PartitionType symbol round trips

src/ImageSharp/Formats/Heif/Av1/Av1Decoder.cs

@@ -205,7 +205,7 @@ internal class Av1Decoder : IAv1TileDecoder
         else if (hasRows && hasColumns)
         {
             int ctx = this.GetPartitionContext(rowIndex, columnIndex, blockSize);
-            partitionType = reader.ReadPartitionSymbol(ctx);
+            partitionType = reader.ReadPartitionType(ctx);
         }
         else if (hasColumns)
         {

src/ImageSharp/Formats/Heif/Av1/Symbol/Av1SymbolDecoder.cs

@@ -32,7 +32,7 @@ internal ref struct Av1SymbolDecoder
         return r.ReadSymbol(this.tileIntraBlockCopy) > 0;
     }
 
-    public Av1PartitionType ReadPartitionSymbol(int context)
+    public Av1PartitionType ReadPartitionType(int context)
     {
         ref Av1SymbolReader r = ref this.reader;
         return (Av1PartitionType)r.ReadSymbol(this.tilePartitionTypes[context]);

src/ImageSharp/Formats/Heif/Av1/Symbol/Av1SymbolEncoder.cs

@@ -8,14 +8,18 @@ namespace SixLabors.ImageSharp.Formats.Heif.Av1.Symbol;
 internal class Av1SymbolEncoder : IDisposable
 {
     private readonly Av1Distribution tileIntraBlockCopy = Av1DefaultDistributions.IntraBlockCopy;
+    private readonly Av1Distribution[] tilePartitionTypes = Av1DefaultDistributions.PartitionTypes;
 
     private Av1SymbolWriter? writer;
 
     public Av1SymbolEncoder(Configuration configuration, int initialSize)
         => this.writer = new(configuration, initialSize);
 
-    public void WriteUseIntraBlockCopySymbol(bool value)
-        => this.writer!.WriteSymbol(value ? 1 : 0, this.tileIntraBlockCopy, 2);
+    public void WriteUseIntraBlockCopy(bool value)
+        => this.writer!.WriteSymbol(value ? 1 : 0, this.tileIntraBlockCopy);
+
+    public void WritePartitionType(Av1PartitionType value, int context)
+        => this.writer!.WriteSymbol((int)value, this.tilePartitionTypes[context]);
 
     public IMemoryOwner<byte> Exit() => this.writer!.Exit();
 

src/ImageSharp/Formats/Heif/Av1/Symbol/Av1SymbolWriter.cs

@@ -25,13 +25,13 @@ internal class Av1SymbolWriter : IDisposable
 
     public void Dispose() => this.memory.Dispose();
 
-    public void WriteSymbol(int symbol, Av1Distribution distribution, int numberOfSymbols)
+    public void WriteSymbol(int symbol, Av1Distribution distribution)
     {
         DebugGuard.MustBeGreaterThanOrEqualTo(symbol, 0, nameof(symbol));
-        DebugGuard.MustBeLessThan(symbol, numberOfSymbols, nameof(symbol));
-        DebugGuard.IsTrue(distribution[numberOfSymbols - 1] == 0, "Last entry in Probabilities table needs to be zero.");
+        DebugGuard.MustBeLessThan(symbol, distribution.NumberOfSymbols, nameof(symbol));
+        DebugGuard.IsTrue(distribution[distribution.NumberOfSymbols - 1] == 0, "Last entry in Probabilities table needs to be zero.");
 
-        this.EncodeIntegerQ15(symbol, distribution, numberOfSymbols);
+        this.EncodeIntegerQ15(symbol, distribution);
         distribution.Update(symbol);
     }
 
@@ -128,12 +128,8 @@ internal class Av1SymbolWriter : IDisposable
     /// [s > 0 ? (CDF_PROB_TOP - icdf[s - 1]) : 0, CDF_PROB_TOP - icdf[s]).
     /// The values must be monotonically non - increasing, and icdf[nsyms - 1] must be 0.
     /// </param>
-    /// <param name="numberOfSymbols">
-    /// The number of symbols in the alphabet.
-    /// This should be at most 16.
-    /// </param>
-    private void EncodeIntegerQ15(int symbol, Av1Distribution distribution, int numberOfSymbols)
-        => this.EncodeIntegerQ15(symbol > 0 ? distribution[symbol - 1] : Av1Distribution.ProbabilityTop, distribution[symbol], symbol, numberOfSymbols);
+    private void EncodeIntegerQ15(int symbol, Av1Distribution distribution)
+        => this.EncodeIntegerQ15(symbol > 0 ? distribution[symbol - 1] : Av1Distribution.ProbabilityTop, distribution[symbol], symbol, distribution.NumberOfSymbols);
 
     private void EncodeIntegerQ15(uint lowFrequency, uint highFrequency, int symbol, int numberOfSymbols)
     {

tests/ImageSharp.Tests/Formats/Heif/Av1/SymbolTest.cs

@@ -186,29 +186,35 @@ public class SymbolTest
         Assert.Equal(expectedValues, values);
     }
 
-    [Theory]
-    [InlineData(0, 255, 255, 255, 158)]
-
-    // [InlineData(1, 255, 255, 255, 158)]
-    // [InlineData(4, 255, 207, 254, 18)]
-    public void ReadPartitionTypeSymbols(int ctx, byte b0, byte b1, byte b2, byte b3)
+    [Fact]
+    public void RoundTripPartitionType()
     {
         // Assign
-        byte[] values = [b0, b1, b2, b3, 0];
-        Av1SymbolDecoder decoder = new(values);
-        Av1PartitionType[] expected = [
+        int ctx = 7;
+        Configuration configuration = Configuration.Default;
+        Av1SymbolEncoder encoder = new(configuration, 100 / 8);
+        Av1PartitionType[] values = [
             Av1PartitionType.Split, Av1PartitionType.Split, Av1PartitionType.Split, Av1PartitionType.None,
             Av1PartitionType.Split, Av1PartitionType.Split, Av1PartitionType.None, Av1PartitionType.None ];
-        Av1PartitionType[] actuals = new Av1PartitionType[expected.Length];
+        Av1PartitionType[] actuals = new Av1PartitionType[values.Length];
 
         // Act
-        for (int i = 0; i < expected.Length; i++)
+        foreach (Av1PartitionType value in values)
+        {
+            encoder.WritePartitionType(value, 7);
+        }
+
+        using IMemoryOwner<byte> encoded = encoder.Exit();
+
+        Av1SymbolDecoder decoder = new(encoded.GetSpan());
+        Av1SymbolReader reader = new(encoded.GetSpan());
+        for (int i = 0; i < values.Length; i++)
         {
-            actuals[i] = decoder.ReadPartitionSymbol(ctx);
+            actuals[i] = decoder.ReadPartitionType(ctx);
         }
 
         // Assert
-        Assert.Equal(expected, actuals);
+        Assert.Equal(values, actuals);
     }
 
     [Fact]
@@ -223,7 +229,7 @@ public class SymbolTest
         // Act
         foreach (bool value in values)
         {
-            encoder.WriteUseIntraBlockCopySymbol(value);
+            encoder.WriteUseIntraBlockCopy(value);
         }
 
         using IMemoryOwner<byte> encoded = encoder.Exit();