在 Java 中发布我基于 Princeton.EDU 版本创建的霍夫曼树的完整实现。普林斯顿包中的版本可以作为学术示例，但在上下文之外并不能真正使用。
下面的示例可以通过提供一个字符串作为输入并获取一个字节数组作为输出来使用。
如果您想将输出编码为人类可读的格式，请使用标准 java Base64 编码器（在空间方面非常低效）或检查我在此处发布的 HumanByte 类
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.util.PriorityQueue;
/**************************************************************************************************************
* Used edu.princeton.cs version of huffman algorithm and made it easily usable outside of example context
* replaced proprietary priority queue by Java standard one
* created binary output/input that has minimal features needed
*
* If human readable encoding of huffman encoded string is desired then look at HumanByte class
* that was posted here
*
* https://stackoverflow.com/questions/4141317/how-to-convert-byte-array-into-human-readable-format/58332208#58332208
*
* @author Stan Sokolov
* 10/9/19
**************************************************************************************************************/
public class BetterHuffman {
private static final int R = 256;
private static final char PARENT = '\u0000';
private static final char EMPTY = '\u0001';
private static final int UNDEFINED = -1;
/**************************************************************************************************************
* Compress string in bytes
**************************************************************************************************************/
public static byte[] compress(final String s) {
final HuffmanOut binaryOut = new HuffmanOut();
final char[] input = s.toCharArray();
final int[] freq = new int[R];
for (char anInput1 : input) {
++freq[anInput1];
}
final BetterHuffman.Node root = buildTrie(freq);
final String[] st = new String[R];
buildCode(st, root, "");
writeTrie(root, binaryOut);
binaryOut.write(input.length);
for (char anInput : input) {
final String code = st[anInput];
for (char ch : code.toCharArray()) {
binaryOut.writeBit(ch == '1');
}
}
return binaryOut.value();
}
/**************************************************************************************************************
* build huffman tree
**************************************************************************************************************/
private static BetterHuffman.Node buildTrie(int[] freq) {
final PriorityQueue<BetterHuffman.Node> pq2 = new PriorityQueue<>();
for (char i = 0; i < R; ++i) {
if (freq[i] > 0) {
//pq.insert(new BetterHuffman.Node(i, freq[i], null, null));
pq2.add(new BetterHuffman.Node(i, freq[i], null, null));
}
}
if (pq2.size() == 1) {//if entire string is just one char repeated
if (freq[0] == 0) {//empty string
pq2.add(new BetterHuffman.Node(PARENT, 0, null, null));
} else {
pq2.add(new BetterHuffman.Node(EMPTY, 0, null, null));
}
} else
while (pq2.size() > 1) {
final BetterHuffman.Node left = pq2.poll();
final BetterHuffman.Node right = pq2.poll();
//aggregate two nodes into one by summing frequency
final BetterHuffman.Node parent = new BetterHuffman.Node(PARENT, left.freq + right.freq, left, right);
pq2.add(parent);
}
//this will be the root node that would have total length of input as frequency
return pq2.poll();
}
/**************************************************************************************************************
* write tree into byte output
**************************************************************************************************************/
private static void writeTrie(final BetterHuffman.Node x, final HuffmanOut binaryOut) {
if (x.isLeaf()) {//if this is a node representing symbol in alphabet
binaryOut.writeBit(true);
binaryOut.writeByte((int) x.ch);
} else {
binaryOut.writeBit(false); //this is an aggregate node used for branching
writeTrie(x.left, binaryOut);
writeTrie(x.right, binaryOut);
}
}
/**************************************************************************************************************
* make substitutes for incoming words
**************************************************************************************************************/
private static void buildCode(final String[] st, final BetterHuffman.Node x, final String s) {
if (!x.isLeaf()) {
buildCode(st, x.left, s + '0');
buildCode(st, x.right, s + '1');
} else {
st[x.ch] = s;
}
}
/**************************************************************************************************************
* Return uncompressed string
**************************************************************************************************************/
public static String expand(final byte[] value) {
final HuffmanIn binaryIn = new HuffmanIn(value);
final StringBuilder out = new StringBuilder();
final BetterHuffman.Node root = readTrie(binaryIn);
final int length = binaryIn.readInt();
for (int i = 0; i < length; ++i) {
BetterHuffman.Node x = root;
while (!x.isLeaf()) {
boolean bit = binaryIn.readBoolean();
if (bit) {
x = x.right;
} else {
x = x.left;
}
}
out.append(x.ch);
}
return out.toString();
}
/**************************************************************************************************************
* get tree from bytes
**************************************************************************************************************/
private static BetterHuffman.Node readTrie(final HuffmanIn binaryIn) {
boolean isLeaf = binaryIn.readBoolean();
if (isLeaf) {
char ch = binaryIn.readChar();
return new BetterHuffman.Node(ch, UNDEFINED, null, null);
} else {
return new BetterHuffman.Node(PARENT, UNDEFINED, readTrie(binaryIn), readTrie(binaryIn));
}
}
/**************************************************************************************************************
* Simple implementation of node
**************************************************************************************************************/
private static class Node implements Comparable<Node> {
private final char ch;
private final int freq;
private final Node left;
private final Node right;
Node(char ch, int freq, Node left, Node right) {
this.ch = ch;
this.freq = freq;
this.left = left;
this.right = right;
}
private boolean isLeaf() {
return left == null && right == null;
}
@Override
public int compareTo(Node that) {
return this.freq - that.freq;
}
}
/**************************************************************************************************************
* class to read bits from stream
**************************************************************************************************************/
private static class HuffmanIn {
private final ByteArrayInputStream in;
private int buffer;
private byte n;
HuffmanIn(final byte[] input) {
in = new ByteArrayInputStream(input);
fillBuffer();
}
private void fillBuffer() {
buffer = in.read();
n = 8;
}
boolean readBoolean() {
boolean bit = (buffer >> --n & 1) == 1;
if (n == 0) {
fillBuffer();
}
return bit;
}
char readChar() {
int x = buffer <<= 8 - n;
if (n == 8) {
fillBuffer();
} else {
byte oldN = n;
fillBuffer();
n = oldN;
x |= buffer >>> n;
}
return (char) (x & 255);
}
int readInt() {
int x = 0;
for (int i = 0; i < 4; ++i) {
char c = readChar();
x <<= 8;
x |= c;
}
return x;
}
}
/**************************************************************************************************************
* Output
**************************************************************************************************************/
private static class HuffmanOut {
private ByteArrayOutputStream out = new ByteArrayOutputStream();
private int buffer;
private byte n;
/**************************************************************************************************************
* @return what was compressed so far in a human readable (no funny characters) format
**************************************************************************************************************/
public byte[] value() {
clearBuffer();
return out.toByteArray();
}
void writeBit(final boolean bit) {
buffer = (buffer <<= 1) | (bit ? 1 : 0);
if (++n == 8) {
clearBuffer();
}
}
void writeByte(final int x) {
for (int i = 0; i < 8; ++i) {
writeBit((x >>> 8 - i - 1 & 1) == 1);
}
}
void clearBuffer() {
if (n != 0) {
out.write(buffer <<= 8 - n);
n = 0;
buffer = 0;
}
}
/**************************************************************************************************************
* write all 4 bytes of int
**************************************************************************************************************/
void write(final int x) {
for (int i = 3; i >= 0; i--)
writeByte(x >>> (i * 8) & 255);//write 4 bytes of int
}
}
}