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/*

* Reference arithmetic coding

* Copyright (c) Project Nayuki

*

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

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

*/

final class PpmModel {

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

public final int modelOrder;

private final int symbolLimit;

private final int escapeSymbol;

public final Context rootContext;

public final FrequencyTable orderMinus1Freqs;

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

public PpmModel(int order, int symbolLimit, int escapeSymbol) {

if (order < -1 || symbolLimit <= 0 || escapeSymbol < 0 || escapeSymbol >= symbolLimit)

throw new IllegalArgumentException();

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 IllegalArgumentException();

Context ctx = rootContext;

ctx.frequencies.increment(symbol);

int i = 0;

for (int sym : history) {

Context[] subctxs = ctx.subcontexts;

if (subctxs == null)

throw new AssertionError();

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 static final class Context {

public final FrequencyTable frequencies;

public final Context[] subcontexts;

public Context(int symbols, boolean hasSubctx) {

frequencies = new SimpleFrequencyTable(new int[symbols]);

if (hasSubctx)

subcontexts = new Context[symbols];

else

subcontexts = null;

}

}

}