Various small PPM-related refactorings/simplifications

java/src/PpmCompress.java

@@ -13,7 +13,6 @@
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
-import java.util.Arrays;
 
 
 /**
@@ -53,7 +52,7 @@ static void compress(InputStream in, BitOutputStream out) throws IOException {
 // its frequency is 1 in the order -1 context but its frequency
 // is 0 in all other contexts (which have non-negative order).
 ArithmeticEncoder enc = new ArithmeticEncoder(32, out);
-PpmModel model = new PpmModel(MODEL_ORDER, 257, 256);
+PpmModel model = new PpmModel(MODEL_ORDER, 256);
 int[] history = new int[0];
 
 while (true) {
@@ -63,17 +62,10 @@ static void compress(InputStream in, BitOutputStream out) throws IOException {
 break;
 encodeSymbol(model, history, symbol, enc);
 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);
-System.arraycopy(history, 0, history, 1, history.length - 1);
-history[0] = symbol;
-}
+history = model.addToHistory(history, symbol);
 }
 
-encodeSymbol(model, history, 256, enc); // EOF
+encodeSymbol(model, history, model.escapeSymbol, enc); // EOF
 enc.finish(); // Flush remaining code bits
 }
 
@@ -87,18 +79,16 @@ private static void encodeSymbol(PpmModel model, int[] history, int symbol, Arit
 for (int order = history.length; order >= 0; order--) {
 PpmModel.Context ctx = model.rootContext;
 for (int i = 0; i < order; i++) {
-if (ctx.subcontexts == null)
-throw new AssertionError();
-ctx = ctx.subcontexts[history[i]];
+ctx = ctx.getSubcontexts()[history[i]];
 if (ctx == null)
 continue outer;
 }
-if (symbol != 256 && ctx.frequencies.get(symbol) > 0) {
+if (symbol != model.escapeSymbol && ctx.frequencies.get(symbol) > 0) {
 enc.write(ctx.frequencies, symbol);
 return;
 }
 // Else write context escape symbol and continue decrementing the order
-enc.write(ctx.frequencies, 256);
+enc.write(ctx.frequencies, model.escapeSymbol);
 }
 // Logic for order = -1
 enc.write(model.orderMinus1Freqs, symbol);

java/src/PpmDecompress.java

@@ -13,7 +13,6 @@
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
-import java.util.Arrays;
 
 
 /**
@@ -51,24 +50,17 @@ static void decompress(BitInputStream in, OutputStream out) throws IOException {
 // 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);
+PpmModel model = new PpmModel(MODEL_ORDER, 256);
 int[] history = new int[0];
 
 while (true) {
 // Decode and write one byte
 int symbol = decodeSymbol(dec, model, history);
-if (symbol == 256) // EOF symbol
+if (symbol == model.escapeSymbol) // EOF symbol
 break;
 out.write(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);
-System.arraycopy(history, 0, history, 1, history.length - 1);
-history[0] = symbol;
-}
+history = model.addToHistory(history, symbol);
 }
 }
 
@@ -81,14 +73,12 @@ private static int decodeSymbol(ArithmeticDecoder dec, PpmModel model, int[] his
 for (int order = history.length; order >= 0; order--) {
 PpmModel.Context ctx = model.rootContext;
 for (int i = 0; i < order; i++) {
-if (ctx.subcontexts == null)
-throw new AssertionError();
-ctx = ctx.subcontexts[history[i]];
+ctx = ctx.getSubcontexts()[history[i]];
 if (ctx == null)
 continue outer;
 }
 int symbol = dec.read(ctx.frequencies);
-if (symbol < 256)
+if (symbol < model.escapeSymbol)
 return symbol;
 // Else we read the context escape symbol, so continue decrementing the order
 }

java/src/PpmModel.java

@@ -7,14 +7,16 @@
  */
 
 
+import java.util.Arrays;
+
 final class PpmModel {
 
 /*---- Fields ----*/
 
 public final int modelOrder;
 
 private final int symbolLimit;
-private final int escapeSymbol;
+final int escapeSymbol;
 
 public final Context rootContext;
 public final FrequencyTable orderMinus1Freqs;
@@ -23,16 +25,15 @@ final class PpmModel {
 
 /*---- Constructors ----*/
 
-public PpmModel(int order, int symbolLimit, int escapeSymbol) {
-if (order < -1 || symbolLimit <= 0 || escapeSymbol < 0 || escapeSymbol >= symbolLimit)
+public PpmModel(int order, int numSymbols) {
+if (order < -1 || numSymbols <= 1)
 throw new IllegalArgumentException();
 this.modelOrder = order;
-this.symbolLimit = symbolLimit;
-this.escapeSymbol = escapeSymbol;
+this.symbolLimit = numSymbols + 1;
+this.escapeSymbol = numSymbols;
 
 if (order >= 0) {
-rootContext = new Context(symbolLimit, order >= 1);
-rootContext.frequencies.increment(escapeSymbol);
+rootContext = new Context(symbolLimit);
 } else
 rootContext = null;
 orderMinus1Freqs = new FlatFrequencyTable(symbolLimit);
@@ -50,22 +51,27 @@ public void incrementContexts(int[] history, int symbol) {
 
 Context ctx = rootContext;
 ctx.frequencies.increment(symbol);
-int i = 0;
 for (int sym : history) {
-Context[] subctxs = ctx.subcontexts;
-if (subctxs == null)
-throw new AssertionError();
+Context[] subctxs = ctx.getSubcontexts();
 
 if (subctxs[sym] == null) {
-subctxs[sym] = new Context(symbolLimit, i + 1 < modelOrder);
-subctxs[sym].frequencies.increment(escapeSymbol);
+subctxs[sym] = new Context(symbolLimit);
 }
 ctx = subctxs[sym];
 ctx.frequencies.increment(symbol);
-i++;
 }
 }
 
+public int[] addToHistory(int[] history, int symbol) {
+if (modelOrder >= 1) {
+// Prepend current symbol, dropping oldest symbol if necessary
+if (history.length < modelOrder)
+history = Arrays.copyOf(history, history.length + 1);
+System.arraycopy(history, 0, history, 1, history.length - 1);
+history[0] = symbol;
+}
+return history;
+}
 
 
 /*---- Helper structure ----*/
@@ -74,17 +80,19 @@ public static final class Context {
 
 public final FrequencyTable frequencies;
 
-public final Context[] subcontexts;
+private Context[] subcontexts;
 
-
-public Context(int symbols, boolean hasSubctx) {
+public Context(int symbols) {
 frequencies = new SimpleFrequencyTable(new int[symbols]);
-if (hasSubctx)
-subcontexts = new Context[symbols];
-else
-subcontexts = null;
+frequencies.increment(symbols - 1);
 }
 
+public Context[] getSubcontexts() {
+if (subcontexts == null) {
+subcontexts = new Context[frequencies.getSymbolLimit()];
+}
+return subcontexts;
+}
 }
 
 }