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/*

* Reference Huffman coding

* Copyright (c) Project Nayuki

*

* https://www.nayuki.io/page/reference-huffman-coding

* https://github.com/nayuki/Reference-Huffman-coding

*/

import java.io.BufferedInputStream;

import java.io.BufferedOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.InputStream;

/**

* Compression application using static Huffman coding.

* <p>Usage: java HuffmanCompress InputFile OutputFile</p>

* <p>Then use the corresponding "HuffmanDecompress" application to recreate the original input file.</p>

* <p>Note that the application uses an alphabet of 257 symbols - 256 symbols for the byte values

* and 1 symbol for the EOF marker. The compressed file format starts with a list of 257

* code lengths, treated as a canonical code, and then followed by the Huffman-coded data.</p>

*/

public final class HuffmanCompress {

// Command line main application function.

public static void main(String[] args) throws IOException {

// Handle command line arguments

if (args.length != 2) {

System.err.println("Usage: java HuffmanCompress InputFile OutputFile");

System.exit(1);

return;

}

File inputFile = new File(args[0]);

File outputFile = new File(args[1]);

// Read input file once to compute symbol frequencies.

// The resulting generated code is optimal for static Huffman coding and also canonical.

FrequencyTable freqs = getFrequencies(inputFile);

freqs.increment(256); // EOF symbol gets a frequency of 1

CodeTree code = freqs.buildCodeTree();

CanonicalCode canonCode = new CanonicalCode(code, freqs.getSymbolLimit());

// Replace code tree with canonical one. For each symbol,

// the code value may change but the code length stays the same.

code = canonCode.toCodeTree();

// Read input file again, compress with Huffman coding, and write output file

try (InputStream in = new BufferedInputStream(new FileInputStream(inputFile))) {

try (BitOutputStream out = new BitOutputStream(new BufferedOutputStream(new FileOutputStream(outputFile)))) {

writeCodeLengthTable(out, canonCode);

compress(code, in, out);

}

}

}

// Returns a frequency table based on the bytes in the given file.

// Also contains an extra entry for symbol 256, whose frequency is set to 0.

private static FrequencyTable getFrequencies(File file) throws IOException {

FrequencyTable freqs = new FrequencyTable(new int[257]);

try (InputStream input = new BufferedInputStream(new FileInputStream(file))) {

while (true) {

int b = input.read();

if (b == -1)

break;

freqs.increment(b);

}

}

return freqs;

}

// To allow unit testing, this method is package-private instead of private.

static void writeCodeLengthTable(BitOutputStream out, CanonicalCode canonCode) throws IOException {

for (int i = 0; i < canonCode.getSymbolLimit(); i++) {

int val = canonCode.getCodeLength(i);

// For this file format, we only support codes up to 255 bits long

if (val >= 256)

throw new RuntimeException("The code for a symbol is too long");

// Write value as 8 bits in big endian

for (int j = 7; j >= 0; j--)

out.write((val >>> j) & 1);

}

}

// To allow unit testing, this method is package-private instead of private.

static void compress(CodeTree code, InputStream in, BitOutputStream out) throws IOException {

HuffmanEncoder enc = new HuffmanEncoder(out);

enc.codeTree = code;

while (true) {

int b = in.read();

if (b == -1)

break;

enc.write(b);

}

enc.write(256); // EOF

}

}