huffdecoder.java

哈夫曼压缩 哈弗曼算法是基本的压缩和解密算法

/****************************

HuffDecoder class
used to decoded a Huffman encoded Bitstream

 ****************************/


package huffmancoding;



import java.io.*;
import java.util.Arrays;
import java.util.HashMap;

public class HuffDecoder
{

	private final int BYTE_LENGTH = 8;
	private final int DWORD_LENGTH = 32;

	// used to normalize the byte range (-128 to 127) to array indicies
	private final int BYTE_RANGE = Byte.MAX_VALUE + Math.abs(Byte.MIN_VALUE) + 1;

	// sentinel values for huffman header
	private final byte SENTINEL_BYTE = 65;
	private final int SENTINEL_FREQ = 0;

	private Bitstream decodedStream;

	public HuffDecoder(Bitstream inputStream)
	{

		inputStream.reset();

		int numCodes = inputStream.readDWord();

		// get the freqs from the file
		int[] frequencies = getFreqs(inputStream);
		System.out.println("got freqs");

		// build a decoder tree from the freqs
		HuffTree decoderTree = new HuffTree(frequencies);
		System.out.println("Decoder: got tree");

		// decode the stream
		decodedStream = decodeStream(inputStream, decoderTree, numCodes);
		System.out.println("Decoder: got decoded stream");
	}

	// returns the decoded stream
	public Bitstream getDecodedStream()
	{
		return decodedStream;
	}

	// pulls the frequency table from the file header
	private int[] getFreqs(Bitstream stream)
	{

		int[] freqs = new int[BYTE_RANGE];
		Arrays.fill(freqs, 0);

		byte theByte = stream.readByte();
		int theFreq = stream.readDWord();

		// pull freq info until we hit the sentinel value
		while( (theByte != SENTINEL_BYTE || theFreq != SENTINEL_FREQ) && !stream.eof())
		{

			freqs[theByte - Byte.MIN_VALUE] = theFreq;
			theByte = stream.readByte();
			theFreq = stream.readDWord();
			System.out.println(theByte + " - " + theFreq);
		}

		return freqs;
	}

	// decodes the stream and returns it
	private Bitstream decodeStream(Bitstream inStream, HuffTree decoderTree, int numCodes)
	{

		decodedStream = new Bitstream();

		for (int codeIterator = 0; codeIterator < numCodes; codeIterator++)
		{
			decodedStream.writeByte(decoderTree.getByte(inStream));
		}

		decodedStream.reset();
		return decodedStream;
	}

}