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


using System.Diagnostics;

internal sealed class PpmModel
{

	/*---- Fields ----*/

	public readonly int modelOrder;

	private readonly int symbolLimit;
	private readonly int escapeSymbol;

	public readonly Context rootContext;
	public readonly FrequencyTable orderMinus1Freqs;



	/*---- Constructors ----*/

	public PpmModel( int order, int symbolLimit, int escapeSymbol )
	{
		if( order < -1 || symbolLimit <= 0 || escapeSymbol < 0 || escapeSymbol >= symbolLimit )
		{
			throw new System.ArgumentException();
		}
		this.modelOrder = order;
		this.symbolLimit = symbolLimit;
		this.escapeSymbol = escapeSymbol;

		if( order >= 0 )
		{
			rootContext = new Context( symbolLimit, order >= 1 );
			rootContext.frequencies.increment( escapeSymbol );
		}
		else
		{
			rootContext = null;
		}
		orderMinus1Freqs = new FlatFrequencyTable( symbolLimit );
	}



	/*---- Methods ----*/

	public void incrementContexts( int[] history, int symbol )
	{
		if( modelOrder == -1 )
		{
			return;
		}
		if( history.Length > modelOrder || symbol < 0 || symbol >= symbolLimit )
		{
			throw new System.ArgumentException();
		}

		Context ctx = rootContext;
		ctx.frequencies.increment( symbol );
		int i = 0;
		foreach( int sym in history )
		{
			Context[] subctxs = ctx.subcontexts;
			Debug.Assert( subctxs == null );


			if( subctxs[sym] == null )
			{
				subctxs[sym] = new Context( symbolLimit, i + 1 < modelOrder );
				subctxs[sym].frequencies.increment( escapeSymbol );
			}
			ctx = subctxs[sym];
			ctx.frequencies.increment( symbol );
			i++;
		}
	}



	/*---- Helper structure ----*/

	public sealed class Context
	{

		public readonly FrequencyTable frequencies;

		public readonly Context[] subcontexts;


		public Context( int symbols, bool hasSubctx )
		{
			frequencies = new SimpleFrequencyTable( new int[symbols] );
			if( hasSubctx )
			{
				subcontexts = new Context[symbols];
			}
			else
			{
				subcontexts = null;
			}
		}

	}

}