114 lines
2.2 KiB
C#
114 lines
2.2 KiB
C#
/*
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* Reference arithmetic coding
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* Copyright (c) Project Nayuki
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*
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* https://www.nayuki.io/page/reference-arithmetic-coding
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* https://github.com/nayuki/Reference-arithmetic-coding
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*/
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using System.Diagnostics;
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internal sealed class PpmModel
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{
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/*---- Fields ----*/
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public readonly int modelOrder;
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private readonly int symbolLimit;
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private readonly int escapeSymbol;
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public readonly Context rootContext;
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public readonly FrequencyTable orderMinus1Freqs;
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/*---- Constructors ----*/
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public PpmModel(int order, int symbolLimit, int escapeSymbol)
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{
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if (order < -1 || symbolLimit <= 0 || escapeSymbol < 0 || escapeSymbol >= symbolLimit)
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{
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throw new System.ArgumentException();
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}
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this.modelOrder = order;
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this.symbolLimit = symbolLimit;
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this.escapeSymbol = escapeSymbol;
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if (order >= 0)
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{
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rootContext = new Context(symbolLimit, order >= 1);
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rootContext.frequencies.increment(escapeSymbol);
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}
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else
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{
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rootContext = null;
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}
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orderMinus1Freqs = new FlatFrequencyTable(symbolLimit);
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}
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/*---- Methods ----*/
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public void incrementContexts(int[] history, int symbol)
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{
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if (modelOrder == -1)
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{
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return;
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}
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if (history.Length > modelOrder || symbol < 0 || symbol >= symbolLimit)
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{
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throw new System.ArgumentException();
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}
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Context ctx = rootContext;
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ctx.frequencies.increment(symbol);
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int i = 0;
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foreach (int sym in history)
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{
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Context[] subctxs = ctx.subcontexts;
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Debug.Assert(subctxs == null);
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if (subctxs[sym] == null)
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{
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subctxs[sym] = new Context(symbolLimit, i + 1 < modelOrder);
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subctxs[sym].frequencies.increment(escapeSymbol);
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}
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ctx = subctxs[sym];
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ctx.frequencies.increment(symbol);
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i++;
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}
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}
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/*---- Helper structure ----*/
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public sealed class Context
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{
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public readonly FrequencyTable frequencies;
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public readonly Context[] subcontexts;
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public Context(int symbols, bool hasSubctx)
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{
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frequencies = new SimpleFrequencyTable(new int[symbols]);
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if (hasSubctx)
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{
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subcontexts = new Context[symbols];
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}
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else
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{
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subcontexts = null;
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}
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}
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}
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}
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