if (bits == 0)
      return 0;

    int r = 0;
    for (int i = 0; i < bits; ++i) {
      r |= (getBit() << (bits - i - 1));
    }
    return r;
  }

  public int peekBits(int bits) {
    int p = pos;
    int r = getBits(bits);
    pos = p;
    return r;
  }

  public int getRemaining() {
    return numOfBits - pos;
  }
}