k-9/src/com/fsck/k9/mail/filter/PeekableInputStream.java

package com.fsck.k9.mail.filter;

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

/**
 * A filtering InputStream that allows single byte "peeks" without consuming the byte. The
 * client of this stream can call peek() to see the next available byte in the stream
 * and a subsequent read will still return the peeked byte.
 */
public class PeekableInputStream extends InputStream
{
    private InputStream mIn;
    private boolean mPeeked;
    private int mPeekedByte;

    public PeekableInputStream(InputStream in)
    {
        this.mIn = in;
    }

    @Override
    public int read() throws IOException
    {
        if (!mPeeked)
        {
            return mIn.read();
        }
        else
        {
            mPeeked = false;
            return mPeekedByte;
        }
    }

    public int peek() throws IOException
    {
        if (!mPeeked)
        {
            mPeekedByte = read();
            mPeeked = true;
        }
        return mPeekedByte;
    }

    @Override
    public int read(byte[] b, int offset, int length) throws IOException
    {
        if (!mPeeked)
        {
            return mIn.read(b, offset, length);
        }
        else
        {
            b[0] = (byte)mPeekedByte;
            mPeeked = false;
            int r = mIn.read(b, offset + 1, length - 1);
            if (r == -1)
            {
                return 1;
            }
            else
            {
                return r + 1;
            }
        }
    }

    @Override
    public int read(byte[] b) throws IOException
    {
        return read(b, 0, b.length);
    }

    @Override
    public String toString()
    {
        return String.format("PeekableInputStream(in=%s, peeked=%b, peekedByte=%d)",
                             mIn.toString(), mPeeked, mPeekedByte);
    }
}