Store AV1 cumulative probabilities in 16 bits

src/ImageSharp/Formats/Heif/Av1/Entropy/Av1Distribution.cs

@@ -34,9 +34,9 @@ internal sealed class Av1Distribution
     private const int ProbabilityBitCount = 15;
 
     /// <summary>
-    /// The inverse cumulative thresholds followed by the required zero sentinel.
+    /// The Q15 inverse cumulative thresholds followed by the required zero sentinel.
     /// </summary>
-    private InlineArray16<uint> probabilities;
+    private InlineArray16<ushort> probabilities;
 
     /// <summary>
     /// The symbol-count contribution to the adaptive update rate.
@@ -273,13 +273,13 @@ internal sealed class Av1Distribution
     /// <param name="speed">The symbol-count contribution to the update rate.</param>
     private Av1Distribution(ReadOnlySpan<uint> props, int speed)
     {
-        Span<uint> probabilities = this.probabilities;
+        Span<ushort> probabilities = this.probabilities;
 
-        // AV1 range coding consumes inverse cumulative thresholds. The defaults are written in the more readable
+        // Q15 thresholds range from zero through 32768, including zero-mass intervals, and fit in unsigned 16-bit storage.
-        // forward form, so convert every real threshold while leaving the final zero sentinel untouched.
+        // Convert the forward defaults before narrowing; the final zero sentinel is already initialized.
         for (int i = 0; i < props.Length - 1; i++)
         {
-            probabilities[i] = ProbabilityTop - props[i];
+            probabilities[i] = (ushort)(ProbabilityTop - props[i]);
         }
 
         this.NumberOfSymbols = props.Length;
@@ -292,8 +292,8 @@ internal sealed class Av1Distribution
     /// <param name="source">The distribution state to copy.</param>
     private Av1Distribution(Av1Distribution source)
     {
-        ReadOnlySpan<uint> sourceProbabilities = source.probabilities;
+        ReadOnlySpan<ushort> sourceProbabilities = source.probabilities;
-        Span<uint> probabilities = this.probabilities;
+        Span<ushort> probabilities = this.probabilities;
         sourceProbabilities[..source.NumberOfSymbols].CopyTo(probabilities);
 
         // The adaptation rate depends on both the alphabet size and prior update count, so copying only the
@@ -329,8 +329,8 @@ internal sealed class Av1Distribution
     {
         // Entropy contexts are created from the same fixed default table shape. Copy only mutable state so resetting a
         // working tile never allocates or replaces the distribution objects referenced by the symbol decoder.
-        ReadOnlySpan<uint> sourceProbabilities = source.probabilities;
+        ReadOnlySpan<ushort> sourceProbabilities = source.probabilities;
-        Span<uint> probabilities = this.probabilities;
+        Span<ushort> probabilities = this.probabilities;
         sourceProbabilities[..source.NumberOfSymbols].CopyTo(probabilities);
         this.updateCount = source.updateCount;
     }
@@ -402,7 +402,8 @@ internal sealed class Av1Distribution
         int tmp = ProbabilityTop;
 
         // Switching tmp to zero at the observed symbol moves the thresholds on either side toward the sample while
-        // preserving their inverse-cumulative ordering in one pass.
+        // preserving their inverse-cumulative ordering in one pass. Arithmetic stays wide until the stored update;
+        // each step moves toward zero or 32768, so narrowing cannot discard a significant probability bit.
         for (int i = 0; i < this.NumberOfSymbols - 1; i++)
         {
             tmp = i == value ? 0 : tmp;