Add tests for ReadUnsignedVariableLength()

src/ImageSharp/Formats/Heif/Av1/Av1BitStreamReader.cs

@@ -70,8 +70,14 @@ internal ref struct Av1BitStreamReader
     {
         // See section 4.10.3 of the AV1-Specification
         int leadingZerosCount = 0;
-        while (leadingZerosCount < 32 && !this.ReadBoolean())
+        while (leadingZerosCount < 32)
         {
+            uint bit = this.ReadLiteral(1);
+            if (bit == 1)
+            {
+                break;
+            }
+
             leadingZerosCount++;
         }
 
@@ -80,9 +86,14 @@ internal ref struct Av1BitStreamReader
             return uint.MaxValue;
         }
 
-        uint basis = (1U << leadingZerosCount) - 1U;
-        uint value = this.ReadLiteral(leadingZerosCount);
-        return basis + value;
+        if (leadingZerosCount != 0)
+        {
+            uint basis = (1U << leadingZerosCount) - 1U;
+            uint value = this.ReadLiteral(leadingZerosCount);
+            return basis + value;
+        }
+
+        return 0;
     }
 
     public uint ReadNonSymmetric(uint n)

tests/ImageSharp.Tests/Formats/Heif/Av1/Av1BitStreamTests.cs

@@ -267,6 +267,23 @@ public class Av1BitStreamTests
         Assert.Equal(expected, actual);
     }
 
+    [Theory]
+    [InlineData(new byte[] { 0x80 }, 0)] // Zero bit value.
+    [InlineData(new byte[] { 0x60 }, 2)] // One bit value, 011.
+    [InlineData(new byte[] { 0x38 }, 6)] // Two bit value, 00111.
+    [InlineData(new byte[] { 0x00, 0x00, 0x00, 0x01, 0xFF, 0xFF, 0xFF, 0xFF, 0xFE }, uint.MaxValue - 1)] // 31 bit value.
+    public void ReadUnsignedVariableLength(byte[] buffer, uint expected)
+    {
+        // arrange
+        Av1BitStreamReader reader = new(buffer);
+
+        // act
+        uint actual = reader.ReadUnsignedVariableLength();
+
+        // assert
+        Assert.Equal(expected, actual);
+    }
+
     [Theory]
     [InlineData(5, 6, 4, -7, -2)]
     [InlineData(7, 26, -8, -19, -26)]