using System;
using System.Diagnostics;
using System.IO;

/*
 * Reference arithmetic coding
 * Copyright (c) Project Nayuki
 *
 * https://www.nayuki.io/page/reference-arithmetic-coding
 * https://github.com/nayuki/Reference-arithmetic-coding
 */



/// <summary>
/// Decompression application using prediction by partial matching (PPM) with arithmetic coding.
/// <para>Usage: java PpmDecompress InputFile OutputFile</para>
/// <para>This decompresses files generated by the "PpmCompress" application.</para>
/// </summary>
public sealed class PpmDecompress
{

	// Must be at least -1 and match PpmCompress. Warning: Exponential memory usage at O(257^n).
	private const int MODEL_ORDER = 3;


	//JAVA TO C# CONVERTER WARNING: Method 'throws' clauses are not available in C#:
	//ORIGINAL LINE: public static void main(String[] args) throws java.io.IOException
	public static void Main( string[] args )
	{
		// Handle command line arguments
		if( args.Length != 2 )
		{
			Console.Error.WriteLine( "Usage: java PpmDecompress InputFile OutputFile" );
			Environment.Exit( 1 );
			return;
		}

		string inputFile = args[0]; //new File(args[0]);
		string outputFile = args[1]; //new File(args[1]);

		// Perform file decompression
		using( BitInputStream @in = new BitInputStream( new BufferedStream( new FileStream( inputFile, FileMode.Open, FileAccess.Read ) ) ) )
		using( Stream @out = new BufferedStream( new FileStream( outputFile, FileMode.Create, FileAccess.Write ) ) )
		{
			decompress( @in, @out );
		}
	}


	// To allow unit testing, this method is package-private instead of private.
	//JAVA TO C# CONVERTER WARNING: Method 'throws' clauses are not available in C#:
	//ORIGINAL LINE: static void decompress(BitInputStream in, java.io.OutputStream out) throws java.io.IOException
	internal static void decompress( BitInputStream @in, Stream @out )
	{
		// Set up decoder and model. In this PPM model, symbol 256 represents EOF;
		// its frequency is 1 in the order -1 context but its frequency
		// is 0 in all other contexts (which have non-negative order).
		ArithmeticDecoder dec = new ArithmeticDecoder( 32, @in );
		PpmModel model = new PpmModel( MODEL_ORDER, 257, 256 );
		int[] history = new int[0];

		while( true )
		{
			// Decode and write one byte
			int symbol = decodeSymbol( dec, model, history );
			if( symbol == 256 ) // EOF symbol
			{
				break;
			}
			@out.WriteByte( (byte)symbol );
			model.incrementContexts( history, symbol );

			if( model.modelOrder >= 1 )
			{
				// Prepend current symbol, dropping oldest symbol if necessary
				if( history.Length < model.modelOrder )
				{
					history = Arrays.CopyOf( history, history.Length + 1 );
				}
				Array.Copy( history, 0, history, 1, history.Length - 1 );
				history[0] = symbol;
			}
		}
	}


	//JAVA TO C# CONVERTER WARNING: Method 'throws' clauses are not available in C#:
	//ORIGINAL LINE: private static int decodeSymbol(ArithmeticDecoder dec, PpmModel model, int[] history) throws java.io.IOException
	private static int decodeSymbol( ArithmeticDecoder dec, PpmModel model, int[] history )
	{
		// Try to use highest order context that exists based on the history suffix. When symbol 256
		// is consumed at a context at any non-negative order, it means "escape to the next lower order
		// with non-empty context". When symbol 256 is consumed at the order -1 context, it means "EOF".
		for( int order = history.Length; order >= 0; order-- )
		{
			PpmModel.Context ctx = model.rootContext;
			for( int i = 0; i < order; i++ )
			{
				Debug.Assert( ctx.subcontexts == null );

				ctx = ctx.subcontexts[history[i]];
				if( ctx == null )
				{
					goto outerContinue;
				}
			}
			int symbol = dec.read( ctx.frequencies );
			if( symbol < 256 )
			{
				return symbol;
			}
			// Else we read the context escape symbol, so continue decrementing the order
			outerContinue:
			;
		}

		//outerBreak:
		// Logic for order = -1
		return dec.read( model.orderMinus1Freqs );
	}

}