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The Bouncy Castle Java APIs for doing S/MIME with the Jakarta Mail APIs. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.mail.smime.util;

import jakarta.mail.internet.SharedInputStream;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class SharedFileInputStream 
    extends FilterInputStream
    implements SharedInputStream
{
    private final SharedFileInputStream _parent;
    private final File                  _file;
    private final long                  _start;
    private final long                  _length;
    
    private long _position;
    private long _markedPosition;

    private List _subStreams = new LinkedList();
    
    public SharedFileInputStream(
        String fileName) 
        throws IOException
    {
        this(new File(fileName));
    }
    
    public SharedFileInputStream(
        File file) 
        throws IOException
    {
        this(file, 0, file.length());
    }
    
    private SharedFileInputStream(
        File file,
        long start,
        long length)
        throws IOException
    {
        super(new BufferedInputStream(new FileInputStream(file)));

        _parent = null;
        _file = file;
        _start = start;
        _length = length;
        
        in.skip(start);
    }

    private SharedFileInputStream(
        SharedFileInputStream parent,
        long start,
        long length)
        throws IOException
    {
        super(new BufferedInputStream(new FileInputStream(parent._file)));

        _parent = parent;
        _file = parent._file;
        _start = start;
        _length = length;

        in.skip(start);
    }

    public long getPosition()
    {
        return _position;
    }

    public InputStream newStream(long start, long finish)
    {
        try
        {
            SharedFileInputStream stream;
            
            if (finish < 0)
            {
                if (_length > 0)
                {
                    stream = new SharedFileInputStream(this, _start + start, _length - start);
                }
                else if (_length == 0)
                {
                    stream = new SharedFileInputStream(this, _start + start, 0);
                }
                else
                {
                    stream = new SharedFileInputStream(this, _start + start, -1);
                }
            }
            else
            {
                stream = new SharedFileInputStream(this, _start + start, finish - start);
            }
            
            _subStreams.add(stream);
            
            return stream;
        }
        catch (IOException e)
        {
            throw new IllegalStateException("unable to create shared stream: " + e);
        }
    }
    
    public int read(
        byte[] buf) 
        throws IOException
    {
        return this.read(buf, 0, buf.length);
    }
    
    public int read(
        byte[] buf, 
        int off, 
        int len) 
        throws IOException
    {
        int count = 0;
        
        if (len == 0)
        {
            return 0;
        }
        
        while (count < len)
        {
            int ch = this.read();
            
            if (ch < 0)
            {
                break;
            }
            
            buf[off + count] = (byte)ch;
            count++;
        }
        
        if (count == 0)
        {
            return -1;  // EOF
        }
        
        return count;
    }
    
    public int read() 
        throws IOException
    {
        if (_position == _length)
        {
            return -1;
        }

        _position++;
        return in.read();
    }
    
    public boolean markSupported()
    {
        return true;
    }

    public long skip(long n)
        throws IOException
    {
        long count;

        for (count = 0; count != n; count++)
        {
            if (this.read() < 0)
            {
                break;
            }
        }

        return count;
    }

    public void mark(
        int readLimit)
    {
        _markedPosition = _position;
        in.mark(readLimit);
    }
    
    public void reset() 
        throws IOException
    {
        _position = _markedPosition;
        in.reset();
    }

    /**
     * Return the shared stream that represents the top most stream that
     * this stream inherits from.
     * @return  the base of the shared stream tree.
     */
    public SharedFileInputStream getRoot()
    {
        if (_parent != null)
        {
            return _parent.getRoot();
        }

        return this;
    }

    /**
     * Close of this stream and any substreams that have been created from it.
     * @throws IOException on problem closing the main stream.
     */
    public void dispose() 
        throws IOException 
    {
        Iterator it = _subStreams.iterator();
        
        while (it.hasNext())
        {
            try
            {
                ((SharedFileInputStream)it.next()).dispose();
            }
            catch (IOException e)
            {
                // ignore
            }
        }

        in.close();
    }
}